1956 Fender Champ High frequency problems

Hello folks
Sorry for my English.
Just got the 1956 Fender Champ 5F1 and have High frequency problems.
The amp sounds dull . Converted to 5E1 with Heyboer chocke.
All caps are new from Spargue and FT.
OT is original from RCA but with 4(yellow) and 8ohm( green)wires.
Voltage is OK, 308v B+
One input channel is disconnected.
Tried many speakers, Magnavox, Heppner,Jensen and always got luck of highs.
Connected the speakers to the Silver face Fender and it sounds much better.
please,Need help!
see pics

New tubes?

New tubes?
YES, all NOS BRIMAR, I guess it could be OT , maybe primary current is to high?

What do you mean by 'high frequency problems'? Could you please be a little more specific? Can you post/link to a soundbyte of it somewhere?

What do you mean by 'high frequency problems'? Could you please be a little more specific? Can you post/link to a soundbyte of it somewhere?
It sounds very dull, like High frequency part was cut . But the low bottom is very present and always sounds very natural . I red about OT current, the people says to hight current is bad for OT.

And you tried another new output tube? (Beware that not all NOS tubes are what they're cracked up to be)

Yes, i tried, the same results. It sound like the tone control of highs were turned down.But here is only volume control, sure

Maybe your replacement coupling cap values are wrong (too big)? You want 22nF. I can't tell from your pics what the values of the big green thing and the smaller silvery thing are - the pics are too blurry to see the markings

Hi!! I'm at the beginning of my journey.
Looking at your pics, where the plate (6) of your 12ax7 connects to the eyelet, it looks like nothing is soldered with the rest, like someone forgot to solder there. Maybe you're not making a connection there. Maybe you took the pic while doing work. But if not, check that.
Also, you mentioned trying several speakers but with one it sounded good, correct? I can tell you that if they are all old worn out speakers and stored poorly they cooooouuld (big could) all just be bad?
On a build of mine I tried two bad old speakers (cause they were free/cheap), but the symptom was just a very low flabby output.

Maybe your replacement coupling cap values are wrong (too big)? You want 22nF. I can't tell from your pics what the values of the big green thing and the smaller silvery thing are - the pics are too blurry to see the markings
Caps Ok 0.022UF and 0.01Uf the small one

Hi!! I'm at the beginning of my journey.
Looking at your pics, where the plate (6) of your 12ax7 connects to the eyelet, it looks like nothing is soldered with the rest, like someone forgot to solder there. Maybe you're not making a connection there. Maybe you took the pic while doing work. But if not, check that.
Also, you mentioned trying several speakers but with one it sounded good, correct? I can tell you that if they are all old worn out speakers and stored poorly they cooooouuld (big could) all just be bad?
On a build of mine I tried two bad old speakers (cause they were free/cheap), but the symptom was just a very low flabby output.

it just the picture ,all soldering is correct.I guess it might be OT problem :roll:
Speaker are ok, sound is very full and loud, tested with the fender silverface amp

it just the picture ,all soldering is correct.I guess it might be OT problem :roll:
Speaker are ok, sound is very full and loud, tested with the fender silverface amp

Oh sorry, I misunderstood. I thought you plugged the champ into the silver face amp speaker.

Is that a vintage amp that someone rebuilt?

Since it has already been modified, it would be greatly improved (more user-friendly by being duly serviced) with some new clean wiring, general clean up and lead dress. Perhaps some new caps and test the resistors.

It would not be a difficult task. There are but 12 resistors and seven caps in the whole circuit.

PM me if you want parts. I can ship parts to Europe and Asia via Priority Mail in a padded envelope for $16.95 (cost of shipping, parts exta). I have a good deal on a parts kit. See Vendor Forum on this site.

Champ requires a 4-ohm speaker load, check.

If you have swapped the OT or changed the NFB circuit, you may have the wrong value NFB resistor for the OT you are using. Disconnect the NFB resistor for a minute and see if that changes the game (NB It will have more gain with the NFB disconnected).

If you have swapped the OT or changed the NFB circuit, you may have the wrong value NFB resistor for the OT you are using. Disconnect the NFB resistor for a minute and see if that changes the game (NB It will have more gain with the NFB disconnected).
You mean those two resistors in series that goes to the speaker jack ?

The NFB resistor goes between the OT secondary winding and the cathode of the pre-amp stage that is driving the output tube (i.e. In a 5F1, its the 22k that connects to with the 1k5 cathode resistor on one side, and the speaker jack tip on the other side).

The one big fat resistor on your speaker jack is a failsafe in case you power up with no speaker connected.

The other is part of the NFB circuit as tubeswell explains.

The NFB resistor goes between the OT secondary winding and the cathode of the pre-amp stage that is driving the output tube (i.e. In a 5F1, its the 22k that connects to with the 1k5 cathode resistor on one side, and the speaker jack tip on the other side).

Thank You, just wanted be sure. Yes I tried it before, disconnected NFB, It sounds louder and fuller , with more bass but not significant changes in high frequency area. But the RCA OT has had a reversed wire , I mean this blue and red wires to 6V6. I rewired it as it must be and rewired the secondary black and green wire.So the black wire is now hot and the green (8ohm) goes to the ground. I would say it sounds right to me now, more balanced, some more highs( but still not as expected) and has better sustain now. So the OT has two 4 ( yellow) and 8 (green)secondary wires what is very seldom for the mid 50´s Fender Champ. RCA OT looks like original 50´s OT. The next question: can I change the output impedance of the OT when I change the Green or Yellow wire that goes to the ground now? Cause the black wire is hot now .

The critical thing in determining the reflected load of the OT is the ratio of the number of turns (Pri:Sec). Therefore as long as you have the correct ratio of number of turns between the chosen ends of the secondary to the number of turns between the ends of the primary, the OT will reflected the desired load to the output tube(s). (If you employ global NFB (from the OT secondary), the signal phase is also going to be important.)

The critical thing in determining the reflected load of the OT is the ratio of the number of turns (Pri:Sec). Therefore as long as you have the correct ratio of number of turns between the chosen ends of the secondary to the number of turns between the ends of the primary, the OT will reflected the desired load to the output tube(s). (If you employ global NFB (from the OT secondary), the signal phase is also going to be important.)

Sorry, my English has switched off now :razz: would You mind describe it on the simple way?

The critical thing in determining the reflected load of the OT is the ratio of the number of turns (Pri:Sec). Therefore as long as you have the correct ratio of number of turns between the chosen ends of the secondary to the number of turns between the ends of the primary, the OT will reflected the desired load to the output tube(s). (If you employ global NFB (from the OT secondary), the signal phase is also going to be important.)
PS my question was : can I change the output impedance of the OT when I change the Green or Yellow wire that goes to the ground now? Cause the black wire goes hot now .

Yes - that was my answer. The reflected load of a transformer is in proportion to the ratio of Pri:Sec turns. If you have x turns between black and green, or between black and yellow, you will still have x turns if you swap the leads around. Provided that the Pri:Sec signal phase is still the same (i.e.: provided you swap both Pri and Sec leads around) you won't have any issue with the global NFB loop either.

Wait! I think I know this one.


When I say "I've tried everything!" I mean I tried everything. I've built at least three of these with vintage RCA output transformers. I hate to admit it because most of RCA's old stuff is the cream of the crop...

...except for those RCA output transformers. Yuck!


If I had to pick out my favorite vintage "Champ" OT that isn't a Triad or a Schumacher I'd go for the OT from a '58 to '63 Revere tape recorder. They used a 6V6 output through Midwest iron if I remember correctly.


If I had to choose between Triad ('50s) and Schumacher ('60s) I'd go with Schumacher.


5F1 Champ notes:

These get thicker and darker as you turn them up.

They get "breathless and outta shape" all the way up. If you've crankin' it to "12" you're gone two or three clicks too far. It doesn't get louder past "9".

Those two inputs are mandatory. It's only a difference of 6db from one input to the other. That 6db makes all the difference in the world sometimes.


Specifics: The speaker swap is a big clue. Plug it into extension speakers until you find one you like. In Germany I'd try an old Grundig or Telefunken. Weird speakers are your friends! Don't be afraid of oval speakers.

V1a is bypassed on your amp. '60s Champs came with bypass caps, some '50s Champs didn't. Lift that bypass cap and see if you like it.


Summary: Lift the bypass cap. Ditch the dull sounding OT and try something else. Wire the other input. Don't crank it all the way up. If it sounds better through a 12" speaker use a 12" speaker.

Try these one at a time so you hear what each one does. I'd eventually do all of 'em.

Wait! I think I know this one.


When I say "I've tried everything!" I mean I tried everything. I've built at least three of these with vintage RCA output transformers. I hate to admit it because most of RCA's old stuff is the cream of the crop...

...except for those RCA output transformers. Yuck!


If I had to pick out my favorite vintage "Champ" OT that isn't a Triad or a Schumacher I'd go for the OT from a '58 to '63 Revere tape recorder. They used a 6V6 output through Midwest iron if I remember correctly.


If I had to choose between Triad ('50s) and Schumacher ('60s) I'd go with Schumacher.


5F1 Champ notes:

These get thicker and darker as you turn them up.

They get "breathless and outta shape" all the way up. If you've crankin' it to "12" you're gone two or three clicks too far. It doesn't get louder past "9".

Those two inputs are mandatory. It's only a difference of 6db from one input to the other. That 6db makes all the difference in the world sometimes.


Specifics: The speaker swap is a big clue. Plug it into extension speakers until you find one you like. In Germany I'd try an old Grundig or Telefunken. Weird speakers are your friends! Don't be afraid of oval speakers.

V1a is bypassed on your amp. '60s Champs came with bypass caps, some '50s Champs didn't. Lift that bypass cap and see if you like it.


Summary: Lift the bypass cap. Ditch the dull sounding OT and try something else. Wire the other input. Don't crank it all the way up. If it sounds better through a 12" speaker use a 12" speaker.

Try these one at a time so you hear what each one does. I'd eventually do all of 'em.

Will try. Thank You a lot!

Yes - that was my answer. The reflected load of a transformer is in proportion to the ratio of Pri:Sec turns. If you have x turns between black and green, or between black and yellow, you will still have x turns if you swap the leads around. Provided that the Pri:Sec signal phase is still the same (i.e.: provided you swap both Pri and Sec leads around) you won't have any issue with the global NFB loop either.

Thank You , will try to change OT , (2-3 weeks) will reply you later

Nine out of ten it's the NOS tubes.
Most of these are overpriced crap even worse than el cheapo new Chinese tubes. Most NOS tubes have spent years in the worst possible storage for tubes or they are used junk stuffed in old boxes and sold as never used.

The NFB resistor goes between the OT secondary winding and the cathode of the pre-amp stage that is driving the output tube (i.e. In a 5F1, its the 22k that connects to with the 1k5 cathode resistor on one side, and the speaker jack tip on the other side).

Hello again,
Just checked the voltage between 1k5 and 22k resistors on NFB = 1,4V but must be 1,5V as by Fender 5E1 schematic. Its ok or to low? Also on the first 1k5 resistor and .02uf caps gives me 1,3v instead of 1,5v as by original 5E1
thanks again for reply and support. Also the resistors measurements on NFB gives me 1,55k and 24k. so it looks like OK.
PS: so 22k resistor on NFB shows me 24k if I separated it from the board but in soldered position it shows me 1,5k, the same like the the first 1,5k resistor of NFB its OK or not? Thank You.

If the NFB resistor is 22k, then the amount of NFB being inserted back into that driver stage (via the cathode) should be okay if you are using a 'stock' spec 5F1 OT.

The NFB loop feeds back a little of the AC signal produced at the OT secondary winding into the pre-amp, in order to improve the fidelity of the amp. But if you don't have the 'right amount' of NFB signal being fed back, it can screw around with the signal (or even cause oscillation).

The 22k, in conjunction with the 1k5 (which doubles as the cathode resistor for the pre-amp driver stage), acts as a voltage divider for the NFB signal - knocking the strength of the AC signal down to about the right level. However as to whether its 'right' or not also depends on the Pri:Sec turns ratio of the OT.

A stock 5F1 with an 8R speaker hooked up to an OT with a ~1000:1 impedance ratio (~31.62:1 turns ratio) produces the 8k reflected load for the 6V6, but it also produces an AC swing on the secondary winding that is 31.62 times smaller than the AC signal on the primary winding. The 22k/1k5 voltage divider in the NFB loop knocks this AC voltage signal further down (by a factor of fifteen or so) so it is only 6.4% of the strength of the AC signal at the OT secondary. This relatively tiny NFB signal (which occurs at the 'knee' of the 22k/1k5 voltage divider (i.e.: and also at the cathode of the pre-amp driver stage, which s directly connected to this 'knee') is in opposite phase to the voltage swing that is being produced on the cathode by the normal functioning of the pre-amp driver stage, and is the right strength to help 'clean up' the main signal.

Now if your OT is not a stock 5F1 OT, but is wound to give (say) a 500:1 impedance ratio (i.e.: requiring a 16R speaker to get an 8k reflected load), or a 2000:1 impedance ratio (i.e.: requiring a 4R speaker to get an 8k reflected load), then the turns ratio of the OT is going to be different either way (i.e.: 22.36:1 or 44.72:1 respectively). If the OT is different, then the OT secondary will produce a different AC signal voltage for a given amount of primary signal voltage, (which will in-turn affect the strength of the NFB signal). Therefore in order to get the relatively same amount of signal at the knee of the NFB voltage divider as a stock 5F1, you would then need to change the NFB resistor accordingly.

Is the OT a 5F1 spec OT? Or has it got a different OT turns ratio? (or, on another matter, is an OT with bigger iron? - bigger iron = more bass)

The OT has A LOT(!) of bass and it looks also bigger then the stock champ OT as I can see from the pics, it has 4 and 8 ohm output impedance wires , but it fits exactly the board and looks pretty original, the soldering looks like it was soldered on the factory. Its is RCA OT. Also i tried to put 1,2 K resistor instead of 1,5K NFB and it sounds even with more buss and muddy.What is Your advise? should I change 1k5 and 22k resistor only? how much higher apr. ?
thanks, Vic

With a bigger iron, that OT will be transforming somewhat more bass frequencies than typical for a stock OT. I'd start by reducing the coupling caps and/or cathode bypass caps in the pre-amp. (You might need to be quite 'drastic' with the reduction 1.e. 1/2 or 1/4 or 1/8th or even 1/20th of the typical values - but try tweaking it one bit at a time).

If you're going to experiment with the NFB loop, don't change the lower (1k5) resistor; try changing the 22k resistor instead to begin with. Another thing you could do, is convert the NFB loop to a 'blackface' spec champ NFB (look at the vibrochamp - uses a 2k7 NFb resistor and 47R in the leg, and then the pre-amps 1k5 cathode resistor sits on top of the 47R - and the 1k5 is bypassed. The beauty of that is that you can reduce the bypass capacitance value in order to get the desired bass rolloff for that stage more easily)

With a bigger iron, that OT will be transforming somewhat more bass frequencies than typical for a stock OT. I'd start by reducing the coupling caps and/or cathode bypass caps in the pre-amp. (You might need to be quite 'drastic' with the reduction 1.e. 1/2 or 1/4 or 1/8th or even 1/20th of the typical values - but try tweaking it one bit at a time).

If you're going to experiment with the NFB loop, don't change the lower (1k5) resistor; try changing the 22k resistor instead to begin with. Another thing you could do, is convert the NFB loop to a 'blackface' spec champ NFB (look at the vibrochamp - uses a 2k7 NFb resistor and 47R in the leg, and then the pre-amps 1k5 cathode resistor sits on top of the 47R - and the 1k5 is bypassed. The beauty of that is that you can reduce the bypass capacitance value in order to get the desired bass rolloff for that stage more easily)
"try changing the 22k resistor " should i go higher ? 30K or more ?

With a bigger iron, that OT will be transforming somewhat more bass frequencies than typical for a stock OT. I'd start by reducing the coupling caps and/or cathode bypass caps in the pre-amp. (You might need to be quite 'drastic' with the reduction 1.e. 1/2 or 1/4 or 1/8th or even 1/20th of the typical values - but try tweaking it one bit at a time).

If you're going to experiment with the NFB loop, don't change the lower (1k5) resistor; try changing the 22k resistor instead to begin with. Another thing you could do, is convert the NFB loop to a 'blackface' spec champ NFB (look at the vibrochamp - uses a 2k7 NFb resistor and 47R in the leg, and then the pre-amps 1k5 cathode resistor sits on top of the 47R - and the 1k5 is bypassed. The beauty of that is that you can reduce the bypass capacitance value in order to get the desired bass rolloff for that stage more easily)

PS: Also I guess the NFB dosn´t realy work properly I disconnected it again, its not louder or more bass just little more highs... hmm, the changing in sound are not significant , almost nothing...

Well then - that's possibly ruling the NFB strength out. The signal should be slightly cleaner and better fidelity with NFB, if the NFB loop is set up correctly. But that won't counteract the extra bass freqs you are getting from OT iron that is larger than usual.

So it would seem to be the result of the bigger-than-typical size of the OT iron. Time to experiment with coupling cap and/or cathode bypass cap values - or changing the NFB to a blackface vibrochamp style (so you can alter the frequency gain boost rolloff point for the pre-amp driver stage).

Well then - that's possibly ruling the NFB strength out. The signal should be slightly cleaner and better fidelity with NFB, if the NFB loop is set up correctly. But that won't counteract the extra bass freqs you are getting from OT iron that is larger than usual.

So it would seem to be the result of the bigger-than-typical size of the OT iron. Time to experiment with coupling cap and/or cathode bypass cap values - or changing the NFB to a blackface vibrochamp style (so you can alter the frequency gain boost rolloff point for the pre-amp driver stage).

Yes, with NFB sounds some cleaner. Also I have 16uf,10uf, 8uf 450v cups instead of 8uf,8uf,8uf like bei original 5e1 layot. I think I should buy the right OT. Would You mind to help me with the change to BF NFB vibrochamp. What I need exactly and were should I place it ? Thank You

Well then - that's possibly ruling the NFB strength out. The signal should be slightly cleaner and better fidelity with NFB, if the NFB loop is set up correctly. But that won't counteract the extra bass freqs you are getting from OT iron that is larger than usual.

So it would seem to be the result of the bigger-than-typical size of the OT iron. Time to experiment with coupling cap and/or cathode bypass cap values - or changing the NFB to a blackface vibrochamp style (so you can alter the frequency gain boost rolloff point for the pre-amp driver stage).

PS: Also i tried 3 alnico speakers all 8ohm. The 2 speakers are guitar speakers from Heppner and Magnavox, the third one is Hi-Fi speaker from the Fisher console and this third one has much more highs, looks like better bandwidth and with this speaker the Champ sound pretty well clean but by the overdrive it makes pretty bad distortions. I guess it cause of better bandwidth of the Hi-Fi speaker?

Yeah - hifi speakers can sound pretty crappy with geetar amps

Here is a schematic (minus the power tranny and rectifier) showing my suggestion about using a blackface NFB loop (so you can alter the bypass cap in order to boost higher frequencies). (It also shows an optional HT choke and extra filter cap for power supply ripple hum reduction - which you can simply omit)

I make this suggestion in case you want to try some modifications without actually going to the expense of changing to a 5F1 spec OT. This concept should give plenty of boost to the mid-high frequencies (depending on the value of the bypass cap that is chosen). JM2CW

Yeah - hifi speakers can sound pretty crappy with geetar amps
just found this
The Champ is supposed to have a 4 ohm speaker so what is happening is that the 4 ohms is seen through the transformer as 5000 ohms. The transformer multiplies the 4 ohms by 1250 to get 5000 ohms. If you were to hook an 8 ohm speaker up to a champ the 6V6 would see 10000 ohms. It would work, but the high impedance would mess up the higher frequencies and the amp would sound muffled. Also, less power would flow through the speaker (because of the higher resistance) and you’d get less sound.
http://www.harpamps.com/transformers.html
I guess its the problem I have... I have 8ohm speaker and connected it to green wire(8ohm) of the OT, I guess I have to change OT or find the 4ohm speaker...
You suggestions ?

You can run a 6V6 on 5k, you just won't get optimum power output. 10k is much closer to the optimum. Try it and see how you like it.

You can run a 6V6 on 5k, you just won't get optimum power output. 10k is much closer to the optimum. Try it and see how you like it.
Thank You, will try.
About OT... I understood that the simple OT like for the Champ is reflecting the impedance of the speaker and multiplying it. So 4ohm he multiply, in the common case for the Champ, as +/- 5000ohms (or depend of the turns ratio of prim.)Right? So when I hook up the 8 ohm speaker to 4ohm sec. OT makes 10000 ohm impedance and it might has the lack of highs and lowered power.?
But whats about if the sec. section has two wires for 4 ohm and 8ohm?
Will the primare section of OT makes 5000om for 4 and 8 ohm outputs either ?
Or will 8ohm sec increase the primare impedance of OT ?

Thank You!

If the OT in that amp of yours reflects 5k with a 4R speaker, then it must have an impedance ratio of 1,250:1. If you hook up 8R (instead of 4R) speaker to the same secondary winding, you'll get a 10k reflected load. If the secondary is multi-tapped, then the reflected load will depend on which tap you hook the speaker up to. Multi-tapped secondaries are designed so that if you hook up the correct speaker impedance to the correct tap, you'll get the same primary load reflected. (However, that is different from hooking up an 8R speaker to a '4R' tap)

The optimum load for your 6V6 in that amp is about 8k. If you run it at 5k it won't have as much power. If you run it at 10k it, will be more efficient than at 5k - and the amp will go into 'overdrive' sooner.

But the frequency response is also related to the amount of iron in the core. Where you have more iron, the transformer is more capable of handling inductance at lower frequencies, so the bass response will be boosted no matter which speaker impedance you run. Hence my suggestion about using a blackface type of NFB circuit at this stage - because the cost of adding a few extra resistors and a capacitor is a lot less than buying a 'period-correct' spec OT for your amp. (But that's your decision)

If the OT in that amp of yours reflects 5k with a 4R speaker, then it must have an impedance ratio of 1,250:1. If you hook up 8R (instead of 4R) speaker to the same secondary winding, you'll get a 10k reflected load. If the secondary is multi-tapped, then the reflected load will depend on which tap you hook the speaker up to. Multi-tapped secondaries are designed so that if you hook up the correct speaker impedance to the correct tap, you'll get the same primary load reflected. (However, that is different from hooking up an 8R speaker to a '4R' tap)

The optimum load for your 6V6 in that amp is about 8k. If you run it at 5k it won't have as much power. If you run it at 10k it, will be more efficient than at 5k - and the amp will go into 'overdrive' sooner.

But the frequency response is also related to the amount of iron in the core. Where you have more iron, the transformer is more capable of handling inductance at lower frequencies, so the bass response will be boosted no matter which speaker impedance you run. Hence my suggestion about using a blackface type of NFB circuit at this stage - because the cost of adding a few extra resistors and a capacitor is a lot less than buying a 'period-correct' spec OT for your amp. (But that's your decision)

If you run it at 5k it won't have as much power. If you run it at 10k it, will be more efficient than at 5k - and the amp will go into 'overdrive' sooner.
The guy in other Forum describe it opposite ,Im confused ,what is right ?
http://www.harpamps.com/transformers.html

"Fender Tweed type amps are supposed to have about 6500 ohms with an 8 ohm load. Fender black face amps have an impedance of 4300 at 8 ohms.
Older amps used the higher impedance to cut down on distortion at the cost of bandwidth and power. Newer Fender amps like the lower impedance for higher power but with some distortion."

"Fender Tweed type amps are supposed to have about 6500 ohms with an 8 ohm load. Fender black face amps have an impedance of 4300 at 8 ohms.

If you look at the fender tweed amp OTs on the market, you will see that most of the OTs for 6V6 amps are designed for 8k reflected load. In reality, you can run them anywhere between 5k and 10k, and 7k5 to 8k2 is about optimal.

If you look at the fender tweed amp OTs on the market, you will see that most of the OTs for 6V6 amps are designed for 8k reflected load. In reality, you can run them anywhere between 5k and 10k, and 7k5 to 8k2 is about optimal.

But what gives more power 8k or 5k prim OT? I mean the guy in the other topic tells different thing as You. : ) please look below ...... Thank You.
"Fender Tweed type amps are supposed to have about 6500 ohms with an 8 ohm load. Fender black face amps have an impedance of 4300 at 8 ohms.
Older amps used the higher impedance to cut down on distortion at the cost of bandwidth and power. Newer Fender amps like the lower impedance for higher power but with some distortion."

The highest output power is delivered from an optimum load. 8k is optimum for a 6V6 in a 5F1. 10k is not much higher - try it and see

The highest output power is delivered from an optimum load. 8k is optimum for a 6V6 in a 5F1. 10k is not much higher - try it and see
OK, thank you! the worses thing is the incorrect information from the topics : )
I´m very thankful to You for helping me!
Maybe the last Question: My PT has 65mA, most OT have 40mA or 50mA max.
The people says in topics that to much current for OT makes less bandwidth, its true? Will the OT really see 65mA from PT,will the 65mA from PT "arrive" the OT or some mA will going "lost on the way"?
Thanks again, : )
Vic

In a cathode-biased SE amp with one 6V6, the 6V6 is biased to idle at 100%. We say it runs in 'Class A' mode because the current through the tube is continually 'on' for each part of the sine (signal) wave. For a 6V6 in Class A mode, the plate current under idle conditions might be about 36mA-38mA. (You should determine what your 6V6 is idling at by proper measurement of the plate current. Alternatively you could estimate plate current from measurement of the tube current - through the cathode resistor - then subtraction of about 2mA to allow for the screen current).

Now when a signal is put through the tube and on the 'down-swing' part of the signal cycle (when plate voltage swings down), the plate current increases. Under 'ideal' conditions with the optimum load, the bias point (idle voltage) is about 1/2 of what the maximum voltage could be, and so we might suppose (from looking at a load line) that we would see 2 x the idle plate current if the plate voltage swung down all the way to a 'bottom peak' of zero volts during signal conditions. However in reality, in an SE amp in Class A, for various reasons which I won't bore you with right now, the plate never swings to lower than about +50V (from idle) at maximum. Accordingly the maximum current draw you might get to see at the 6V6 plate is about 1.2 x the idle current when the plate voltage has swung to the lowest (nadir) voltage. If the idle current through the plate is 36mA, then the maximum plate current draw using this assumption would be about 1.2 x 36mA = 43.2mA, which is the maximum that the OT primary would see. This is converted into magnetising current in the core and thereby inducted and transformed into a proportionately higher current in the secondary winding.

In a cathode-biased SE amp with one 6V6, the 6V6 is biased to idle at 100%. We say it runs in 'Class A' mode because the current through the tube is continually 'on' for each part of the sine (signal) wave. For a 6V6 in Class A mode, the plate current under idle conditions might be about 36mA-38mA. (You should determine what your 6V6 is idling at by proper measurement of the plate current. Alternatively you could estimate plate current from measurement of the tube current - through the cathode resistor - then subtraction of about 2mA to allow for the screen current).

Now when a signal is put through the tube and on the 'down-swing' part of the signal cycle (when plate voltage swings down), the plate current increases. Under 'ideal' conditions with the optimum load, the bias point (idle voltage) is about 1/2 of what the maximum voltage could be, and so we might suppose (from looking at a load line) that we would see 2 x the idle plate current if the plate voltage swung down all the way to a 'bottom peak' of zero volts during signal conditions. However in reality, in an SE amp in Class A, for various reasons which I won't bore you with right now, the plate never swings to lower than about +50V (from idle) at maximum. Accordingly the maximum current draw you might get to see at the 6V6 plate is about 1.2 x the idle current when the plate voltage has swung to the lowest (nadir) voltage. If the idle current through the plate is 36mA, then the maximum plate current draw using this assumption would be about 1.2 x 36mA = 43.2mA, which is the maximum that the OT primary would see. This is converted into magnetising current in the core and thereby inducted and transformed into a proportionately higher current in the secondary winding.

OK, sorry,Its to complicated for me... Just assume, my PT has 65mA and the OT i want to buy has 40mA max. and it might be still OK ? Or better to buy the OT with the 50mA? Also I have the choke installed instead of 10k resistor. I´m not tech, just the musician with the very simple knowledge about the amps,it might be boring for You to explain those all stuff to me, sorry

the 40mA OT will give you a little more saturation, the 50mA one will be slightly cleaner. Either one will run okay for a 5F1 amp that uses 1 x 6V6 output tube.

BTW I'm just a muso myself. I stumbled into this hobby as a way of being able to make my own tube amps. As I've gotten deeper into it, my understanding has inevitably gotten more abstract of necessity. So when you ask me a question, that's the answer I come up with. There is no absolute good or bad in the zen of tube amps, only greater or lesser evils depending on how extreme you wanna get.

Just and aside, but nobody is bored, AFAIK! :grin:

Certainly not you, tubeswell! Man, you seem like you're just getting warmed up. :wink: While a lot of this is waaaay over my head, I'm surprised to say that I can (at least) get the gist.

And Oregon, I hope you get your amp to sound just like you want!

charisjapan

the 40mA OT will give you a little more saturation, the 50mA one will be slightly cleaner. Either one will run okay for a 5F1 amp that uses 1 x 6V6 output tube.

BTW I'm just a muso myself. I stumbled into this hobby as a way of being able to make my own tube amps. As I've gotten deeper into it, my understanding has inevitably gotten more abstract of necessity. So when you ask me a question, that's the answer I come up with. There is no absolute good or bad in the zen of tube amps, only greater or lesser evils depending on how extreme you wanna get.

OK I see, but how it works together ...65mA from PT and 40mA on OT?
I mean...the OT just will see only 40mA or he will have a "hard work" with the 65mA from PT with worse sound or how it works? : ) Please use the simple explanation as possible for the young Grateful Dead fan : )

The 65mA rating on the PT's HT (High Tension) winding is a recommended (usually conservative) rating from the factory as to how much current can be safely drawn from the winding without causing problems.

Similarly, the rating on the OT is a factory recommendation on how much current can be drawn through the primary winding without causing problems.

In either case, the ratings don't equate to the maximum current that will be drawn. The current draw is determined by the load (the tubes, and any DC resistance path) between the power supply and the ground return.

The 65mA rating on the PT's HT (High Tension) winding is a recommended (usually conservative) rating from the factory as to how much current can be safely drawn from the winding without causing problems.

Similarly, the rating on the OT is a factory recommendation on how much current can be drawn through the primary winding without causing problems.

In either case, the ratings don't equate to the maximum current that will be drawn. The current draw is determined by the load (the tubes, and any DC resistance path) between the power supply and the ground return.

OK ,thank You, I changed the original 117v PT to Custom ordered 230v PT by Heyboer USA. And the Heyboer has 65mA instead of 60mA or even 50mA as it must be on the original PT. Just have had the worries about not properly working OT due to higher current on PT.

OK, I just checked this way see below,to find out prim impedance and my OT has 16k prim for 4 and 8ohm!
I have an AC wall wart that does 13.7 volts. You have to measure the AC voltage because DC voltage won’t pass through the transformer. Hook the output from the wall wart to the tube side of the transformer. Measure the speaker side with a digital VOM set on AC volts. This will give you a number like 0.4 volts (actually somewhere between .4 and .5 as my VOM doesn't show more precision than 1 decimal point). Divide this number into 13.7 and you should get number like 34.25. This is the turn ratio. If you square this number you get the impedance multiplier which in this example is 34.25 * 34.25 = 1173. If you are using a 4 ohm speaker then the impedance will be 4 * 1173 = 4692. This is just right for a champ. (Remember champs like to see around 5000 ohms impedance.)

A better way is to use a lower VAC source on the OT secondary and then (carefully) measure the VAC on the primary. That way you get more measurement accuracy.

The 5VAC or 6.3VAC winding on your PT is handy in this regard and is ideal (once you have the PT installed in the chassis and the mains fuse properly hooked up and the mains earth ground connected to the chassis). Be careful not to let the PT secondaries touch each other, or the chassis, or you or anything else for this test. You only want to use one of the windings hooked up to the OT secondary.

Also, you ideally want to use 2 x VAC meters so you can get simultaneous readings. The resulting Pr:Sec VAC ratio, when squared, gives you the Pr:Sec impedance ratio. You can then multiply this by your chosen speaker load to get what the reflected load would be.

Say you used the 5VAC winding on your PT, and measured 158VAC across your primary OT winding. 158/5= 31.62. That would be a Pri:Sec VAC ratio of 31.62:1. 36.62 x 31.62 = 1000, so the impedance ratio would be 1000:1. Multiply that by an 8R speaker load, and you have 8k reflected load.

A better way is to use a lower VAC source on the OT secondary and then (carefully) measure the VAC on the primary. That way you get more measurement accuracy.

The 5VAC or 6.3VAC winding on your PT is handy in this regard and is ideal (once you have the PT installed in the chassis and the mains fuse properly hooked up and the mains earth ground connected to the chassis). Be careful not to let the PT secondaries touch each other, or the chassis, or you or anything else for this test. You only want to use one of the windings hooked up to the OT secondary.

Also, you ideally want to use 2 x VAC meters so you can get simultaneous readings. The resulting Pr:Sec VAC ratio, when squared, gives you the Pr:Sec impedance ratio. You can then multiply this by your chosen speaker load to get what the reflected load would be.

Say you used the 5VAC winding on your PT, and measured 158VAC across your primary OT winding. 158/5= 31.62. That would be a Pri:Sec VAC ratio of 31.62:1. 36.62 x 31.62 = 1000, so the impedance ratio would be 1000:1. Multiply that by an 8R speaker load, and you have 8k reflected load.
Hello,
OK, I´ve got the 133v/4,9v= 27*27= 730*8 = 5,9K by 8ohm sec. and 183/4.9 ........* 4 = 5,5 k bei 4 ohm sec.impedance, looks like right now, but its huge difference with first 16k I´ve got used the other way of measurements...

The OT has A LOT(!) of bass and it looks also bigger then the stock champ OT as I can see from the pics, it has 4 and 8 ohm output impedance wires , but it fits exactly the board and looks pretty original, the soldering looks like it was soldered on the factory. Its is RCA OT. Also i tried to put 1,2 K resistor instead of 1,5K NFB and it sounds even with more buss and muddy.What is Your advise? should I change 1k5 and 22k resistor only? how much higher apr. ?
thanks, Vic

Sorry to throw a curve into the current discussion, but this might be a worthy consideration.

Your amp may not truly be lacking in treble signal, but instead may be suffering from a excessive amount of bass.

Human ears are not absolute measurement devices - everything you hear is relative to whatever else is in the mix.

A Champ that is producing a lot of bass signal may sound like a (loud) Champ that has no treble. The solution is not to add treble, the solution is to attenuate (reduce) bass.

The simplest way to do this is to try lower value coupling caps.

Sorry to throw a curve into the current discussion, but this might be a worthy consideration.

Your amp may not truly be lacking in treble signal, but instead may be suffering from a excessive amount of bass.

Human ears are not absolute measurement devices - everything you hear is relative to whatever else is in the mix.

A Champ that is producing a lot of bass signal may sound like a (loud) Champ that has no treble. The solution is not to add treble, the solution is to attenuate (reduce) bass.

The simplest way to do this is to try lower value coupling caps.

Thank You, But i try to keep the original design also the .02 uf caps.

My experience with (real) old Tweed Fenders is that they are not exactly the go-to amp if you're looking for clear, or indeed any, high end.

All midrange, all the time. And that's what a lot of folks love about them.

My experience with (real) old Tweed Fenders is that they are not exactly the go-to amp if you're looking for clear, or indeed any, high end.

All midrange, all the time. And that's what a lot of folks love about them.

Yes , i know, i like it too, but its sounds too dull compared to common tweed champ sound

Hello,
OK, I´ve got the 133v/4,9v= 27*27= 730*8 = 5,9K by 8ohm sec. and 183/4.9 ........* 4 = 5,5 k bei 4 ohm sec.impedance, looks like right now, but its huge difference with first 16k I´ve got used the other way of measurements...

A quick way of experimenting with this would be if you had a 16R speaker and an 8R speaker lying around, and you wire them in parallel with each other (giving 5.3R) and connect them to the '4R' tap - should give you 7k4 reflected load.

You would notice some increase in output power, but it might not make a lot of difference in bandwidth if the OT iron is bigger anyhow. Which takes us back to shaping the bandwidth in the pre-amp to compensate, as suggested earlier (either by coupling cap or cathode bypass cap changes - which will reduce bass, or converting to blackface type NFB to V1a, which you can selectively boost the bypass cap of - which will boost the gain overall but from which you can then selectively reduce the bass). Or trying a smaller 'stock' OT

A quick way of experimenting with this would be if you had a 16R speaker and an 8R speaker lying around, and you wire them in parallel with each other (giving 5.3R) and connect them to the '4R' tap - should give you 7k4 reflected load.

You would notice some increase in output power, but it might not make a lot of difference in bandwidth if the OT iron is bigger anyhow. Which takes us back to shaping the bandwidth in the pre-amp to compensate, as suggested earlier (either by coupling cap or cathode bypass cap changes - which will reduce bass, or converting to blackface type NFB to V1a, which you can selectively boost the bypass cap of - which will boost the gain overall but from which you can then selectively reduce the bass). Or trying a smaller 'stock' OT

Changing the coupling cap or cathode bypass cap will reduce also the gain. The amp sound pretty loud and i like it. I guess the converting to blackface type NFB will also change the typical tweed sound.... maybe a new OT....

A quick way of experimenting with this would be if you had a 16R speaker and an 8R speaker lying around, and you wire them in parallel with each other (giving 5.3R) and connect them to the '4R' tap - should give you 7k4 reflected load.

You would notice some increase in output power, but it might not make a lot of difference in bandwidth if the OT iron is bigger anyhow. Which takes us back to shaping the bandwidth in the pre-amp to compensate, as suggested earlier (either by coupling cap or cathode bypass cap changes - which will reduce bass, or converting to blackface type NFB to V1a, which you can selectively boost the bypass cap of - which will boost the gain overall but from which you can then selectively reduce the bass). Or trying a smaller 'stock' OT

Interesting thing, I told You I disconnected the second input on the amp first, it gave me more gain and headroom, so I disconnected one 68k resistor and wire between 2-nd input( the leg in the middle) and 1meg resistor /ground leg on 1-st input. Now I "restored" it back and got much less low bottom but lost also some gain. The amp sound now better .How it works? Maybe should I change the 68k, to less or more value? Also I used 5R4 GY JAN RCA tube as rectifier , it has more Hi-End sound but also more bass . With 5Y3 it sounds now with more midrange and less bass.
Thanks

Thank You, But i try to keep the original design also the .02 uf caps.

If you wish to keep to the original design then you might want to consider changing to a more original type of output transformer...

Going from a 5y3 to a 5r4 will increase all operating voltages a bit. Probably not a huge amount, but you should still measure the 6v6 plate voltage to be safe.

You could probably get similar results by keeping the 5y3 and lowering the v1 plate resistors.

If you wish to keep to the original design then you might want to consider changing to a more original type of output transformer...

Going from a 5y3 to a 5r4 will increase all operating voltages a bit. Probably not a huge amount, but you should still measure the 6v6 plate voltage to be safe.

You could probably get similar results by keeping the 5y3 and lowering the v1 plate resistors.

I guess its not original OT there, looks also like from 50´s but from RCA,oversized and has two sec outputs 4 and 8 ohm, orig.Champ OT has only 4ohm and is smaller

That's what we keep sayin'