5e3 ( VHT Standard 12) voltages question after Sovtek --> JJ rectifier...

bluesholyman

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DISCLAIMER - I properly discharge voltages before digging inside the amp (unless I need it live) and verify with a meter that voltage levels are safe/gone.

I neglected to measure any voltages before I swapped rectifiers as I heard the Sovtek is higher than it should be, so I just put in the JJ and chunked the sovtek - it rattled anyway and didn't want to bother with it.

This VHT has a ~460 Ohm (470 nominal) bias resistor instead of the standard 250 that shows on a regular 5e3 schematic - even the VHT schematic shows it as 250. I suspect for amps running Sovtek rectifiers, they beefed that up to get bias back down? I am looking at all this to change my bias resister to ~250 as is in a stock 5e3

Voltages with 460 resistor and JJ 5y3 with 120v at the wall:

B+ - 366
B+1 - 360
B+2 - 330
B+3 - 253

Bias: 23.5v

Plate current: 23mA
Plate Voltage: 340v

This seems like its presently biased really cold...


So my question is, should I switch that bias resister to something smaller, such as 250? These voltages seem a bit low, but I am not really sure. The amp currently runs fairly clean for a good bit on the volume and it just doesn't have the grind that it did with the sovtek (and chinese tubes). I really have to turn it up to get any sort of non-clean tweedy-ness with a P12Q in it. I'd like it to cook a little sooner if I can.

Would appreciate thoughts/insight. Am I even thinking about this correctly?
 
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nickmm

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JJ can take a bit more current than most current production 6V6s they are more like a 6L6.

Check your voltage drop across the Cathode bias resistor.

This is a good place to get some voltage and current numbers
 

bluesholyman

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JJ can take a bit more current than most current production 6V6s they are more like a 6L6.

Check your voltage drop across the Cathode bias resistor.

This is a good place to get some voltage and current numbers

I think that 23.5v up there is the voltage drop but I'll double check. I think that measurement was at TP12 (on schematic) to ground, but not specifically across the resistor itself.

I like JJs 6v6s for a modern rock sound, but not for vintage 6v6 sound - that is more RCA, Genelex, EHX, Tungsol, etc.
 

Dacious

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A 5Y3 is a 5Y3 is a 5Y3. I've swapped many brands in and out and the appearance of them can vary but Trigon to RCA to Sovtek to Phillips Miniwatt voltage never varied by more than a volt. They're just a pair of diodes. If they've still got cadmium on the cathodes and vacuum in the envelope they work. The NOS and even old used ones usually last much longer. I had a Hammond branded one that rang sympathetic A in a Champ but still worked.

470 is an odd value for a 5E3 - there may be other changes in the VHT circuit. Often though putting a smaller value cathode resistor heats everything up and drops B+ in a cathode-biased circuit. Depends on the power and output transformer. It might have a larger value to limit current and protect them.

Normally 5E3s will grind up on around half volume depending on guitar. The Shuguang 6V6s will have likely have lower headroom than JJs. If you raise the other volume unjumpeted you'll get more headroom too. Ditto if you have a 12AY7 in V1.
 
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Wally

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If you have 23ma of current draw in each 6V6 with 340Pv, your plate dissipation is around 55%, which is a bit cool for cathode bias, ime. Do you like the sound?
You could go to the next lower resistance number…330ohms????….to see what happens. Then try the next lower resistance if you are curious. Or….you could spend more money and buy hotter tubes. That rebiases the amp, too.
I would want to see something closer to 100% of max plate dossipation for my ear. Ymmv..
 

bluesholyman

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If you have 23ma of current draw in each 6V6 with 340Pv, your plate dissipation is around 55%, which is a bit cool for cathode bias, ime. Do you like the sound?
You could go to the next lower resistance number…330ohms????….to see what happens. Then try the next lower resistance if you are curious. Or….you could spend more money and buy hotter tubes. That rebiases the amp, too.
I would want to see something closer to 100% of max plate dossipation for my ear. Ymmv..

Thanks for the suggestions Wally. The clean sound is nice, almost 3 dimensional - strange how it does that, but indeed it does. Its the "grind" that I am not getting enough of early on, and I think that is because it is running so cold. I could genuinely be wrong because I have not worked with a 5e3 circuit before, playing or tinkering, so maybe it is what it is. I believe a hotter bias will indeed change things for the better, so thats kinda the basic goal.

I am currently running Genelex Gold Lions in there and both sets I have bought recently came in pretty cool (identical idle currents actually,) so I think I will rebias the amp for the tubes to what a stock 5e3 would be - those Gold Lions should sound sweet when they are at a good operating point.

The 250 ohm 5w resistor(s) arrive today so I'll be able to do that adjustment by this weekend. It is as much the education of how the 5e3 works as it is getting a sound I like. Being half engineer, half musician is a blessing and a curse - one is always distracting the other.

I'll try a set of JJs too, once the bias is adjusted and see what I prefer.
 

Wally

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Thanks for the suggestions Wally. The clean sound is nice, almost 3 dimensional - strange how it does that, but indeed it does. Its the "grind" that I am not getting enough of early on, and I think that is because it is running so cold. I could genuinely be wrong because I have not worked with a 5e3 circuit before, playing or tinkering, so maybe it is what it is. I believe a hotter bias will indeed change things for the better, so thats kinda the basic goal.

I am currently running Genelex Gold Lions in there and both sets I have bought recently came in pretty cool (identical idle currents actually,) so I think I will rebias the amp for the tubes to what a stock 5e3 would be - those Gold Lions should sound sweet when they are at a good operating point.

The 250 ohm 5w resistor(s) arrive today so I'll be able to do that adjustment by this weekend. It is as much the education of how the 5e3 works as it is getting a sound I like. Being half engineer, half musician is a blessing and a curse - one is always distracting the other.

I'll try a set of JJs too, once the bias is adjusted and see what I prefer.
Note that when you find a resistor that does what you like with the Genelex tubes that you have any other set of tubes....even another set of Genelex unless they are matched to what you have there…will rebias the amp to that particular tubes’ operational parameters…..so the exercise might begin again!! Lol….. The thing is to find what works whether that is through changing the resistor or finding tubes that work for you there and then maintaining your sonics by running matched tubes from one set to the next.
 
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bluesholyman

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Ok, so cathode bias resistor swapped out and replaced with 250 Ohm - what I find most interesting, is that the low end is more under control now, all other things remaining the same. Where the P12Q was flapping pretty hard before, audibly so, it is controlled now, even at loud volumes - much more inline with what I was expecting from the speaker in this amp.

Clear chinese glass that came with it were bassiest of all, JJs have a good tight low end, Genelex are pretty loose with some unpleasant artifacts, which surprised me and for now, I seem to have settled on some black glass ruby's that I pulled from a Vox Tony Bruno amp 5 years ago - they are possibly same tube used for EHX.

Amazing amp (5e3's) and I am thoroughly enjoying the education this thing is giving me as well as playing it - the wife does not appreciate its loudness nearly as much as I do.
 
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Wally

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So…what are your numbers now? Plate, cathode, current draw if you are measuring that in some manner?
 

nickmm

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A nice and simple mod on these amps are .022 coupling caps after the phase inverter.
Helps clear up the bottom end.
"The Edge mod"
I note the VHT runs a 12ax7 in tube 1, I prefer the 12ay7 also easily reversed.
 

bluesholyman

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So…what are your numbers now? Plate, cathode, current draw if you are measuring that in some manner?

I want to say 330V plate voltage and 33mA plate current, but I might be wrong on the voltage. I'll plug in the probe for a quick check and see what it says this afternoon. Current is in the low 30s for sure at this point.

I am beginning to get an idea of what is/was going on. In another post I had asked about an extra filter cap I had seen, wondering why it was there. I figured I'd leave it alone, but based on the bias resistor change, I think I am going to remove it. It is at B+2 and adds 22uF to the 16uF that is there giving 38uF for B+2 - I think that is just too much based on more reading I've done since - maybe it doesn't matter a lot, but I think it was done to tame the low end on those stock chinese tubes and also what was provided by the bigger bias resistor, although I don't quite understand the effect of the cathode bias on the low end - need to read more. In any case, here is a pic of what was previously done and I think later today, snip, snip so that b+2 filter cap is back to 16Uf - I should check them all to make sure they are within value. This picture is before the bias resistor was dropped to 250 Ohm

565157D8-E74D-4B3A-B5A1-8814E7D80308_1_201_a.jpeg
 

Wally

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Ime, if one wants to up the filtering to tighten up low end, one might want to double up that first stage of filtering instead of the second stage. A 5Y3 can handle 30-40mfds there.
Also, imho, the use of a 12AX7 along With a local NFB loop is creating a gain problem with the 12AX7 that then needs to be dealt with by using the NFB to cancel gain. Ommv….
IF your 330vdc is good for the plate and you have only 33ma of current draw, you have some cold tubes in that amp.
 

bluesholyman

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IF your 330vdc is good for the plate and you have only 33ma of current draw, you have some cold tubes in that amp.

Those numbers were with the Genelex and I do think they are cold. I have to measure again with the Ruby's that are currently in there, but I am inclined to think I will see some hotter numbers.
 

Wally

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Those numbers were with the Genelex and I do think they are cold. I have to measure again with the Ruby's that are currently in there, but I am inclined to think I will see some hotter numbers.
One never knows until one gets the numbers. Imho, those numbers are what inform one as to what an amp is do8ng and therefore what one likes or doesn’t like that amp to do. P
 

bluesholyman

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Yes return the amp to stock.

Yes, leaning strongly that way, even the NFB in that first stage. I am wondering if that was causing some sort of oscillation I heard with the volume all the way up and a 5751 I tried - it was odd - tube was new but I figured it was microphonic.

After I remove that 22uF cap, going to see how things are and then attack the NFB resistor and try the 5751 again.
 

Wally

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I have owned a few vintage 5E3s. I have never had a need to use anything except a 12AY7 in V1. Many people complain of a loose low end in these amps. Imho, keeping that first stage of gain under control is important in utilizing this type of amp To its fullest ability. Ommv.
 

nickmm

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I have owned a few vintage 5E3s. I have never had a need to use anything except a 12AY7 in V1. Many people complain of a loose low end in these amps. Imho, keeping that first stage of gain under control is important in utilizing this type of amp To its fullest ability. Ommv.
Yeah changing the coupling cap is a tonal choice, shifting the amp more toward brownface/blackface sound.
 

bluesholyman

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So…what are your numbers now? Plate, cathode, current draw if you are measuring that in some manner?

So quick check with Eurotubes bias meter show 310v on plate and 35mA current. I didn't really wait for amp to warm up - that is about after 2 minutes, no playing.
 




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