Maximum clean volume SE octal 6L6 amp?

[Jerry garrcia]

Briefly checking it looks Vitaly does not show any Local Cathode Feedback unless he has updated the book?

I thought it would be easy to find more info so I have not bookmarked any pages but it wasn't. Here it is discussed in guitar amps and on post 6 is first "correct" circuit and better drawn on post 28.

Adding NFB to power tube cathode?

Hoffman Amplifiers Tube Amp Forum, Adding NFB to power tube cathode?

el34world.com

CFB using OT secondary in cathode current circuit is not very effective but better than nothing.

Perhaps you should buy Ultralinear OT because it work!

If you don't like amp coming heavy then Mennovanderveen toroid SE OT are lightest and they have UL option. Menno used to stock some 12W SE in Holland but two other OT I have bought came direct from Romanian factory so postage was bit higher and tax increased the price some more. He reply emails fast so you can ask if any are on EU sell?

I have used VDV-2512-SEE (only 955g) in few circuits and test UL as well. UL did not improve "my guitar sound preference" but if low distortion is needed it work very well. I did test EL34 for about 12W, KT66 about 13W and KT88 about 15W and all made very loud sweet sounding amp. So I bought VDV-1324-SEE (only 1750g) and going to drive it using KT150 but have not build it. It should be able to produce even 30W when impedance is set to 2,5k, bias is 160mA and B1 450VDC. It lists only 99 euros but it came from Romania which increased its price but I don't remember how much?

Toroidy.pl OTs are heavy but for guitar frequency range should be able to produce double the rated power. Not to exceed max bias using higher voltage and possibly doubling the impedance ot using parallel tubes is needed and then their weight comes reasonable. Anyone who want a monster SE guitar amp the TTG-6C33CSE 600mA bias is enought for even six parallel KT88

Best buy conventional SE OT should be Tubetown 2k5-5/4-8-16 for only 61 euros if it is good for 160mA bias. That OT is very high on my build order after I finish two builds.

And further down the rabbit hole I go.
Since I’m sitting with my 125ESE OT I might as well use it and try the UL tap if not satisfactory. You are just an endless well of knowledge.

Now I’m trying to find which of my PT’s that can handle two preamp tubes and a 6L6 with +420VDC. Don’t want to use one that has a 5V winding since I’m going SS.
An EAM86 would be fun to use in some way in the circuit. @2L man

Edit. You are correct. He described the UL configuration not local cathode feedback. But you delivered it to me though the EL34 world. Thanks!

 

[Jerry garrcia]

Briefly checking it looks Vitaly does not show any Local Cathode Feedback unless he has updated the book?

I thought it would be easy to find more info so I have not bookmarked any pages but it wasn't. Here it is discussed in guitar amps and on post 6 is first "correct" circuit and better drawn on post 28.

Adding NFB to power tube cathode?

Hoffman Amplifiers Tube Amp Forum, Adding NFB to power tube cathode?

el34world.com

CFB using OT secondary in cathode current circuit is not very effective but better than nothing.

Perhaps you should buy Ultralinear OT because it work!

If you don't like amp coming heavy then Mennovanderveen toroid SE OT are lightest and they have UL option. Menno used to stock some 12W SE in Holland but two other OT I have bought came direct from Romanian factory so postage was bit higher and tax increased the price some more. He reply emails fast so you can ask if any are on EU sell?

I have used VDV-2512-SEE (only 955g) in few circuits and test UL as well. UL did not improve "my guitar sound preference" but if low distortion is needed it work very well. I did test EL34 for about 12W, KT66 about 13W and KT88 about 15W and all made very loud sweet sounding amp. So I bought VDV-1324-SEE (only 1750g) and going to drive it using KT150 but have not build it. It should be able to produce even 30W when impedance is set to 2,5k, bias is 160mA and B1 450VDC. It lists only 99 euros but it came from Romania which increased its price but I don't remember how much?

Toroidy.pl OTs are heavy but for guitar frequency range should be able to produce double the rated power. Not to exceed max bias using higher voltage and possibly doubling the impedance ot using parallel tubes is needed and then their weight comes reasonable. Anyone who want a monster SE guitar amp the TTG-6C33CSE 600mA bias is enought for even six parallel KT88

Best buy conventional SE OT should be Tubetown 2k5-5/4-8-16 for only 61 euros if it is good for 160mA bias. That OT is very high on my build order after I finish two builds.

So finally got my thumb out of my a**.
Been having an inner conflict to walk the easy path with a well tried driver as the 6J5, EF86, 6SN7 or 6SL7.
- Couldn’t resist the triode connected 6SK7. Half day trying to find info about that tube and how it operates and what value of the cathode resistor to start with. Since it’s a remote cut off pentode with a low mu I’ll start with an un bypassed 4.7K (for the 6SK7) and
- aiming for 250V on the plates of the two 6S*7’s and
- 425V Vp and 404V Screen of the 6L6 (probably around a 30V drop over the OT).
- 125ESE OT and running the 5K primary for a 16 ohm speaker (actually an 15 ohm).
- Of course @printer2 bias switch for the preamp tube.
- Four filter stages including an 15uF for the preamp.
- Princeton tone stack and a master volume.
- will wire the output tube socket (pin 1 to cathode) so it will be able to handle EL34’s.
- skipped the grid leak for the 6SK7 and 6L6 since there is a pot before them. Some still uses it but can’t really find any support for it.
- I only have a regular 6SK7 but a couple of the ones with the plate on the top of the tube. Need to find a connector for that. Haven’t found it yet.

Got me a new PT since non of the once I had at home could support the +400 Vp on the 6L6. Went with a C.T. Toroidal 325-0-325VAC, 150mA and a 5A 3.13-0-3.15VAC (a bit over powered but might change it later).
Calculated with 488VDC unloaded after rectification and around 460VDC when running with tubes.

Need to do some more reading on the Local Cathode Feedback before I dare to add it so you all don’t laugh at my mistakes.

Any inputs appreciated and now time to continue on my hollow body, P90 in neck position, six string cigar box guitar. Hope it will be a good match for this amp.
I want this as loud and clean sounding as possible with the 6SJ7 characteristic.

 

[Jerry garrcia]

Thanks all for your inputs. Starting a build thread. Its gonna happen

 

[2L man]

6L6GC max grid leak resistance is 500k for cathode and grid biases so if only Master Volume pot does that change it to 500k. There is a danger if "run off" if control grid has too "loose" connection to cathode.

When also grid bias grid leak is quite high perhaps 6L6 is not too prone to run offs. Often other tubes grid bias grid leak resistor value is half or even only fifth what it is for cathode bias.

 

[Jerry garrcia]

6L6GC max grid leak resistance is 500k for cathode and grid biases so if only Master Volume pot does that change it to 500k. There is a danger if "run off" if control grid has too "loose" connection to cathode.

. Missed that. Went with a 1M pot without thinking. Any inputs s on just going with a 330R cathode resistor?
Or the 6SK7 hook up?

 

[2L man]

I make a loadline using those numbers you posted!

 

[2L man]

125ESE primary resistance is 103 ohms so there comes less than 10V drop to Anode voltage.

This loadline produce lots if 2nd harmonics because it is not linear both side of the Operating (bias) Point. This is typical when OT impedance is to too high side of the "optimum". On this ULLC the Headroom define voltage sweet to the left from bias point and what comes to the right comes what grid input tries to make. Sometimes sweep to the right comes higher and sometimes lower and sometimes equal

However off optimum bias it is often used for guitar amps because 2nd harmonics sound sweet and it is possible to get more power out of the OT. Also power tube run bit cooler and when driving it harder make it work harder when it pull higher current from left side of the loadline.

OTs which can be biased higher weight and cost more. For HiFi it does not usually matter how much amplifier weights but low distortion does matter.

Your loadline might come something like this:

 

[Jerry garrcia]

125ESE primary resistance is 103 ohms so there comes less than 10V drop to Anode voltage.

This loadline produce lots if 2nd harmonics because it is not linear both side of the Operating (bias) Point. This is typical when OT impedance is to too high side of the "optimum". On this ULLC the Headroom define voltage sweet to the left from bias point and what comes to the right comes what grid input tries to make. Sometimes sweep to the right comes higher and sometimes lower and sometimes equal

However off optimum bias it is often used for guitar amps because 2nd harmonics sound sweet and it is possible to get more power out of the OT. Also power tube run bit cooler and when driving it harder make it work harder when it pull higher current from left side of the loadline.

OTs which can be biased higher weight and cost more. For HiFi it does not usually matter how much amplifier weights but low distortion does matter.

Your loadline might come something like this:

So for the layman this all looks good with a high wattage, lots of headroom and a bit of 2nd harmonics? The end goal! (Depending on if that 6SK7 is doing its job).

Was trying to find the DCR of the OT but couldn’t find it. Thanks. Will recalculate the voltage dropping resistors in the power stage to get it more accurate.

Wouldn’t the OT’s impedance differs a bit since I’m planning on running a 15ohm speaker?
15/16= 0.9375
0.9375*5= 4.68K
So maybe even better

 

[2L man]

On 125ESE datasheet there is written resistance between brown and blue primary wires. There is a possibility that I did look the 125GSE because I wrote about it to other thread?

15 and 16 ohm loudspeaker effect should be minimal. Guitar speakers can have huge high reactance resonance peak around 100Hz which in practice concerns only few notes. Then impedance increase after 600Hz or so and often is considerable at 5kHz.

All this kind of throw loadline operation outtawindow but I think loadline principle is essential for tube amp designers and tuners. Resistive pre amp stages as well although they are not as interesting.

So far I have measured mostly single ended operation and I don't remember what was the best method but I recall I made a voltage divider to anode and it went to oscilloscope X-channel. Cathode current turns voltage over cathode resistor and it went to Y-channel. I recall when I detached the cathode bypass capacitor it did "strengthen" the current signal.

If LL is calculated using loudspeaker high impedance frequencies it comes much more horizontal.

In practuce loadline first spread to an oval obviously because there comes phase shifting between cathode current and anode voltage. When wide frequency noise is amplified it fill the space both side of loadline. When power stage is driven to high distortion the loadline turn messy and look interesting and scary same time especially if, instead if resistor, a loudspeaker is used to load the power amp. Obviously speaker reactabce reflect back thru OT to power tube anodes? Loud amplified guitar chords look so bad that it is difficult to understand how good they sound

 

[Jerry garrcia]

On 125ESE datasheet there is written resistance between brown and blue primary wires. There is a possibility that I did look the 125GSE because I wrote about it to other thread?

15 and 16 ohm loudspeaker effect should be minimal. Guitar speakers can have huge high reactance resonance peak around 100Hz which in practice concerns only few notes. Then impedance increase after 600Hz or so and often is considerable at 5kHz.

All this kind of throw loadline operation outtawindow but I think loadline principle is essential for tube amp designers and tuners. Resistive pre amp stages as well although they are not as interesting.

So far I have measured mostly single ended operation and I don't remember what was the best method but I recall I made a voltage divider to anode and it went to oscilloscope X-channel. Cathode current turns voltage over cathode resistor and it went to Y-channel. I recall when I detached the cathode bypass capacitor it did "strengthen" the current signal.

If LL is calculated using loudspeaker high impedance frequencies it comes much more horizontal.

In practuce loadline first spread to an oval obviously because there comes phase shifting between cathode current and anode voltage. When wide frequency noise is amplified it fill the space both side of loadline. When power stage is driven to high distortion the loadline turn messy and look interesting and scary same time especially if, instead if resistor, a loudspeaker is used to load the power amp. Obviously speaker reactabce reflect back thru OT to power tube anodes? Loud amplified guitar chords look so bad that it is difficult to understand how good they sound

Once again thank you! I will need to dig
in to LL theories.
Hopefully the build starts Monday

 

[Jerry garrcia]

Any inputs on the 6SK7? That tube seems like a black hole in the guitar tube amp society
Updated schematic

 

[2L man]

I don't know any other adverse features but 6SK7 logarithmic grid voltage vs anode current is obvious issue despite where the OP is set.

 

[Jerry garrcia]

I don't know any other adverse features but 6SK7 logarithmic grid voltage vs anode current is obvious issue despite where the OP is set.

Once again thanks. Will try with this configuration and probably will do some tweaking later. The high value cathode resistor value was picked from a forum discussing this tube for audio purposes.
Found this lecture on triode connected pentodes. Interesting

 

[mountainhick]

Once again thanks. Will try with this configuration and probably will do some tweaking later. The high value cathode resistor value was picked from a forum discussing this tube for audio purposes.
Found this lecture on triode connected pentodes. Interesting

Nice understandable article!

 

[Jerry garrcia]

Nice understandable article!

I think so too. Just realised that I have a choke, Hammond 194A. Maybe before the screen or preamp? Since the PT and OT weighs a ton a small choke won’t do any major difference. Do they really make a difference in that position?

 

[2L man]

In your amp 4H 50mA Choke should be best before 6L6GC SE power tube screen. For OT supply its current rating is not enough. For preamp its inductance is low and current rating high.

 

[Jerry garrcia]

In your amp 4H 50mA Choke should be best before 6L6GC SE power tube screen. For OT supply its current rating is not enough. For preamp its inductance is low and current rating high.

 

[Jerry garrcia]

6L6GC max grid leak resistance is 500k for cathode and grid biases so if only Master Volume pot does that change it to 500k. There is a danger if "run off" if control grid has too "loose" connection to cathode.

When also grid bias grid leak is quite high perhaps 6L6 is not too prone to run offs. Often other tubes grid bias grid leak resistor value is half or even only fifth what it is for cathode bias.

Hi and sorry to bother you again. Would it be any advantage of going for a 1 M master volume pot and add a 470K grid leak before the 6L6 or to go with a 500K pot and skip the grid leak R?

 

[2L man]

Hi and sorry to bother you again. Would it be any advantage of going for a 1 M master volume pot and add a 470K grid leak before the 6L6 or to go with a 500K pot and skip the grid leak R?

I think both prevent "runoff" but fixed resistance parallel with potentiometer change its operation. When volume potentiometer usually is logarithmic it move slightly toward more logarithmic or slightly toward linear? I don't remember or figure it just now

Potentiometers sometimes are tuned connecting one, two or three resistors between its terminals and there are instructions in inet which I have to read to remember details.

 

[jsnwhite619]

I meant to chime in here earlier and forgot. Don't have much tech advice on the preamp choice, and overall design, but I've built a few 5881 5f2a Tweed Princeton amps. With a loud speaker they can blow your mind with the volume and the added mids of the vol/tone setup & no tone stack scoop/loss. I built one for a friend last year that wanted something mean and loud. His is always turned up enough to start getting the overdrive, not "clean" volume. But, honestly, I used my dB meter on his a lot to see what it was doing. There was only maybe 1/2 a dB difference in edge of breakup at 6 to cranked on 12. It sounded louder, felt huge, dirtier for sure, but the actual output what not that much.

I think this was measured from 12" away from the speaker, but you can see what the C-Rex was doing with it.

Keeping the preamp voltage up to Blackface level and lower value dropping resistors in the power section helped a lot in clearing up the mud. It was easy to sound good with single coils, humbucker presented the challenge. Personally, I like using a 12ay7 in the V1 and using more gain in circuit instead of a 12ax7 in V1. But he has plenty of 12ax7's, and I didn't want to throw a wrench in there, so I lowered the gain - mainly the bass with 2uf bypass caps instead of 25uf - and used a 12ax7 in V1. It's easy to make that kind of amp dirty, but the hard work is keeping it clean. If you want max. output from it, pick your speaker accordingly. High efficiency will add some horsepower in the end.