Design help needed for 4x6v6 amp

[Tonegeek]

I posted recently about the 4 6v6 Magnavox amp I was given.

I have decided to build a guitar amp using the trannies. I will reuse the tubes (including the 2 5y3 tubes) if possible.
At this point I am just trying to determine what bias resistor/bypass cap to use.

I took some measurements before I gutted this thing:

one side of the HV tap to ground = 300V
B+ after the rectifiers = 325

Just looking at dual 6v6 designs it seems 2 X 250 ohm 5 watt resistors (1 for each pair of tubes) might get me in the ball park for my quad amp. I may allow a choice of SS or tube recto so in that case I would bump the resistors up a bit since the SS recto would have a higher output V. I know the math to calc the bias current on an existing amp, but not enough to pick a resistor from scratch. Seems I need a load line for that.

I am guessing 25uF might be a good bypass cap for each pair. I will work on the preamp after I get this part settled. I am not even sure what type of phase inverter I will use at this point. My overall goal is to have an amp that Tweedy clean at low volume and old Fender Bassman distorted at lead (but lower) volumes. I may use a VVR to make the amp more versatile. I don't need bedroom distortion, just want it to work in small rooms and larger rooms at gig/rehearsal volumes. Really 4 6V6 tubes is a bit much for my needs, but I figured between going cathode biased and the lower PT HV I should be able to tame this a bit.

[kwijabo]

never built a 4x6V6, but built a 2x6V6 that i started out with 25uF/240 ohms, as the old tweed tremolux basically shows. i used 1k screens. B+ after the rec was about 370 VDC and the cathode was about 20-25Vdc.

it sounded great clean, but the low strings (low E, A, D) sounded terrible when pushed. like, really muddy.

i changed the cap/resistor to 10uF/330ohms, the cathode DC went to 25-30Vdc. changing those made a dramatic improvement in sound.

[tubeswell]

If you've got 300-0-300VAC on the HT winding with 4 x 6V6s, that would be perfect for a 5G9 with 4 x6V6s (Tweed front end/LTP/fixed-bias 6V6s). And you get the tweedy sound and fantastic trem as well! (You might want to use a 5AR4 rectifier if you cant get 2 x 5U4G onto the 5V winding)

[Phil_S]

If you've got 300-0-300VAC on the HT winding with 4 x 6V6s, that would be perfect for a 5G9 with 4 x6V6s (Tweed front end/LTP/fixed-bias 6V6s). And you get the tweedy sound and fantastic trem as well! (You might want to use a 5AR4 rectifier if you cant get 2 x 5U4G onto the 5V winding)

Great suggestion. I just built a 5G9. I haven't had much time to play it, but the test drive led me to believe it's going to be a favorite. I am guessing with 4x 6V6 you'll get 25-35W from it. This is sort of wattage is loud, but can be managed. You might think about wiring it to allow you to pull the inner or outer pair, bringing it into the 15W range. I don't think this is at all like pulling a pair from a 100W quad, which won't be much of a change.

[Tonegeek]

If you've got 300-0-300VAC on the HT winding with 4 x 6V6s, that would be perfect for a 5G9 with 4 x6V6s (Tweed front end/LTP/fixed-bias 6V6s). And you get the tweedy sound and fantastic trem as well! (You might want to use a 5AR4 rectifier if you cant get 2 x 5U4G onto the 5V winding)

Thanks for the suggestion. The 5v winding was designed for the 2 5Y3 rectos but I could use a 5AR4 tube as well. I doubt 2 5U4GB tubes would work since they draw 3a each on the heaters. I don't know the specs on this PT, only that it did work with the original equipment that it came out of. It has massive heater wires on the order of 12 or 14 gauge wire.

Not sure I need tremolo, but I would like to have a tube driven FX loop.

@Phil_S I put a 50/100 watt switch on my Twin and never liked it. It made the amp too mushy. It was still loud, but lost something with the cathodes lifted on 2 of the tubes. I am guessing the OT impedance had something to do with it. I have had better luck with a VVR. You may be right that the amp will still be too loud even with cathode bias, and B+ of 325-335. I do want the amp to clean up with the guitar volume knob and get fat and dirty when cranked. Up till this project I have built channel switching amps to do that but now I want to try a simpler design and get the power amp to breakup when needed. I don't mind a boost relay being part of the design but really want to get flexibility with the power section first.

[tubeswell]

If you've got 300-0-300VAC on the HT winding with 4 x 6V6s, that would be perfect for a 5G9 with 4 x6V6s (Tweed front end/LTP/fixed-bias 6V6s). And you get the tweedy sound and fantastic trem as well! (You might want to use a 5AR4 rectifier if you cant get 2 x 5U4G onto the 5V winding)

Thanks for the suggestion. The 5v winding was designed for the 2 5Y3 rectos but I could use a 5AR4 tube as well.

2 x 5Y3G in parallel should provide enough current for 4 x 6V6s and 3 pre-amp tubes with just the right amount of sag.

[gingertube]

For what it is worth.
I built a 4 x 6V6 amp with full power scaling and each pair switchable between triode mode and Pentode mode AND between cathode bias and fixed bias.
The power scaling had a variable curent limit which was called the "SAG" control.
I scaled only the main B+ to the OT centre tap (6V6 anode supply) and 6V6 screen supplies and ran a tracking regulator on bias supply.

Here are my set to work notes for that amp.
Set to Work of the Power Amp.

Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).

Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts

Set Power Scale Control to 10 (full volts)
Set Compression (SAG) Control to 0 (no current limit)
Set Fixed Bias
Set pentode Mode

A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.

Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.

Set the 4 bias contrls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.

Checks:
B+ = 345 Volts
Bias Current/Voltages
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode - Actually I got the Pentode/Triode measurements backward. That is Triode Mode current was HIGHER than Pentode mode.

Set Power Scale Control for B+ = 250V
Checks:
B+ = 250V
Bias Current/Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V

Set Power Scale Control for B+ =150V
Checks
B+ = 150V
Bias Current and Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V

Set Power Scale Control back to 10 => B+ back to +345 Volts

Wind Compression Control from 0 to 10 => B+ drops to +105V
This looks a bit "drastic" but it works.

Set Compression back to 0 and switch to Cathode Bias.

Checks:
B+ = 343V
Bias Currents / Voltages
Pair#1 Cathode Volts = +23.9V
25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V
25.2mA, 25.4mA tube currents

Other notes:
In cathode bias mode I used a common unbypassed 470 Ohm cathode resistor for each pair of 6V6. This was done to get a different sound between fixed and cathode bias. With bypassed cathode resistors there was practically no difference in sound between fixed and cathode bias.

Hope there is something useful in this.
Cheers,
Ian

[Tonegeek]

@gingertube-

Thanks for sharing the data.
Did you have a good experience with the pentode/triode switch? What was your favorite configuration, the one that you would use the most in regard to the screens, bias type, VVR, etc.

I am always tempted to build in the most versatility into every amp, but then I find I just settle in on one or two favorite settings and wish I had left the other stuff off. what I do now is start out with a list of all the goodies I want and then whittle it down. For instance on this amp I am not sure I need a VVR and a cathode, fixed switch and the SS/tube recto switch. I am leaning toward a simple design with maybe just the VVR and an FX loop. And since I already have the setup for the 2 5Y3 tubes, just keeping that. I think the amp will be plenty loud, its the situations where I need it to breakup without being too loud that worry me. If I also do cathode bias, then I won't have to worry about the bias tracking components on the VVR. Keeps the parts count down.

[gingertube]

This amp was designed and built for a friend - his brief was that he wanted it to be able to everything, money no object. So I "cheated" - it was largely based upon a London Power "Standard" from TUT5 to the extent that I used one of Kevin's Power Scaling Kits. I had a bit of a play with it before delivering it to him and played a bit with it since, as he and I and a few others get together every few Sunday afternoons to "jam" and share a bottle or 2 of wine.
I like it with one output tube pair running fixed bias pentode mode and the second pair running cathode bias triode mode. I was'nt a big fan of the VVR (powerscaling). I initially tested it with a spare Marshall Quad box I had but it really sounded much better with the box I designed and built for it. HiFi dimensions closed box with baffle cutouts for 2 x 12" speakers with just one Weber P12B loaded and the other speaker hole left open as a Very loose and broad tune port.

Suffice to say that after delivering it to him I promptly set about building another amp for myself, leaving out most of the "bells and whistles". My amp runs a single pair of 6V6, fixed biased with an Ultralinear/Pentode switch which I leave in Pentode mode all the time, so the only switchable feature I implemented for my amp I don't actually use. I also changed from the CV4004 (premium 12AX7) stacked schmitt phase splitter to a concertina splitter using a 6SL7.
For my amp I re-used a Sanyo HiFi Speaker Box - ripped out the 10" driver and the 3" tweeter and just fitted a vintage Aussie ETONE 10" Guitar speaker with the 3" hole left open.

I can't empahsize enough that for any guitar amp build you need to spend say 30% of your build budget on a decent speaker.

One of the other guys we jam with has now asked for an amp build - he wants a copy of my simpler amp rather than the full bells and whistles job.

Hope that is the info you wanted.
Cheers,
Ian

[Tonegeek]

I started on a design and I had an idea for a sag control using the 2 5Y3 rectos with option to use 1 GZ-34 (see schematic below) and just need a reality check and comments. Also look at the output section and see what you think. I would like comments on screen resistors, should I use them, what value and how will it affect the sound. My desire is to build a really touch sensitive amp, but I don't want it to be too loud to get the goods. The low 300 V HV tap with the drop of the 5Y3s and cathode bias should tame this thing a bit and give me some good breakup when guitar volume is cranked for leads. I am not stuck on the cathodyne PI, but it is something I have wanted to try. THe PI is set up similar to a 5e3 at the moment but I am wondering if it will have enough drive to push the 4 outputs.

So, I need ideas on what:
screen resistor values
feedback resistor value (not sure I will even use feedback. May go with no presence or feedback and use a cut control instead)
cathode bypass cap values

I will get to the preamp later

[tubeswell]

I started on a design and I had an idea for a sag control using the 2 5Y3 rectos with option to use 1 GZ-34 (see schematic below) and just need a reality check and comments.

The rectifier looks feasible. (Not sure how 'well' it would work in practice to have 1 x 5Y3 powering 4 x 6V6. Suck it and see I guess)

The plate-to-screen dropping resistor in teh power rail, would need to be 470R if you were trying to emulate a 'double BF Princeton' (just guessin' - going by the PI). And it would need to be double the power rating of a typical screen dropper.

[gingertube]

You don't have any bias for the concertina splitter. It won't work like that. Either direct couple it to the input stage or PREFERABLY provide self bias for it or a divider between rail and ground.

Cheers,
Ian

[roberto]

Concertina needs to have its cathode voltage to about 1/4th of B+ to have best balance and swing on the output. You can dc couple with a 220k 470R previous stage, or use a voltage divider to bias it, like you have read before.
Also a 470k grid stopper on concertina's input is a good choice.

I'd use 1k screen resistors too, and connect all 6V6s' cathodes together with 120-130 ohm resistor and 220µF in parallel.

[gingertube]

Concertina bias point recommended by Roberto is good.
Screen resistors depend on how hard you run the 6V6 but 1K is good. At lower voltages (say 300 to 320 Volts) you can get more dynamic sound with smaller screen resistors of say 270 Ohms. If your rail is greater than 320V then stick with 1K.

Personal preference only but I prefer to either individually cathode bias each out put tube or at least one bias network per pair. I think connecting all 4 cathodes together with a single resistor and capacitor is tempting fate and just about requires using a matched quad.

I recently did a restoration of an old copper front panel AC30. The power stage had to be entirely rebuilt. I fitted individual cathode bias resistors and bypass caps for each of the 4 output tubes. I was a bit worried about a change in tone from doing this but the customer reported that he could not notice any change - its for damn sure its more reliable that way and matched quads are not required.

Cheers,
Ian

[Tonegeek]

You don't have any bias for the concertina splitter. It won't work like that. Either direct couple it to the input stage or PREFERABLY provide self bias for it or a divider between rail and ground.

Cheers,
Ian

Good catch. I updated the shemo posted earlier to reflect the changes. Basically just borrowed the 5E3 version and added the 470K grid stopper as suggested by Roberto and Merlin. Also added screen resistors, value TBD, but leaning to 1K.

I have read different discussions about shared cathode components(shared has sweeter sound, separate is safer for the tubes, better for matching, etc.) My experience with an 18 watt amp that I tried both ways, is the shared cathode sounded slightly better. I don't know why. I thought I would share in groups of two, that way I don't have to run everything through one big resistor/cap.