36 Watt EL84 and 6V6 - Black and Tan amp

[csbradley]

Well, I have the 12AX7 channel of my EL84/6V6 amp wired up and going. It sounds good right now, but I have some tweaks to make. I'm going to wire up the EF86 tonight.

The amp started out as a Weber 6M36 Head Kit. I have it set up with two "Master" Volumes. As you can see from the picture, I need to work on the front Plexi Panel, but since it will be in a head cab, I really wouldn't need to trim it down at all if I don't want to.

I can turn Master one down and run it as a 18Watt with EL84's, or I can turn the other down and run it with 6V6's, or crank them both for using both sets of tubes at once.

One channel is somewhat similar in topology to Hoffmans "Stout" amp. There's some changes to let some more low end through and some ideas taken from the Deluxe , so this channel still needs to be tweaked. The other is more of a Vox/Matchless style EF86 channel. Two Channels and two different output tubes mixed. So far I just call it a "Black and Tan". Seemed to be appropriate.

So far, the amp seems very stable. No oscillations of any kind at any volume. I am impressed with it when I have the either (or both) of the Masters up and the preamp volume set below half. Above that, the low end gets a little mushy. I need to decide how I want to finish voicing this channel, but I'll wait till I get the other channel wired up.

Just thought I would share. I should have more pictures and schematics soon.

CSB

[rjgtr]

That looks really nice and is a cool idea. Glad it is working out for you.

I have an idea on the back burner for a two channel amp with a 6V6 clean channel and an EL84 dirty channel. I might actually just do a 36 watt that will disable two tubes for dirty sounds, but I'm still thinking about it.

I like the fact that in your design all 4 output tubes can work at the same time. Sort of like a built in two amp system. Very cool.

[stu]

Once you get finished tweaking, PLEASE, PLEASE, and PLEASE post audio clips if you can.

[Turmoil]

Oh, I'm gonna do that!

Can you tell us how you layed out the controls? Does anybody have a source for a dual concentric 1meg pot?

I just added a 6v6 to my P1 this weekend and it sounds awsome in cunjunction with the el84. I did nitice though, that the el84 is doing the majority of the work. Do you notice this when you have both masters full up? It would be awsome to have a single knob to blend the two outputs, I just can't figure out how to go about it yet.

Tim

[csbradley]

I've been playing with it a little today. I added the matchless style tone switch instead of the 18 Watt tone control. I think I like it a little better. It has a nice overdrive that way. May need to tweak the values, but I'll wait till I have a few others play through the amp.

I have also tried it with 2 EL84's and 2 5881's, and with just 2 EL34's. I still need to try it with EL34's and EL84's. That puts me right up at the 5A heater current limit, but the PT should be able to handle it. I think I like the sound of 2 EL84's and 2 5881's best. Biggest contrast between tubes. The PI contributes so much of the distortion (18Watt PI) so the distortion characteristcs stay very much the same through out.

Since I'm running only around 300V on the plates, I don't think there will be too much of a problem with the mismatch of OT impedance (6V6's -vs- 6L6's, etc...). Things seem to work fine so far.

Also, one channel is the standard 18Watt normal channel, but with a 5G3 tone control. Its similar to the 5E3 tone control, but the volume and tone are more independant. The other channel is the EF86 channel biased with a 1.5k resistor and 50uF cap. Standard 220k on plate and 1M on G2. The "LOW" input disconnects the cathode cap.

I am using the 18Watt Pi. It seems to control the distortion characteristics of the amp. I may set up the LOW channel on the normal channel so that I can lower the Tail resistor and hopefully get a little more headroom from the PI.

From there I use use the standard .01uF coupling caps for the EL84's and .1uF coupling caps for the 6V6's. Dual 1Meg pots for the two master volumes.

For the B+ to the power tubes, I send the B+ first to the 6V6's plates, and then to the EL84 plates via an Inductor and Resistor in parallel. This is just a 2W 10k resistor, with about a 10 turn wire wrap around it. Just wanted to make sure I didn't get any oscillations between the two sets of tubes.

I'm just letting the heaters run right now to make sure I'm not drawing too much heater current with various tubes.

CSB

[Turmoil]

After looking at the RCA tube manual it seems that the 6v6 can handle almost twice the input signal voltage. I would assume that this means the el84 is more sensitive and more easily driven compared to the 6v6.

CSB, do you find that you attenuate the signal (via master vol.) going to the el84's?

Tim

[csbradley]

Well, I'm still playing with it. I have it set up with two master volumes, so I can attenuate the signal to the EL84's if I want to. If you are sending enough signal in to clip both the EL84's and the 6V6's, then the volume balances pretty well. If your not yet clipping the 6V6's, then the EL84's are a little louder, but I can blend them any way I want.

CSB

[csbradley]

Tried to draw out a schematic of the Power Amp section. It's a little hard to follow, but I figured I would share anyways.


CSB

[markh]

Very interesting concept and implementation.

Does the tiny inductance and big resistance in the '84 plate circuits actually prevent oscillation? Did you actually have oscillation problems, or is just 'insurance'?

--mark h

[csbradley]

The inductance and resistor is a "just in case". Its just a 10k 2 watt resistor with about 10 turns of wire around it. Its something that a friend (and old HAM guy for many years) suggested when doing parallel tubes to insure no problems. But he typically worked with tube Radio's, where you are dealing with much higher frequencies, so in that case, unwanted oscillations would be more likely. Probably doesn't do much, but the amp is very stable, so I'm not taking them out just to see.

CSB

[markh]

That makes sense, 10 turns is, like, micro Henries, and would have a lot more effect at RF.

--mark

[Turmoil]

Okay, I've finally gotten around to doing the 'Black and Tan' thing. I didn't use the resistor/inductor, and I've had no problems so far. It also uses the standard 12ax7, not EF86.

It sounds "frikin' awsome! On the TMB channel with my Strat, at around 3, it's a beutiful clean full sound, at 5, you can really hear the '84 getting hairy while the 6V6 brings up the clean low end, and at 8 and above, it's crunchy bliss with a bit more focused low end than with the quad of '84's. I can't wait to throw some different octals in there for more taste testing.

A highly recomended modification indeed!

Tim