Parallel single ended 6v6 amp with a half power switch

[fopoman]

I'm looking for a parallel single ended 6v6 poweramp with a half power switch... example. I've looked at the old Angela super single ended champ. Just needed a switching example.

Thoughts on a pair of output transformers for the project???

I really appreciate y'all and the help I've received on this site. It's amazing how far we've come since the old message board days.

Thanks again,
Nate

[Stevem]

You do not need a pair of output transformers unless you want to build it that way!

With single ended OTs the same primary impedance that works for one tube works for two in parallel.

If your running the output tubes biased by a Cathode resistor(s) then all you need to do to have a half power setting is to switch off the drive signal to one output tube.

[fopoman]

Thanks

[martin manning]

You do not need a pair of output transformers unless you want to build it that way!

I agree with that statement.

With single ended OTs the same primary impedance that works for one tube works for two in parallel.

Here's where I start to disagree.

If your running the output tubes biased by a Cathode resistor(s) then all you need to do to have a half power setting is to switch off the drive signal to one output tube.

Doing it that way in half-power mode you have the same quiescent operating point for both tubes, but the active one will be seeing half the load impedance it did when both tubes were working. I would switch (un-short) a large resistance (10k, say) in between the cathode of one output tube and a shared cathode resistor and cap to put it in cutoff, and switch the speaker connection down to an OT secondary tap with half the impedance used for two output tubes (8Ω speaker from 8Ω tap to 4Ω tap e.g.). A single DPDT switch will work for this. You could also use separate Rk and Ck for each tube, which would preserve the frequency response and bias excursion in single or parallel mode. A 4k OT designed for SE 6L6GC would be fine. The other way to go about it would be to get an 8k OT and mismatch the secondary one click up for parallel tubes (8Ω speaker on a 16Ω tap). Either way you need a 10W OT, and for 8k/4k load, primary voltages should be around 310 plate and 250 screen to center the operating point.

[FUCHSAUDIO]

FWIW: The Weber WSE-15 is a clone of a Hammond SE output and it's pretty darn good. The original Lucky-7 (one EL-34) used them, and they held up and sounded great.

If you make a half power switch, you want either an R/C network for each tube and shut one tube off (lift from ground), and move your speaker tap accordingly. 4-ohms becomes an 8 ohm output, 8 becomes 16 and so so forth.

Shutting off the drive to one tube and leaving it biased and on, isn't optimal. If you want to have one R/C network, you can change the resistor value when you ditch one tube. Either way works really.

[Phil_S]

Build it and pull one tube to see whether you are really interested in half power. I think you'll find that you can't really hear much difference in terms of perceived volume. And if that's the case, you can decide if it's worth doing anything about it.

[Colossal]

Build it and pull one tube to see whether you are really interested in half power. I think you'll find that you can't really hear much difference in terms of perceived volume. And if that's the case, you can decide if it's worth doing anything about it.

I was going to suggest this as well. However, by way of example, the Dr. Z Joe Walsh inspired Z PLUS PSE 2x6V6 does have a half-power switch (7/15W is the published output value) and there is an audible difference (which is mentioned in a review, IIRC). So the OP may have actually utility for the change in feel and tone. But volume savings? Not much.

To do this, I would shunt one tube's grid leak to ground, killing the input signal, and just let the electrons flow otherwise.

[Firestorm]

I built something along these lines with parallel Hammond 125ESEs wired to three sets of taps for 2K5, 5K, and 10K primary impedance (secondary load is always 8R). There's a volume pot in front of each output tube so you can turn them up, down or off. I used the "E" OTs to get a high enough DC current rating to run 6V6, 6L6 or EL34s in any combination. 10K was for a single 6V6; 5K for dual 6V6 or 6V6 plus a 34 or 6L; 2K5 for dual EL34s or 6L6s. Most flexible for me was the 6V6/EL34 combo at 5K.

[statorvane]

Here's my take - a pair of 6CA7s driven by parallel Hammond 125E OTs. Preamp is derived from a Hiwatt DR103. Sounds nothing like the Hiwatt, but sounds great. My favorite amp.
I used two OTs as I had one from a previous build and wanted to use it on another build - but different. So I paralleled another Hammond OT. I have tried EL34s, 5881s, 6L6GCs, and 6550s. The 6550s (new production Tung Sol) sounded pretty good, but the 6CA7s - something about those tubes I really like them in this build.

No 1/2 pwr switch, but I have an attenuator when my wife is at home.

PSE103_Schematic.pdf

[bepone]

i like it!
i've built many S.E. amps in my life..2xkt88 in parallel SE 30W is the beast..
this topology with 2 output OT's i never tried, but now when i see yours, immediately arrived idea of un-equal biasing of output section / hot-cold/ to fatten the sound (possible-not possible all the questions, searching the end in the labirint )

[statorvane]

If I were to build another one, I'd leave out the cathode switch on V1. The shared cathode sounds so much better than split.

[Reeltarded]

I built something along these lines with parallel Hammond 125ESEs wired to three sets of taps for 2K5, 5K, and 10K primary impedance (secondary load is always 8R). There's a volume pot in front of each output tube so you can turn them up, down or off. I used the "E" OTs to get a high enough DC current rating to run 6V6, 6L6 or EL34s in any combination. 10K was for a single 6V6; 5K for dual 6V6 or 6V6 plus a 34 or 6L; 2K5 for dual EL34s or 6L6s. Most flexible for me was the 6V6/EL34 combo at 5K.

You are a witch.

[Firestorm]

I built something along these lines with parallel Hammond 125ESEs wired to three sets of taps for 2K5, 5K, and 10K primary impedance (secondary load is always 8R). There's a volume pot in front of each output tube so you can turn them up, down or off. I used the "E" OTs to get a high enough DC current rating to run 6V6, 6L6 or EL34s in any combination. 10K was for a single 6V6; 5K for dual 6V6 or 6V6 plus a 34 or 6L; 2K5 for dual EL34s or 6L6s. Most flexible for me was the 6V6/EL34 combo at 5K.

You are a witch.

It was two EL34s at 2K5 that had the townfolk showing up with pitchforks and torches.

[Reeltarded]

You know all the magic Marshalls are 2k2/2k5... thats for quads, but also PP..

heh.. i said pp

[fopoman]

Thank you Folks!