Deluxe reverb style buid OT impedance question.

[Dingleberry]

Hi.
Finally I have time to start my next project. It's going to be a single channel blackface AB763 style front end into 2x6V6 power amp in a 2x12" combo cab with reverb and power tube bias vibrato.
An amp that I've been longing for years...
Combo cab and the chassis are recycled from an old blown up Italian FBT amp I bought few years ago as a project platform.

The output transformer I have on hand is From another Italian amp,
Geloso G213-A, a "Hifi" amp of it's own times. The G213-A has an 6V6 p-p output stage and sizewise the iron looks really hefty for a 2x6V6 amp, bigger than vibrolux reverb OT.
It has 10K ra-a so it's bit on a high side for fixed bias 6V6 amp with something like 400Vdc on plates. Fortunately it has multiple output taps so I have two options at the moment.

8 ohm load into 14 ohm tap gives 5K7 : 8ohm ratio, a bit on the low side.
16 ohm load into 18 ohm tap gives 8k9 : 16ohm ratio, a bit on the high side.

I have tried to read everything there is about this subject, but of course single answer does not exist. Have you guys tried 6V6 p-p with different primary impedances and which of them have you preferred? Lower for sweeter clean and higher for sweeter drive sounds?

-T

[Roe]

5k7 should work if the voltages are somewhat low. 8k9 also works but you may need to raise plate voltage and lower screen voltage somewhat

[Dingleberry]

5k7 should work if the voltages are somewhat low. 8k9 also works but you may need to raise plate voltage and lower screen voltage somewhat

That's what I've read, BUT. Fender did run deluxe reverb plates somewhere around 410V-420V into 6k6 primary and that's not somewhat low voltage imho for 6V6's and also lower Z than the data suggested 8k. Hifi amps usually run 6V6's lower voltages and higher primaries, so that's the total opposite...

There seem to be lots of examples of both ways doing it. My PT will put out something like 410 Vdc so I just probably experiment and go with the one that sounds best for my needs.

-T

[martin manning]

At 400V+ I like 8k9 much better than 5k7 from a plate dissipation point of view.

[Stevem]

If that OT is the size of a vibro,then it was a class A output stage and I would not have any reservations about running it hard in straight up class A/B!

[billyz]

I am with Martin on this one. The higher primary will be much easier on the tubes at the voltages you specified. From my experience the tone will be very rich in the all important midrange. The lower primary impedance will give it a more lively tone , more apparent volume. Why not make it switchable . I think either load will work.

[Dingleberry]

If that OT is the size of a vibro,then it was a class A output stage and I would not have any reservations about running it hard in straight up class A/B!

It was actually fixed bias amp. You can find the schematics if you google geloso 213-a.

[Dingleberry]

If that OT is the size of a vibro,then it was a class A output stage and I would not have any reservations about running it hard in straight up class A/B!

It was actually fixed bias amp. You can find the schematics if you google geloso 213-a.

[Stevem]

Just because it's fixed bias does not mean it was not biased class A!

[Dingleberry]

Just because it's fixed bias does not mean it was not biased class A!

Good point.

-T

[Roe]

At 400V+ I like 8k9 much better than 5k7 from a plate dissipation point of view.

agreed on this. I'd consider 5k7 if voltages are around 380v. If voltages are higher than a deluxe reverb, then 8k7 and 1k screen resistors may work

[Dingleberry]

I'm starting to lean towards making the amp ~40-50W.
I might want A bit more headroom and bigger clean sounds than 2x6V6 can offer.
I have one set of recycled transformers that would suit well for that kind of project. OT has primary Z somewhere around 5k-5k2 and PT puts out something like 420-430Vdc under load. Something in the spirit of EZG-50. Big blackface tone and 2-knob reverb with power tube bias vary trem.