best multi tube OT impedance

[makepeace]

Hello again reverent tube gurus.

I'm in the process of designing a guitar amp with a super adaptable push-pull output stage. My plans are to have switchable fixed/cathode bias, both adjustable, and a choice of a myriad of output pairs (6K6, 6V6, 6L6, 5881, EL34, KT66, KT77, KT88, 6550's, KT120's), focusing on getting the best tone from 5881's and EL34's.

I'll be working with a B+ adjustable from 300V to 480V.

I'm looking for a recommendation for an OT primary impedance. I've done some research and popular values for the various tube pairs seem to be:

6V6: 8000ohms
6L6: 6600ohms
EL34: 3400ohms
6550: 3500ohms
KT66: 5000ohms
KT88: 4500ohms

I know the effects of this vary greatly, but would a safe bet be something like 6000 ohms? Don't really want to mess around with multi tapped primaries and things.. there are already going to be too many controls and switches on this.

Also, feel free to shoot down my idea (with valid reasoning).

Thanks,
Jono

[pdf64]

EL34 are more normally ~3k4, 6L6GC ~4k, KT66 6k6-8k.
An OT with several secondary taps, eg 4/8/16, which can also be used for 8/16/32 for alternative primary impedance, eg 4k / 8k, may be useful.
Bear in mind the primary dc/ac current of KT88 (static and max signal) when selecting an OT.
Consider limiting the max VB+ to ~400V in cathode bias, to avoid large value cathode bias resistors and consequent bias shift / crossover distortion at high signal levels.

[makepeace]

Thanks for the reply. Yep, the EL34 one was a typo. Have 3400 written down in front of me. Yep that also crossed my mind.. When swapping tubes I wouldn't mind flicking switches/swapping plugs at the same time.

Would a safe bet then be to go for a 4k into 4,8,16? I assume its also ok to parallel secondary taps for more primary options?

[pdf64]

'Would a safe bet then be to go for a 4k into 4,8,16?'
Yes. Though 6k:4/8/16 is probably equally as valid a compromise.
Another option would be a JTM 'RS' type which has loads of primary/secondary options; but it may not cope with KT88 at 480V

'I assume its also ok to parallel secondary taps'
Hmm - almost certainly not, as (guitar amp) tapped secondaries are effectively one winding; to put them in parallel you'd need to access the start and end of each secondary section. More of a hifi OT thing.
The RS type, or Hammond 1650 series, would enable that though.
Pete.

[makepeace]

thanks again pdf64. i realize now that paralleling the taps would be a veeeery bad idea.

on second thoughts, a 6k6:4/8/16 would probably be the best bet.. then i could satisfy the EL34's and 6L6's nicely. i was under the impression that the 'correct' output transformer for a 6L6 DR output stage is 6k6? is 4k a better idea for that?

6V6 tone would suffer a bit with a 6k6, but I can deal with that as I have a 5E3 for those tones .

[Chuck H]

FWIW...

It does depend somewhat on plate voltage, but, 6k6 is about as common as it gets. 6L6, 6V6, EL34 and KT66 tubes have all been run into 6k6 (or near enough) on good sounding guitar amps. As noted, you could impedance switch the secondary to get 3k3 for 6550 and KT88 tubes (which seem never to be ran into the generic 6k6 load.

[pdf64]

No, you're good, 6k6 is correct for a DR.
But 6L6 / 5881 may be short changed at 3k3 or 6k6 IME.
However the Hammond 1650PA (6K6ct 04/08/16, 60 watts) has UL primary taps, which could be used to achieve a 4k primary I think.

[Chuck H]

However the Hammond 1650PA (6K6ct 04/08/16, 60 watts) has UL primary taps, which could be used to achieve a 4k primary I think.

That's a clever idea. I do think the taps are 40%. What that means to the actual impedance I'm not sure. But a bench test would be easy enough and I'll bet that some combination of taps and secondaries could be found that is more ideal for each tube type. It could be a PITA if you wanted to make it switchable on the fly though.

[Enzo]

I think it valuable to keep in mind that while we can do all manner of calculations, there are boocoo amplifiers on the market that switch between say 6L6 and EL34 with nothing more than the change of a bias voltage switch. "Optimal" OT impedance aside, the necessary value is SO flexible that these amps work just fine. It isn;t rocket surgery, we don;t have to figure it all that closely. And sometimes the "wrong" impedance winds up sounding better.

[Gregg]

I think it valuable to keep in mind that while we can do all manner of calculations, there are boocoo amplifiers on the market that switch between say 6L6 and EL34 with nothing more than the change of a bias voltage switch. "Optimal" OT impedance aside, the necessary value is SO flexible that these amps work just fine. It isn;t rocket surgery, we don;t have to figure it all that closely. And sometimes the "wrong" impedance winds up sounding better.

+1.
Don't spend too much time into wondering what the best impedance is and save yourself a lot of trouble figuring out something that others have already figured. Theory is one thing guitar amps are another.

[Chuck H]

Case in point would be the Trainwreck Express. It uses EL34's or 6V6's into a 6.6k load. That's "too high" for EL34's in a guitar amp and "too low" for 6V6's. If you can find one of these rare amps for sale you might want to buy it for $30,000 (US).

[makepeace]

Thanks for all of the replies fellows. Yes I remember posting a while back about output impedance of an output stage and realize that its a little bit like quick sand. Just want to make the 'right' decision on an OT, or rather - not the wrong one. Especially based on the number of tubes I'd like to try in this amp.

Case in point would be the Trainwreck Express. It uses EL34's or 6V6's into a 6.6k load. That's "too high" for EL34's in a guitar amp and "too low" for 6V6's. If you can find one of these rare amps for sale you might want to buy it for $30,000 (US).

Eish! Think I'd rather build a clone

[Steve Conner]

Well, anything on the order of a few thousand ohms should work. You can also move the speaker impedance up or down one tap, which multiplies or divides the load seen by the tubes accordingly.

So for instance I have one homebrew that has (I think) a 6.6k to 16 ohm OT. I have tried everything from 6V6s to KT88s in it. For KT88s I connected an 8 ohm speaker, and the OT seemed to work very nicely as a 3.3k to 8.

I'd be inclined to go for an oversized, hi-fi-ish OT like the Hammond ones. Maybe even the hi-fi ones, not their new guitar line. The better the quality, the easier it can tolerate being used at a different impedance level than what it was designed for. For instance, a small, cheap 3.3k OT might give very little bass when used with 6V6s, because the designer skimped on the magnetizing inductance.

In short, if you go for a "characterful" undersized guitar-style OT, then it might start showing a less desirable character if used with different tubes than what it was designed for.

Chuck: Impedance goes as the square of the turns ratio, so 40% taps have an impedance of 16% of the main winding, which is too low to be useful.

[makepeace]

Thanks for your input Steve. What would you say to something like this: Edcor 6K6: 4, 8, 16 ?

[tedmich]

at 3.22" high couldn't those Edcors fit in a 2U box on their sides? Cant find the horiz bells at edcor...