Best OT Primary Z for 6BQ5 in AB1?

[bob p]

I'm thinking about building an AC-15 “lite” amp (no tremolo). Basically, it would be a stripped-down/half-power AC30 with the normal channel and maybe the top boost channel. Before getting started I'd like to ask if anyone who has already done this can offer me practical advice about changes in the amp's behavior with an OT primary Z of 6k6 vs. the 6BQ5 “standard” of 8k.

I haven't picked out iron yet, but I'm considering the following:
(I really prefer a transformer that has the multiple secondary taps.)

Hammond 1760H; 6k6 primary with 4/8/16 secondary taps.
Hammond 1760E; 8k primary with 4/8/16 secondary taps.
Hammond 1750Y; 6k3 primary with 8 ohm secondary tap.

Also, if anyone has a plot of power & distortion vs. OT primary Z for a pair of 6BQ5 in push-pull AB1, I'd appreciate it. I haven't had much luck finding one.

Thanks!

[Dave H]

I’d expect the difference between 8k and 6k6 to be subtle in the AC 15 circuit so I’d just go with the better sounding tap if your transformer can do both values. My OT won’t do that but it sounds fine at 8k to me. If you don’t make any other adjustments to the AC 15 circuit I’d expect the crossover distortion to be greater at 6k6. The power will be about the same.

If I can find the right value load resistors I’ll measure the power and distortion at both values and post the plots.

[jazbo8]

I’d expect the difference between 8k and 6k6 to be subtle in the AC 15 circuit so I’d just go with the better sounding tap if your transformer can do both values. My OT won’t do that but it sounds fine at 8k to me. If you don’t make any other adjustments to the AC 15 circuit I’d expect the crossover distortion to be greater at 6k6. The power will be about the same.

If I can find the right value load resistors I’ll measure the power and distortion at both values and post the plots.

+1 The difference between the two will be negligible.

Jaz

[Dave H]

If I can find the right value load resistors I’ll measure the power and distortion at both values and post the plots.

There's more power at 8k for the same distortion level, 17W vs 15.5W at 6k6.

AC15 6k6 Scope.pdf
AC15 6k6 Spectrum.pdf
AC15 8k Scope.pdf
AC15 8k Spectrum.pdf

[trobbins]

The STC report is the most comprehensive manufacturer data around that I know of.
Retrovox Data page

My AC15 clone works beautifully on 8k, although I haven't really tried any good comparative testing with 6k taps I have - but to my mind, that difference is minor unless I know I have to work with say an 8 ohm speaker instead of the rated 15 ohm output winding I have.