6K6 primary Z for 6L6pp

[ben-horn]

I saw that some older tweeds and trainwreck clones use 6k6 output transformer for two 6L6 pp Amps while newer Fender use 4k ot's.
What is the difference in sound ?
Will 6k6 gives a tighter bass and better damping factor ?
I want to build a low power ( 400 V B+ /cathode bias)6L6pp Amp
with very low or no NFB but don't want a flubby bass response.

Thank you

[tubeswell]

higher load resistance = more output power (to a point)

[Old Tele man]

Po = Zo * (gm*Vg)^2

...which is nothing more than a variantion of P = I-squared times R

[Krwkka]

I'd like to see this question answered.

What is the difference in sound???

[R.G.]

Every power device has a maximum voltage it can withstand, and a maximum current it can draw. In addition, every power device has a maximum waste power it can dissipate without melting down. And each one has a different operational distortion, the subtle distortion it makes before it clips.

A great deal of work was done back in the Golden Age of Tubes on how to get the most power, least distortion, etc. out of a particular power tube type. Over time, they recognized some things by inspecting charts and graphs of power versus load impedance, distortion versus load impedance, second harmonic versus loads, etc. Those boil down to
- there is a maximum power load for a given tube type, where it will give the best power for the available voltage in a given circuit.
- there is a minimum distortion load for a given tube type, where it will give the least distortion in a given circuit.
- these two loads are similar but not the same.

The 6L6 in a class AB push-pull circuit tends to give lowest distortion for a given supply voltage/circuit at a load of about 6K to 6.6K. The same pair of tubes will produce a higher power before clipping in the same circuit if the load is adjusted to about 4K to 4.4K plate to plate loading. This second condition produces higher second and third harmonics from the tubes before the output runs in to clipping.

What that means is that a pair of 6L6s that make 50W into a 4K plate to plate for a given power supply will produce maybe 30-35W for the same power supply if the plate to plate load is adjusted to 6K or a bit higher; and it will have less distortion before feedback is applied.

Historically, the hifi guys went for the low distortion loads, the guitar amp guys went for all the power they could advertise and let the distortion go hang.

Difference in sound? Subtle second and third harmonic distortion makes guitar guys happy and hifi guys sad. The exact mixture of second and third varies by the exact load relative the tube characteristics, and this is different for the 6L6, EL34, KT88, EL84, and every other power tube. They all have their own flavors.

Whether you load the plates to 6K or 5K or 4K makes not much difference to load damping. Remember that transformers don't have impedances, they have ratios. Changing from a 4K:8 ohm to a 6K:8 ohm will not make a huge difference in damping. And changing a speaker will have massive differences in the low and high frequency character of the sound on the same amplifier.

[tubeswell]

Thanks RG - I stand corrected

[EFK]

VERY informative post!!! Thank you RG - that was priceless and even a layperson like myself can understand it. It also makes me wonder if there may be some truth to the long-debated rumors of early JTM45 amps with the RS transformers wired up for 8K at 16 ohm, but many of them being sold and sounding better if run into an 8 ohm load (which would make 4K, correct?).