Converted 6V6 amp to use 6L6s... odd results

[octal]

I have a single channel BF Fender Deluxe clone... started out fairly stock. 6V6s into a 6K6 OT, B+ around 400V. I never could never get what I'd consider a good overdriven tone out of the amp... glassy cleans turned into a harsh OD. On the scope, it had a lot of X-over distortion with the tubes biased for 80% dissipation or so.

I opened the NFB loop. A bit better. I converted the PI to paraphase (using a 12AU7), reduced the size of the coupling caps to the output tubes to .047uF and increased the value of the grid stoppers. Perhaps a slight improvement. I then swapped the output tubes for 6L6s, converted to cathode bias with a shared 300 ohm cathode resistor bypassed by 20uf. Presto! The thing is that the clean tones on the amp are dramatically different- it sounds like it has much less treble. I haven't measured the freq response yet, I really wasn't expecting a big difference in clean tone but there it is. I wonder if I was hearing crossover distortion even when the amp was running clean and that was perceived as extra brightness/ glassiness?

Any thoughts? I was thinking that maybe there was a significant difference in the control grid to cathode capacitance of 6v6s vs 6l6s, but the difference (about 2pf) doesn't seem large enough to cause a frequency response change, even with my large 15K grid stoppers....

Nathan

[booj]

Could be the 6L6's are a better match to the o.t.

[MWJB]

Unfortunately, "single channel BF Deluxe clone" tells us that the amp is possibly very different from any reference that we would otherwise have. Does it have the reverb circuit & associated gain stage, does it have the LDR tremolo & associated losses, how have you refigured the wiring to the PI to simulate the same gain as a 2 channel BF Deluxe, assuming your design had less preamp tubes, did you remember to adjust the power supply dropping resistors to correct plate voltages?

If you need 15K grid stoppers on the power tubes it seems that you have gone awry on the layout somehow.

At 80% plate current, in fixed bias, crossover distortion should not have been a factor. At what output wattage were you getting this crossover distortion, were you able to get 22-25W clean from the amp?

In a direct comparison (well as close as you can get anyway), in the same amp, 6V6 do tend to have a more detailed, foward treble component than 6L6 but a softer envelope, but if the above points were all dealt with, it shouldn't have been any harsher than a regular Deluxe. Did you AB against a Deluxe?

I guess that if you like the sound as it is now, then you know what works for you, with your amp (as opposed to BF Deluxes in general)...how you got there seems less relevant.

The BF Deluxe Reverb is a well proven amp, however, variations in running conditions, tubes & speaker from one to the next, means that there can be wildly differing sounding amps.

[pdf64]

A 6K6 OT? That's a big unknown, eg what's the primary impedance - 12k p-p?
My understanding is that the load value will affect the frequency response, with 12k being a lot higher than is usual for 6L6GC.
Also I'd be wary of running 6L6GC on an OT intended for 6K6 - the currents will be way higher than the wire was specced for, possibly resulting in overheating, leading to shorted turns etc.
And the magnetic circuit will likely saturate easily, thereby losing bottom end (as I understand it).

[MWJB]

Hi Peter,

I assumed (never a good idea) that Octal meant a 6600ohm OT primary, rather than an OT designed for 6K6 tubes? It was a bit ambiguous.

[octal]

Just a few points of clarification.

My main question was basically “what possible reasons might there be to explain an apparent significant difference in clean tone (specifically brightness) between the same amp running a pair of 6V6s in fixed bias versus a pair of 6L6s in cathode bias.” It was more of a theoretical question based on an observation than a “why doesn’t my amp sound like a real Fender Deluxe” or “I don’t like Fender Deluxes based on my experience with this amp” question/ statement. I just used the words “Fender Deluxe” to give general idea of what kind of circuit topology we’re dealing with… it’s a loose interpretation, not designed to be a clone.

-Yes, I meant 6600 ohm plate to plate primary impedance on the OT. It's a decent quality OT, rated for more power than this amp could ever put out.

-The output section was stock Fender Deluxe originally- 470ohm screen resistors, fixed bias, 1.5k grid stoppers. I did eliminate the NFB loop.

-I’ve noticed that when running 6V6s in this amp, the tops of a sine wave look slightly “pinched” rather than rounded as the signal level begins to approach clipping. The signal coming from the PI looks like a proper sine wave and the balance is decent. If I had to guess, this “pinching” is a form of non linearity that I’m perceiving as extra brightness.

-Swapping in 6L6s and switching to cathode bias results in sine waves that look like sinewaves up to the clipping point.

-Changing the PI from LTP to paraphase did not really make any significant difference in the output waveform. The change to 15K grid stoppers was not an attempt to cure any stability problem… it was an attempt to reach achieve a specific bias shift/ recovery time constant.

I'll try to measure the freq response over the weekend to see if I can figure anything out. I'm happy with the way it sounds now, but I'd like to learn something from the experience.

Nathan

[MWJB]

Well put simply, the 6V6s in fixed bias might put out 22-25W, even in cathode bias the 6L6 should be putting out a little more than that, with a less strident high end/slightly darker tone. But you're not really comparing like with like, because you haven't just changed one perameter. You have changed power tube type, phase inverter design & PI tube type, enlarged the grid stoppers, dropped plate voltage (what is it now 370-ish from plate to cathode?), method of bias & idle plate current. Your question is therefore rather more complex than "why do 6V6 in fixed bias sound different to 6L6 in cathode bias"...even if it was that simple, the answer is largely already in the question...

It's quite unusual to totally eliminate the NFB loop in a BF design, sometimes an increase in NFB dropping resistor to over 5K can make some amps unstable.

Switching to cathode bias will lop off some plate voltage due to the rise in cathode voltage & increased idle current, plus the 6L6 will draw even more idle current than the 6V6 would in cathode bias, pulling the voltage down further - lower plate voltages typically reduce high end. So your 6V6 were pulling 28mA per tube @400v in fixed bias, the 6L6 are now probably in the region of 55-60mA @ <370v in cathode biased. This alone would explain a very noticeable shift in tone.

You should be able to try the 6V6s as a drop in comparison with the 300ohm cathode resistor, they'll be quite hot but will be fine for a quick AB test at least, even possibly bias up OK for permanent install.

[octal]

Thanks for your response, MWJB.

I get what you're saying about changing multiple parameters, but I did the changes one at a time and listened after each change. I thought I heard a little difference with the PI change, a little difference when I changed this or that, but when I changed the output tubes and bias type, it made a major, easily detectable difference, both on the 'scope and to the ear.

I think you're on to something with the lower effective plate to cathode voltages in cathode bias mode... it would be a good experiment to temporarily swap the 6V6s in.

It is interesting that eliminating NFB can cause a BF Fender to be unstable. My layout must be better or perhaps I'm just lucky!

Nathan