cathode bias question - ampeg m15

[ilyaa]

couple questions about this amp, just wanna get some opinions and thoughts -

here's the scheme: http://www.unofficialampeg.com/schematics/j-12-b.gif

it was sounding kind of honky and felt like the tubes were running really hot. i measured and got ~32VDC at the cathodes of the power tubes, so 128 mA running through the 250 ohm cathode resistor - 64 mA per tube at idle. too hot!! right? i was hesitant to f!!* with the design - almost all original parts in here, BUT they were new JJ tubes and i figured well maybe they pull more current so itd be worth it to tame it down - i raised the cathode resistor 330 ohm now im about 50 mA per tube, still hot but seems a bit better. i know this is a tried and true ampeg design - does it make sense to try and tame it like i did?

other question: even with the amp turned up all the way, its really not as loud as 2 6L6s should be - i checked the output of the PI to the power tube grids and im only getting about 30Vp-p sine wave with volume all the way up. now the voltage at the cathodes, with the new 330 ohm resistor, is about 35VDC, so shouldnt i be getting closer to 70Vp-p at the power tube grids? or is this amp designed to not push the tubes all the way? i dont have any 6SL7 tubes around, but i tried rotating the three that are in the amp and there was no change. please disillusion me if im applying grid bias principles to a cathode bias amp and therefore barking up the wrong tree.

[sa230e]

Is that the right schematic? The schematic is for an Ampeg Jet J12B but you put the M15. The schematic has 6V6s but you mentioned 6L6s.

Anyway, a few ideas:

Cathode current includes screen and anode current so it will appear as if the tubes are slightly hotter than they are. It also seems kind of optimistic to just assume the currents through both tubes are equal (128/2 = 64). Ideally the tubes should be well matched but you never know, one might be pulling way more than the other and that imbalance could be the source of the bad tone. I think it would be worth it to measure just the plate current for each tube.


As for the phase inverter, did you check the input voltage? It's only being driven by one 6SL7 stage and that's being attenuated by the volume and tone network so  it's possible it's just not driving the PI hard enough to drive the output tubes to full volume. A 6SL7 has a mu of what, 70? So that's the maximum possible voltage gain of one stage. In an actual circuit it will be significantly less so just by looking at the circuit, 30Vpp sounds like a plausible figure.

I'm guessing the phase inverter is some sort of paraphase circuit? It doesn't look like a long tail pair.

[ilyaa]

oh yeah wrong here's scheme. here's the right one:

http://www.freeinfosociety.com/electronics/schematics/audio/ampegm15.pdf

[Enzo]

So what is the plate voltage on those power tubes?  Subtract the cathode voltage from that for the voltage across the tube, then times 50ma.  If you had 332v on the plate, that leaves 300 across the tube, and at 50ma that is only 15 watts dissipation.   You have to do the whole calculation, not just look at some current number and assume it is high.

And I agree, only one triode in the preamp, so not a ton of signal hits the phase inverter.   Loudness is usually about the preamp, not the power tubes.

[ilyaa]

oh i see i see

about 415V on the plate - so 415 - 35 = 380. 380 * 0.05 = 19 watts dissipation.

and yeah just one triode before the PI so i guess it's supposed to be a lower key amp. and that's what it is!

thanks guys

[Enzo]

And old Ampegs were designed for clean, they did not embrace overdrive and distortion.  In fact just because so many people were starting to want distortion, they came up with that horrible distortion feature on some later amps.  My old B15 was a darned hifi sounding amp if anything, really nice sound.  Not gainy.