Took out the bias pot and measured with nothing attached and it was the same - so I grabbed a new pot from my 2nd Gen build pile, reconnected everything, and it all works as it should. Yes!
With the 26k resistor and new pot I get a sweep from -30 to -42, so I set it to -42 for the next phase of testing.
That would be the 3k9 on the rectifier/bias board if I understand correctly?
I sure can, how much smaller? I’ve got 3k3, 3k, 2k7, and 2k2 on hand in 2 or 3 watt ratings.
Not sure if it works this way, but if it’s proportionally “1 to 1” I’d guess either the 3k or 2k7 would be the choice?
But keep in mind I know nothing of the mathematical parts of this, lol. Just trying to ask how the math works.
Correct. It's pretty nonlinear since you are dealing with a spiky charging pulse. Maybe just temporarily tack the 3k3 across the 3k9 (that will be 1.8) and see where that gets you. Once you get tubes in and try to bias them you'll find out what value will work.
Understood, thanks Martin - that’s exactly what I’ll do.
I’ll do that and finish the rest of the power amp testing tomorrow, but have a few questions starting with this part of your power amp test process:
“Install power and PI tubes
Connect a load to the speaker jack
Short PI input (Power Amp In) to ground with a clip lead”
Am I literally clipping a test lead from the switch tip to chassis ground?
Also, I went back and read your post breaking down Gil’s implementation of the “Ampeg PI process” and wanted to know if following his steps would be applicable to my project?
Raoul Duke wrote: ↑Thu Jan 11, 2024 1:52 am
“Short PI input (Power Amp In) to ground with a clip lead”
Am I literally clipping a test lead from the switch tip to chassis ground?
Yes. Doing that insures that the PI input doesn't pick up random noise from the room. See in the yellow shaded part of the schematic here: https://ampgarage.com/forum/viewtopic.p ... 94#p459994 If you have the master volume pot wired in, turning it full CCW does the same thing, and doing that would be sufficient.
Raoul Duke wrote: ↑Thu Jan 11, 2024 1:52 am
Also, I went back and read your post breaking down Gil’s implementation of the “Ampeg PI process” and wanted to know if following his steps would be applicable to my project?
Raoul Duke wrote: ↑Thu Jan 11, 2024 5:12 pm
1. Doesn’t apply because I only have 2 power tubes? Correct
2. Pin 8 on each tube - separate measurements, so 2 DMMs make it easier. No, this measurement is from one cathode to the other, pin 8 - pin 8
3. This is the audio adapter/signal generator app part (400 to 1000Hz sine wave). Yes, but injected at the PI input
4. This is the measurement from step 2 set-up. Yes, monitor the AC voltage across the load as you increase the signal level, looking for ~12V RMS
5. Straightforward - no questions. Remember, DC mV
Last question: do you still tweak by ear after final assembly, or should this be “it” (in your experience)? Personally, I call it good when the measurement indicates balance
All of the above is done after you set the static operating point (bias).
martin manning wrote: ↑Thu Jan 11, 2024 12:02 am
-42 seems a little high. Can you reduce the value of the resistor between the PT and the bias rectifier? I think you might need another 10V or so.
Martin,
The additional 10V would increase the sweep on the -42 end then, correct?
I'll add another point of view on the Last Question. Once my amp was completed I did the full procedure as outlined, then hooked up a speaker. I had my son playing random licks ( we both like Rock) while I made adjustments on the FET, OD entrance and PI pots. On the PI I did hear a difference as I adjusted it but really didn't have much of a preference of one way or another. I think this is mostly due to the fact I was (am?) still in the "honeymoon stage" with this newly completed amp. I simply went through the procedure again and left it.
I'm still amazed that it sounds as good as it does right from the beginning once I got a good set of power tubes in it (had a faulty set at first...). I know this is primarily due to the wonderful resources provided by the members of this forum! I've been following this thread and must compliment Martin on the consistent knowledgeable assistance he freely provides.
Most likely I'll revisit the PI adjustment in the future once the newness has become a thing of the distant past.
Right on Glenn, I feel the same way you do about all the help I’m getting from everyone and especially Martin’s patience given my probably very basic questions.
I’m sure I’ll be in the honeymoon phase for a while as well - really looking forward to seeing how I did all things considered. My original plan was to assemble as much as I could, then give it to my local amp guy to fire up and QC; but now - with everything I learn each day - I feel pretty confident I can bring this to the finish line myself (well, with you guys pushing me along, lol).
