I'm trying to set the trimmer on my #124 clone. I've tried hooking up a software spectrum analyser, to a 10 ohm dummy load on the 8ohm output. I used a testsignal of around 440 Hz.
On the analyzer I see a spike at around 400 and one a bit smaller at 800 Hz. Turning the trimmer I can turn up or down the spike at 800 Hz. What puzzles me is, that I get the tallest spike when the trimmer is turned all the way to one side. I've tried different tubes but it's the same. I tried changing the trimmer from 5k to 10k but with no change. I've also changed the resistors to 100k both, and used a 20k trimmer as described by Andy, but the result is still the same.
Last, I've measured the AC signal with my DMM, from each plate to ground, adjusting for equal volts. This way, the trimmer is set about 1/3 to one side.
I haven't tried the listening test yet. I probably should , but I want to know if I'm doing something wrong with the spectrum analyzer method.
I usually run a 1Khz signal through and use my scope to look at the signal, then adjust for balance on either side of the sine wave, but I've used the ear method as well- sounds better a bit off balance imho
I've not put my amp (D'Lite 44) on a scope to see the effects, but I have adjusted the trimmer and I can't say I can really hear any difference at any setting, so I've left it at 12:00. What should I be listening for? Should I hear a difference more when playing chords or single notes, bass or treble?
Bill
Once again I'll answer myself by saying I should have read the threads linked above before asking my dumb-*ss question I guess I expected the PI trimmer to have a bigger effect than it actually does...
I take the signal off the dummy load, I typically run the amp clean, Rock mode, mid and brite off, controls dimed, then I turn my signal generator up to just at the point of clipping (Handy time to measure output power as well) and adjust. I find that the amp makes the most power when the sine wave is balanced positive and negative.
I use a balanced long plate tube, 100k and 110k plate resistors and a 10k trimmer. I remember the 6v and 3v plate difference spots on the trimmer, play a note that wants to sustain into the octave and adjust in that range to find the spot that allows it to happen the easiest. Doesn't matter if it's in OD or not. This is probably not the most scientific method.
BTW if you can't find any notes that want to sustain into the 2nd harmonic adjusting the PI is not going to help, you need to tweak elsewhere.
soje wrote:I tried changing the trimmer from 5k to 10k but with no change. I've also changed the resistors to 100k both, and used a 20k trimmer as described by Andy, but the result is still the same.
Have you measured the values of your resistors? The tolerances may be stacking against you such that you are on the wrong side of the balance point to get anything approaching null using the trim pot.
On the analyzer I see a spike at around 400 and one a bit smaller at 800 Hz. Turning the trimmer I can turn up or down the spike at 800 Hz. What puzzles me is, that I get the tallest spike when the trimmer is turned all the way to one side. I've tried different tubes but it's the same.
Interesting. I observed exactly the same with a non-HRM build. Used 1% resistors for the PI.
Ended up with adjusting like Funk
I usually run a 1Khz signal through and use my scope to look at the signal, then adjust for balance on either side of the sine wave
, but I want to know if I'm doing something wrong with the spectrum analyzer method.
