So perhaps a 100k pot would be a better idea so as not to load down the signal generator too much? Or bigger, like 250k?
EDIT: Or maybe try that 220k resistor instead of the 10k, Jay, and see how your signal behaves.

Now, why did I suggest that attenuator contraption? As I remember from one of your previous posts, your signal generator can't put out less than 100mV p-p. Because the amplifier stage has gain, you will get a higher amplitude signal at the output, whereas g1 suggested (in reply #28 or thereabouts) the following

Quote from: g1 on October 23, 2024, 09:05:54 PMSet the bias to the minimum mV reading across each 0R6 (no signal applied).  What reading do you get?
Now get a nice clean sinewave at the output like the first pic in post #24, but with less amplitude, maybe 100mV peak to peak.  Do you still have a nice clean sinewave like that, or is there a crossover notch in the middle of the sinewave?
For solid-state amps, we just increase the bias til the crossover notch just disappears, no more is needed.

Therefore, you need less signal at the input, hence my idea for the attenuator.

If you wired it as per my diagram, the highest resistance on the sub box means the least amount of attenuation. It's like a high value potentiometer turned up, and if you set the resistance to 10k, you should have half of the signal at the junction. Is it like that? I need to check whether we're on the same page.

Quote from: RG100ESROX on December 07, 2024, 09:50:39 PMHere is the result of the signal input into the FX return, biased just out of xover at 16mV,  at the highest resistor sub box setting (1M) with a 10K in series with the sub box.

The oscilloscope is hooked up to the speaker output / dummy load box, right?

QuoteAs you can see, we have clipping at the highest resistance setting with a .1mV/1KHz injected signal.

Highest or lowest? I'm asking because the file name says 'lowest'.

Quote from: RG100ESROXSine wave (is it just me, or does this sine wave look pretty bad? Is that cover distortion we're seeing?)

Yeah, it does look bad. AFAIC there is crossover and clipping. Reduce the signal perhaps?

Quotehow do I do that since the front panel controls do not function while we're plugged into the FX return jack. Do I just increase the signal level? If so, to what level? How many volts? I'm not clear on this

First, bias should be right so there's no crossover. Once that is done, and, if I remember well, it is, move on to the next steps.

Btw, did you place that resistor in series with the signal generator like I suggested earlier, in reply #258?

Do this experiment:
Unplug your signal and resistor box combo from the amplifier. Disconnect your scope from the amp too, just in case.
Now, hook up your scope probe to the output of the SG / resistance box, the point where it says 'to plug' on the diagram I posted in reply #258.
Set the signal level on your generator to, say, 1V. What can you see on the scope screen?
Now change the resistor value on the substitution box. What can you see on the scope? Try again and look at the scope.
 
This is your makeshift volume control now. Use it to adjust the signal going into the amp's return jack. Then, on your oscilloscope, watch the waveform coming out of the speaker jack and see when clipping starts. Full clean power is just before the onset of clipping, as I understand it. Then post the signal voltage needed to get there, as requested by g1.

Quote from: RG100ESROX on December 07, 2024, 06:41:02 PMI measured the DC at the speaker output jacks, and it read -.88mV...

Did you take this reading just now? If so, I understand that you put the theory to the test and it turned out to be true, right?

Bad solder joint somewhere?

EDIT: I don't see how injecting signal into the effects loop return could have done any damage. After all, that's where your effects go in if you use any. Also the preamp out is internally connected to it so...

Quote from: RG100ESROX on December 07, 2024, 03:09:46 AMPreamp settings were: P10-B0-M0-T10 (and pulled for clipping)-V3~4-G10

The distortion I'm hearing sounds like if you took a distortion pedal, and on top of a good sounding amp with the gain set to 10, you add just a tad bit more distortion on top of the amps gain, and it sounds like you've turned the gain up just a little too much to the point where it no longer sounds good any longer. It's just too much distortion. The only other way I know of describing this distortion is, kind of like Xover distortion. It does not sound like a good distortion while you're playing. It's just....too much distortion.

Sorry, it's just one of those things that's hard to describe, but we all know what 'Good' distortion sounds like. Well, this is just too much distortion to sound good.

I understand it's not easy to describe tone in words. You've done a pretty good job anyway, afaic.

Another question, how are YOU (you specifically) realizing channel switching?
*Looking at the schematic, the switching works by simply shunting either channel's output to ground, so only one of them puts out signal.

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMYES. I RECORDED A SOUND DEMO OF THE DISTORTION I AM GETTING FROM THE AMP. TRUST ME, SOME MIGHT SAY IT SOUNDS FINE, BUT IT'S NOT. IT'S THE BAD KIND OF DISTORTION SITTING ON TOP OF THE GOOD DISTORTION.

I've listened to your demo again just now. I'd like to ask you to post your preamp settings for it, if possible of course.

EDIT: Also, as difficult as it may be to do using words, could you please describe the distortion you hear?

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMQ:Also, do you have another dummy load resistor to try?
I DO NOT. JUST THE ONE (disregard how you see the dummy Load box connected in the photo. it's for the image of the box only.)

I asked that question because you'd said that your current readings were too high at ZERO signal, so I figured there might have been something wrong with the dummy load. Because you took what looks to be correct (based on info from this thread) readings later on, it seems the DL is fine and you had most likely taken those alarming readings with signal in. (Although at some point you did write that the current jumped up WITH AND WITHOUT signal applied, so I'm confused).

Therefore, I encourage you to refrain from using the term IDLE CURRENT for the time being because you sometimes tend to apply it to working / under-signal conditions. I can't speak for other people here, but it gets me confused more than it should. It is critical to be as precise as possible and post all necessary details, such as whether the generator is on or off, where it's plugged in, signal level, possible attenuation etc. As sir Francis Bacon put it, 'The ill and unfit choice of words wonderfully obstructs the understanding.' I have been guilty of it a few times in my life too.

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMI AM ASSUMING THAT THIS WILL WORK IN PLACE OF THE 22K POT THAT I WAS SUPPOSED TO BUILD...? I BROUGHT MY RESISTOR SUBSTITUTION BOX WITH ME. I'LL START OUT AT THE 1M, AND ADJUST ACCORDINGLY FOR THE OUTPUT SIGNAL INJECTED AT THE FX RETURN?? AND YES, I AM GOING TO HAVE TO DO SOME CREATIVE WIRING IN ORDER TO USE THE SUBSTITUTION BOX.

I would add a resistor in series between the signal generator and the resistor box so as to have a resistive divider. Diagram attached.

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMYES. I RECORDED A SOUND DEMO OF THE DISTORTION I AM GETTING FROM THE AMP. TRUST ME, SOME MIGHT SAY IT SOUNDS FINE, BUT IT'S NOT. IT'S THE BAD KIND OF DISTORTION SITTING ON TOP OF THE GOOD DISTORTION.

I listened to it and, you're right, I can't hear it. Can this nasty distortion be heard on cleans too? If possible, could you plug your guitar in to the FX return and play it to see if it's there? You might have to use the volume knob on your axe to control the loudness.
**Do it only when you have gotten your bias right. You don't want to damage your amp.

Quote from: RG100ESROX on December 06, 2024, 07:29:33 PMThe ground connection that connects to the tip, is completely disconnected with the phono plug plugged into the FX return jack.

I have already pointed out that the tip of the RETURN jack is NOT grounded with nothing plugged in. It's connected to the preamp out. Your front-end input jack is tied to ground with nothing plugged in. See the schematic attached.

Quote from: RG100ESROX on December 06, 2024, 03:01:57 AMAND...we're injecting the signal in at the FX return

This shouldn't make a difference other than the signal being processed (amplified, clipped, shaped etc.) when going through the preamplifier. You can plug your guitar straight into the loop return and play it like that, no problem. Not that you will like the results, but you can. I have done this a number of times.

Did you build that attenuator to control the signal level going into the return jack?

Also, do you have another dummy load resistor to try?

Have you tried playing the amp at your parents' house before you took those alarming measurements?

Quote from: RG100ESROX on December 05, 2024, 09:39:40 PMIf I remove the dummy load. The idle current with or without an injected signal is about 5mV.

If I understand it correctly, when you inject a signal, it's no longer idle. Idle is when NO SIGNAL is being applied, so the device is not amplifying anything except noise, it's idle.
When you put a signal in, the output devices will conduct, so the current through the output devices and anything in series with them will rise. Correct me if I'm wrong, anyone.

What is different from the previous setup when it was more or less functional?

Quote from: RG100ESROX on December 05, 2024, 03:15:38 PMWith initial power up this morning, and with a 8Ω dummy load plugged in. My idle bias current rose extremely fast and way too high to proceed (trim pot set to FCCW.)

Could this be an indication of the issue with the amp?

How to proceed?

Did it rise at idle or with signal applied?

Jay, the dummy load will get hot. It soaks power in, so that's the way it is. If you haven't done it yet, put a heatsink on it.

If you plug your signal generator into the FX loop return, the preamp should make no difference. Personally, however, I would turn every knob on the preamp down to zero in case any noise were to bleed through. Maybe I'm paranoid, I don't know.

I don't know what circumstances you are under at the moment, but there are a few questions and requests that you haven't responded to.

From g1
1. If you connect generator to the FX return jack, can you get full power with a clean sine wave into a dummy load?
If so,...
2. what signal level is required from the generator to get there?

From me
3. Are you sure the distortion is not mechanical rattle? You know, from a loose screw, bolt or nut on a transistor, socket, switch, etc.? Or maybe something in the room is vibrating? If it's definitely not mechanical...
4. Could you record a sample of the distortion you're talking about so the more experienced guys can get an idea of what we're dealing with? Even with your phone perhaps?
*I, as an inexperienced guy, would like to learn from that as well.