Assistance Using Scope to Troubleshoot

[Perkinsman]

I’m starting my first troubleshooting of a homemade amp head using a scope & signal generator & will be asking a lot of stupid questions of the experts here so please indulge me. The amp has switchable dual preamps and the complaint is feedback at the 4-5 volume level that doesn't decrease when volume is turned down on the JCM800 preamp side. My plan was to send a signal thru the amp & try to isolate the problem. I’m going to post pics of the schematic and my gear, Techtronix 465 scope & HP 204b oscillator & cables. First question is where on the amp to input the signal & what settings to use on the scope.

[Chuck H]

Put 100mV @ 400Hz to 1000Hz into the input. Amp controls set as you would when playing through it and the problem occurs.

You'll be probing along the signal chain starting from the input and moving toward the output.

Setting the scope is another matter and depends on where in the signal chain you're probing. I'd suggest a 10:1 probe and just set the division so you can see the wave clearly enough. The right division setting for each node will be different. Perhaps after the distortion is identified a more finite setting for accurate measurements will be recommended. But for now you just need to "see" the wave form.

[Chuck H]

Oh, and...

You really should hang a resistor from the downstream side of that preamp switch to ground. The arrangement as it is leaves the grid without bias. Only for a moment, I know, but it's bad practice. It makes more noise and if anything happens to the switch the tube could be damaged.

2.2M would drop signal about 1dB at that grid. Which is no great shakes.

[nosaj]

I’m starting my first troubleshooting a homemade amp head using a scope & signal generator & will be asking a lot of stupid questions of the experts here so please indulge me. The amp has switchable dual preamps and the complaint is distortion at high volume on one of the two. My plan was to send a signal thru the amp & try to isolate the problem. I’m going to post pics of the schematic and my gear, Techtronix 465 scope & HP 204b oscillator & cables. First question is where on the amp to input the signal & what settings to use on the scope.

I would suggest hooking the scope up to the signal generator and then playing with the knobs to get a feel for how things should operate.
For one you won't be learning how it operates near high voltages and 2 your still learning.

I got a handle on mine that way . The kids really love it when I hook a microphone up to the scope.
Jack Darr guitar amp bookhttps://nartlof.com.br/livros/JackDarr1973.pdfSee page 47 for general test points.

nosaj

[Perkinsman]

Put 100mV @ 400Hz to 1000Hz into the input. Amp controls set as you would when playing through it and the problem occurs.

You'll be probing along the signal chain starting from the input and moving toward the output.

Setting the scope is another matter and depends on where in the signal chain you're probing. I'd suggest a 10:1 probe and just set the division so you can see the wave clearly enough. The right division setting for each node will be different. Perhaps after the distortion is identified a more finite setting for accurate measurements will be recommended. But for now you just need to "see" the wave form.

I edited my intial post to read feedback at low volume, not distortion. I'm going to start in on this tomorrow, thanks Chuck.

[Chuck H]

I edited my intial post to read feedback at low volume, not distortion. I'm going to start in on this tomorrow, thanks Chuck.

That IS an entirely different matter. Fortunately the advice is still the same.

[Perkinsman]

That IS an entirely different matter. Fortunately the advice is still the same.

Sure is....so what are the most common reasons for feedback in a circuit like this?

[g1]

Preamp tubes themselves are probably the most common cause of feedback. While it is occurring hold them with a rag (or something to prevent burns) one at a time to see if anything affects the feedback/squeal/microphony.

[Chuck H]

This^^^^^^^^^^^^^^^^^^^^^^^^^

is the first go to. Especially if the amp was known to work correctly at some point.

If the amp has always had a "feedback" problem then I might suspect parasitic oscillation due to layout and/or lead routing. This is where you'd want to have a scope.

EDIT: This amp doesn't have a master volume so it's going to be loud when turned up. Other things it could be might include microphonic guitar pickups and if an attenuator is being use some have inductors that can emit enough of a field to interact with the guitar when playing too close to the unit. Too close being relative. One attenuator I made had to be modified because I couldn't get within about eight feet of it.

[Pedro Vecino]

The two connections that usually generate more problems (feedback, bad sound...) in the 2204 circuit are the input (to V2A grid) and the output (from potentiometer to V2B grid). It īs advisable to use shielded wire in them.

In that design, another problem will be the different final volume on each channel. If you only have one master volume you can assign it to 2204 channel exclusively. That way you can find a balance between both. It can be done with a relay or switch synchronized with the previous one.

[The Dude]

Agree and I'll add. Often tubes are microphonic in one position and not another. In other words and for example, a tube that might be microphonic in V1 position may be just fine in the V2 position. I often find that simply swapping tubes from one spot to another will solve a microphony problem.

[Perkinsman]

Im going to try & simulate the problem first. Im using an 8 ohm dummy load & have the amp switched to 8 ohm. Since this is my virgin scope&signal gen project, I need to start slowly on this. If I hookup my signal gen black lead to chassis, where does my red lead go? Told u i was gonna ask stupis questions. Also, remember that this a dual preamp so why would the preamp tubes cause a feedback problem on the 2204 (jcm) side & not the 1987 (jmp) side? I'm thinking seņor Vecino is on to something with the wiring shielding. I've never used shielded wiring, what type do I need?

[The Dude]

The easiest way to apply signal would be to just get a 1/4" plug and connect your leads to the plug. That also opens the shorting part of the input jack.

On the preamp tubes: If it were not preamp tubes and were in the power amp, it's likely both sides would feedback, so it is indeed likely a preamp problem. If it's a microphonic issue, you should be able to tap lightly on the tubes and see if one of them is causing the feedback/oscillation. That would be a simple first check.

[Perkinsman]

I get sound at both the middle & one leg of jcm volume control. Is there a preference?

[Chuck H]

The project might be getting ahead of itself. You may not even need to hook up the scope. As you already deduced, the first thing to do is instigate the problem. After that you should tap on the tubes and see if any of them sound particularly microphonic. A thunk or ping is usually ok. A crunchy sound or if the tube wants to break into a longer noise like a shriek or whistle would be a clear indication of a microphonic tube. If it's not a microphonic tube Then...

You should discover what controls affect the problem. What makes it worse and what makes it less or stop. It's probable that Pedro's shielded lead solution is all that's needed and it could be indicated by this information.

As to where to put the red lead, are you ok with transposing the schematic? That is, if I were to mark places on the schematic could you locate them on the board? Making marks on the board would be harder for me because then "I" would have to trace the circuit from the one photo angle.