New JCM800 Mini Again

[boostedsuby]

Hi all,
I posted a while back about some problems I had with my newly built (and my first build as well) JCM800 Mini. I built is per Rob Robinette's drawing's (https://robrobinette.com/RR2104_Master_ ... JCM800_6V6). Bass notes seem to cause a feedback as the note is struck. It happens on low E and A, particularly with the bass up or the presence rolled off. I don't know if I need to swap out my OT leads? I did have to replace the power socket because it got damaged on my bench. At that time I realized that I had the narrow blade wired as neutral so I corrected that. Would that cause me to also have to swap the OT leads? Thanks in advance for any help you can provide. After some changes recommended by another member in the initial post, it sounds amazing other than this issue.

[boostedsuby]

After more experimentation, if I max out the master volume and use the preamp volume to adjust overall volume, at about 1 or 2, I hear a scratch in the pot and things get fuzzy and begin to make all sorts of bad noise.

[xtian]

Sounds like parasitic oscillation in the preamp. Too much gain, and/or poor lead dress. Try removing (temporarily) the 470p treble peaker cap between the Low jack and the Preamp pot, and the bright cap on the Preamp pot.

Can you post photos?

[boostedsuby]

I removed the low input from this circuit, as well as the bright cap. The 470k/470pf treble peaker between V1 and V2 has also been replaced with a 100k resistor. Oscillation was what I landed on after some research, but narrowing down the source is where I have some weakness.

[xtian]

I'd concentrate on the first two stages, because you told me the issue starts when you open the Preamp pot past one or two. Replace that 100K back to 470K--that will drop a little gain (back to stock value). Replace the wire that runs from the input jack to V1a with a shielded cable (ground the shield at the input jack only), and move the 68K grid stopper right to the tube pin for best RF rejection.

[alkuz1961]

I once faced a similar problem. There was a strange low-frequency level in the sound of the amplifier. There was no oscillation.
But that amplifier had a GNFB switch, and I noticed that turning on the feedback increased the volume level of the amplifier. So I realized that the feedback is positive.

[boostedsuby]

Thanks for the advice. I will make the changes and advise. I have tried another 2x12 cab that I built. It is loaded with Avatar M65s and is closed back. It still has the issue, but the onset is much higher in the volume range and even then, much less serious.

[Littlewyan]

What's going on here? Looks like you have the Low input jack wiring mixed up with the high input jack wiring?

[boostedsuby]

While researching the proper way to remove the low input, I found a thread here that discussed just that, so I used it as a guide. What do you see wrong?

[Littlewyan]

Not wrong per say, but instead of routing the wire through the input jack I would route it directly from the coupling cap to the 470K resistor. Less chance of interference then.

[Littlewyan]

Definitely do what Xtian said as well. Run shielded wire from the input jack straight to the grid of v1a with the 68K grid stopper soldered directly to the valve pin.

I would also try swapping the grid wires for your output valves, see if you have got positive feedback instead of negative. (Note, swapping the output valve grid wires has the same effect as swapping the OT primaries).

[boostedsuby]

I was using shielded wire for that connection, but I see the point about that resistor hanging out there. I ended up bypassing that turret entirely and running a longer shielded wire to the resistor and then to the pin with minimal exposed lead. I did try swapping OT leads while I was waiting for some guidance and I am pretty clear now on what that sound is. I will tidy up that unnecessary diversion through the jack and see what goes. It is good enough now that I could live with it. I have noticed that the V1B grid wire is pretty microphonic. I replaced the tube and the condition persists. Again, thanks to you all. I came here ignorant and asking for help and you have offered it. All I can offer for now is deep gratitude. Archaeology and CNC machine tools are my fortes, so if you find yourself needing help in those areas, I'm all yours!

[Littlewyan]

Glad you're happy with it. You could also replace the wire from the volume pot to the V1B grid with shielded wire, that should help with that. A good test I like to do with my amplifiers is turn every control on 10 to make sure it's stable (With the guitar unplugged). If it starts feeding back or oscillating then I keep working to stabilize it.

[boostedsuby]

I was thinking about using a shielded wire there, as it happens. I didn't want to do it "just because" though so I was going to research that a bit. The weird bit is that I have done as you suggest and put everything to 10 and all I hear is some hum, but it is present even with volumes down, so I am assuming that is not related to my issue. Again, thanks.