Amp repair help….

[Norton666]

Ok I’m officially stumped. I have a 78 JMP that started whistling on me at my last rehearsal. I assumed it was a preamp tube, so I replaced them. Now, with nothing plugged in…it sounds like a dang air raid siren! Suggestions?

 

[japetus]

Try going tube by tube in the v1, then v2 and ect. I have tried replacing a tube before only to have replaced it with another microphonic tube on accident.

 

[Mikeyboyeee]

Check for ground loop. My Delisle switcher makes my high gainers squeal like that. Once I plug direct into amp Output still going thru Delisle, it goes away/ That and typical extra hum.

 

[blackba]

also check if any of the knobs (like volume for example) change the level, that will help you narrow down where it is in the signal path of the amp.

 

[glip22]

Check your input jacks. Whistle is usually a grid. Follow your first stage grid wires. Move them and see if it stops

 

[Findthetone1]

Check for ground loop. My Delisle switcher makes my high gainers squeal like that. Once I plug direct into amp Output still going thru Delisle, it goes away/ That and typical extra hum.

Really? Is that at high volume or all volumes? I was considering one of their switchers down the road but it's just for home use and ease of use with different amps.

 

[Mikeyboyeee]

Really? Is that at high volume or all volumes? I was considering one of their switchers down the road but it's just for home use and ease of use with different amps.

All volumes.

 

[Findthetone1]

All volumes.

Well, that sucks..

 

[Mikeyboyeee]

Well, that sucks..

It does. Reading up on it some others have had same issue. Related to ground loop. Trying to solve for, got it a bit better, but still there.

 

[Findthetone1]

It does. Reading up on it some others have had same issue. Related to ground loop. Trying to solve for, got it a bit better, but still there.

I hope you can figure it out. I really like what they have and the prices. I like they have an option to put my PS-100 into it.

 

[Mikeyboyeee]

I hope you can figure it out. I really like what they have and the prices. I like they have an option to put my PS-100 into it.

That is how I use it.

 

[glpg80]

Is it modded?

 

[scottosan]

Divide an conquer. It’s definitely oscillation and does it with no signal. So, pull the phase inverter tube. If it still whistles you know power amp. If it goes away likely preamp. Then perhaps some further suggestions can be given

 

[psychodave]

Could be lead dress to the preamp tubes as well. Like was already said. Pull preamp tubes one at a time.

 

[glpg80]

Is it modded?

I’ll ask again. Is the amplifier stock or not?

 

[panhead]

One of my marshalls did that and it was capacitor? i think that was under the board. Someone else fixed it

 

[Norton666]

Update! I finally got around to troubleshooting and with no preamp tubes installed, there is now air raid siren. The only time it happens is when I put a tube in V2. Tried many different tubes, with no avail. Suggestions? Thanks fellas!

 

[Norton666]

I’ll ask again. Is the amplifier stock or not?

The amplifier is completely stock.

 

[glip22]

Did you replace the power tubes? If you did not, you should

 

[Beyond Black]

Really? Is that at high volume or all volumes? I was considering one of their switchers down the road but it's just for home use and ease of use with different amps.

I’ve got the V2, and you have to manually change the cord to the preferred amp’s input, and it does have that issue. It’s a great unit.