Norton666
Member
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
Moderator
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.
M
Mikeyboyeee
Well-known member
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
Well-known member
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
Well-known member
Check your input jacks. Whistle is usually a grid. Follow your first stage grid wires. Move them and see if it stops
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Findthetone1
Well-known member
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.
M
Mikeyboyeee
Well-known member
All volumes.
Findthetone1
Well-known member
Well, that sucks..
M
Mikeyboyeee
Well-known member
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.Well, that sucks..
Findthetone1
Well-known member
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.
M
Mikeyboyeee
Well-known member
That is how I use it.
glpg80
Well-known member
Is it modded?
scottosan
Well-known member
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
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psychodave
Well-known member
Could be lead dress to the preamp tubes as well. Like was already said. Pull preamp tubes one at a time.
glpg80
Well-known member
I’ll ask again. Is the amplifier stock or not?
panhead
Well-known member
One of my marshalls did that and it was capacitor? i think that was under the board. Someone else fixed it
Norton666
Member
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
Member
The amplifier is completely stock.
glip22
Well-known member
Did you replace the power tubes? If you did not, you should
Beyond Black
Well-known member
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.