'78jmp humming loudly.Any suggestions on a fix.

[bennieusmaximus]

I have a '78 jmp non master that has a loud hum that changes when either volume is moved.the hum goes away when the 2nd preamp tube is removed.The tube has been replaced with a known good one and humm is still there.any sugestions?Thinking about going ahead and swaping the cans out anyway.thanks!

 

[core]

I'm not a tech but I can tell you about my experience and something to consider. I recently sent my 77 JMP in for a retube and check up. It was pretty loud as well. The hum is normal but on these amps it should not be as loud as it was. It increased a lot as you turn the master up to very annoying levels.

My local amp tech looked over the amp with a loupe and fixed some solder joints that were cracked and replaced the power tubes with good ones that I gave him. Now that I have the amp back it is almost dead quiet as I crank the master. I can't tell you for sure what was causing the excess hum but having someone qualified go over the circuit board and bias up new power tubes may do wonders for you.

Also while the amp is on and off standby you can tap the tubes lightly with an eraser on the end of a pencil to see if you get tapping noise coming through the speakers. If so you have a microphonic tube that will need to be replaced.

 

[bennieusmaximus]

Yea I should go over all the joints.Ive fixed a few like that,but didn't even think of that.DUH.

 

[charveldan]

Time for a cap job and a new Matched quad of SED Winged =C='s.

 

[bennieusmaximus]

I recently bought this amp and it has kt88's in it.Sounded good until the noise came.Looks just like the amps in your pic.

 

[Jonathan Wilder]

I have a '78 jmp non master that has a loud hum that changes when either volume is moved.

Teach it the words!

In all seriousness...if the filter caps have never been replaced now's the time to do that. Also, if you're comfortable working inside of the amp, open it up and find the wire that is connected to pin 2 of V2 and pick it up off the chassis as far as you can. That fixes the all too common V2 grid hum issue in these amps.

 

[core]

Dang, now what can you do for radio static/noise??!!

 

[Jonathan Wilder]

Dang, now what can you do for radio static/noise??!!

Have you tried doing what I stated above? Using shielded cable throughout the preamp as well as a proper grounding scheme in the preamp works wonders too (well...as proper as it CAN be with multisection filter cap cans...figured I'd state that before Joey nails me on it ).

 

[bennieusmaximus]

Thanks for the info Wilder.I am going to do that as soon as the Blackhawks beat the Flyers.By the way howd ya get so smart?

 

[MajorNut1967]

Have you tried doing what I stated above? Using shielded cable throughout the preamp as well as a proper grounding scheme in the preamp works wonders too (well...as proper as it CAN be with multisection filter cap cans...figured I'd state that before Joey nails me on it ).

Would you two stop blowing each other in public! Enough already.

 

[core]

LOL, not yet but I will open it up once I get home from work today and try the wire trick. The caps will have to wait unfortunately.

 

[MajorNut1967]

By the way howd ya get so smart?

By Listening to his Dad!

 

[core]

Jon, not only did I move pin 2 wire on V2 off the chassis, I went ahead and set every wire on the preamps and power tubes (except the red and black wires), off the chassis. Just for good measure! Makes a big difference I think! I don't hear near as much hum. I'll have to play it for awhile to see if there is radio noise as it was intermittent but I'm confident there won't be any. I thought it was quiet before but now it's REAL quite! Even at at high volume. Thanks for the tips man.

 

[kanders1]

I'm sorry, I'm not sure what you mean by picking the pin 2 wire up off the chassis. Are you saying it's connected to ground and you're disconnecting it? I feel so ignorant sometimes, not being an amp tech.

My V2, the big resistor across it, really the whole socket makes noise if I tap on it, which doesn't seem right to me (especially since both channel volumes are turned down to zero). Is this fix something I should try?

Thanks,
Kevin

 

[Jonathan Wilder]

I'm sorry, I'm not sure what you mean by picking the pin 2 wire up off the chassis. Are you saying it's connected to ground and you're disconnecting it? I feel so ignorant sometimes, not being an amp tech.

My V2, the big resistor across it, really the whole socket makes noise if I tap on it, which doesn't seem right to me (especially since both channel volumes are turned down to zero). Is this fix something I should try?

Thanks,
Kevin

No...the wire lays down on the chassis. Just physically pick it up so that it's "flying"...i.e. "suspended" off the chassis.

If that socket makes noise when you tap it you more than likely have a microphonic valve in that socket or the socket needs to be retensioned/cleaned if not replaced.

 

[kanders1]

Ahh, I see. Thanks, Jon.

 

[bennieusmaximus]

Ok,so Installed new filter caps 50 x 50 500. I had 3 of them.Not a real big deal,but the amp is doing the same thing.When I pull valve 2 the humm goes away.and the humm can be changed when moving vol 1 and vol 2 around.I checked the wire on pin 2 and found it had allready been done.The volume and sound of the amp is normal exept for the loud humm.There isnt any techs around here so any help would be appreciated.

 

[Adwex]

<snip> By the way howd ya get so smart?

My guess would be education, years of experience, and a "knack".
A winning combination.

 

[bennieusmaximus]

I got little education,little experience,but alot of knack.

 

[Adwex]

I got little education,little experience,but alot of knack.

Fortunately, you have the one thing that you have no control over. You can always acquire the other two.