No sound on high input of JCM 800!

[Ricky Rockhardo]

So, I was messing around with preamp tubes in the V1 of my Mojotone 800 and all of the sudden I lost sound.
Going back to my original tube it looks like only half of the first preamp tube lights up and I only get sound in the low sensitivity input, but not the high.

All other preamp and power tubes light up just fine.

Any suggestions are greatly appreciated!

 

[glpg80]

Wiggle the tube in the socket. Does sound come back? Betting you need new sockets as they’re worn out.

 

[Ricky Rockhardo]

Wiggle the tube in the socket. Does sound come back? Betting you need new sockets as they’re worn out.

Winner, winner, chicken dinner!

Yep, compared to the other sockets, the troubled one has more give. I can get sound back for a few seconds before it fades away.

Upon further inspection it looks like a few of the metal contacts are popping up, so perhaps I can try to tighten those before replacing the whole socket?

Thanks for the help!

 

[glpg80]

Winner, winner, chicken dinner!

Yep, compared to the other sockets, the troubled one has more give. I can get sound back for a few seconds before it fades away.

Upon further inspection it looks like a few of the metal contacts are popping up, so perhaps I can try to tighten those before replacing the whole socket?

Thanks for the help!

No don’t go poking on the sockets. There’s B+ on them even immediately after the amp is turned off. If you don’t know what you’re doing you need new sockets installed by a technician.

When I modded my 74 I completely gutted all of the sockets and replaced them first. It’s a common Marshall issue that occurs over time. I’m not a fan of pin squeezing because the actual socket material that holds the pins in place also wear out over time causing risk of pins to short together.

 

[Ricky Rockhardo]

I was going to discharge caps and resistors with a resistor and wire to ground, but duly noted.

As is, I wouldn’t mind getting my inputs shielded, so might as well have a tech take care of everything.

 

[FourT6and2]

You can use a DMM to check the filter caps to make sure they're discharged. Unplug amp from wall, flip both power/standby to on, wait a minute or so, check filter caps. If they're still charged, discharge them with an insulated wire and insulated alligator clip from the main cap on the B+ line to chassis ground somewhere. And leave it connected while you work. A resistor can help too.

As far as the sockets go, you would need to desolder the connections on V1 that are loose, then you can remove the individual pins by rotating them accordingly (they're held in by twisting against the slot in the socket or sometimes little tabs you can push to release them). You can then tighten the pins and reinsert.

Alternatively, you can just replace the socket. But if the sockets are riveted into the chassis, it's a PITA.

If you're going to install shielded coax then yeah, might as well have a tech do it all.

 

[Ricky Rockhardo]

So, here are some pictures of the underneath of the socket and then the top. I’m pretty sure it’s an issue with a few of the metal pins, but might as well and get the socket replaced.

Is there any harm in playing the amp through the low input like this?

 

[FourT6and2]

So, here are some pictures of the underneath of the socket and then the top. I’m pretty sure it’s an issue with a few of the metal pins, but might as well and get the socket replaced.

Is there any harm in playing the amp through the low input like this?

Can't tell much from the photos other than the soldering looks fine and the pins all look intact. But if they're loose, you can literally just use a needle nose pliers to reach the pins from the top/outside of the chassis and squeeze them together a bit to tighten. Make sure caps are discharged before doing this. Alternatively, you can do like I said above and desolder each connection, remove each pin, and tighten. No need for a new socket. But a new socket might just be easier if you have to desolder the connections either way. Easy fix though.