Fender Blues Junior with NOISY Power Tubes!

[nevetslab]

A new client....same one with that somewhat unique Ampeg SVT-CL Road Case I just worked on.....I set their Fender Tweed Blues Junior up on the bench for general check-out. First thing I spotted was a broken input jack, which, of course, requires removing the main PCB from the chassis enough to fold it down and do the unsolder tasks. Not dissimilar to pulling the Hot Rod series apart, only these having cheap pots and those shaped plastic bushings to center the pot shafts. Finally got the board down, replaced the pot, all else looked ok, and put that back together.

Installed my Compu-Bias probes into the EL-84 tube sockets, and the tubes into those, and powered up the amp. As the power tubes came up, so did lots of random tube noise, like a very noise preamp tube (Volume and Master were CCW). I removed the preamp tube, powered back up. Same noise, shut down. Removed the Driver tube. Same noise, powered back down. Removed the middle tube, powered back up, now only having the EL-84 power tubes in place. Same noise. Powered back down. The plate currents were a bit out of balance...40mA and 28mA, 327VDC Plate voltage on the power tubes. Removed the bias probes, powered back up, same noise. Nothing wrong with the bias probes.

I drove over to the other building to fetch another matched pair of EL84's and brought those back. Plugged them in, with the bias probes back in place. Nice and Quiet. Put the preamp and driver tubes back into place, nice and quiet. And functioning like a normal amplifier again.

I can't recall hearing just one or two EL84 power tubes make noise like that before. Is that uncommon? Replacing them solved the problem, so didn't have to spend time chasing component issues.

[mozz]

Weren't some of those Fenders know to have underrated plate resistors?

[xtian]

Are you sure it's not poor contacts? Did you wiggle the power tubes around?

[Leo_Gnardo]

I can't recall hearing just one or two EL84 power tubes make noise like that before. Is that uncommon? Replacing them solved the problem, so didn't have to spend time chasing component issues.

Don't hear that kind of noise often in EL84 or other power tubes but anything's possible. Usually noises in power tubes amount to low pitched rumble & clank, sounds like dragging an old fashioned galvanized steel garbage can down pavement in an alley. Rumbledy-clankety-racket.

[Jon Snell]

Noisy output valves is more common in incorrectly biased Fender Blues Juniors and other fiddled with amplifiers and of course worn old valves that have seen a lot of roadies.

[Leo_Gnardo]

Noisy output valves is more common in incorrectly biased Fender Blues Juniors and other fiddled with amplifiers and of course worn old valves that have seen a lot of roadies.

True that. Excess heat warps metal parts. Happens in any amp. Made only worse by multiple heat & cool cycles, such is the life of a vacuum tube.

Rough handling doesn't help either, although most tubes seem to survive shipment that involves being dropped from one conveyor belt onto another as much as 12 feet below in sorting centers. Putting "FRAGILE" stickers on packages doesn't help, sometimes invites abuse instead.

[nevetslab]

Weren't some of those Fenders know to have underrated plate resistors?

No plate resistors....just screen resistors, Plates connected to the primary of the OT of course. I didn't measure the IR drop across the screen resistors.

[nevetslab]

Are you sure it's not poor contacts? Did you wiggle the power tubes around?

Tube socket contacts were nice and tight, so wasn't that.

[nevetslab]

Noisy output valves is more common in incorrectly biased Fender Blues Juniors and other fiddled with amplifiers and of course worn old valves that have seen a lot of roadies.

Once I had the replacement matched EL84 tubes installed, plate current settled in around 35mA. Stock amp, so no mods in place to tweak the bias.

[nevetslab]

The amp was packed into a relatively new road case, nice and cushioned, amp itself in like-new condition, apart from the stripped input jack threads. Just an odd set of tubes, I guess. Never have heard that sort of noise from power tubes....sounded like what one normally hears from a really noisy preamp tube. Odd one.