I think my Marshall is a very sick puppy

felim

Last week at rehearsal the sound cut out. I looked over at the amp and saw the pilot light on the power switch fading back on and with it came a hugely reduced volume level. A quick glance 'round the back and one of the EL34s was glowing orange on the plates, then the sound was gone completely. So I turned the amp off and ordered new EL34s, a bias probe and a mulitmeter.

These arrived today and I just went home at lunch and installed the new valves. Kept an eye on them to make sure nothing was glowing that shouldn't be. All okay. Then I tried to read the plate voltage (ground to pin 3), but I'm only getting a reading of 192 volts...I was expecting at least 350. With the probe in, I can't get a bias reading at all on the multi meter (set correctly).

Also, there's still no sound at all from the amp.

In case it makes a difference, the amp's a Marshall Silver Jubilee 2554. Any ideas?

Paolo_M

felim wrote: »

Last week at rehearsal the sound cut out. I looked over at the amp and saw the pilot light on the power switch fading back on and with it came a hugely reduced volume level. A quick glance 'round the back and one of the EL34s was glowing orange on the plates, then the sound was gone completely. So I turned the amp off and ordered new EL34s, a bias probe and a mulitmeter.

These arrived today and I just went home at lunch and installed the new valves. Kept an eye on them to make sure nothing was glowing that shouldn't be. All okay. Then I tried to read the plate voltage (ground to pin 3), but I'm only getting a reading of 192 volts...I was expecting at least 350. With the probe in, I can't get a bias reading at all on the multi meter (set correctly).

Also, there's still no sound at all from the amp.

In case it makes a difference, the amp's a Marshall Silver Jubilee 2554. Any ideas?

This is a bit of an odd one. It sounds (no pun intended) like a valve shorted, red-plated and may have damaged your OT causing the voltage drop. However you'd have hoped/expected the HT fuse to pop before the OT got seriously damaged. Also, even at a reduced B+, you'd still expect some faint sound and some bias reading.
Have you checked the screen resistors for damage, it's looks like R55 and 56 from the 2554 schematic?

felim

Paolo_M wrote: »

This is a bit of an odd one. It sounds (no pun intended) like a valve shorted, red-plated and may have damaged your OT causing the voltage drop. However you'd have hoped/expected the HT fuse to pop before the OT got seriously damaged. Also, even at a reduced B+, you'd still expect some faint sound and some bias reading.
Have you checked the screen resistors for damage, it's looks like R55 and 56 from the 2554 schematic?

Turns out it was the HT fuse. I was under the mistaken impression that if the fuse blew, the amp would not power up. Nice new set of JJ e34Ls in there and biased up now.

Paolo_M

felim wrote: »

Turns out it was the HT fuse. I was under the mistaken impression that if the fuse blew, the amp would not power up. Nice new set of JJ e34Ls in there and biased up now.

The power light would light up (primary side of the PT) and the heating filaments of the tubes would get power (6.3V PT winding), appearing as thought they were operating normally. The the HT fuse only fuses the high voltage DC line, after the rectifier and filter caps, which in turn ends up on the OT, plate and screen grids. It's just really weird that you saw 192V at the plate with a blown HT fuse?!? Must be the amp gremlins...

Eoin Madsen

I would say the 192Vdc was from the charge on the filter caps - the discharge path was probably broken when the fuse blew.