Low Volume 5150

[Valvehead]

Easy enough to check. But usually if those fail, it blows fuses.


So did you ever remove Q7? That is my main go-to for this symptom.

yes on Q7 -- no change

ill try to test those diodes

[The Dude]

It's not likely the diodes, but easy enough to test, as Enzo says. FWIW, you can just unsolder one leg of each and run the amp without them as a test. They are just there for protection.

Edit: Do you have a spare OT laying around that you could temporarily sub in?

[Valvehead]

It's not likely the diodes, but easy enough to test, as Enzo says. FWIW, you can just unsolder one leg of each and run the amp without them as a test. They are just there for protection.

Edit: Do you have a spare OT laying around that you could temporarily sub in?

diodes test good---

i have a few amps here i could use.....ive got a jcm 900 (el34) apart already

[g1]

Post #6 there was question about the signal at the grids. Your response in post #8 was incorrect. Measure AC volts at pin5 with black probe to ground.
Post result.

[Valvehead]

Post #6 there was question about the signal at the grids. Your response in post #8 was incorrect. Measure AC volts at pin5 with black probe to ground.
Post result.

ok with or without 1k test tone into input...amp cranked up

all pin 5 make the ac meter jump all over the place from 0vac up to 16vac

[g1]

That makes me suspect lack of drive to the power tubes, rather than the OT.
Maybe try at the plates of the PI tube to see if you can get a more stable reading.

[Valvehead]

That makes me suspect lack of drive to the power tubes, rather than the OT.
Maybe try at the plates of the PI tube to see if you can get a more stable reading.

Ahhh------------------found something!!

tube removed and PI tube socket has 450vdc at pin6........and 100mv dc at pin 1 .......thats in socket and also thru the ribbon cable at the main pcb . I should have a 200-400 vdc at the PI plates...yes ?

[The Dude]

With no tube in the socket, you have no current, so plate voltage will go high.

[Valvehead]

FOUND IT !!!

Thanks to all you guys for your time........i think what was throwing me off was...i didnt consider where the fx loop was in the path......(before the PI )

It was an open 100k plate resistor on the PI tube !!

Amp is properly loud now and sounds great ...............cheers mates !!!!! Thanks G1 !!

[g1]

Thanks G1 !!

Just rehashing what got lost in the shuffle from post #6 by Jazz P Bass.

[Randall]

"It was an open 100k plate resistor on the PI tube !!"

Pin #1 I'm guessing?