5150 problem

[Enzo]

OK, then you have a red plating tube, and it is a particular socket, not the tube itself? Pull THAT tube only, power up the amp and let it run as long as it would have taken the old tube to red plate. Now measure the DC voltage on pin 5 of that socket.

[The Dude]

Dammit!......I was going there, but I lost my internet connection!
Yes, I agree with Enzo. My bet is that your bias voltage is missing on that socket. It could even be the socket pin itself (burnt, corroded, etc.)

[Valvehead]

Hmm...

as i pulled each tube , the next one red plate.... all 4 would .

so i pulled all 4 and i have -034 mv at pin 5 , all sockets

got to be something in the bias supply ??

[Enzo]

Unless your mete is set for AC. Looks like you have lost your bias. The supply is down or the supply is OK, but the circuit to the tubes is broken.

[g1]

Not sure which version you have, but the schematic I have shows a fuse for the bias circuit.
Also, was the 2.2K you replaced a grid resistor for the socket where you first saw the red-plating tube? Better recheck that work.

[Valvehead]

hmmm...

heres what i have.

right out of the trans i have 56 vac for the bias
ive swapped out the 3 caps - c41/41/43
swapped out the diode CR12
resistors r66/67 are ok

I have very low voltage on the positive sides of those 3 caps ( all 10uf) ---but good bias voltage across the negatives of the caps

[Enzo]

OK, but it isn;t getting to the tubes, so somewhere between those caps and the pins 5 of teh tube sockets, there is an intrruption. Bad ribbon cable or connection?

[guitardad]

Yeah. 5150's are notorious for bad ribbon jumper cables causing intermittent problems. Verify the continuity of those pin to pin board to board. Jump out any opens. I know others just hard wire them.

[Valvehead]

ok---ill try hard wire

[Valvehead]

ran a jumper from board to board. bias is all over the place and the end tube is red plating.

look like i have to drill out the tube board....again...

[Enzo]

There is nothing on that board you cannot get at while it is riveted in place. If you want to take voltages and stuff, do it in the amp. Is the bias voltage missing from that end socket? follow the traces back from that pin 5 though any resistors etc to find where the bias stops. if the bias is present for the other tubes, that localizes the issues to the socket board.

The only time to pull the rivets is when you KNOW what part you are changing. And even then, only after you know it cannot be done from the solder side.

[Valvehead]

I just cant "see" where the parts are with the pcb upside down . Ill give that a try . the socket that red plated---i cant get to the pin due to one of the plugs. maybe i can pull it.... i will try again and get some measurements.

Also--I just remembered, the resistor that burned--was a 2.2k on the tube pcb . The closest I had was 2.7k . Would that cause this catastrophic cluster F . ??

[Enzo]

If you can't you can't, I don;t try to force you.

But what schematic are you using? The factory schematic file, which customer service at Peavey will send you, includes not only schematic but also parts layout, which on the 5150 includes the copper traces. Now the drawing looks at the parts side so it will be "backwards," but it is easy enough to adapt.

Yes, seems to me something on the rear panel gets in the way - jacks, impedance switch, something. So sometimes when I stick a screen resistor up there, I have to mount it offset a bit to clear the switch.

[g1]

Also--I just remembered, the resistor that burned--was a 2.2k on the tube pcb . The closest I had was 2.7k . Would that cause this catastrophic cluster F . ??

No, the value could not cause that. But a solder bridge could. That's why I said earlier you should double check your work there. From what I can see, all the 2.2K's on that board are grid resistors.

[Valvehead]

Thanks guys-----ill have to call PV enzo---i dont have a layout. Anyone on here have one ?

I just took some measurements =

This is with NO power tubes...and the amp is IN standby mode .......I shouldnt have bias in standby...or am i nuts ? maybe this amp does...

at the neg of C41 -048mv

junction of the 2 220k's -048mv

the out of the 2nd 220k -025 mv

all the way to the tube pcb -025mv

heres is the weird part...
socket 1 and socket 2 have -025mv at pin 5

socket 3 and 4 have -048mv at pin 5

got to be something on that board--yes ?