ampeg V4 - red plating!

[methodofcontrol]

this amp exploded one of it's plate resistors recently.
i replaced all of the plate resistors (R39,40,45, 46) and while i was in there i replaced most of the dropping resistors as well,
(R52, 53, 55, 217).

http://www.ampegv4.com/images/schematics/V4.jpg for reference

i turned the amp on and checked voltages. point "A" measured about 555vdc.
point "B" also measured 555vdc. all other major points were pretty spot on.

when i put the tubes in, V7 red plated really bad. aside from checking the solder joints at the socket and the board,
is there any component in particular i should be double checking? (although i replaced the plate resistors, i didn't actually
put them on a meter first. i suppose it's possible one came to me shorted or mislabled.)

[Enzo]

SO you have B+, but did you check the bias voltage at that socket? WHat is on pin 5?

If you don;t have -52v or so, then either the socket has lost its grip, cracked its solder or there is an open 47k or 100k resistor or C13 is leaky/shorted.

[methodofcontrol]

bias voltage is -62 on all sockets. (measured w/ tubes out)
i've found two of my grid resistors are way out of spec, R48 is only 84 ohms and R41 is 370 ohms.
i'm going to replace the grid resistors before i go any further.

(R41, of course being attached to V7 socket and R48 being attached to V8 socket.)

[methodofcontrol]

i took the two good grid resistors and routed them to the inside pair of power tube sockets. retested and one power tube was still red plating.
(same tube, different socket) retested again with new pair of tubes and all is well. grid resistors are on order.
now i just have to figure out why the 2nd input is acting all fuzzy and this amp will be in business!
as always...thanks, enzo!
will post again if i have more questions about the 2nd input.

[Diablo]

I'm curious what you are using for tubes that will stand up to the 560 volts on the plate. I'm afraid to use anything but NOS 7027 in my V4.

[Prairie Dawg]

I just retubed a V4B with JJ 7027s and the plate voltage is running at 530. Crossed fingers emoticon here.

[Zouto]

Six(?) years ago I completely restored an old broken down '74 V4B, added a bias pot and retubed it with russian NOS 5881(6P3S ) as power tubes.
http://www.ampegv4.com/forum/viewtopic.php?id=727&p=2
Help, My Amp Has 7027's!
The head's running fine and sounds absolutely great ever since.
Zouto