erm, 380v to pin 1 of a 12ax7?

[bakerlite]

So i am getting off my ass and sorting out this jcm 2000. (maybe with a bit of help)

It smoked a power transformer - new one installed i have done the light bulb limiter tests no thing shows up.

so after that i did a normal voltage test power up and just sat over the amp taking voltage measurements from everywhere i knew how to.

V3 is showing 380v on the anode so according to the schematic below would it be a good guess that c32 is possibly faulty? or is it a case of "eh yeah buddy and everything else downstream of or connected to that is "possibly faulty"!

thanks folks

http://www.lydian.ca/Marshall_DSL_files/jcm2-60-02-iss5_TubeBoard100w.pdf

[bakerlite]

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11239

Sorry here is a voltage data sheet to give a better idea of whats going on.

[VMS]

1-2-3 of V3 is a cathode follower, therefore pin1 is connected to B+ and should show a high(ish) voltage.

Pin2 should have same voltage as pin6, but your voltage data shows 0?

[bakerlite]

my error - pin 2 of v3 reads 91v
and pin 2 and pin 7 of pin v2 of v4 reads 14v

v1 and v2 have no reading on pin 2 or 7.

[sluckey]

my error - pin 2 of v3 reads 91v

There's still a problem with that reading. Pin 2 is connected directly to pin 6, yet you have 91v on pin 2 and 158 on pin 6.   

and pin 2 and pin 7 of pin v2 of v4 reads 14v

I'm not sure what that means, but if you're talking about pins 2 and 7 of V4 (phase inverter) those voltages are probably OK.

Do you have a problem with this amp? Any symptoms?

[bakerlite]

yes:

It blew a power transformer and i dont know what caused it so thats why i did the lamp limiter tests and then took voltages after they showed no short.

At the moment it has a nasty hum from the clean channel.

[PRR]

> would it be a good guess that c32 is possibly faulty?

Why?

C32 could be good or open, you'd still get 380V there. (If C32 were open the amp might motorboat, but still show ~~380V there.).

If C2 were dead short you'd get dead zero there, and smoke in the string of resistors back to the main caps C38 C39,

If C32 were part-short you'd get some fraction of 380V, and excess current in the string of resistors back to the main caps, so C32 and/or these resistors would smoke in minutes.

C32 is not your problem.

Marshall _put_ +380V on that plate, this is correct.

It does not violate the 330V rating on 12AX7. This section's grid is held up at (I figure) +230V, so its cathode is near +232V, this section feels just 150V.

> It blew a power transformer

C2 and everything downstream is fed through R62 1K R63 4K7 and R71 270, total 6K. Assuming raw supply is 500V, the most can flow is 500V/6K= 83mA. This dead-fault current is much less than the roughly 400mA that the total amp takes, so would not stress the PT; anyway the 500V*0.083A= 40 Watts in less than 10W worth of resistors would smoke before the PT.

Your PT-blowing problem is the rectifier, EL84s, maybe OT, or of course the PT. Nothing else is able to suck enough power long enough to strain the PT.

If it is holding +380V at C32, and not idling hot, maybe the old PT just internally shorted.

> it has a nasty hum from the clean channel.

How about the other channel?

(I don't actually see two channels. I see a basic path plus switchable added-paths, some not on this sheet.)