Massive current draw

[firemedic]

I'm wrapping up my Pro reverb/vibroverb build. So far with no tubes the voltages appear OK, fine w/ preamp tubes only....When I plug in my power tubes the plate voltage drops to 226V and they appear to be drawing @150mA each. Then the fuse blows quickly. My bias circuit is the same as the Hoffman AB763 and the bias voltage is right; 52VAC, dropped to -65VDC max but I have it trimmed down to the minimum, -25VDC. At pin 5 both tubes are -11,-12V.
It does this whether the tremolo tube is in or out. I tried 2 sets of tubes that I know are good. The intensity pot is at its lowest setting. 
What could be causing this massive current draw? I'm running out of fuses! Help!

[phsyconoodler]

Set your bias pot so you have -50v at pin 5 of each power tube and try again. -25v will cook those tubes.

[firemedic]

Okay, I tried maxing my bias voltage & found -66vdc at the insertion point (junction of the grid leak resistors) and -23vdc at the pins, with the amp on standby. The Fender schematic shows nominal          -51vdc at the junction, so I must be in the ball park as far as bias voltage goes.
But the same thing happens when I take it off standby- the grid voltage drops rapidly, the current skyrockets and the fuse dies. This is very frustrating.
Some mods I made, which I wouldn't think would cause problems:
- Grids 1 & 8 are tied together.
- 2.2k grid resistors, I ran out of 1.5ks
- Pentode-triode switch, drawing screen voltage from the plate
- 2.2k 5W screen resistors
- Power supply nodes: A- OT CT, choke, B- Screens & reverb trans, C- tremolo, D- driver, E- V4, V3, F- V2, G-V1.
- 100uF (2 in series) totem pole main filter (OT), 20uf for all other nodes

I'm stumped. And desperate. Please help.

[sluckey]

found -66vdc at the insertion point (junction of the grid leak resistors) and -23vdc at the pins

That's a bad sign! You should have exactly the same voltage on the pins as you have at the junction of the grid resistors. Pull the tubes and make the voltages the same before putting the tubes back in. There has to be a wiring error or the coupling caps from the PI are leaking badly.

[jjasilli]

Do you have grid leak resistors on the power tubes?  Are their connections solid on both ends: G1 pin & ground?

[firemedic]

I've stopped for the nite but I wouldn't be surprised if my .1 coupling cap is leaking, I'll fix that tomorrow. I've had a few .1s go bad before (chinese metal polyester).
Dumb question: the 220k doesn't drop the voltage at all?

[sluckey]

the 220k doesn't drop the voltage at all?

There is no current flow thru the 220K unless something is wrong, ie, wiring error or grid drawing current (not normal).

Look at page 5 of this pdf for a visual...
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

Don't put the big tubes back in until you fix this.

[sluckey]

Do you have grid leak resistors on the power tubes?  Are their connections solid on both ends: G1 pin & ground?

It's fixed bias. The grid leaks better be connected to the bias supply.

[firemedic]

It was leaky coupling caps. I took out all the cheap 0.1uFs & replaced them with big ODs- problem solved. This is my 1st fixed bias amp so I'm at the bottom of the learning curve.
Stay tuned for more tweaking issues- I replaced the normal channel with a 5879 and a low-loss tonestack. So more on that soon...
Thanks sluckey for knocking the rust loose!