crate blue voodoo 120 bias problem

[danger-russ]

I picked up an crate amp cheap at the pawn shop with blown power tubes (6l6).
I got new tubes for it but the bias is way off. b+ voltage is 466. I figure bias should be around 37 ma. On my prob I measure .15 - .13 amps, It changes randomly when turning the bias pot.
I figure the pot is bad. Tonight I'm going to check out the entire bias circuit. Anything else I should check. The bias circuit from the pot to the power tubes looks good.
The schematic is attached

[danger-russ]

Also wondering if this could be a sign of a shorted ot. I will check that as well tonight when I get off work.

[plexi50]

I have worked and modified my fair share of these amps. First thing is the power tube PC board that has the pins on the left and right of this board sometimes dont make a very good connection

With all power tubes out of the amp, confirm and measure the plate and grid voltage on pin#5
If no grid voltage on pin #5 then to start with you have the tedious task of doing a continity check on all of the board pins to make sure they are all making a connection to main board

The traces as well are very small for the heater filament circuit and can burn the traces
But that has nothing to do with your problem at this time
Confirm all board pins are making connection with the main board and then report back

It's all a matter of the process of elimination

[danger-russ]

plate voltage without tubes is 490v on pin 3,
grid is at -59 bias turned up / -38 bias turned down
all of the sockets the same.

the whole bias circuit from the pt to the power tubes looks good.

I checked the Ot primary
blue-brown 102.4 ohms
brown to red (center tap) 57.3 ohms
blue to red 45.0 ohms
no conection to grnd or secondary

Ot secondary
black (common) to orange (16 ohm tap) = .4 ohms
black to green (8 ohm tap) = .2 ohms
no conection to grnd or primary

The halves of the primary are a little off but add up right.

Not sure what to check next

[danger-russ]

Ok bais problem fixed, turned out to be a bad connection on one the pins on the power tube board, a bad solder joint on the bias pot, and operator error my dmm didn't want to read right on the DC ma setting, worked fine on the dc 10 amp. I think the power surge when I switched it out of stand by threw the dmm off.

Ok Plexi50 any idea how to get the angry bees out the amp now

Any suggestions on where to start?
I'm thinking the tone stack and drop some more highs in the preamp

I plan to change the Ot aventually and add a lamar ppimv.

[plexi50]

I have done the same thing working to fast and forgetting to change my meter settings

Make these changes:

Change R18 & R22 Plate resistors from 220K to 100K  
Change R32 (Feedback) Resistor to 47k
Change R24 from 68K to 100K
Add 25/uf 25V Cap across R17

Tone EQ Caps
C14 390pf = Black Cap
C15 .1=104J (Brown Cap)
C16 .047=473K (Smaller Brown Cap)

Remove These Parts
(just cut them off the board)
R101 - C5 - TR1 in photo (Big Black Sidac)
Making filament voltage float / Can cause bad hum

Buss These Resistors
R7-R25-R26-R28 (solder wire across parts)

[danger-russ]

thanks for the help/ advice
I'm wondering about changing some of the cathode bypass caps going a little bigger maybe going 1u or larger on the second stage in the distortion channel. Seems like a little too much bass is getting droped from nfb. I've been looking at different amp schematics that I have found, looks like the slo clone has the same value pots as the voodoo but more of a marshal style stack, might give that a try. I think I'm going to change the vol pots to logs. The lins that are on it don't do so well at low volumes.
Yeah the hum isn't too noticeable until the volume is up to 3 or 4 it gets pretty bad. I think I might take the whole heater circuit of the board, seems kind of counter productive to run the heater wires from the transformer across the board, then back across it through the traces.
I have some time to think about it not going to make any changes untill I get the new OT, would a 2.1k primary be about right for it?

[plexi50]

4K primary OT

[danger-russ]

Thanks again for the advice plexi, I got the mods done on wensday. I ended up leaving the tone stack alone, I changed out the the 2 plate resistors to 100k's and droped the nfb resistor to 47k. I replaced most of the caps in the signal path with Mallory's, and changed the two .68u cathode bypass caps to 1u bipolar electrolytics, changed the 22u on the clean to a bipolar electrolytic, and added the 22u you suggested in the dirty channel and removed the sidac. Plays like a whole new amp. The distortion goes from crunchy to fizzy with lots of inbetween. The bass is way better. All the hum is gone. Only thing left is the hiss, which sounds worse now that the hum is gone and the distortion isn't so buzzy. By I can manage that.

[equinity]

I have done the same thing working to fast and forgetting to change my meter settings

Make these changes:

Change R18 & R22 Plate resistors from 220K to 100K  
Change R32 (Feedback) Resistor to 47k
Change R24 from 68K to 100K
Add 25/uf 25V Cap across R17

Tone EQ Caps
C14 390pf = Black Cap
C15 .1=104J (Brown Cap)
C16 .047=473K (Smaller Brown Cap)

Remove These Parts
(just cut them off the board)
R101 - C5 - TR1 in photo (Big Black Sidac)
Making filament voltage float / Can cause bad hum

Buss These Resistors
R7-R25-R26-R28 (solder wire across parts)

plexi,

does the cap going in with r17 should that parallel or in series? And when you say "buss" should I just remove and jump those connections or leave the resistors there and short them?

thanks.

[plexi50]

does the cap going in with r17 should that parallel or in series? And when you say "buss" should I just remove and jump those connections or leave the resistors there and short them?

Parallel the cap across the resistor from end to end.

Short,Buss them means you just (solder wire across parts) R7-R25-R26-R28

You can remove them if you have the board out of the chassis and replace the resistors with a piece of wire

Either way once a wire is across a resistor the resistor is no longer functioning as a resistor