Crate Blue Voodoo 6212 Low Output

[Axecutioner]

Hey guys, I'm new here and had a question regarding troubleshooting a Crate. I recently acquired a 60W Blue Voodoo for next to nothing as the seller stated that the power tubes were bad. When I got the amp the output volume was very low with the channel volumes and master cranked and the sound had some noticeable irregular distortion to it. Swapping power tubes yielded the same weak output, so I decided to crack the thing open and start probing.

Almost everything seems to be in order except for one thing. The B+ voltages are quite a bit higher (more than 50V in many instances) than what they should be according to the manufacturer's schematic. The power tubes (6L6s) measured over 500V on the plates which has me worried. I checked the signal on the grids of the power tubes with an oscilloscope and they seemed to be within the proper range on both tubes. The power section is supposed to be self biasing.

At this point I have no idea exactly what is wrong. Does anyone have any thoughts on this one? Could it be a bad OT? Is the higher than spec voltage causing issues?

[Twist]

It's been a while since I've been inside one of those but I think there were fuses on the cathodes of the output tubes on the PC board. Also those amps were many times subject to poor or cracked solder joints. If you pull out the main board for reflowing solder joints you also need to clean the contacts of all input and return jacks. Let us know what your voltages are on pins 3, 4 and 5 of the output tube sockets.
Good Luck

[Robert M. Martinelli]

Hi and welcome to the forum!

+B voltage higher than expected at the plates=current lower than expected ( or even zero ).

I'd check that current is actually flowing through the power tubes' cathodes ( you stated the power output stage is cathode biased, measuring the voltage drop across the bias resistor(s) and dividing it by the resistor(s) value you get the cathode(s) current ); if not, then the possible reasons for this are:

1 - missing heaters' supply.

2 - defective ( burnt ) tubes.

3 - circuit from cathode to GND open. ( if this is the case, +1 on checking the cathodes' fuse(s) if any, and/or the tubes' sockets/solder joints )

Hope this helps

Best regards

Bob

[Enzo]

And try this: plug your guitar into the effects return jack. Is the sound clear and reasonably strong?

There are those fuses Twist mentioned, and the power tube board is connected to the main board by a couple ribbon cables. Ribbon cable connections are always worth checking.

And another angle. Ther are those 12 ohm resistors in series with the fuses. One of them opens and it won;t be right either.

Also, look into the action of the little clamp circuit based around Q16. Look just below the OT on the schematic drawing.