blues jr - 20v difference on each side of OT

[wagdog]

Greetings,

Creme board blues jr, 2003.

Added a bias pot.

Taking measurements on both sides of the OT, I'm getting 97.7 ohms on the blue side and 96.3 ohms on the brown side.

With tubes out, I'm seeing approx 358v on both sides of the OT (at the blue and brown connector).

With the tubes in, I'm seeing 335v on the plate of the brown side (v5) and 313v on the plate of the blue side (v4). Wondering why the difference, I checked that I'm seeing a bias voltage on each of the grids and am (-11.5v), the cathodes are connected to ground, and the screen resistors are good (and seeing 313v on the screens of each tube measured at the top of the socket).

Also, when checking the voltage on the plates, I get a loud hum on the blue sound and a small mild click on the brown side.

Did switch the tubes, the blue side remains 20v less on the plates.

Wondering why the difference of 20v on the blue side? D11 and D12 seem good (not open or shorted).

[Steve Conner]

One of the above statements must be false. Are you sure the power tube socket connections are OK? Check the solder joints between sockets and board.

[pdf64]

Maybe your meter probe at the plate is sending it into oscillation?
Try repeating the plate measurement with the phase splitter tube removed.
If no different, then consider 1 ohm series resistors in the plate or cathode circuit of each power tube, so there's a way of independantly verifying their static conditions.
Pete

[wagdog]

One of the above statements must be false.

I was thinking the same thing, but I triple checked all of it. I was thinking the screen resistor was maybe open, but only partially, or something like that. Nope, it's all good.

Maybe your meter probe at the plate is sending it into oscillation?
Try repeating the plate measurement with the phase splitter tube removed.
If no different, then consider 1 ohm series resistors in the plate or cathode circuit of each power tube, so there's a way of independantly verifying their static conditions.

That was it.

Took out the PI and measured, it's fine. I believe the "Also, when checking the voltage on the plates, I get a loud hum on the blue " is an indication of that.

I'd like to put some 1 ohm cathode resistors in at some point, much easier to bias like that.

Thanks for the replies!

[soundguruman]

In the BJ, the output tube sockets tend to burn the circuit board. This forms a carbon trace across the fiberglass.
If there is any evidence of burning, around the screen grid pins, you have to cut the burned spots out of the fiberglass.
The burn spots are carbon. This forms an extra resistor on the circuit board.
Hence: the voltages go goofy...
Also, it is a really good idea to replace the plastic sockets with ceramic ones.

[wagdog]

Also, it is a really good idea to replace the plastic sockets with ceramic ones.

Thanks for the reply.

In this case I do believe it was the el84 going into oscillation when the meter probe touched the plate. I did look at the board about the tube sockets and they do not show signs of burning. I will definitely consider replacing the sockets w/ceramic. Was actually considering converting to octal sockets and running 6V6's at some point.

[g1]

Thanks for the reply.

In this case I do believe it was the el84 going into oscillation when the meter probe touched the plate.

This seems fairly common for the blues jr (maybe also other blues/hotrod amps with the ribbon cables). Also can be hard to get proper dc readings from the phase splitter plates. Make sure to keep hands away from the ribbon cables.

[Prairie Dawg]

This seems fairly common for the blues jr (maybe also other blues/hotrod amps with the ribbon cables). Also can be hard to get proper dc readings from the phase splitter plates. Make sure to keep hands away from the ribbon cables.

Billm has a video of him playing an oscillating BJR like a theramin. They are very prone to it and you can, as you say, get them to oscillate easily with any sort of stray capacitance like a finger.

[soundguruman]

Billm has a video of him playing an oscillating BJR like a theramin. They are very prone to it and you can, as you say, get them to oscillate easily with any sort of stray capacitance like a finger.

the ribbon cables should be stretched out in a nice smooth arch, up, away from the boards,
not bent into an "s" shape or 90 degree "L"
This has been recommended to minimize noise & oscillations on BJ.
The capacitance 5 -7 pf 1000V, on V1A, between plate and grid, will usually stop those stray oscillations.
The methods used are twisting the plate and grid wires together,
installing silver mica or other capacitor directly on the socket,
or using a length of 1000V coax cable, the grid to the center, and the plate to the screen. (The screen is hot, not grounded.)
The coax method, used by Marshall, is often highly regarded in heavy metal circles...

Afterall, many of your oscillations originate in the first stage, and are amplified, by the stages that follow.
And not all of them are hear-able, either. Quite often the oscillation will only be verified with yer o- scope.