Blues Jr III Anomaly

[lowell]

This amp came in with a blown fuse. Replaced it, and both power tubes. I tried 2 sets of new tubes, because I was getting different bias readings for each tube. I checked cathode ground, grid bias voltage, screen voltage etc... all check out good. Yet, 65ma on one side, and 40ma on the other. Pulled the tubes and got ~100ohms from each primary to CT. Swapped tubes with each other, same result. Both flyback diodes seem ok. Again, both tubes are getting identical negative bias voltage on their grids.

So the very odd anomaly that is happening as well - I cannot get a voltage reading on the 40ma side. When I touch the plate, the amp hums, and my meter reads OL. My meter is a 600vDC meter. I get a 330v reading on the other plate. I'm confused by this. Anyone had this happen? I can guess that it's the OT, but wondering if anyone has experienced this.

http://support.fender.com/schematics...atic_Rev-D.pdf

[Jazz P Bass]

Sounds like a bad OT.

[Dave H]

Did you check screen grid voltages on the actual screen grid pin of each tube? If you only checked the common screen voltage a screen grid resistor could be high value on one side.

It could be bursting into oscillation when you connect the meter to the side reading OL. Try clipping a short leaded 100k 'stopper' resistor to the red meter lead and probe the plate with the other end of the resistor.

[lowell]

I checked cathode ground, grid bias voltage, screen voltage etc...

..

Also yes the Rscreens are good.

[g1]

You could disconnect the NFB and reverse the OT primary wires. Then you will know if the problem stays with one side of the OT primary.

[Pedro Vecino]

These measures (40 and 65mA) are extremely high in this amp. 30 / 32mA per tube should be sufficient.
In this amplifier I have observed increases in intensity in one side once connections are extended from sockets to measure bias current. Almost certainly a product of oscillations. Try to keep the power cord and speaker cable away and try orient the tester wires in the best way.

[drewl]

The right side primary will oscillate when you put your meter in it.
Most do it.
Turn master volume down and try probing at the socket.

They run the tubes very hot in these. I always put in a bias adjust circuit.

[J M Fahey]

IF you suspect oscillation reduce system gain: pull the PI tube and remeasure power tubes.
Please post results.

By the way, how are you measuring tube current?

IF using a Bias Probe, junk that clumsy amateurish invention and do it like Real Men do: add a 1 ohm cathode resistor and measure voltage across it.
And you must also be able to read plate voltage.

Bias probes are Rude Goldberg devices invented for OCD Musicians (not Techs) to let them obsess endlessly over tube swapping and 70% dissipation without actually knowing how to solder.
Not forgetting that adding lots of extra (unneeed) contacts and random wires in the path of everything can only complicate things.

[Dave H]

..

Also yes the Rscreens are good.

Sorry, I missed that. I think that the 40mA reading is the real bias current and 65mA is the result of oscillation. If you were using a bias probe try the transformer shunt method. It shouldn't oscillate with the OT shorted.

Edit: Simulpost

[lowell]

No bias probe here guys. Transformer shunt method. The 65ma side is NOT the side that won't read when measuring voltage.

I'll try pulling the PI and see how it goes. I thought something seemed odd here... and the oscillation totally makes sense. Thanks! I'll report back.

[lowell]

Yep, pulled the PI. Getting all readings now, and 40ma on both sides.

It seems EL84 amps have issues with oscillations. I remember years ago, now that I'm running into this problem, that I had a problem biasing one of these for similar reasons. I'm gonna see if I can snub some uber high frequencies and get rid of this issue. I'll report back.

[lowell]

Increasing the PI plate to plate snubber didn't help. However, I have 2 meters, and only one of 'em (cheaper one?) has the problem. Turning the Master Volume down helped as well.

[g1]

Is there still a fault you are trying to cure?
Or just sorting out meter issues?

[lowell]

Just the meter issue.