Magnatone 480A Bias + Plate issue

[FPALB]

I have this 480A that just came in that the owner says is red plating after playing for a half hour or so.

I put in some 1ohm resistors to check the cathode currents and put in a bias pot replacing the 47K resistor in the bias circuit.

The funny thing im seeing is that the plate voltage is changing when I adjust the bias - haven't seen this before.

Any ideas here?

100k grid to bias resistors are spot on
Changed the coupling caps

 

[CJ]

Plate voltage will come down as the rubes are biased to draw more current,

What brand tubes? They might run hot when the amp is in the cabinet,

Or there might be something else going on, like a conductive circuit board,

If that is the case, you can stiffen up the bias supply by lowering resistor values and you could also install cathode resistors line 50 ohms that will help regulate current runaway. And you can also lower screen volrage.

 

[FPALB]

Plate voltage will come down as the rubes are biased to draw more current,

What brand tubes? They might run hot when the amp is in the cabinet,

Or there might be something else going on, like a conductive circuit board,

If that is the case, you can stiffen up the bias supply by lowering resistor values and you could also install cathode resistors line 50 ohms that will help regulate current runaway. And you can also lower screen volrage.

Thanks CJ

Electro Harmonix

480A is Point to Point. I did find an intermittent short between the bias point between the 100Ks and one of the screens. That would definitely shoot the bias through the roof. No issues yet.

Is there anything wrong with using the "remote" outputs for dummy loads? Remote connectors on the front of the 480A break the speaker output connections when used.

 

[trobbins]

Episodes of red-plating may have caused some of the output tubes to significantly out-gas. That may upset the bias from abnormal grid leakage, which may or may not recover with time, and move idle levels around over time. And the 5U4 may be aging, which could make an idle bias adjustment more noticeable for B+ voltage change.

There are some subtle bullet proof changes that could be made, but the owner may not want that.

 

[abbey road d enfer]

The funny thing im seeing is that the plate voltage is changing when I adjust the bias

Seems pretty normal to me. By how much?
Ohm's law says that increasing current also increases voltage drop across the primary's DC resistance.

- haven't seen this before.

You didn't look hard enough.

 

[FPALB]

Episodes of red-plating may have caused some of the output tubes to significantly out-gas. That may upset the bias from abnormal grid leakage, which may or may not recover with time, and move idle levels around over time. And the 5U4 may be aging, which could make an idle bias adjustment more noticeable for B+ voltage change.

There are some subtle bullet proof changes that could be made, but the owner may not want that.

Purchased a new old stock 5U4 just in case. thanks

 

[FPALB]

Seems pretty normal to me. By how much?
Ohm's law says that increasing current also increases voltage drop across the primary's DC resistance.

You didn't look hard enough.

It was changing by 40V ish. But that short was really messing a lot of things up, so im considering that issue solved for the moment. I have it running on some dummies and monitoring the current.

"You didn't look hard enough" - I accept this dig, lol. The short was really moving the bias voltage a lot, and I was just monitoring the cathode current so I was missing some real time information. Ive also never pushed the bias voltage that far off (which is what the short was doing) so ive never seen the B+ vary that heavily. Another one for the lab notebook.

Appreciate it yall.