Twin Reverb reissue Power trans question.

[lo-watt]

Revisiting this amp:
The customer has had it now for a little while. I had put a new Fender 65 Twin Reissue power Transformer in it and all was well. Customer has returned it with redplating on his tubes. Also redplates on another set. Tested secondary voltages and they seem within spec. Checked,470 ohm and 1.5k ohm resistors on sockets,good. Bias pot does nothing. Stays the same. Checked all components of the bias circuit,pot resistors,caps,all test well. Any thoughts on this?

[tubeswell]

Revisiting this amp:
The customer has had it now for a little while. I had put a new Fender 65 Twin Reissue power Transformer in it and all was well. Customer has returned it with redplating on his tubes. Also redplates on another set. Tested secondary voltages and they seem within spec. Checked,470 ohm and 1.5k ohm resistors on sockets,good. Bias pot does nothing. Stays the same. Checked all components of the bias circuit,pot resistors,caps,all test well. Any thoughts on this?

regardless of whether the bias supply caps ‘test well’, Replace then and see what the working result is.

(In fact, if it is an RI amp with ILLinois caps, might as well replace them all)

also, does the bias pot measure for DC resistance fully through its rotation?

[nickb]

regardless of whether the bias supply caps ‘test well’, Replace then and see what the working result is.

(In fact, if it is an RI amp with ILLinois caps, might as well replace them all)

also, does the bias pot measure for DC resistance fully through its rotation?

and add, remove the tubes while you isolate the problem. Take some DC voltage measurements around the bias cct - that should point you in the right direction.

[lo-watt]

regardless of whether the bias supply caps ‘test well’, Replace then and see what the working result is.

(In fact, if it is an RI amp with ILLinois caps, might as well replace them all)

also, does the bias pot measure for DC resistance fully through its rotation?

All of the filters were changed with F&T caps previously as well as all of the components in the bias section save the pot. I tested the pot from 0-100 and 100-0 for resistance but not DC voltage as the amp wasnt on.

[tubeswell]

All of the filters were changed with F&T caps previously as well as all of the components in the bias section save the pot. I tested the pot from 0-100 and 100-0 for resistance but not DC voltage as the amp wasnt on.

then pull the tubes and measure the VDC on the grid pins

(edit: a common issue I’ve found with a lot of RIs that I’ve fixed, apart from the Illinois caps, is dodgy PCB traces)

[J M Fahey]

Agree, besides *parts* testing fine, measure voltages, specially what reaches power tube grids, as suggested above.

Alo not sure what you mean by

bias won’t adjust...

and

Bias pot does nothing. Stays the same.

I googled "same" to "volts" conversion scale and couldn´t find it

I tested the pot from 0-100 and 100-0 for resistance but not DC voltage as the amp wasnt on.

So?
Turning it ON and measuring bias voltage would have taken 1 extra minute.

Please do.

[g1]

Like the others said, we need to know the voltage at pin5 with power tubes out. It must be grossly wrong if the tubes red-plate.
And the bias pot is supposed to be 10K, so if it is reading 100 that is probably a significant clue.

[lo-watt]

Thank you for the responses,I’ll take some voltage measurements when I get back to work to tomm. I’ll have a lot to report.

[lo-watt]

I forgot to follow up;I discovered a blown fuse in the bias secondary circuit(this one is one of the old ones with all the fuses on the secondaries from the early 90’s). Replaced it and all was well. Thanks for all the input and advice. Always a learning experience here.