Fender Super Reverb red plate issue

[goldtop5]

I have a Fender super reverb (1970s) that's having an issue with red-plating (both tubes) and current draw. Any hints on what to look for that may be causing the issue?
I've replaced the 1.5ks and 470s on the power tube sockets and installed new power tubes (6l6) and a new rectifier tube (5u4). The re-cap job looks like it may have been recent.

[TomCarlos]

May I offer a checklist I put together (originating from geofex.com - thanks RG).

Red Plating Checklist - https://music-electronics-forum.com/...ting-checklist

Super Reverb Schematic - Super_Reverb_AB763.pdf

[Helmholtz]

Most certainly too hot bias.
Do you have a bias probe?

What's the exact model number like AA270 or is a later UL model?

It would be best to find and post the correct schematic.

[goldtop5]

Helmholtz, yes I do have a bias probe. I've looked around the amp to see if I could find what model it is. I found a number stamped on the chassis (A600782). If that means anything. All I know is that the owner told me it was a 45W amp.

[TomCarlos]

Does the amp still have the Tube Chart stapled inside the cabinet?

Perhaps you should pull out the power tubes and measure the voltage on the grids (Pin 5). It should be somewhere around -52 volts. From there, you can use the bias probe. Tell us what you are measuring for the Plate Voltage (Pin 3). If your bias probe reads both plate voltage and current, that is one way to do it. Be careful if taking these measurements by going directly to the pins.

After that, you can refer to a Tube Bias Calculator for bias points - https://robrobinette.com/Tube_Bias_Calculator.htm

By the way, what type of Power Tubes are you using?

[Helmholtz]

Helmholtz, yes I do have a bias probe. I've looked around the amp to see if I could find what model it is. I found a number stamped on the chassis (A600782). If that means anything. All I know is that the owner told me it was a 45W amp.

Ok, then go ahead and measure idle currents.

45W most likely means model AA270.
Find the schematic and post it here, so we know which components we're speaking of.

[g1]

Many times the bias filter cap(s) gets reversed during a re-cap, and red-plating is the result. The filter caps for the bias circuit should have their (+) end to ground.

[christarak]

Helmholtz, yes I do have a bias probe. I've looked around the amp to see if I could find what model it is. I found a number stamped on the chassis (A600782). If that means anything. All I know is that the owner told me it was a 45W amp.

Some excellent tips there, Goldtop. Only if you are confident around high voltages, lift the two 0.01uF coupling capacitors from the phase inverter to your new 1K5 grid stoppers and check the bias voltage again. You can leave your 6L6s out. If the bias voltage on pin 5 drops to a negative voltage as Tom suggests after lifting the coupling caps, you will have leaking 0.1 coupling caps. If it makes no difference to the bias voltage keep on with the other suggestions.

[Helmholtz]

Please let's start with measuring the actual idle currents.

No sense jumping to conclusions without data.

[The Dude]

I don't disagree, but I think if tubes are red plating, it's pretty likely bias current is high. Still, a good start is to check idle current and/or bias voltage.

[Helmholtz]

I don't disagree, but I think if tubes are red plating, it's pretty likely bias current is high. Still, a good start is to check idle current and/or bias voltage.

Please see my post #3.

I have a reason to proceed as suggested.

[Helmholtz]

Please identify amp model/schematic/layout here: http://www.thevintagesound.com/ffg/super_reverb_sf.html

[g1]

I'll be contrary and say the first thing you should do with red-plating power tubes is remove them and verify that you have an acceptable bias voltage at the grids before you install them again.
Doing any kind of measurements while the tubes are installed and red-plating will certainly damage the power tubes and may allow further damage to other components.

[Randall]

Yes please, what is your bias voltages at pins 5? First place to look.

[Helmholtz]

I'll be contrary and say the first thing you should do with red-plating power tubes is remove them and verify that you have an acceptable bias voltage at the grids before you install them again.

You're right, with strong replating that order is certainly safest.