Red Plating Checklist

[TomCarlos]

MEF Team...

I know we have many threads on the subject of "Red Plating" but I could not find a checklist. So for myself and others, maybe it would be helpful to develop one. I invite you to add offer edits, additions, etc. To help organize the list, I have it broken down by Steps. If the Steps do not make sense, I am open to suggestion. For individual checks, please reference the Step. You can copy the entire step and alter if that is easier to do.

Thanks in advance for your contributions!!


Checklist Step 1

1. Turn off amp and remove tube(s)
2. Check the line voltage – most households are 117v.
3. Correct tube type installed?
4. Inspect Plate Resistor (if applicable, preamp tubes) for proper value or out of spec
5. Inspect Cathode resistor (if applicable, preamp tubes and Cathode Biased output tubes) for proper value or out of spec
6. Tube socket (tight pins, dirt in pins)
7. Inspect tube for damage, broken or dirty pins
8. Check all solder connections at tube(s), resistors, power supply

Checklist Step 2

1. With tube(s) removed, apply power
2. Heater filament voltage
3. Bias voltage circuit (for output tubes)
4. Grid Voltage (measured at the grid pin)

Checklist Step 3

1. Install tube(s) and apply power
2. Plate voltage
3. Plate current and adjust bias for proper
4. Check Screen Voltage
5. Check for leaky coupling / grid caps allowing DC voltage from previous stage
6. Is tube oscillating?
7. Try another tube or swap with another and see if red plating follows tube or remains with socket

[Enzo]

Tom, go visit RG's site at geofex.com On the left see Tube Guitar amp tech pages CLick that and find Tube Amp Debugging PAge, the second item.

Look through that for some insights

[TomCarlos]

Got it.... thanks. That is a great resource.

A few days ago, Mick Bailey started a thread titled Red plating preamp tubes? That is what triggered me to create a check list, specifically for that topic. I am sure there must be one out there, I just didn't find it.

I will be visiting the GEOFEX.com site again for the other Debug steps and lists.

[gui_tarzan]

[*]Check the line voltage – most households are 117v.

Tom, I live in Central Michigan in a rural area and our voltage has been 122-125v for twenty years. This of course raises all other voltages in the amp by quite a bit depending on the transformer and circuit, but it can cause issues with old equipment that wasn't designed for voltages that high. It doesn't sound like a lot, but it can make a difference. Just adding that into the equation.

[Jon Snell]

If the mains voltage rises, the HT or B+ or whatever you wish to call the high voltage supply rises, as does the negative bias voltage, cancelling out the rise in dissipation. Assuming the amplifier is designed properly and hasn't got a regulated bias supply.
Cathode biased amplifiers can handle the dissipation by increasing the cathode voltage.
A high, or indeed low mains voltage, fed to a properly designed amplifier, should be able to cope with +10% and -20% (which is the standard due to harmonisation by the EU).

Red Plating or Over Dissipation is caused by a number of issues; worn out valves, mis-biased valves, leaky coupling capacitors, too low a frequency feeding the output transformer that is saturating or HF oscillation. In fact many more causes can be found.

[Enzo]

I fear it is wishful thinking that rising mains lifts B+ and bias so they track.

[Jon Snell]

I fear it is wishful thinking that rising mains lifts B+ and bias so they track.

So are you saying the bias tapping voltage does not increase with the HT winding voltage ... how would that work I wonder.

[g1]

I think he just means we can't be certain they rise in a perfectly linear relationship (B+ and bias voltage)
Or even if they do, that the idle dissipation will still be exactly the same at this other operating point. That is expecting a lot.

[TomCarlos]

By the way..... If anyone would like to take the original list in #1, reorganize, add, edit, etc etc etc, please feel free to repost. I can see where G1's suggestion to check the polarity of the Bias cap (from a different thread) would be good to add to the list.

[Helmholtz]

I remember an earlier thread where we discussed that "compensation" effect using a tube data conversion table.
IIRC, the result was that the increased bias voltage compensates about 50% or 60% of the increased plate dissipation by higher B+-.
The effect of increased bias voltage is partly counteracted by the increased screen voltage.

Maybe pdf64 remembers better than I?

[Enzo]

So are you saying the bias tapping voltage does not increase with the HT winding voltage ... how would that work I wonder.

I think he just means we can't be certain they rise in a perfectly linear relationship (B+ and bias voltage)
Or even if they do, that the idle dissipation will still be exactly the same at this other operating point. That is expecting a lot.

Exactly. Of course both supplies go up and down together, but That is no guarantee the relative "settings" would stay the same.

In defense of that premise though I would agree that the differences probably won't matter. The difference between say 70% versus 65% are likely meaningless.