Peavey Combo Tube Replacement

[Frank De Vincenzo]

I have a Peavey Valve King50 II that the power tube "health" lights on the front panel are telling me the tubes are not working correctly(they have gone green to RED.) I will be replacing the two 6L6 power tubes when they arrive this week. To be safe, I also ordered three 12AX7 tubes for the preamp, although I am not sure if I am supposed to change all tubes at the same time.
What's the normal procedure when doing this? Anything I should be looking out for when I remove the old tubes? I have never done this before, having always been a solid state guy.
Thank you in advance,
Frank

[ajm]

I was not able to find a schematic that showed the tube LED lights.
I was able to find a manual on the Peavey site, but it may not help you.

Based upon what we have to go with, here's what I'd do. Others can chime in to vote yay or nay.

The preamp tubes should have no effect on this, so you don't need to change them for this issue.

Remove the power tubes with the power off.
Change the power tubes one at a time and see if the problem goes away or follows a specific tube.
Mark them with a Sharpie or tape or something so that you don't get them mixed up.

If the problem goes away, great.
If not, it's probably off to a tech.
It could just be that the amp needs a simple minor bias adjustment. As I was not able to find a schematic that seemed correct, I can't advise much beyond this point.

Do not throw the old tubes away unless you are absolutely sure that they are totally bad.
There may not be anything wrong with them other than they need a slightly different bias voltage to work properly.

[Frank De Vincenzo]

Can I "bias" the tubes? What tools are needed to do that? Do all tube amps need that procedure? And also, thank you AJM, for getting a dialogue started.
fd

[Jim Sliff]

As I recall you can't bias the power tubes yourself, even with a "plug-in" bias tool ( a tube adapter that has a digital readout) as there is no externally-accessible bias pot. Even if there was you'd need to spend around $120 for a bias tool that displays both plate voltage and bias current.

Setting bias by current alone - even if you understand what the numbers generally mean - is meaningless. You need to know the % dissipation based on the plate voltage to know whether the tubes are "biased" in a safe operating range - within that range most bias by ear to get the best sound.

The bias should be checked by a tech - and probably adjusted - any time you replace power tubes. It's an inexpensive job. And if the amp is an original Valve King (not a "II") and is 15+ years old it's due for replacement of electrolytic filter capacitors and the bias capacitor. those have a service life of 12-15 years and can cause expensive problems if one blows - and it can happen without any warning.

Something to be aware of even if it's not that old as manufacturers do not notify tube amp owners that regular service is required.

Preamp tubes don't need replacement unless you notice specific problems - gain issues, noise etc. But unless you understand tube amp electronics those problems will normally require a tech visit to determine whether it's a tube or some other problem.

[Frank De Vincenzo]

I can't remember where it was I read that not all tube amps need to be biased when the power tubes are replaced, as long as the replacements are a matched set. Also, that same article said that biasing tubes is a means of getting peak performance and longevity from the power tubes, but not necessarily needed for the amp to work well. How true that is I don't know so I'll change the tubes and see what happens. I'm already dismayed that I'm putting money into an amp that is two years old. I am not looking to put out money for a tech to repair/fix/adjust it so soon. Am I wrong for thinking this way?

[Jerry Erickson]

Looking at a schematic for the 50 watt ValveKing, the bias supply is fixed,non adjustable, just like a Mesa Boogie amp. I'm unsure if Peavey added this feature( adjustable bias) to the MKII models. When you install your new tubes, it would be a good idea to watch the new tubes for 30 minutes or so, to make sure that they don't "red plate". Red plating is a condition where the output tubes draw too much bias current and their plates turn red/orange.

[Frank De Vincenzo]

OK Jerry. Will do. Lets see what happens tonight.

[Mark Fowler]

According to Peavey manual:

Peavey Valve King
(18) TUBE STATUS INDICATION (T.S.I.â„¢) LEDS
These are LEDs that light green or red depending on the status of the power tube they are representing. These are merely the visual part of the wider status indication, fault detection and tube protection circuits.
All models have one LED for each power tube, therefore all have two except for the ValveKing® 100 which has four due to it having four power tubes. From left to right these LEDs are associated with the corresponding power tube in the chassis.
The simple explanation is that the LED will be green in normal working mode and red in any other mode including: Standby, low bias, low current (tube wearing out) or high current fault condition that has switched on the protection circuit.
The more detailed explanation is as follows: (This applies to all models but see below for important differences on ValveKing® 100.)
At Standby the LEDs should be red. This is due to the tubes not yet being fully on. When switching from Standby to On these should then turn from red to green. These should basically remain green in normal playing conditions.
If one or more LEDs goes red it means that tube is not working properly for one of the following reasons:
- Tube is ‘under current’: This could be due to incorrect bias, low current due to aging, open circuit due to structural/physical fault or missing filament heater supply.
- Tube has gone ‘over current’: In this case the resettable protection circuit will be switched on to protect against further damage and to allow the amplifier to carry on working with the remaining tube(s). This could be due to bias failure, over-heating of the tube or other fault condition resulting in excessive current draw.
Reset: In some conditions, with an adequate gap in playing, the protection circuit will auto-reset and allow the tube to be turned back on. If the fault remains then the LED will stay red. In these situations, at a convenient point the amp should be turned off for a few minutes then back on again.
If the fault is still there then the amp should be checked by a factory authorized technician for correct bias or faulty/worn out tube(s).
Differences on ValveKing® 100:
In the normal ‘100% power’ and ‘speaker enabled’ modes, this will work exactly the same as mentioned earlier. However, as the 25% and 5% power modes as well as the speaker defeat setting intentionally turns off one pair of power tubes (to reduce unnecessary heat), the inner two LEDs will be lit red when set to any of these settings. This is the T.S.I.™ circuit operating as it should, by indicating that these two tubes are not currently active.

[Frank De Vincenzo]

The best laid plans of mice and men...changing the power tubes did nothing. Off to a tech it goes. I’ll let you know the remedy when fixed. Thanks boys for the support.
Frank