6L6's Glowing Red Fender HRD 1x12

[Chumly]

I have a 20 year old USA made Fender Hot Rod Deluxe 1x12 that I bought new and have babied. A while ago the amp started making a modest amount of hum and the sound quality was not quite as pristine (on the clean channel).

I looked in the back and much to my horror both 6L6 output tubes were glowing red. I do not mean simply the filaments which would be expected, but the actual plate/grid assemblies themselves.

I quickly turned off the amp, and after letting the 6L6 tubes cool I pulled them and did a simple continuity test using my DMM. Everything checked out according to the 6L6 pin-out diagram i.e. there were no dead shorts between the plates or between the plate and the grid etc and the filament continuity was fine.

I bought two new 6L6 tubes but I am reluctant to install them without first confirming that the bias resistor and/or other components are within spec and that there isn't excess plate current etc.

Also given the age of my amp and the fact that I can buy a perfectly fine used replacement HRD 1x12 for only $400, I am not really sure if it's worth my time and effort to repair it and keep it running going forward.

Do you-all have a step by step procedure for diagnosing/repairing the Fender HRD 1x12 when you cannot run the amp due to (in all likelihood) a faulty screen resistor or some other component causing excessive plate current?

I am fully aware of how to discharge capacitors and the high plate voltages available and all the other safety concerns. I am a skilled solderer and great with tools. I have all the modern conveniences including a scope, dummy load, DMM etc.

So all the usual newbie criteria need not apply.

However I am more of a solid-state guy and my time is rather limited and the amp is rather old and easily replaced.

Thanks indeed for having me on this cool forum; it's my first posting but I've been around for a spell (I'm a well preserved 56) but that does not mean I don't have lots to learn!

[Jazz P Bass]

Red plating usually indicates a loss of bias voltage or simply bad tubes.
Pull the 6L6 tubes.
With the amp unplugged & drained, make sure that the cathode is at ground potential (pin 8).
Run a continuity test to the chassis.
Check both sockets.
Power up the amp & verify that pin 5 (grid) has a negative voltage on it. (-50 something or other)
Check both sockets.
Also check that you have full B+ on the plate pin (3) & maybe a slightly lower voltage on the screens (4)
If everything checks out, install new tubes.
If you are not sure, install (temporarily) a 1 amp fuse in the main fuse holder.

[pdf64]

'I can buy a perfectly fine used replacement HRD 1x12 for only $400'
Problem is that it may develop a problem too. As your's has been babied, many of the issues resulting from hard use should be at a very early stage, so it may be better to try to fix yours rather than buy another.
A useful initial step is to power the amp up with no output tubes in places, and measure the voltages at the various socket terminal numbers.
An obvious suspect with your symptoms is a failure in the bias supply to terminal 5, see http://tubedata.itchurch.org/sheets/093/6/6L6GC.pdf
So compare the voltages to the schematic and report back.
Loads of great info (including schematics etc) at The Unofficial Fender Hot Rod Deluxe Owner's Guide!
Pete.

[Chumly]

Thanks so much guys, I'll have a look-see! The reason I mention replacement is that a guy is selling a 1.5 year old HRD 112 III (the newest iteration) and it's been babied as well. The downside is that it's a three hour drive in total and with the price of fuel and my limited time...

[Chuck H]

I bought two new 6L6 tubes but I am reluctant to install them without first confirming that the bias resistor and/or other components are within spec and that there isn't excess plate current etc.

Also given the age of my amp and the fact that I can buy a perfectly fine used replacement HRD 1x12 for only $400, I am not really sure if it's worth my time and effort to repair it and keep it running going forward.

It's correct that you shouldn't just install the new tubes and fire the amp up. Other parts in the amp are likely failed or out of spec. And even though it seems cost effective to buy a working used amp, it's really not. Any used amp will also need service. If not now then soon. Better to fix the amp you have for the same money and have an amp that will work like new for another fifteen or twenty years.

There are other parts in a tube amp (besides tubes) that go bad with age. So ANY old amp will have the same issues. What's more is that these parts go bad whether or not the amp is being used. In fact they go bad faster if the amp is NOT used regularly. I'm talking about the electrolytic capacitors used in the power supplies, bias circuit and a couple in the preamp. The quality of these capacitors seems to be a shortcomming in some years for the HR series Fenders. Not likely yours though since it has worked for twenty years. Electrolytic capacitors for tube amp power supplies typically last from ten to twenty years. As they go bad you will notice the amp sounding a little weak on the bottom end and overall power, increasing hum and sometimes failed bias supplies. I don't doubt your in need of new tubes. But your probably also in need of a "re-cap". If you choose to go to a shop for this be sure to stipulate that you want new electrolytic caps in ALL the power supplies, the bias supply and the preamp. Often times a shop will replace only the main poer supply caps only to have the bias supply or some peripheral power supply fail shortly after. You can do yourself, of course. Just replace all the caps that look like cans with the same value and the same or higher voltage rating.

After that you'll want to power the amp up without tubes in it and take voltage readings. If all the voltages look right you would plug the tubes in and check/adjust the bias. This may require changing a resistor.

[Chumly]

Great info and much obliged! If it wasn't for the HRD's beautifully voiced clean channel and the lovely over-the-top springiness of the full length reverb tank, I would stick exclusively to my: Line 6 modelers, Roland modelers, Korg modelers and Fender modelers (yep the new Mustang).

[Enzo]

It's a good amp, and easily repaired. WHile it is open, CLOSELY look at the solder on the power tube socket pins for cracks. I automatically resolder them when one comes into the shop. THEN as they said, power up without the power tubes, and check for B+ on pins 3 AND 4 of each ower tube, and importqant, the -50 or whatever it is on pin 5 of each.

If those voltages are present, then you can try the new tubes.