Fender Bandmaster Reverb Red-plating/low B+

[Matt D]

I took an old Bandmaster Reverb head I haven't used in a year or so to band practice a couple of nights ago. I never really liked the sound of that amp but for whatever reason is was sounding great this night.....until the last tune when one of my bandmates said "Do you guys smell something burning?" I looked at the back of the amp and both power tubes were red-plating like crazy and there was a burning electronics smell. I quickly switched the amp off.

Tonight I had a chance to take it apart and make some measurements. If anyone has any suggestions about the causes of what is happening in this amp and what I can measure to pinpoint the fault I would appreciate them.

Here is all the evidence I have so far:

None of the components in the chassis look burned, but the power transformer has what looks like some dried melted lacquer on the sides and a drop that dripped to the bottom of the end bell. Partially shorted HT winding? Do 70's fender transformers show lacquer drips from manufacturing?

The amp hadn't been turned on in a while. How do I know if a power filter cap failed?

The amp had a cheap Chinese 5ar4 in it. I replaced that with a tested RCA 5u4 as the schematic calls for to rule out a bad rectifier.

Whoever had the amp before me put diodes on the anode wires of the rectifier.

The B+ starts around 440 when the amp is switched off standby and drops rapidly to about 325. Once the voltage settles the tubes begin to red-plate.

To find the bias or the tubes I measured the DC resistance of each side of the output transformer and measured the voltage drops across each side under load. I got 80R from the ct to the blue lead and 71R to the brown lead. With the amp on I measured 316v on the center tap, 302 on the blue and 300 on the brown. That makes the tubes biased at 175ma and 225ma!?! Uh oh. If anyone wants to double check my math that would be much appreciated. The tube with the higher bias red-plates more quickly/brightly.

The bias supply has been modded from the balance control to the standard adjustable bias.

I'm trying to figure out if I have bias supply failure, a bad power cap, or a fried power transformer or some combination of these. Any suggestions about what to check next or probable fails?

[rooster]

Um, and you have not tried reducing the ma draw on the power tubes? Noticing that they are burning up? This seems odd.

Adjust the bias voltage to -48VDC and start your process over.

[Structo]

Once you smell a fried transformer you never forget that smell.

Most likely what you are seeing is lacquer from potting the transformer.

How are the screen resistors looking?

To me it sounds like you lost the bias on it and the tubes ran away.

If it were me I would solder some low tolerance 1 ohm resistors from the cathodes to ground then measure the voltage (millivolts) from pin 8 to ground.

You can also measure the negative voltage at the junction of the 100K resistors that feed the grid stoppers to see what's going on with the bias voltage.

[John_P_WI]

Before you do anything, take the power tubes out and make sure you have a negative bias voltage. There should be approximately 15% of the plate voltage available as bias BUT NEGATIVE.

Make sure the bias diode and cap polarity are correct.

[Leo_Gnardo]

Classic bias failure symptom. With output tubes removed, check bias voltage in the supply, on the adjustment pot, and on pin 5 of each output tube socket. Common problem on old Fenders & other fixed bias amps - bias filter cap packs up, output plates go red. I'm also a bit worried that the modification from balance to bias may have been botched. Make sure your bias voltage is some reasonable value say -45 to -55 VDC before re inserting output tubes. The ones you had in there may be fried by now but worth a try. Don't wreck a new pair of output tubes until you have bias supply sorted out.

If you notice a major drop in bias voltage after reinserting output tubes then you have a failure in one or both. There's also a small possibility of a leaky capacitor in the grid feed circuit.

Also make sure you have the proper fuse in your amp. Fuse is supposed to pop in case of failures like this. Sometimes it doesn't but often I find someone's put in a larger fuse than what's supposed to be there.

Lacquer drips on transformers, uneven ohms readings from CT to plate windings of OT, all normal & not worth worrying about. Do have a look at the high voltage caps in the "bathtub". Not likely a failure here would cause the symptom you describe. 40+ year old caps should be replaced anyway. If you don't see any signs if imminent failure here, think about getting them replaced anyway. You should be able to troubleshoot the bias / output tube problem with the old ones in place.

Now you know where this came from: "Sounded great! Just before it blew up!" Attributed to Joe Walsh.

[Firestorm]

Fenders use carbon pots for bias set (and bias balance). The way the circuit is designed, if the pot gets dirty, the bias goes away (the wiper goes open). Also removes the ground reference for the output tube grids. Both conditions will make the tubes run away. Clean the heck out of the bias pot (or replace it) and check to see it you have appropriate negative voltage range. The bias cap in Fenders is often slightly underated (50V cap for a-52V supply. Get a good cap in there, most important cap in the amp.

[Matt D]

Thanks to all of you for your help. When I get home from practice tonight I'll pull the output tubes and check the bias voltage, then report back.

[Matt D]

So with the tubes out I have a nice, stable -46v on the bias. With the tubes in it drops to less than 1 volt. I take it this means tube failure?

I would also put my meter on the bias voltage and vary the bias pot to see if it changes smoothly with no drop outs along it's rotation.

[Firestorm]

So with the tubes out I have a nice, stable -46v on the bias. With the tubes in it drops to less than 1 volt. I take it this means tube failure?

Probably. Replace the .1 caps to be sure they're not leaking.

[Matt D]

Update: I cleaned out the bias pot, replaced everything in the bias supply and the problem seems solved. The resistor on the bias board was the 1.2k from the balancing version of the bias supply. I changed it to 470r as is used on the ab763. Thanks again to everyone for the help.

[tubeswell]

Whoever had the amp before me put diodes on the anode wires of the rectifier.

Leave those on there for protection against rectifier tube shorting. They don't affect the functioning of the rectifier tube in terms of the B+ voltage it delivers. (Just sayin')