Tubes red plating on Fender Concert

[beasleybodyshop]

Hey folks, I have another head scratcher here for me.

I am working on a 1979 Fender Concert....ew. Not my ideal amp but my friend purchased it new and he has a lot of sentimental attachment to it. I won't come on, and pops fuses.

Long story short, its the power switch popping the fuses. I replace the switch, and also recap the power supply as it has original mallories in it. now it fires up and stays on.

Now when i fire it up, it works, however each tube is drawing like 110 ma (!) and it steadily climbs. Now I double checked my bias probe. It's working fine. I also swapped in a new set of known working power tubes, it still draws tons of current. I converted the bias balance system over to a blackface style bias control to provide even bias. the bias comes from a capacitor in the doghouse, unlike older systems that had a separate board under the indicator light. The max bias voltage that it gives me is -29 volts....that seems a little low, doesn't it? I double checked the voltage drop across the plate resistors, they are within spec and not cracked, so ive eliminated that...

Any other ideas? Im a bit lost here. Attached is a gut shot.

[Jana]

The bias voltage is much too low. For 6L6 tubes, your range should be able to go to -55 volts or so at that end of the adjustment. Check the bias caps. If they are okay, check other components in the bias circuit.

[Stevem]

How much ac voltage do you have coming into the bias diode even before the bias filters?
If I recall right it's about 70 something.
Also make sure that bias pot is tight to the chassis.

[DonMoose]

It's not the switch - that's a symptom, not a cause.

It sounds like a super-leaky bias cap. An amp that old is about due to have all its electrolytic caps replaced anyway, if they haven't been yet.

I'd also look at the rectifier and dropping resistor (if any).

Another spot to check is the grid pin terminals on the sockets - they may be getting loose (less likely).

Hope this helps!

[beasleybodyshop]

It's not the switch - that's a symptom, not a cause.

It sounds like a super-leaky bias cap. An amp that old is about due to have all its electrolytic caps replaced anyway, if they haven't been yet.

I'd also look at the rectifier and dropping resistor (if any).

Another spot to check is the grid pin terminals on the sockets - they may be getting loose (less likely).

Hope this helps!

Thanks Don. I recapped everything inside the doghouse - I believe the bias cap for this amp is under there as well.

Im not at my bench, so I can't post any different photos but I believe you are correct. I will check to see if any of the other caps are leaking.

[beasleybodyshop]

Oh, I also forgot to mention - Death cap WAS bad and shorted to chassis.


These things were a super bad idea. I think of the last 5 or 6 amps ive had come across my bench with ground switches, the last 3 had faulty ground switch caps. But I digress.....when I get back to my bench i will double check. Earlier this morning when I was looking at it I noticed that the bias voltage dropped very, very slowly from -29 to -13 volts....guess this would indicate a leaky cap somewhere?

[martin manning]

Earlier this morning when I was looking at it I noticed that the bias voltage dropped very, very slowly from -29 to -13 volts....guess this would indicate a leaky cap somewhere?

That would be increasing voltage, and yes possibly.

[Leo_Gnardo]

I noticed that the bias voltage dropped very, very slowly from -29 to -13 volts....guess this would indicate a leaky cap somewhere?

How fast can you solder a new one in? That's how fast you'll know... On rare occasions there's a resistor in series with the bias rectifier, and that goes bad. Some amps use a cap instead of resistor (Ampeg 70's, Marshall 900, early Sundowns) same t'ing....

Something else to look for besides. That series of amps used a 47R half-watt resistor plus rectifier to produce voltage to run the switching system. Often the 47R is roasted, and could be turning to all-carbon, partially shorting out low-voltage lines. Replace the 47R with a higher-wattage resistor anyway.

[gingertube]

Maybe wrong but of all the Fenders I've worked on - none had the bias supply filter cap under the tin lid ("in the dog house"). Check under the chassis.

If the output tubes have been redplating then I would also check the screen resistors. The 470 Ohm 2 Watt are not really big enough. If they need to be replaced use 1K 5 Watt.

Cheers,
Ian

[Leo_Gnardo]

Maybe wrong but of all the Fenders I've worked on - none had the bias supply filter cap under the tin lid ("in the dog house").

Not wrong at all if you've never seen a Fender from that time frame. Late 70's Fender moved bias filters there. I call it the bathtub - what it looks like - others call it the doghouse.

[pdf64]

I recapped everything inside the doghouse - I believe the bias cap for this amp is under there as well

Yes, the very late 70s / early 80s amps had the bias caps in the doghouse. The last amps to use eyelet boards.
Bathtub - ha, yes!

My guess is that at least one bias supply cap has been installed the wrong way around (we've all done that at least once)!

I noticed that the bias voltage dropped very, very slowly from -29 to -13 volts....?
That would be increasing voltage

Martin, I think dropped / decrease is correct.
It wouldn't make sense for something to increase as it moves towards zero, towards a lower energy state.
The decrease is therefore because the magnitude of the voltage is moving toward zero, the polarity here just indicates which side of zero it's coming from!
Pete

[beasleybodyshop]

Thanks for all the info people, and Leo, I probably owe you at least a six pack for all the help you've given me over the last six months lol!

I haven't been back to my bench, but I am thinking I installed that bias cap backwards - doesn't that usually destroy the capacitor? Or not because it's the bias control? Either way I will know as soon as I get back to it and swap her around.

[Leo_Gnardo]

Thanks for all the info people, and Leo, I probably owe you at least a six pack for all the help you've given me over the last six months lol!

Well, garsh... Say one of my customers dropped off a sixer of Shiner varieties and it was very very good indeed! Recommended to anyone who likes barley suds. Shiner Bock was the only choice up this way before. Had no idea the others existed.

I haven't been back to my bench, but I am thinking I installed that bias cap backwards - doesn't that usually destroy the capacitor?

If it's reversed it's cursed. I'm sure just about all of us ...