'68 Bandmaster Reverb- one tube red plating-then fuse blows

[Stratz]

Hi all. I have a '68 BMR that's now black faced. It started blowing fuses while sitting at idle.
Finally had some time to take a good look at it and the 6L6 closest to the PT is red plating within 30-60 seconds of throwing the standby switch to play mode then the 2A Slo Blo fuse blows. I hadn't realized one tube was getting hot at first because I had the chassis on the bench with the board side up.

I sprayed all 9 sockets with contact cleaner, tightened pins, tried different tubes both brand new and known working tubes from other amps (rectifier, V8 and V7) but shortly after I flip the standby switch the one tube (V8) starts to run away. 450*F (~200*C) or more while V7 stays @ 200*F (~100*C) or so.

Pin 3 on the tube that's getting hot reads ~300v and the other 6L6 reads 330v or so. This is about 100v lower than I recall them reading under good working conditions. Should read ~ 420-4350v on pin 3

I have all new coupling caps (I ordered all new caps for the entire pre-amp board except for the cathode bypass caps which are new) on the way so I'll try to get them installed tomorrow.

I measured all resistors on the board and they're measuring mostly high but they're 50 yr old CC resistors so I suppose 15%- 20% or so above/below their rating is to be expected.

I suppose my main question is how can I diagnose an amp that wont stay on?

Thanks all.

[Jazz P Bass]

That tube that is 'redplating' is shot.

Remove the tubes and check the bias pins of both tubes.
It should read about -50Vdc.

[bob p]

it sounds like you said that any tube that you put into the V8 position cherries out in that position, and that the problem is not related to one tube in particular. is that right?

and all this started happening right after the BF mod?

I'm not exactly sure what you mean by "Blackfacing" as it means different things to different people. most people end up focusing on the bias circuit and the PI and little else, while other people do other things.

by any chance did you mod the bias circuit to change it from non-adjustable offset/adjustable-balance to make it adjustable-offset/non-adjustable balance? if you could clear that up, and let us know exactly what changes you made, that would be helpful. in cases where someone revises the bias circuit and one tube starts redplating, it's probably a good idea to double check your work to make sure that a mistake didn't creep into the bias circuit revision. I'm thinking that you've got a wrong connection, a wrong part, or a part that's way out of spec.

[Stratz]

Will check the bias pins of both tubes for -50v and report back.

Yes the bias pot was converted. Please see pics. Thats a new 50uf/100v cap on the bias board with the + side towards the pilot lamp.
I only had the amp a few days before the black facing was done (not by me). The volume was dropping off to about 30%-50% or so with distortion so I took it to a pro to repair it. He asked me if I wanted it black faced and I said yes. The bias pot operation was one of the few changes made along with new filter caps and cathode bypass caps (it still had cardboard sleeved original Mallorys) which were just routine maintenance.

Thanks guys.

Your help is very much appreciated,
Rob.

[Stratz]

That tube that is 'redplating' is shot.

Remove the tubes and check the bias pins of both tubes.
It should read about -50Vdc.

Any tube that put in the V8 slot will start to run away.
Thanks
Rob.

[Enzo]

Then leave it empty and measure pin 5 for bias.

[Stratz]

Ok, while checking for voltages I found the bias wire for pin 1/ V8 flapping in the breeze. It read about the same as my bias pot center lug which is ~ -49v
I soldered it back on to pin 1 and I now have 460v on pin 3 of both power tubes again rather than 300 or 310v I was reading before.

I'm out of 2A Slo Blo fuses (I used a 1 amp Slo Blo to test the plate and bias voltages with no tubes installed) and I cant imagine where I could find fuses any locally so I'll have to report back when I get some more fuses.

The caps will be here tomorrow so I may change them over the weekend or wait until I have more fuses to rule out the problem.
I think I may have power switch that's on it's way out as well.

Thanks all. I'll let you know the outcome.
You guys are the best!
Rob.

[Stratz]

Guys, will my polarity switch work in place of the On/Off switch until I can order a new one?
Thanks
Rob.

[Enzo]

Sure, why not, it is a switch after all. But check teh fit, the ground switch is mounted sideways while the power switch is vertical. Make sure that ground switch doesn't put its solder lugs against the chassis.

Most hardware stores and places like Home Depot Menards or Lowes have fuses, and they also have toggle switches.

[bob p]

Ok, while checking for voltages I found the bias wire for pin 1/ V8 flapping in the breeze. It read about the same as my bias pot center lug which is ~ -49v

Pin 1 = No Connection.
Some Fenders with fixed bias tied Pin 1 to Pin 8 and grounded it.
Other Fenders used Pin 1 to Pin 5 as the mount point for the grid resistor.

Connecting to Pin 1 makes no sense unless youv'e got the 1k5 grid resistor spanning 1 to 5. If that's the case then that should fix your bias problem.

Menards might have the right fuses.

edit: Dammit Enzo, you beat me again. don't you ever sleep?!?

[Stratz]

Pin 1 = No Connection.
Some Fenders with fixed bias tied Pin 1 to Pin 8 and grounded it.
Other Fenders used Pin 1 to Pin 3 as the mount point for the grid resistor.

Connecting to Pin 1 makes no sense unless youv'e got the 1k5 grid resistor spanning 1 to 3. If that's the case then that should fix your bias problem.

Menards might have the right fuses.

edit: Dammit Enzo, you beat me again. don't you ever sleep?!?

Pin 1 and pin 5 of the power tubes are tied together along with a 1.8k 1/2 watt CC resistor
The Silver Face AA1069 Bandmaster Reverb schematic says it should be 1.5k but the 1.8k resistor sure looks original.
The AB763 circuit shows the same. Pins 1 and pin 5 of the power tubes are tied together along with a 1.5k resistor.

Pin 3 of the power tubes are the brown and the blue wires that go to the OT and they are reading ~460v

I've never heard of Menards. The only local place I can find has 2A but they're not Slo Blo. Will they still work?
Thanks.

[Justin Thomas]

Hey Bob,

I think you meant pin 1 to pin <5> for that resistor mount? Otherwise, I would imagine your negative bias supply being hooked right up to the plates...

Rob, any chance that while that bias wire was "flapping in the breeze, that it was in contact with the metal shield on the top inside of the cabinet? It <should> have one... Or any other exposed metal - chassis, wires, etc.?

Also, a lot of times, once a tube redplates badly, it's suffered internal damage and may not work again. It's one thing if a spot or two showed up, but if the whole plate is heating up that quickly and repeatedly, I'd chuck that tube...

I bet once you verify all yout connections and put in two tubes that didn't redplate, you'll be okay. No need to wait for "matched" tubes to come in just to test; just use ones that haven't toasted yet. And quadruple-check the wiring on the bias supply. And even though you have probably found the problem, I'd check the entire system, to make sure that bias is getting to the OTHER side of the socket - not just at the pin side. Just because the pin has bias doesn't mean it comes out the other side of the socket. I've had sockets break inside the plastic/ceramic housing. Yes this involves cramming a meter probe into a live socket... But this IS one of the most important circuits in your amp, so to me it's worth the extra attention.

Justin

[Stratz]

Hi Justin. I meant to write pins 1 and 5. I had corrected it while you must have been writing your reply.
I seriously doubt that the loose wire was touching anything other than pin1 occasionally. It wasn't exactly flapping in the breeze, just a figure of speech. It was just out of the hole in the pin 1 solder tab.

I've since tried good tubes that haven't red plated but I blew another fuse. I think I had a bad on/off switch because i was getting a pop through the speaker and a flash on the power tubes so I replaced both the on/off and standby switches. No more pop or flash but I'm out of 2A Slo Blo fuses.

I did locate standard 2A fuses locally but they're not Slo Blo s I dont know if I should try them or not.

I had a set of Groove Tube 5881's in the amp with the loose wire soldered back in place and the new on/off and standby switches installed for a good 10 minutes at idle with no issues. I also have a brand new set of JJ 6L6GC's that I can put in the amp along with quite a few sets of used JAN 6L6's but I dont have any more fuses and I dont want ruin any more tubes. Especially any of the JAN's.

I may also have some DC leaking from the coupling caps which I can change out for Mallory's today when the new caps get here.

But at this time I can only check the tube socket for continuity from pin to tab.
I cant believe that there are no more local stores where a person can buy a glass fuse.......

Rob.

[bob p]

Hey Bob,

I think you meant pin 1 to pin <5> for that resistor mount? Otherwise, I would imagine your negative bias supply being hooked right up to the plates...

Uh-oh, looks like I have my foot in my mouth. Yeah, grid resistors connect to grids not to plates. Dunno how I managed to screw that up, other than it being 3:40 am when I wrote that. I should have just gone to bed.

If you're still popping fuses then it might be a good idea to wait on diagnostics until you do the cap job. If you've got a cap that's shorting to ground you'll just blow more fuses. You might get lucky and the caps might fix the problem, but if you keep blowing fuses you're going to need a variac or a light bulb current limiter. Standard 2A fuses won't last if you're popping slow-blows, but you might be able to use them with a current limiter. Once the amp goes back into service be sure to put in Slow Blow fuses. The fast-blow fuses won't last long under the high inrush conditions when the transformer gets switched on. They may work for a while but they will eventually fail.

[jbltwin1]

Sounds like it's time to build the light bulb limiter! Essential tool when diagnosing fuse pops. Mike.