76 Deluxe Reverb fried at a gig...

[Bob Simpson]

Saturday night.
The push on terminal(s) to the speaker became unhooked.
I play intermittently during a gig, with the standby switch in operate mode.
I smelled electronics burning, but couldn't locate the smell. First guess from the folks on stage was lighting.

Eventually it became obvious it was MY amp, of course.

Anyway, the 470 ohm CC screen resistor on V8 ( closest to the GZ34 ) burned ( black ), cracked in half, and bubbled the insulation on the filament wire.
Slowblow fuse blew.
I don't know if the Mazda 6V6's or Sovtek GZ34 fried or not...

My guitar player says take it to a tech...
We're blessed with several here in the Denver area. He uses Lord Valve.
Now that I've built a couple of amps, I think I can fix it, but I'm not sure what to expect...

What should I fix first ( resistor is right up there, of course )?
What do you think the chances are the OT survived?
Can a replacement '76 OT be found?
Should I try to find carbon comp replacement resistor?
Any words of wisdom, or instructions on testing transformers?
Should I bring the repaired DR up via the light bulb limiter?

Thanks in advance, guys.

Bob Simpson

[tubeswell]

Never trust push-on terminals.

I'd start with changing the smoked resistor, then R-metering the circuit looking for shorted filter caps. If that looks okay, then plug the speaker back in and fire it up, watching for more smoke and/or cooking/frying noises, and measure the voltages

[TheGimp]

Worst case what has happened is you lost the low impedance to the plate of your output tubes and they went red-plate, in addition you may have experienced excessive screen current on at least one output tube (are there seperate resistors for each screen?), which can distort the screen and destroy the tube.

Before powering it up I'd measure the OPT for shorts to frame, DC resistance of the primaries, etc. Smell the opt. Does it smell burnt? Do the leads going into the OPT look burnt? If so the OPT may be shot.

If the OPT seems ok, then I'd replace the bad resistor and and inspect everything for other damage(shorted caps, etc). Then, remove all the tubes except the rectifier tube. Add a load resistor on the B+ that is roughly equal to the load of the output tubes plus 10% to account for the missing pre stage tubes.

Bring it up with a fast blow fuse instead of a slow blow, and measure the B+ and filament/heater voltage. The heater should slightly be high. B+ should be close to spec but without the tubes may be slightly high.

If the voltages look ok, shut it down remove the temporary load from B+ and put the tubes back in and put a dummy load on it. Bring it up and watch for smoke. Be ready to kill it if anything out of the ordinary happens.

If it comes up ok, measure all your plate voltages and watch the output tubes for red plate.

[Bob Simpson]

Continuing saga of the SFDR...

So I took the 6v6's and rectifier tubes to Lord Valve.

One of the mazdas had a shorted screen. Shot.
One tested very good. Now all I need is an SE amp project...
I have a quad of Tungsol 6v6's...

The Sovtek GZ34 tested bad on one side...
I got a NOS JAN phillips 5U4GB at LV's suggestion.

Fistell's had replacement CC 470 ohm 1W screen resistors.
And a bunch of 2A slowblow fuses.
I plan on replacing them both.

I made a light bulb limiter. I needed to do that anyway...

Unsoldering the fried screen resistor I broke off pin 6 on the 6v6 socket.
I'll have to fix that.

OT secondary reads .6 ohms.
OT Primary blue to brown 428 ohms
blue to red 215 ohms
brown to red 214 ohms
No primary or secondary wires have continuity with the OT frame.
A wall wart providing 16.5 vAC across the unsoldered secondary
produced a 377.4 vAC reading on OT primary blue to brown...

ratio 1 : 22.8722727

No smoke smell to the OT. Burned resistor was pretty rank.
No burned OT wires...

I guess I'll not worry about the OT right now...

I checked voltages on the filter caps to ground...

Any thoughts or cautions? Things to look for?

TIA

Bob

[martin manning]

Everything about the OT sounds good except that a 22.87 voltage ratio suggests a 4k2 ohms a-a using an 8-ohm speaker... DR should have 6k6 (?). Almost sounds like the tube with the shorted screen started the trouble. If the speaker leads came off you'd have no sound, and be alerted to the problem right away.

[Bob Simpson]

The Deluxe Reverb went back together OK.

Except...

When I flip the on switch, the left ( from the back ) 6v6 flashes pretty bright, then settles down.
When I flip the stdby switch, it crackles once or twice, then plays just like it should.

If I go to standby, then off, then back on again within a few seconds, no initial flash...

Any ideas?

TIA

Bob Simpson

[Gibsonman63]

If you haven't replaced the filter capacitors, you are due. It is probably just the inrush current. When you flip the power switch off an on, quickly, the capacitors are still charged, so the tubes don't see it. You may want to replace all of the electrolytics and just be done with it.

For the crackling, I would check the 470 ohm resistors on the power tube sockets. I generally go with 3W metal film resistors there because they handle the heat very well. Also check the tube sockets and retension if needed.

[mlp-mx6]

The flashing would be the heaters. Some heaters flash as a normal behavior. Many NOS European 12AX7s, for example. That flash (with standby off) cannot possibly be related to filter caps in that circuit.

[jaysg]

The layout of those Fender amps is wrong in the grid and screen department. When you replace the 470's, don't go across the middle of the socket...go around. 3W is good...1W is a bad idea.

[67plexi]

I had one cross my bench last year same cause no load to the speaker
did the same dammage but the crackle was the power transformer arc inside
the transformer after time it failed but it was ok at first.

Steve.

[Richie]

also look for an arc on the tube socket. the spade terminals always suck. I always solder the connections. Some of the switches themselves are not very high rated. Might look into changing those if you have had a problem.

[Bob Simpson]

I had one cross my bench last year same cause no load to the speaker
did the same dammage but the crackle was the power transformer arc inside
the transformer after time it failed but it was ok at first.

Steve.

How would I test for something like that?

Or is it just a wait until it fails type of thing.

Was there other damage caused by the PT?


Bob

[billyz]

I prefer the 1 watt screen resistor on a fender. I would rather replace a resistor than an Output transformer. I have yet to see these resistors fail unless the tube screen shorted first.

[marcoloco961]

The flashing would be the heaters. Some heaters flash as a normal behavior. Many NOS European 12AX7s, for example. That flash (with standby off) cannot possibly be related to filter caps in that circuit.

I have been wondering about that. I have a couple of tubes that do the same thing. First fire up after it sits off for a while, and the one 12ax7 glows almost white for a few secs. before dimming to orange. The tube itself sounded great in the V1, so I just kind of kept an eye on it. Thanks for that info, I can quit fixating on it now.

[C Moore]

C
OT secondary reads .6 ohms.
OT Primary blue to brown 428 ohms
blue to red 215 ohms
brown to red 214 ohms
No primary or secondary wires have continuity with the OT frame.
A wall wart providing 16.5 vAC across the unsoldered secondary
produced a 377.4 vAC reading on OT primary blue to brown...

ratio 1 : 22.8722727

Bob

Hey Bob -
Can you tell me what kind of wall wart you have that produces low voltage AC...?
I was in need of something to test transformers, and with the help of the guys here, built a 10VAC device from a filament transformer. I guess I am kind of jealous that you have something like that; something that you could just grab, that already gives some kind if low AC.....
Thank You