super reverb intermittant fuse blowing

[pontiacpete]

A bandmate has had fuses blowing in a blackface super for the past few years. On gigs a fuse would blow and he'd put a fuse in and finish the gig no problem. He had a cap job done by someone else about 2 years ago and still the problem persists until it got so bad that it wouldn't hold a fuse. Now I have it. I'm a novice, at amp repair and amp building, been do this for only a bit more than 2 years.
I tapped around alot with chop stick and check voltages. Checked PT with the mentor I'm working with. That measured up fine
He suggested changing paper caps and checking resistors for the appropriate value. Changed alot of everything on the board. Almost a new amp. So I run it for a good long while and played it hard and loud and a fuse blows. Now, I've always suspected the tube sockets, having read this form and others, so I finally change them and put in ceramic octals, all 3, none of the 9 pins. I had checked them all by moving them and tapping all tubes and no apparent problems but put the new octals in anyway.
(BTW, I put in ceramic sockets and as many of you know they are alot stiffer and harder to work the tube in and out. In the process I snapped off the plastic guide at the center of the tube base. Has anyone done this and is it possible to glue it back on[with what?] and will it hold? of course he gets the new one.)
The other problem I came across was that one of the bolts holding the PT to the chassis was stripped and the nut couldn't be tightened down. So not having an exact replacement I bought one from a hardware store that fit and put the cardboard sleeve around it and put it in. However, I'm a bit concerned that the cardboard sleeve was damaged at the top and therefore not covering a 1/4" of the bolt. Could this have an effect on the amp? Damage the PT in the long run?
thanks in advance.

pete

[bnwitt]

Pete,
I doubt the new trannie bolt will be a problem. As far as the broken tube base Key, you can put an adapter on it to fix that. Go to:

http://www.tubesandmore.com

and search "p-stky-1" that is an octal tube base keyway replacement.

By the way, what are you setting the idle current of the 6L6GC tubes at and at what plate voltage?

[stratomaster]

When's the last time you changed the rectifier tube on it? My amp did this for a while, and it ended up just being a bad rectifier.

[52 Bill]

When's the last time you changed the rectifier tube on it?

Yes, the one thing that you don't mention in all of this is how old are the tubes?

The first thing to look for are bad rectifier or power output tubes.

[pontiacpete]

Pete,
I doubt the new trannie bolt will be a problem. As far as the broken tube base Key, you can put an adapter on it to fix that. Go to:

http://www.tubesandmore.com

and search "p-stky-1" that is an octal tube base keyway replacement.

By the way, what are you setting the idle current of the 6L6GC tubes at and at what plate voltage?

Thanks bnwitt.
That's great that they have such an adaptor, whew.. too bad I just placed an order last night with AES. I'll get'em on the next one.

I have the plate voltage set a 460 vdc, the grid voltage is set to -52 vdc and the idle current on the is 93mA. Does this sound correct?
The screens are also measuring at 460 vdc.
thanks again.
pete

[pontiacpete]

When's the last time you changed the rectifier tube on it? My amp did this for a while, and it ended up just being a bad rectifier.

The owner claims to have change the rect. tube. When I got the amp I put a new one in and I'm sorry to say that I can't remember if the new one was in or his old one when it popped the fuse.

regards
pete

[pontiacpete]

Yes, the one thing that you don't mention in all of this is how old are the tubes?

The first thing to look for are bad rectifier or power output tubes.

None of the tubes are new. However the owner has put many different tubes in. He has quite a collection of tubes(he's old man worked on antique radios).
When I got it I tested all of them and I'm sure had done so too. All were measuring fine. But I went ahead and put a new rectifier in and another couple of 6L6's in(not new). Anyway I can't remember if this particular new rect. was in when it popped the fuse last time.
The rect. is new, now. The 6L6's are his old ones(don't have any new ones around at the moment). I guess I will continue to run the amp, which is running and sounding fabulous, and see what happens.

thanks
pete

[pontiacpete]

Thanks bnwitt.
That's great that they have such an adaptor, whew.. too bad I just placed an order last night with AES. I'll get'em on the next one.

I have the plate voltage set a 460 vdc, the grid voltage is set to -52 vdc and the idle current on the is 93mA. Does this sound correct?
The screens are also measuring at 460 vdc.
thanks again.
pete

sorry to respond to myself, but was thinking about my 93mA for idle current and was wondering if what is meant by idle current is the total current on B+.?
That measurement is taken from across the lugs of the standby switch, 93mA. On each of the 6L6's there is 37mA, with, now 470 vdc on the plates(turned my variac to my house ac voltage of 123vac).
Does this sound correct for this amp?
thanks
pete

[RWood]

Does this sound correct for this amp?

Pete,

Take a look at this bias chart for 6L6s.

Find your amp's plate voltage in the second-to-last column. The recommended "Class A/B" current at that voltage should be no more than 25.5mA!

I think you are on the right track (the long, circuitous track) to curing this amp, especially when you tame this bias issue. You'll need to increase the negative voltage on the grid. If you can insert your ammeter in series between the plate wire and pin 3 of the power tube socket, you'll get an accurate reading of the plate current.

RWood

[Bruce / Mission Amps]

sorry to respond to myself, but was thinking about my 93mA for idle current and was wondering if what is meant by idle current is the total current on B+.?
That measurement is taken from across the lugs of the standby switch, 93mA. On each of the 6L6's there is 37mA, with, now 470 vdc on the plates(turned my variac to my house ac voltage of 123vac).
Does this sound correct for this amp?
thanks
pete

Just to comment... No.
I think if you do that standby switch current shunt check you'll be also measuring the idle current of all the preamp tube's triodes too.... that is 6 more tubes worth of current... probably around 12ma -18ma extra you are measuring in conjunction with the power tube's quiescent, no signal current.
How you got 37ma for each 6L6s is not obvious to me.
However, if the power tubes are all good, these amps sound excellent with both 6L6s running around 32ma- 38ma each at 450v-470v and should never blow the main fuse because of it.
Have you considered watching the current of the tubes while the amp is hot and has been making 40-50 watts of output power?

[pontiacpete]

thanks for the responses.
I'm not sure which to go by here, 25.5mA that rwood suggests from the bias chart. (BTW thanks for that, I found the web page that comes from also)

To answer your question Bruce, are you suggesting that I send a signal from an audio generator? If so , no I haven't done that yet. I've set up a test like that before with help but probably have to scratch my head a bit to figure out how much signal would give me 40 to 50 watts of power and how long should I run it for.
Would you mind helping me with this?
oh, I read the 37mA from where the red wire on the OT meets the one of the choke wires on the board, in series, or across to each of the pin 3's of the 6L6's.
pete

[yann]

The owner claims to have change the rect. tube.

I currently have a VOX AC30CC2 that was doing the same thing, intermittent main fuse blowing.
Despite the rectifier has been replaced one month ago by the owner, a new one seems to have fixed the problem.
One thing I have noticed is that when the amp was on limiter, when it was having the issue, I was able to see sparks within the rectifier tube. A very short one then things go back to normal. Completely intermittent.