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FDP Forum / Amp Mods, Repairs, and Projects / Fuse blowing after about 5 minutes

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vanceen25
Contributing Member
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United States

Vance Erickson
Sep 17th, 2017 04:37 PM   Edit   Profile   Print Topic   Search Topic

My 64 Super Reverb has started blowing fuses after about five minutes of normal play.

This happened a couple of times before I noticed that I had done a stupid thing - the internal speakers were plugged into the extension speaker jack, and the speaker cable in the main speaker jack was going to an attenuator that was connected to nothing.

For a while I thought this might be the whole problem, but after a few minutes of playing it blew a fuse again. I pulled out the fuse right away, and noticed that it was really hot. So I have to pull out the chassis and have a look. Obviously I've done some kind of damage that needs fixing.

Can any of you suggest a place to start looking? Screen resistor, perhaps? Bias circuit? It seems odd that that it sounded OK up until the moment the fuse blew.

Leftee
Contributing Member
*******

VA

One foot on the brake, one on the GAS
Sep 17th, 2017 05:47 PM   Edit   Profile   Print Topic   Search Topic

Have you tried a fresh set of output tubes? I'd start there with a known-good set of tubes.

pdf64

UK

Sep 18th, 2017 07:16 AM   Edit   Profile   Print Topic   Search Topic

I can't see that the cab / attenuator connection arrangement you describe would have stressed anything (other than yourself :-)) unduly.
So the root cause may be coincidental.
Usual causes of fuses starting to blow include failing power / rectifier tubes, failing HT ecaps, unsuitable bias, failing parts in the bias supply.

vanceen25
Contributing Member
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United States

Vance Erickson
Sep 18th, 2017 09:41 AM   Edit   Profile   Print Topic   Search Topic

Thanks. I'll try a new rectifier and 6L6GC's before doing anything else.

The big electrolytics are only a couple of years old, as are the bias cap and resistor. Of course, that doesn't mean they aren't failing, but if they are, I'm guessing it indicates another problem.

pdf64, I assumed that configuration was a bad idea because I have no idea what load this attenuator presents to the OT when there isn't a speaker plugged in. I assumed (perhaps wrongly) that it would be significantly less than 2 ohms, which would mean that the parallel load (with the 2 ohm speakers in the extension jack) might be well below one ohm. The attenuator was set on only a mild attenuation level, but had no speakers plugged in.

Leftee
Contributing Member
*******

VA

One foot on the brake, one on the GAS
Sep 18th, 2017 12:52 PM   Edit   Profile   Print Topic   Search Topic

It's an old Fender.

There was a load.

You were probably good.

6G6

Texas

Fender power to the people!
Sep 19th, 2017 06:31 AM   Edit   Profile   Print Topic   Search Topic

The fuse is blowing because something is drawing extra current, beyond what it is designed for.
Most of the time you will find voltge drop when the current draw is up.
Fender schematics from this era will hve voltges noted for different points in the circuit.
If you can take a volt meter and compare readings with what it says it should be, you may be able to find where the problem is.
Since Leo used a lot of parts with a 20% tolerence, there will be some difference even where everything is good.
Look for large differences and signs of heating.

vanceen25
Contributing Member
***

United States

Vance Erickson
Sep 20th, 2017 04:29 PM   Edit   Profile   Print Topic   Search Topic

Just an update in case it gives anyone a clue.

I ran the amp without the power tubes for about a half hour, and the fuse didn't blow.

I then put in some new Mullard 6L6GC's and both the rectifier and the power tubes got very hot, very fast. I turned it off before the fuse blew, but was definitely not normal.

I replaced the rectifier with a new Mullard GZ34, and the same things happened. The power tubes redplated.

vanceen25
Contributing Member
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United States

Vance Erickson
Sep 20th, 2017 04:53 PM   Edit   Profile   Print Topic   Search Topic

Seems like it might be something in the bias circuit.

willie
Contributing Member
********

Too Near Atlanta GA

Amp Tech Emeritus
Sep 20th, 2017 06:20 PM   Edit   Profile   Print Topic   Search Topic

Check the bias voltage with the output tubes removed.

w

vanceen25
Contributing Member
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United States

Vance Erickson
Sep 20th, 2017 07:08 PM   Edit   Profile   Print Topic   Search Topic

Thanks, Willie. I'm guessing it will be nowhere close to -52V.

pdf64

UK

Sep 21st, 2017 07:31 AM   Edit   Profile   Print Topic   Search Topic

Err, in the case of the fuse blowing, it's good to check on that before cooking another tube set.
Redplated tubes may be considered bad until proven otherwise.

The load placed on the amp by an attenuator at a low attenuation setting was probably minimal, ie resistance well above 2 ohms.
So in conjunction with the 2 ohm speaker load in the ext socket, not a problem.

rfrakes331K
Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Sep 21st, 2017 03:09 PM   Edit   Profile   Print Topic   Search Topic

Seems like I had this problem back in December 2008. Can we search that far back. You guys helped me fix it. Just off the top of my head, R46 or 47 comes to mind. I ordered some 1% resistors from Lord Valve and it was fixed.

vanceen25
Contributing Member
***

United States

Vance Erickson
Sep 22nd, 2017 04:43 PM   Edit   Profile   Print Topic   Search Topic

Thanks, pdf64.

The tubes were only powered up for about two minutes, so perhaps they're OK. If not, lesson learned.

My uncertainty about the attenuator comes from the fact that it was not connected to any output. I'm sure you're right.

pdf64

UK

Sep 23rd, 2017 04:18 AM   Edit   Profile   Print Topic   Search Topic

"My uncertainty about the attenuator comes from the fact that it was not connected to any output"

You wrote previously that the attenuator was connected to the amp's main speaker jack; that is an output socket.

Do you mean above that the attenuator's output wasn't connected to anything?

willie
Contributing Member
********

Too Near Atlanta GA

Amp Tech Emeritus
Sep 23rd, 2017 08:12 AM   Edit   Profile   Print Topic   Search Topic

...and the bias voltage reads...what?

w

vanceen25
Contributing Member
***

United States

Vance Erickson
Sep 23rd, 2017 03:00 PM   Edit   Profile   Print Topic   Search Topic

pdf64,

The amp's main speaker output was connected to the attenuator. The output of the attenuator was not connected to anything.

Willie, I haven't checked yet. I don't have a proper bench, so pulling the chassis and working on it means moving out some other things I'm working on. I suppose I could check pin 5 to ground with no power tubes and the volume at zero.

willie
Contributing Member
********

Too Near Atlanta GA

Amp Tech Emeritus
Sep 23rd, 2017 04:18 PM   Edit   Profile   Print Topic   Search Topic

That'll work...just be careful doing so.

w

ECS-3
Contributing Member
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USA / Virginia

Oct 3rd, 2017 04:56 AM   Edit   Profile   Print Topic   Search Topic

I would connect a voltmeter to the bias supply and then monitor it.

I had a 67 Showman that would intermittently lose power and red-plate the power tubes, and that turned out to be a bad bias capacitor. Changed that cap out and no more problems.

vanceen25
Contributing Member
***

United States

Vance Erickson
Oct 4th, 2017 11:49 AM   Edit   Profile   Print Topic   Search Topic

Thanks ECS-3. I'll be pulling it and checking it put it the next few days.

vanceen25
Contributing Member
***

United States

Vance Erickson
Oct 6th, 2017 04:55 PM   Edit   Profile   Print Topic   Search Topic

OK, finally pulled the chassis. Thanks all for your patience.

I get 53V AC on both sides of the bias resistor, as expected. But I don't find any AC on the input to the bias diode, nor do I find any DC on the output side. No DC on either side of the bias cap, no DC on the input or center lug of the bias pot (rotated it to confirm), and no DC at pin 1 of either power tube socket.

So, as expected. I have no bias voltage. Now the question is why. The resistor reads 470 ohms in circuit. Everything seems to stop at the junction of the resistor and the diode, which seems weird to me unless I have a really, really bad solder joint there.

So I'll reflow that solder joint, and replace the diode if that doesn't work.

Any advice would be most appreciated!

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FDP Forum / Amp Mods, Repairs, and Projects / Fuse blowing after about 5 minutes




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