'68 Bandmaster Rev. Blows Fuses, GZ34 Arcing

[vintageguitarz]

Was about to demo for sale my ol trusty '68 Fender Bandmaster Reverb head, had just shown it a couple months ago and it worked fine. This time I turn it on with switch in Standby and it immediately blew the 2A Slow Blow fuse. Put in another fuse and this time watched the GZ34 rectifier tube and the two 6L6's and on power-on I noticed the rectifier tube was arcing like a Mid-west Lightening Storm at the top inside and one 6L6 (the 2nd to the right) was dead with white powdery residue on the top of the (upside down) 6L6 tube and on the side of the tube across from the cathode.

I've never had this happen to any of my vintage amps, so I'm shocked (excuse the pun).

Any Amp Techs have an idea what happened?? Blew a Cap? Xfrmr?
Can I diagnose and fit this my self - pretty good with a DVMM. Any diagnosis / repair guides online?

[buckaroo]

I am NOT a tech. Others, such as Kap'n, will inform you much better than I. But as a start, I would unplug the amp. Pull the chassis out. And do a safe visual inspection. Don't touch anything yet!

I am curious if you noticed the tubes condition before you replaced the fuse? Were they already bad? Have you checked the power delivered to the AC outlet you had the amp plugged into at the time? Was the amp connected to a load (speakers) properly? Just some background questions to ask yourself to fill in the diagnostic picture as to cause of the problem.

If you know how, and only if you know how, with one hand free I would discharge the power supply caps. Then look deeper at the chassis for any obvious parts gone bad. Based on what you have said, I would suspect a blown resistor (and or capacitor) somewhere geographically near the power transformer side of the amp. The blown component may have caused the tube failures or the tube failures may have caused the component to blow.

Sometimes the failures are visually obvious and someone with less knowledge (like me working on my own gear) can replace a part and then measure the voltages to make sure all else is normal. Other times a tech is the best answer.

A reputable tech with respect for vintage gear is never a bad choice!

Times like these are when you will be happy to have a bin or two of NOS Fender parts that you have stashed away over the years! Good condition examples of the original caps and resistors keep the sound the same.

[buckaroo]

This has been here before but it is a bit of an amp repairman's bible:

http://www.diyguitarist.com/Misc/J_Darr.htm

[vintageguitarz]

Thanks for your reply, nope things like wall outlet is used all the time for other amps, had not long ago demoed this amp and no problems, sounded damn good in fact, didn't look at the tubes until after the blown fuse was replaced and noticed 6L6GC #2 was DOA and the rectifier tube was arcing. This was completely unexpected, but what isn't.

Now one question I have is, with the GZ34 arcing like that, is it likely bad now?

[buckaroo]

Based on what you have written, and not having seen it, I would say that the rectifier is best discarded. Even if it might still "work" I would consider it damaged and potentially unreliable.

Now you have to discover the cause. Which possibly could be the rectifier but could be another component.

I think Kap'n would be a much better asset to you than myself. Hopefully he will jump in sometime here. I would hate to steer you wrong. I am not a tech. I just tinker enough to keep my own stuff going.

[Kap'n]

Yep, the Darr book is the go-to for this kind of thing.

If you know your way around the inside of an amp, and know some electrical safety, I would

give a visual inspection, check the power tube sockets for arcing, and measure the value of the screen resistors - replace if drifted.

If you have a current limiter this is the time to use one for the following steps...

pull all the tubes, including the rectifier, put in a fresh fuse, and check for voltages on the rectifier socket.

Put in the rectifier and check voltages down the line

Put in a known good (but expendable) power tubes, bias and see what happens.

Add V6-V1 in sequence, powering up and down each time to see if anything happens.

[vintageguitarz]

Guys I really appreciate your ideas and procedures on tracking down the bad boy or boys inside the amp. I'm no electronics engineer, but I've taken a few courses over the years and know my way around a DVMM and basic scope. But you know diagnostics is an art for pros, so hence the suggestions on where to look for likely problems.

Anything else you can suggest will continue to be appreciated, taking it to an expensive Amp Tech is my last choice in this terrible economy.

Thanks!

[buckaroo]

Open it up and look at it. A careful look might reveal exactly what is wrong without much question.

[vintageguitarz]

Kap'n-

Thanks for putting in some much needed 2 cents. I don't have a current limiter, not even sure what that is, for this purpose. Is there an alternative?

V6-V1, what are those? This may be a "duh" moment for me.

[vintageguitarz]

Open it up and look at it. A careful look might reveal exactly what is wrong without much question.

Yep, that's my first step tomorrow under some good day light.

[Kap'n]

Kap'n-

Thanks for putting in some much needed 2 cents. I don't have a current limiter, not even sure what that is, for this purpose. Is there an alternative?

V6-V1, what are those? This may be a "duh" moment for me.

A current limiter isn't necessary, but it's a load wired in series with the primary. It's used as a diagnostic tool to allow a failed piece of equipment to run by limiting the amount of power to it, making it easier to find the failure. It usually uses a light bulb.

V6 through V1 are the preamp tubes, in the order from phase inverter (closest to the power tubes) to the normal channel preamp tube (furthest from the preamp tubes).