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Post subject: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcing!
Posted: Mon Aug 20, 2012 5:27 pm
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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 6L6GC'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. All my tubes were NOS vintage Sylvania and GE's

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?


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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Mon Aug 20, 2012 6:48 pm
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The main filter electrolytic caps need to be inspected. Perhaps, replaced. When you have an amp in storage for a awhile... Then want to fire it up after this period of inactivity --- I'd highly recommend ramping it up slowly with a Variac or a home-made light bulb-current limiter. Sounds like the caps may have been boarderline original items. And sudden turn-on, after awhile of inactivity, pushed them over the edge.

Heat, discharging while not in use, drying out of the electrolytic fluid from hot weather --> failed cap after first inrush, after much time with no use. Failed cap can lead to popped fuses. Arcing rectifiers. Other bad things.

You need to go through the power supply section of the amp. Prolly replace caps that are dried out or show damage. OOS (Out-of-spec) resistors. Clean the sockets and tube pins with good contact cleaner. Rewire the power cord to the wall for three-prong. Patent green line grounded to chassis. Bypass the polarity (ground) switch and remove the "death cap."

Be sure that the PT & OPT are good (they usually are ---- being one of the tougher devices in the amp).

Check & replace OOS items in the Bias Power Supply circuit. Check all grid stopper and screen resistors for OOS issues.

Check all voltages, once OOS items are replaced with quality new parts. Check plate voltages, idle bias current draw by all tubes, at idle.

Check wiring looks for induced hum. Good contact on all jacks and plugs.

Should be go to go.

If you wish, a good amp tech can do all of the above. Idea is to have a reliable amp both tone and electricity-wise afterwards.

Good luck! Keep us posted.


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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Mon Aug 20, 2012 6:51 pm
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Hazarding a "WAG", I'm gonna posit that the DOA 6L6GC finally gave up the ghost and shorted, possibly between the heater and cathode. It might also be a component related to that specific tube, namely the 470-ohm 1-watt screen resistor on the bottom of the tube socket. You'll need to open the patient's "chest cavity" to confirm this diagnosis, doctor.

Best of luck, HTH

Arjay

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Mon Aug 20, 2012 8:27 pm
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Concur with Beemer and Arjay. If the the power filters caps(bias cap) are original, I'd say that they have provided enough service. Same for the power rail resistors, bias resistor, screen & grid stopper resistors. Also check the diode on the bias board. I would do all the static testing and any necessary R&R before firing up the amp. Hope you dodged the bullet on that rectfier tube! I've lost a low hour NOS GE power tube in my Bassman. But it was my own damn fault for cranking the current draw to the 60% level. Be thorough and meticulous, and the amp will be in great shape! Art

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 2:15 am
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Your 's 6L6's could be short.

I first do a test , removing those 6L6 , put a new fuse and try if it blow and if GZ34 arcing.

If no , put new 6L6's and adjuts bias

If yes, do some test as Aclemmpoppi , Retroverbial and BMW suggest


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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 4:01 am
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Yeah, my Canadian brother is accurate, in his diagnosis of possible problems. Art

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 6:00 am
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stratele52 wrote:
Your 's 6L6's could be short.

I first do a test , removing those 6L6 , put a new fuse and try if it blow and if GZ34 arcing.

If no , put new 6L6's and adjuts bias...


+1

And for goodness sake, do not use a variac with the tubes installed! :roll:

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 6:46 am
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All good advice. I prefer to check the power supply voltages at the tube sockets (without tubes installed) before putting in a new tube and risking blowing it. The voltages will measure a little high, but not outragiously high.

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 9:35 am
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vintageguitarz, you might have several issues going on.

1.) Pull all tubes. Replace fuse with correct value. Put in a known good rectifier. Have the speaker connected. Volume zero. No input. Turn on amp. Does the amp pop the fuse? Better start looking at the PT.

Alternative is to pull tube, except the suspect (arc-ing) tube. Pop a fuse? Try a new rectifier ---- no popped fuse? Bad rectifier. Popped fuse? --- prolly something drawing too much current downstream of the rectifier.

2.) If the fuse doesn't pop. Prolly could be due to bad output tube (6L6GC). Or main filter caps. I'd rule-out those two before venturing further.

Good luck!


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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 10:33 am
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BMW2002Ti wrote:
...Prolly could be due to bad output tube (6L6GC)...


Cracked glass.

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 4:27 pm
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Holly Smokes guys - ALL of you - have given me a lot of great suggestions .... about a month worth of testing and trying! BMW2002Ti of course is ultimately right with your first posting to check everything against standard spec for the component. Of course then I've got to figure out WHICH circuit this 68 Bandmaster Reverb is? (Any suggestions on ascertaining which circuit it is?)

I really don't want to sell it to another musician as a "time bomb", so I want to make it right without spending a fortune and then not be able to get back my investment in a selling price.

I think I may try some of your other suggestions just to narrow down a range of potential problems. Like I said in the OP, it worked sweet just a few months ago and sounded as beautiful as ever. I had it worked over, replacing a lot of solid state components about 12 years ago when I was more active in the studio, then it sat dormant for 6 years until I decided to sell it in January. A lot of Caps and resistors were replaced with NOS where it could be found - a lot easier 12 years ago. :(

I like bluesky636 voltage check thought first, and aclempoppi and stratele52 (eh) procedure after checking the socket and voltage. And again BMW2002Ti add-on post yesterday.
BTW: "PT & OPT" are what "power tube" and out-put transformer? Just want to be sure.

"shimmilou" - You're right, it cracked the 2nd 6L6GC tube glass and there was a white power on the side of the tube and at the bottom (top / upside down). It's def "roasted".

A couple questions: I've heard some say before, used variable 120V PS on an amp that's sat too long, but I've never heard a Amp Tech agree with that and "shimmilou" wrote not too with Power Tubes installed. So is using a Variac an old wives tale or what and how to use it?? And where the hell do I find one (cheap)?

The GZ34 isn't dead, but is it likely that the mid-west style thunder storm arcing in the tube mean it's bad and/or DON'T use it? Just plan to toss it? I have a good but older GZ34 in my orig 65 Super Reverb I could use to test with, and just plan to buy a new NOS for the 65 SR instead of buying a cheapo Russian for the 68 Bandmaster Rev just to test with.

Lastly, suggestions for good and cheaper sources for Caps and Resistors online? The level of quality a good Amp Tech would buy. Anyone have opinions on JJ Tubes (from Slovakia I hear) for power tubes and rectifiers? How are the Russian tubes lately?

Again, THANKS! to ALL of you for your sincere suggestions .... real life savers. :D

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 4:51 pm
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vintageguitarz wrote:
Lastly, suggestions for good and cheaper sources for Caps and Resistors online? The level of quality a good Amp Tech would buy. Anyone have opinions on JJ Tubes (from Slovakia I hear) for power tubes and rectifiers? How are the Russian tubes lately?


There are many places on-line for caps and resistors. A few of my favorites:

http://www.amplifiedparts.com/

http://www.tubesandmore.com/

http://www.tedweber.com/

I use JJ tubes in my Weber 5F6A clone, my Blues Deluxe Reissue, and my Champion 600:

http://www.eurotubes.com/

For vintage tubes I go here:

https://www.kcanostubes.com/

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 8:40 pm
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Think this will do for a schemo,http://www.elworld.com/charts/Schematics/files/fender/bandmasterreverb_aa768_schem.pdf . HTH Art

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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Tue Aug 21, 2012 11:01 pm
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aclempoppi wrote:
Think this will do for a schemo,http://www.elworld.com/charts/Schematics/files/fender/bandmasterreverb_aa768_schem.pdf . HTH Art


aclempoppi - that links not working. I had to copy n paste it into the browser, the way you inserted it didn't make it clickable.

You think it's the AA768 circuit and not the AA568 circuit? Circuit models aren't marked on the Tube Charts after 67 like the old Black Faces were. So where did Fender mark them?

bluesky636 - thanks for the many store links, most of these guys I didn' know about. I had heard that JJ (formerly Tesla I think) had better quality than the Russian tubes like Sovtec. I used to use Ruby tubes decades ago, now I hear they are Chinese! And All the Chinese tubes are crap.


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Post subject: Re: My '68 Bandmaster Reverb now blowing fuse, GZ34 is Arcin
Posted: Wed Aug 22, 2012 12:37 am
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To find correct OEM circuit, prolly best to pull amp from cab and inspected layout:

http://vintagefenderamprepair.com/wp-co ... Layout.pdf

and schematic (of the AB768 BMR):

http://vintagefenderamprepair.com/wp-co ... ematic.pdf

Some nice detailed photos of your amp under the hood. Of the wiring ----be still my pounding heart, sorry it's kinda peccadillo I have for these older amp layouts --- would really help narrow down the specific circuit type.


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