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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Mar 10, 2013 10:47 pm
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Hi Art

Yes, It Flickered then it went out, as the sound was dying (there was a burning smell that all the musicians on stage noticed as well)

My guess is that some sort of thermal shutdown was triggered inside the amp? My tech agreed that some amps have something like this.

After the incident, the Light turned back on along with the tubes lighting up. But no sound.

Pulled the speakers. No movement with a 9 volt battery. No reading with the meter set to Ohms.

Thanks
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Mar 11, 2013 1:19 am
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OK, Dave. So the speakers are broken down, correct? I'm thinking that the output trans needs a Hi-Pot test. The trans would have to be removed and taken to any electronic business that can run the test(ie. electric motor builders, auto starter rebuilders, transformer bulders). As long as you tell them ahead of time, what you have and the problems you've experienced. Take the schemo, if you decide to go this route. I'd be looking at $30, it's a real simple test. This would certainly eliminate a possible problem. The smell and shut-down: I'm going with a fusible link and of course the speakers. JMHO. I did read the entire thread, again. If I'm not mistaken, everything has been brought back to original component values and configurations. On another front, I find the ground scenario to be low-brow. But that's a subject, that can wait(I think?). Art

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Mar 12, 2013 2:29 am
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Art,

This scenerio sounds way too familiar & painful. Prolly a good idea to check the power tranny as well as the output tranny. My experience is whatever cooks the output tranny, cooks the PT.

Such as putting a 20 amp fuse into an amp, as a "temporary" replacement.

:(


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Mar 12, 2013 7:26 pm
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Yeah Beemer!!! The pilot light flicker, triggered the PT concern to me also. Dave might as well pull both transformers and have them tested. Either way, this helps narrow the failure down. The bad bass response does point to a lack of substantial power. Must admit, I'm not familiar with this particular amp. So my comments are just general repair suggestions!!! Art

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Mar 14, 2013 12:46 pm
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Hey guys

Thanks for the info re PT and OT, i'll get back to that in a second.

Installed 2 new speakers today (Eminence Red white and Blues, 75w a piece)

Turned it on, and she sang like she always does when I first turn it on...then slowly but surely, the fuzz returned. In my heart I knew those speakers were old, but I also knew there was something else going on.

I hooked up some distortion pedals and just shredded some power chords for about a minute or so at low volumes. They sounded nasty, so I turned all my pedals off. And its back to being the same old fartbox it was before I changed the speakers.

The amp cannot handle a high current coming in. Its good, then you crank your incoming signal, and it turns bad.

The techs aren't seeing the issue. A) because it doesn't register on their scope for some reason. and B ) Because when you first turn the amp on, the problem is fixed. I fell pray to this at my last visit to the tech, sounded pretty good at his shop... (sounds good for about 5 to ten minutes before returning to mud)

Does this sound like a Power transformer or Output transformer issue?

The Repair list on this amp to date:

All new power tubes, and the pre amp tubes have been tested/changed out to no effect
All new filter caps
New Speakers
New Phase Inverters
Power tube socket replaced

Im not too sure where to turn with regards to getting a HI-Pot Test. I googled "Hi-Pot test Toronto" and the results were a bit confusing. A lot of folks selling Testers, or letting you rent them.

Thanks guys
Dave


Thanks again


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Mar 14, 2013 1:04 pm
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The symptoms (as you describe) are telling me it's a thermal issue. Thus, it could very well be a transformer. You have to actually work to kill a Schumacher PT or OT though. OTOH, blowing out a choke in an older Fender is a pretty common occurence. You might give that look-see.

HTH

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Mar 14, 2013 1:44 pm
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Ha!

I just called tech #2 to ask if he knew where to get a Hi Pot test done. He told me that it was extremely unlikely that was the problem.

He told me he was sorry that he couldn't look at my amp the last time I brought it in, but he's insanely busy. Told me not to bring my amp back to him...no time.

His advice was to change the caps in the tone stack. I said "Uh ok, so to test those caps do I need to have the amp on, to test the voltage drops?" (I mentioned the schematic and the voltage etc) ...He kind of lost him temper at this point.

"what??? We are talking about 2 different this. Have the amp on??? of course not!!!...Im just telling you to change the tone stack caps.....good luck....goodbye"

-I don't know whats wrong
-Whatever you hear on the internet is moronic
-I don't have time to help you
-Trying doing this
-The way you think you should do it, is stupid

In conclusion, don't do it yourself, and don't bring it to me.

:mrgreen:


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Mar 14, 2013 1:56 pm
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Methinks you need a new tech......somebody who knows WTF they're doing.

:shock:

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Mar 14, 2013 2:02 pm
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Haha. I know ART...but remember, this was tech #2 of 3. There's only so many.

The first one I don't trust at all. He tack soldered all the filter caps in. A problem that was repaired by Tech 2

Tech 2 is too busy to work on any more amps. Also If you make the mistake of saying "I heard on the net..." he basically loses his mind.

Tech 3 is a really nice guy. But, he has had the amp on the bench twice. His understanding of this seems to be "the scope don't lie" Scope says its fine (though I'm not sure how long he's letting it warm up before testing...)

I put a call in to tech #4.... see how that plays out.

It can't be the tone stack caps because the problem exists in both the vibrato and non-vibrato channels right?


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Mar 14, 2013 3:18 pm
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The tone-stack caps have *dick* to do with your amp's problem.

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Mar 14, 2013 5:36 pm
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I googled "Hi-Pot test Toronto"

Prolly got hits for legal MJ. :lol:

Your issue sounds similar to one that I'm dealing with right now. It's most likely a thermal and/or instability issue(s).

First thing... pull the fuse and check the amp rating. If it's way out of range (like 10-amps or more) --- you may have severe tranny and wiring damage in your unit. Along with solid-state items (like diodes) which cannot tolerate excess power.

If fuse is ok, I would do is pull all tubes. And test the output section. First, see how stable that negative bias voltage is (after the bias supply's diode and at the tied grid-to-ground resistor point). This MUST be stable. read the negative voltage in the socket of pin 5 of the 6L6GC.s It MUST be a steady negative voltage. (prolly around 5-10 VDC different than at the grid-to-ground resistors, due to the 1500ohm grid stopper resistors tied to the signal grid).

All this can be read without the 6L6GC in the sockets. Just be sure that the output tranny is seeing a load (connected to speakers).

If bias voltage is in-spec and steady... put in and test the idle bias of those 6L6GCs. Are they steady? Or wandering around? No load (no guitar plugged in and volumes at zero). Speaker connected. Keep the bias meter reading for a good 10-15 minutes. Watch for sudden red-plating in those tubes.

If the bias is unsteady, check your socket contacts. And the ground strap from the cathode to the chassis. Check any stubber caps off the 6L6GCs. And the general layout and solder of the wires to the sockets. This is where most amp instabilities occur.

If steady bias, turn off amp & plug in only the 12AT7 phase inverter. Recheck the bias. If it now starts to wander around ---- check the negative feedback circuit for out-of-spec component. Check the coupling caps between the 12AT7 and the 6L6GC. Check voltages on the 12AT7 anodes & cathodes.

If no matter what you do the amp is unstable --- you prolly have a severe oscillation issue (maybe seen only under load --- or volume). Or bad tranny(s) which are shorted and saturating too early. Be sure the secondary side of the output tranny has a good ground contact.

Good luck. I'll be going through an amp this weekend with a similar issue(s). Try to post some results.


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Fri Mar 15, 2013 1:35 am
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No useful techs in my area is what got me started too.
These guys here were not only able to get me through a refurbishing and an update to a Bassman head, but then the second one through the door with similar troubles was a very fast fix, just by measuring voltages I quickly found the issue.
I'm now the owner of two very nice 60's Bassman heads one of which was headed for the dump :shock:

I did find that buying a few books on the subject of tube amps helped to cement the ideas and the trouble shooting methods the guys here on the forum where trying to teach me (long distance) !

Hang in there and best of luck ! The prize at the end of it all is a sweet sounding vintage Fender !

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