It is currently Mon Mar 16, 2020 3:34 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 27 posts ]  Go to page 1, 2  Next
Go to page 1, 2  Next
Author Message
Post subject: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 5:20 am
Offline
Hobbyist
Hobbyist

Joined: Tue Feb 09, 2010 9:55 pm
Posts: 78
Location: Perpetual State of Bliss
This is taking a shot in the dark cause I know trying to troubleshoot electrical problems by proxy is a near-impossible task. But....

My 65' Bandmaster started making what sounded like an oscillation with the volume turned to 0.

I was plugged into 'Normal' 1 and it started about 20 minutes in. It got progressively louder but not run-out-of-the-room loud. Plugged into each of the other inputs and there was no change. Even with no guitar plugged in, it was still there. It only stopped when I turned the Stand-by switch off. No controls had any effect at all.

I powered it down and pulled the first two tubes (V1 & V2) and replaced them with a couple brand new JJ ECC83S tubes I had lying around. Turned it on and it was quiet while I let it warm up. As soon as I flipped the stand-by, I heard the oscillation start out slow and gradually pick up speed and become more pronounced again. I shut her down and called it a night.

This Bandmaster was modded before I got it with what I was told were "Marshall" mods.
The tech who did the cap job said that the Tremelo circuit is bypassed and used as an extra gain circuit.

I love the way it sounds when it breaks up and the tech said the mods looked like they were done very professionally and could easily be changed back if I desire.

Any help would once again be greatly appreciated.

This links to a photobucket account which has pics of both my Bassman and Bandmaster. The second half of the pics are the Bandmaster which shows the mods and the paper caps before the new cap job.

https://plus.google.com/photos/11156929 ... banner=pwa

_________________
1964 Bassman, 1965 Bandmaster,
1978 Fretless Precision, 1982 Bullet Bass USA,
1989 MIJ '54 Re-issue Strat


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 9:16 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
You definitely have a microphonic tube somewhere in the amp. Try this. Pull V3 (12AX7). Turn on amp. Does the problem go away? If not, put V3 back in & pull V4 (12AT7). If you still have the problem, it's prolly one or both 6L6GC. Do you have any spare, good, 6L6GC to substitute?

http://bmamps.com/Schematics/fender/ban ... _schem.pdf


Good luck!


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 9:56 am
Offline
Hobbyist
Hobbyist

Joined: Tue Feb 09, 2010 9:55 pm
Posts: 78
Location: Perpetual State of Bliss
Just the 2 in my bassman. Can I put them in without biasing?


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 10:06 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Have you tried pulling V3, then V4, first? It sounds like it maybe V3, 12AX7 that operates the tremolo/vibrato circuit.

If V3 or V4 are not the culprit, you can pull the 6L6GC out of your Bassman --- for a temporary check. No problem. You should be able to rule-out if the original 6L6GC's in the Bandmaster are bad within a few seconds.


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 10:19 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
Remove and discard the "mods". Likely, the alleged extra gain is creating unwanted oscillations that are being fed directly into the phase inverter.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 12:14 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Feb 09, 2010 9:55 pm
Posts: 78
Location: Perpetual State of Bliss
I pulled V3, flipped on the stand-by, there it was. Sounds like a spinning plate.
I put back V3 and pulled V4, quiet as a church mouse.

There was a GT-12AT7 in there that I'm on my way to replacing ASAP.

Thanks for the help, I'll see how it goes with the replacement tube.
So THATS what micro phoning sounds like?


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 2:41 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
swerve,

What you could try is putting a good 12AX7 TEMPORARILY into V4 and see if the noise goes away. Or any good 12AT7 from another amp to rule-out the bad 12AT7. Before buying a new tube. But, it sure sounds that that GT-12AT7 is bad.


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Tue Apr 15, 2014 8:17 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Swerve, you might want to pull V2&V3 and run the amp. That would give you the Normal preamp channel and the power amp section. By pulling V4, the power amp section of the amp is taken out of the picture(no sound at all). I don't see the purpose of that test( subbing V4, yes---- out of the amp, no). IMHO the amp sections need to be isolated as best they can by the tube removal method. If the noise continues after removing the Vibrato channel tubes, then reverse the scenario....V1 out..... V2&V3 back in the amp. If the noise is about the same, then I'd suspect the power amp section(V4, V5, and V6). Just an idea, Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Wed Apr 16, 2014 2:50 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Art,

Pulling the phase inverter and leaving the output tubes in will rule-out the PI as the cause of the microphony versus the 6L6GC's. If you pull the PI (V4) and the amp is still microphonic (and I've seen this maybe a half-dozen times) = bad output tubes.


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Wed Apr 16, 2014 1:02 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Feb 09, 2010 9:55 pm
Posts: 78
Location: Perpetual State of Bliss
Acquired a new ECC81, installed.
Still there. Sounds a little different, but still there.
Noticed that when tapping the top of the cabinet, it alters the oscillation effect.
So I very lightly tapped the glass of the 6L6 with the handle of a wooden spoon and I could hear it altering the noise thru the speakers, big time. A whole lot more pronounced on the first 6L6 than the last one.
Not supposed to do that are they?


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Wed Apr 16, 2014 1:34 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
No they're not.

Have you got a known-good pair of 6L6's to try?

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Wed Apr 16, 2014 1:34 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Dunno. Try subbing the 6L6GC from your Bassman. If it is still making the same noise, you have other issues. Like what Arjay stated. Maybe the mod effected the stability of the amp. Or other components related to the PI or output stage is microphonic. Like a disc cap.

Get back to us about whether changing the 6L6GC's made any difference.

Good luck!


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Wed Apr 16, 2014 2:52 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah you're right Steve!!!! My Bad!!! I've done that when checking for arcing in the output section and checking the NFB loop. The memory bank was operating on a deficit!!! Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Wed Apr 16, 2014 4:59 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Feb 09, 2010 9:55 pm
Posts: 78
Location: Perpetual State of Bliss
That didn't go well.
In the process of removing the 6L6's, I depressed the retainer clips and got the first one out easily. The second one was more stubborn and thus my tugging, tugged the bottle top right in to the cabinet bottom crushing it. Sweet.

Expensive lesson.

So now I need 2 sets of power tubes. One apparently for the bandmaster (since one seems microphonic), and another to replace the one I just man-handled.

What a dumbass.....

I'm afraid this story will have to resume in a couple pay days.


Top
Profile
Post subject: Re: Leaky Tremelo Circuit?
Posted: Wed Apr 16, 2014 5:54 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Little tip for next time. If the clips are super tight, don't bend them back. Just remove the two screws holding them down and remove clips with the tubes.

Tip #2. Use good contact cleaner/lubricant on the sockets. Like DeOxit GN5, will help insertion and extraction. Kinda like K-Y of the vacuum tube world. :mrgreen:

It's cool. We've all done that. Either scratch the bejeebers out of the base or even pull the bulb off the base, in really old tubes. If you are looking for new 6L6GC's. The Electro-Harmonix or the new Tungsol 6L6GC-STR are very good tubes.

I feel your pain. :(


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 27 posts ]  Go to page 1, 2  Next
Go to page 1, 2  Next

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: