It is currently Mon Mar 16, 2020 2:20 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 8 posts ] 
Author Message
Post subject: 68 Twin Reverb Noise
Posted: Wed Aug 27, 2014 5:35 pm
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 4:52 am
Posts: 56
Location: Connecticut
Hi everyone,
I have an early 68 twin reverb with a AB763 chassis that I'm working on. It's been sitting idle for 5 years because I didn't have the time to mess with it. It originally had an extremely loud hum. I replaced the diodes in the bridge (3 of which were shorted) and the bias cap which was bad. The loud hum was gone.

I still have a problem with the amp making kind of a whooshing sound when the reverb pot is turned up along with a horrible loud squeal when the reverb gets turned up about half way.

I swapped out the V2, and V4 tubes with no change.
If I pull V4 out all problems are gone.
I took some dc voltage measurements:
.6vdc on the side of the .02 cap where it connects to the 500pf and 10pf caps that comes off V2 P-6.
The caps that go to V4 all have less than .05 vdc.

The .6 volts seems high to me but I don't know about the .05 volts. I measured the resistance of the plate resistors and they seem ok if a little on the high side.
Before I go ripping resistors and caps out I thought I would seek out some greatly appreciated advice.


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: 68 Twin Reverb Noise
Posted: Thu Aug 28, 2014 9:09 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Try replacing V3 (12AT7). This is the reverb driver tube. And often time these go noisy.


Top
Profile
Post subject: Re: 68 Twin Reverb Noise
Posted: Thu Aug 28, 2014 11:55 am
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 4:52 am
Posts: 56
Location: Connecticut
Hi, Thanks for the suggestion.
I tried replacing V3 with a different 12AT7 tubes as suggested with no luck.
I'm going to replace the .02uf and 500pf coupling caps that come off V2 since that is where the most unwanted dc seems to be.
I ordered a few OD caps. The closest thing to .02uf was .022. I hope that value will be ok.
My capacitor theory is kind of rusty.


Top
Profile
Post subject: Re: 68 Twin Reverb Noise
Posted: Thu Aug 28, 2014 3:19 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
0.022mfd will be fine. Be sure both this cap and the 500 pico-farad one are rated for al least 500VDC.


Good luck!


Top
Profile
Post subject: Re: 68 Twin Reverb Noise
Posted: Thu Aug 28, 2014 6:45 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah, I agree with the cap R&R. If the tubes, tranny, and tank look good.... I'd suspect that RC isolator circuit. This assumes the rest of the components come in close to spec. I'm sure you're treating this section like a seperate amp, right? Good Hunting! Art

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


Top
Profile
Post subject: Re: 68 Twin Reverb Noise
Posted: Thu Aug 28, 2014 8:15 pm
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 4:52 am
Posts: 56
Location: Connecticut
Thanks for the reply.
Yeah I hope the caps do the trick too. I've had this amp since 1980 and it was always a work horse. It had a meltdown in the middle of a gig. Smoke was pouring out of it.
I had it repaired when I moved to FL for a few years and only used it a couple times until a few years ago and that's when it started making this extremely loud hum which turned out to be the shorted rectifier diodes (1N4004) and bias cap.
In the process of working on the amp I discovered some really weird things that were done by the tech in FL.
He changed parts of the amp to a silver face when he replaced the burned caps and resistors. I've seen lots of people turn a silver face into a blackface, but it's the first time I ever saw a tech turn it the other way.
The resistor values in the filter section were a 10k and 2.2k, there was a 1k coming from the PT to the diode in the bias section.
My guess is, he gave the amp to one of his techs and the saw the silver face on the amp and grabbed an AC563 Schematic. I'm just thankful there wasn't any more parts to replace than there was.
I've been away from electronic for quite a few years so this has been a re-learning experience for me. I have a ways to go with it still so I hope I can count on you guys for some more advice as I need it.


Top
Profile
Post subject: Re: 68 Twin Reverb Noise
Posted: Thu Sep 04, 2014 1:59 pm
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 4:52 am
Posts: 56
Location: Connecticut
Hi everyone,
well I got rid of the loud squeal from the reverb. It was the 25/25 double cap going to V4.
The reverb is now real quiet.
Thank BMW and aclempoppi for your input.


Top
Profile
Post subject: Re: 68 Twin Reverb Noise
Posted: Thu Sep 04, 2014 6:42 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Your are welcome. Those OEM caps prolly dried out and became microphonic. Nice catch.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

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: