It is currently Mon Mar 16, 2020 8:27 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 4 posts ] 
Author Message
Post subject: Dual Showman bias setting help
Posted: Wed Jun 27, 2012 4:20 am
Offline
Hobbyist
Hobbyist

Joined: Wed Jun 27, 2012 4:09 am
Posts: 1
Just got a 1968 dual Showman amp ( no reverb ) Circuit AB763
If anyone could tell me the recomended bias adjustment please ?
Also considering a Cap job, probably using Sozo Blue Modded Vintage Caps
I would like opnions from experienced techs / users.

thanks !

Claude


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Dual Showman bias setting help
Posted: Wed Jun 27, 2012 5:07 am
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah Hep, I'd set it at 26 to 30ma and forget about it! Actually, I run my RCA and GE's at 27ma or so. But these are early iteration tubes and(like your amp) '67& 68 amps. There are many opinions on this subject, all worth consideration. Art

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


Top
Profile
Post subject: Re: Dual Showman bias setting help
Posted: Wed Jun 27, 2012 7:19 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
hebaton wrote:
Also considering a Cap job, probably using Sozo Blue Modded Vintage Caps


Do you understand what a "cap job" generally means?

We're talking about the electrolytics under the doghouse and on the bias board -- not the critical tone caps on the eyelet board. Tired Mallory's should be replaced with F&T's or Sprague Atoms.

And before proceeding with any maintenance or servicing, you should ascertain that your amp is indeed an AB763. The Showman reverted to the AB568 revision in May of '68. Do not rely on the tube chart -- look for an ink-stamped build date on the underside of the chassis to confirm.

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: Dual Showman bias setting help
Posted: Thu Jun 28, 2012 1:58 am
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah Hep, Arjay makes a good point. It's the large electrolytic caps that should be considered. Looks like (2) 70uf 350VDC and (3) 20uf 525VDC for the power. I'd go with a 50uf 150VDC for the bias supply. If you have the blue molded mallory's on the circuit board, I wouldn't mess with them, unless you find a leakage. Art

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


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 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: