Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently May Mon 28, 2018 4:31 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 5 posts ] 
Author Message
 Post subject: Help with Lafayette PA-268 Tube Amplifier
PostPosted: Feb Sun 04, 2018 12:06 am 
Member

Joined: Apr Sat 30, 2011 2:14 am
Posts: 607
Location: Mississauga, Ontario
Hi all,
Really would like to find a schematic, no mention of this amp anywhere on the web.
I recently got this old PA amp in a package deal and want to try it out. It works just fine but after a minute or so I noticed 2 of the 4 6BQ5's were glowing red, so I shut it down.
The tube compliment is 1-12ax7, 1-6av6, 1- 6ag8 and 4- 6bq5's.
The rectifier is a 5ar4.
It is a mono amp so the 4 output tubes are strange to Me, they must be using them as preamp or something?
Anyway, I'd like to troubleshoot it but without a schematic it's difficult as there is nothing obvious. The only thing I found strange that on G1 (6bq5 pin 2) the dc voltage on the red plating tubes is about 17-20vdc and on the ones running normally it is about 1.7vdc.
Any help here would be appreciated and If there is a schematic out there a copy would be the ticket!
Thanks, Mauro


Top
 Profile  
 
 Post subject: Re: Help with Lafayette PA-268 Tube Amplifier
PostPosted: Feb Sun 04, 2018 12:48 am 
Member
User avatar

Joined: Nov Sat 07, 2009 11:37 pm
Posts: 1858
Location: Sayreville, NJ 08872
I have it; see your PM.

_________________
Pete, WA2CWA - "A cluttered desk is a sign of genius"
http://www.classicradiomanuals.com


Top
 Profile  
 
 Post subject: Re: Help with Lafayette PA-268 Tube Amplifier
PostPosted: Feb Sun 04, 2018 12:55 am 
Member
User avatar

Joined: Nov Sat 26, 2011 4:09 am
Posts: 8760
Location: Texas. USA
Without a schematic it's hard to say but it sounds like the 4 output tubes are parallel push pull. I.e. two tubes in parallel per side. That's done to get more power, which would be consistent with it being a PA amp.

The grid volts are obviously bad on the red plating tubes. Could be they're shot (excessive grid leak possibly due to grid emissions or a grid cathode short). Also could also be bad coupling caps leaking (assuming they're separate and the two tubes aren't sharing one coupling cap).


Top
 Profile  
 
 Post subject: Re: Help with Lafayette PA-268 Tube Amplifier
PostPosted: Feb Sun 04, 2018 3:39 am 
Member

Joined: Apr Sat 30, 2011 2:14 am
Posts: 607
Location: Mississauga, Ontario
Thanks guys for the replies!
The configuration of the amp threw me off as I've never seen one with this circuit. Turns out it was the coupling capacitors.
Sounds great now! I may just use it as an amp for my Seeburg library unit.
And, now I know where to get a manual for it - thanks Pete.
Enjoy the rest of the weekend!


Top
 Profile  
 
 Post subject: Re: Help with Lafayette PA-268 Tube Amplifier
PostPosted: Feb Sun 04, 2018 4:17 am 
Member
User avatar

Joined: Nov Sat 26, 2011 4:09 am
Posts: 8760
Location: Texas. USA
That's great news! Glad you got it fixed.


Top
 Profile  
 
Post New Topic Post Reply  [ 5 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Exabot [Bot], Google [Bot] and 3 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB