I bought an old Bogen Presto tube P.A. amp off Ebay with the intention of rebuilding it into a guitar amp. I haven't gotten around to that yet but I started a thread on Ampage about Bogen transformer codes that resulted in good information:
From: Regis Subject: Output transformer color code question
Hi Guys,
I have an old Bogen Presto I am going to rebuild, and need some help on identifying the color codes on the Output transformer. I was partially dismantled when I got it.
Here are the wires:
Black: Ground ?
Yellow: 4 ohm tap ?
Green: 8 ohm tap ?
Blue: Plate ?
Brown: Plate ?
Red: B+ ?
Blue/White: ?
Brown/White: ?
I am going to try to rebuild it into a 5E5 Pro.
Thanks for any help.
Regis
From: Steve Subject:Re: Output transformer color code question
Regis,
Schematics are notoriously hard to find for Bogen tube amps. I couldn't find any OT colors that matched the ones you listed exactly.
Does your OT have a part number on it? For example the CHB-100 PA amp has 83-340-000 for it's OT. The MO200A has part number 83-349-010 and 83-349-000 for both of its OTs.
Also, what tubes are in the amp?
Steve
From: Regis Subject: Re: Output transformer color code question
Hi Steve,
The number on the transformer is 83-320-000.
The amp originally had two 12AX7'S, 7355 power tubes, and a GZ34/5AR4 rectifier.
From: Steve Subject: Re: Output transformer color code question (long)
Regis,
None of the Bogen schematics I have has that part number.
Here's the info on the ones I have:
From: Enzo Subject:Re: Output transformer color code question
Your guesses are probably right. The white wires are probably the CV windings for 25v and 70v.
From: Regis
Subject:Re: Output transformer color code question
Steve, thanks for the info.
Enzo, if that is the case, can I just shrink wrap those up and fuggedabout them?
thanks,
Regis
From: MBSetzer Subject:Re: Output transformer color code question
I would check for continuity of the white striped wires with the OPT primary blue & brown in case they are ultralinear taps.
Otherwise if they are the 25V & 70V secondary taps, the 70V one especially makes a very nice low impedance output to a slave power amp.
Coming straight from the OPT this line out has much more of the full amp tone (when the amp is driving a real speaker on one of its regular 4, 8, or 16ohm taps at the same time rather than a dummy load) than you would normally expect from a line out.
Mike
From: Enzo Subject:Re: Output transformer color code question
Good call on the UL taps. I wouldn't expect them on a Bogen, but you never know. If the amp has 25 and 70v terminals on the rear panel, then they have to come from somewhere.
And yes, you can tape them off if not in use. Continuity tests will tell you if the white wires are part of the primary winding or not.
