CHAMP ELECTRONICS -" THE VINTAGE VALVE
SERVICING A c1968 SELMER ZENITH 50 Watt AMPLIFIER HEAD
This came in for a general service as one of the channels had started to crackle, along with a request that could it be converted to a GZ34 tube rectifier? Of course……no problem!
To be honest, I think the best explanation regarding this amp is actually in the customers (Ian) testimonial at the end of the article.
I changed the speaker output jack nuts for red ones.
This is usual for amps nowadays. (You just tell the roadies to “plug the speaker in one of the red sockets!”).
This has had the output transformer changed at some point in its life. The one fitted was however correct
and worked fine.
In order to convert her to tube rectification, I needed to remove the twin cap that was on a parallel line to the rest of the tubes….for the GZ34.
I replaced the twin cap with two separate axial-type mounted under the chassis….shown here.
I also had to make a small power transformer for the 5 volt GZ34 heater,
as there wasn’t a winding available for this on the main power transformer.
I did replace the pair of 6CA7’s for a pair of EL34’s. Yes, I know they are equivalents but the 6CA7 is a
Beam-Tetrode and just doesn’t have the same sound as the EL34 with this being a pentode. Anyway….all done here.
Ian’s testimonial tells the whole story:
“Hello, this is Ian from Bolton. Sorry for the delay but this has been the first week
that I've been able to get back to you.
Anyway, to the Selmer - it's nothing short of spectacular. It was always a good reliable amp until the power transformer burnt out in 2005 (even though in 1990 it was run for 1/2 an hour with no speaker plugged in by an IDIOT I
lent it to at school who didn't know shit from clay!). I still gigged it for another 15
years, then in 2008 I had it rebuilt for 385 quid with the new transformer and reverb tank, and those weird bottles that you pointed out (should have been EL34's I think you said).
At that time the guy said it had been converted to a master volume and that the soldering was 'disgusting' and had I done it!! (no) so he re-soldered it.
(We have no idea who converted it. He said it had been redesigned but was done OK). I had asked him if while it was in bits he could convert it to valve rectification, but he said it would be a 'radical re-design and too expensive'. Although I'm not an engineer, I felt in my heart this was wrong. So then when a channel started to crackle after only one gig I looked around for someone else to do the work, and found your
website. Then you did the amp, and here we are.
After getting the amp back from you with the conversion to the GZ34 tube rectifier and general service....I am well pleased with everything. Many thanks for the good work.