CHAMP ELECTRONICS -" THE VINTAGE VALVE
AMP HOSPITAL"
NOTTINGHAM, ENGLAND
SERVICING A TWEED FENDER BASSMAN
COMBO
I don’t think many of you out there will need telling what this is worth!
Belonging to Damian, a customer of whom I have done previous work before.
Here she is finished, and again on a final soak-test. Now, before
you all start e-mailing me and telling me that the type of 6L6’s
I have chosen to use are not very good…..or even
“crap”, then I am afraid you would all be completely wrong!
I am probably going to let the “cat out of the bag” here
but……what the heck. OK, the type I have used are Russian
1960’s – 1970’s NOS (new old stock) selected military
spec. Being manufactured to the original RCA 6L6 spec, I am well aware
that these don’t take the higher voltages that some of the newer
amps produce and that require heftier, modern 6L6’s
but……use these in the right application and they are
superb. (Just check-out Damian’s e-mail at the end of the
article!). The same thing applies to the two rectifier tubes which I
have replaced with 5U4M’s. These were only ever manufactured in
Russia and are the best of any of the 5U4 range of tubes! They
are indirectly heated (as opposed to directly heated 5U4G’s,
5U4GB’s, etc), they are only a 1.9 amp heater as opposed to 3 amp
on all the other type 5U4’s, and finally not only smaller and
lighter but they actually give about 10 – 15 volts more HT!
In-fact these 5U4M’s only fall slightly short of the performance
of a GZ34! Anyway, God-knows why two 5U4 rectifiers were used in these
amps? One 5U4 of any variant alone can easily supply enough current for
a quad 6L6’s……..let alone just two! I guess it must
have been a “sales pitch” at the time? You can buy these
6L6’s [6P3S is the Russian number] and 5U4M’s from a few
sellers in the Ukraine. I bought a large stock of them both, knowing
how good they were likely to be! I would recommend anybody to do the
same before they run-out!
The base of the cabinet was so “scrapped” it was like a
bloody “see-saw!” I put four new feet on her and then she
stood still….upright!
With the back first removed, the two tired 5U4GY’s can be clearly
seen here along with the two Sylvania 6L6’s. However one of the
6L6’s had a “grid 1” problem, causing the bias to
keep disappearing! A good few of the various caps had been replaced
before and were done OK (ish). Why the engineer at the time just
didn’t do the rest of the caps I don’t really know?
All original transformers and choke.
Close-up before I commenced with any work. The previously replaced caps
can be seen here. Also, note the broken leg on the cap, on the bass
control (second from the left) just hanging there in mid-air
for……well…..who knows how long? Also, when driving
this amp up on the test gear, she wasn’t doing too-well into 2
ohms. ( Which is correct……the four 10” 8 ohm
drivers [speakers] are all paralleled equalling 2 ohms). The final
reason for this is that some previous engineer had changed the two
output tubes’ grid return resistors for 100k whilst in reality
they should have been 220k’s…….hence the drive was
loaded too much and “giving up”!
After all my work on her.
Close-up as received, God……I hate cobwebs and spiders, where the hell has this thing been stored?
I had to replace the “bear-clamps” on the two output tubes
as the type of 6L6’s I used had a wider base and wouldn’t
fit the original clamps. Good thing that I did though, as these bases
and clamps are rubber mounted……to give a slight
cushioning effect to the tubes. However, all the rubbers had perished
with time and were hard/brittle…….I replaced all these
for new, soft ones, of course!
Damian’s e-mail after receiving his baby tweed amp back from service by myself:
"Dear John. Thank you
so much for the excellent work you did on my Tweed Bassman amp, it
sounds completely revived...it has had the kiss of life and is now like
a Bassman on steroids!! It’s a real comfort to meet someone who
goes the extra mile.....
Cheers Guv', Damian."
