CHAMP ELECTRONICS -" THE VINTAGE VALVE
AMP HOSPITAL"
NOTTINGHAM, ENGLAND
SERVICING OF A HIWATT PA150
AMPLIFIER HEAD
The front panel of this one was badly marked, but the camera flash does makes it look worse than it really was.
The
guys who own this one go to gigs & rehearsals in taxis! They asked
me if I could do something with the long power cable as it was always
getting in the way…….simple solution……done.
OK, now to the amp itself. This
one was a real “pain in the butt”! Correct me if I’m
wrong here, but I don’t recall Hiwatt ever actually making PA
amps. (?) I think they were actually farm'd-out to Simms-Watts but they
did carry the Hiwatt brand name. This one certainly is a
Simms-Watts……all the PCB’s state this underneath.
Also, though this is only a four channel PA with 150 watts, if you
compare it with the six channel 200 watt PA amp from Simms-Watts, you
will see that they are pretty much identical. This is how it looked
when first received. The two main smoothing caps had been replaced but
badly soldered & actually were not hefty enough for the job!
Here
I have now changed a good few components. This includes a new standby
switch and “pop suppressor”, all three smoothing caps,
the bias cap plus the inclusion of a splitter balance pot.
Earlier, I said that this one was
a pain in the butt and here’s why. This amp is 150 watts from a
quad of EL34’s…….fair enough…..this can be
done, but there are some rules that need to be adhered to. The most
important one being the screen (grid 2) voltage. EL34’s are quite
happy with 800 volts on the plate & 400 on the screen…..they
will give 100 watts from a pair like this….no problem! But, this
thing is in ultra-linear with a 600 volt HT rail which means that the
screen grids are at this potential too! This is asking for trouble and
it makes these very unreliable! After I had done all the work on her, I
still wasn’t happy about this….regardless of which brand
of tubes I had used. I reluctantly let the customer have it back, now
all working correctly but, within a week, it had popped a fuse and was
dead. On receiving the amp back, I could confirm that one of the
EL34’s had flashed across…..not surprisingly!
Part of the original servicing required a lot of the potentiometers replacing…..as can be seen here.
The input jacks were no better so a complete new set went in here too.
All the knobs re-aligned.
OK, time now to do something
about this EL34/600 volt malarkey! One thing that Matthew (the
customer) had originally asked about was a 4 ohm speaker tap (these
only have 8 & 16 ohms). I had informed him that this would require
an output transformer re-wind to achieve this, so originally we passed
on that. However, with the amp having these high voltage problems, I
now suggested that I did indeed re-wind the output transformer, add the
4 ohm tap and change the plate load for a quad of KT88’s. These
will be quite happy on this voltage and far more reliable! I got the
go-ahead from Matthew and proceeded on those lines.
The re-wound output transformer.
A small change to some of the components and the added 1 ohm cathode resistors, for bias/balance checking.
Still to this day working perfectly, with the quad set of “tried & trusted” good ole’ KT88’s!
