CHAMP ELECTRONICS -" THE VINTAGE VALVE
MEASURES TAKEN ON A c1955 RSA-TRUVOICE TV15 AMPLIFIER
TO MAKE IT SAFE & WELL
When Selmer first designed this amplifier back in the fifties,
that was a time when here in the UK we still had many households with DC mains power. This amp is designed to run on AC (Alternating Current) or DC (Direct Current). There is no power transformer so there is no full isolation from the power source. Although getting a shock from any power source is not a pleasant
feeling, DC is far worse and certainly more lethal!
1. AMPLIFIER PRIOR TO WORK COMMENCING:
Because of the lethal prospects of DC, Selmer got around this issue by using the negative part of the 2 core incoming power cable connected to the ground buss part for all the electronics of the amp itself……but not the metal chassis! The chassis is isolated from the negative rail by a .05uf cap @ 400 volts.
To get around the negative supply being on the input jack sockets and ultimately the
guitar, all three inputs have
small isolating transformers on them.
The big blue dropper resistor shown here is to bring the mains 240 volt down to the correct voltage for all the tube heaters which are wired in series on a 300m/a chain.
The three input isolating transformers on the rear/top of the side control panel.
The output transformer is mounted underneath.
The buss-bar which carries the negative of the mains
supply; isolated from the metal chassis by the .05uf cap @ 400 volts. When new this is all well-and-good
but, as the years go by, this cap becomes leaky or even short. When this happens, the chassis becomes fully live! There’s still only a small chance of shock from the guitar though as this would still be isolated by the three transformers.
2. WORK UNDERTAKEN:
There was a problem with the on/off power switch, so this needed removing for repair.
Apart from changing most of the old caps
and some resistors, I decided to make a power transformer for the amp. This does two
First: the amp becomes completely isolated from the mains power and the chassis can then be connected directly to the negative
buss-rail, removing the .05uf cap along with the fitting of a correct mains ground using a 3-core power cable.
Second: by taping the HT secondary at the correct voltage for the series’d
heaters, we can loose the big dropper resistor and also the unnecessarily generated heat from it too!
The transformer in place.
Nice job done….even
though I say it myself!
.............AND AN EVEN EARLIER VERSION WITH 2 x ECC83's, 1 x
6BR7, 1 x 80 RECTIFIER AND A PAIR OF 25L6 / KT32 OUTPUT TUBES:
Serial # 15/-279