CHAMP ELECTRONICS -" THE VALVE AMP
HOSPITAL"
NOTTINGHAM, ENGLAND
LINEAR CONCHORD 30 AMPLIFIER
WITH 807 TUBES
This one belongs to another tech-guy/friend of mine, John Beer
of Amp-Fix. On having a clear-out, John found this under loads of other things & kindly sent me the photos.
I can’t quite get my head around what exactly this one really is, for various reasons. One thing for sure it is almost certainly a kit-form unit. On the 30w & 50w models, these were available in this kit-form but the output tubes supplied were 807’s….not EL34’s as-in the factory assembled versions. This one definitely has the 807’s but, the power switch is on the
front…..it is normally on the back. It has two front-mounted inputs, but it looks to me that the front facia has been cut near these inputs (If you look at one of the other Conchord’s on this site you will see that the two lined arrows are longer!)
My gut feeling is that this is most probably a Northcourt or Sonas kit?
Notice on the kit-form versions that no tag (group) boards were
used; just simply point-to-point wiring that was left for the builder to do. Also, all the kit-form versions had a Bakelite base for the
GZ34.
Another point well-worth mentioning here is the use of 807’s instead of EL34’s. The Linear 30 runs on about 430 volts whereas the L50 is about 500. Both these amps are in ultra-linear which puts the screen grids (grid 2’s) at this same potential. This is fine for EL34’s, but 807’s don’t like a grid 2 voltage of much above 300 – 320! I don’t think I have ever actually seen any data on using 807’s in
ultra-linear. However, there is data out there with them triode connected. This data suggests that as triode connected they are OK up-to 450
volts! I have never tried this myself but if this is correct then…..ultra-linear is roughly the same,
from a voltages potential point of view. This would suggest that in the 30 watt, at around 430 volts & fixed bias they should be OK…..putting them under a bit more stress though in the L50 at 500 volts!!
A crafty but simple solution to obtaining the fixed bias supply, for the 807’s, on all the kit form amps was
this - The power transformers centre-tap is lifted from ground & grounded instead through a large, 10 watt wire-wound resistor. (I can’t remember the
value.) Then by placing a large cap (about 250uf) across this resistor it now gives us the negative bias supply. This also lowers the main HT by the given voltage amount of the negative supply.
Not the same amp as in the photos above. This is definitely a 30 watt kit with 807’s. Note the gold Hammerite
chassis; not grey like the one in the above photos.
