On Dec 2, 3:00*pm, (Scott Dorsey) wrote:
ron_ayling wrote:
Hi guys, I need some advise on an old British R109 receiver I'm trying
to get going, I bought it recently, and it has been modified, the mod
was to replace all the octal valves with small B7G valves, it has been
done very well each valve has its own adapter, however the detector
pre amp valve was originally an AR8 which is a triode plus two diode
and this has been replaced with a 6AM6 which is a straight RF
Pentode!!! first of all I thought the wrong valve had been used BUT it
seems to have been wired up as a triode, screen grid and anode
strapped together, and the suppresor grid used as a detector diode?
would this work?

Strapping a pentode into triode mode is very common when you need something
with particular characteristics and can't find it. *

The question is what the two diodes on the AR8 were used for. *If they
weren't used at all, you'd be home free. *If one of them was used as a
detector and the cathode tied to the cathode, using the suppressor grid
might work, but it might also be noisy. *I'd tend to disconnect the
suppressor and use an external silicon diode.

as I'm having trouble in this area of the set, I
suppose I could replace the 6AM6 with a DH77 and rewire it as that
more closely resembles an AR8 electrically, but don't want to go to
all that trouble if the suppressor idea should work and the fault is
elsewhere.

I don't suppose you could find an AR8 somewhere? *If not, I'd first try
a silicon diode just for test purposes.
--scott
--
"C'est un Nagra. *C'est suisse, et tres, tres precis."

AR8 is a battery 2v direct heater valve , was the detector/amp in a
WS/18 rx and oscillator in the tx....

G ...