Post by David GinsbergI just scored a few really old 845 triodes, (at least one has a brass
base). I know it's easy to check filiment continuity, and I believe
these things need 10 volts at 3.25 amps to light them up. At some
point I'll probably sell them on eBay but I want to try and rig up a
test set so I can be honest about their quality. I own an AVO Mk. IV
and am wondering if anyone knows if it is possible to make an adapter
to enable me to check how good these are. I do know that the 845 needs
lots of B+, on the order of 900 volts and I understand that there are
some circuits that run it on 500 volts, though.......
Genuininely new old stock 845 would be about as rare as rocking horse
shit.
Its likely that any buyer of these old tubes will assume
they are pulls from a modulator or transmitter set as part of regular
service.
Many tubes with plenty of life for amateurs were available from the guys
who were
contracted to maintain gear using 845 including regular tube
replacements.
Not now though; all the good old tubes were mainly used up, and the gear
junked for something new.
So to get a better than $5 each price for the tubes you'd need to set up
a test rig
with 1,100V B+, and an adjustable bias supply of -180V, +/-50V.
Then run the tube with 65mA and see if the grid bias voltage required
for each sample
tallies with tube data.
Use about a 20k bias R max, and keep a close watch while testing, and
have a 10ohm R
between cathode and 0V to monitor Ik.
Monitor the voltage across the biasing resistance.
If the grid is a bit more positive than the actual bias pot ( 5 watt
wire wound!), ie,
some DC current appears to flow towards the grid, then check to see if
its
not more than maximum allowable.
This should be watched carefully as the tube warms towards its normal
temperature
for class A operation which is how the tube will most likely be used,
ie, in some dude's SET audio amp.
The 1,100V and 65mA gives a Pda = 75 watts approx , and is higher than
what it would be if used for class AB in an old modulator.
After 3 hours the bias should remain steady and the grid current causing
slight voltage across the bias R should be less than 0.1V.
If its several volts, probably the tube is stuffed, and not worth a
cent.
If -250V is required to hold the anode current to 65mA, the tube is
stuffed.
There should not be huge variations away from the operation points found
on the
data curves.
If the gettering has large brownish shaded edges and looks kinda messy,
then the tubes are very used,
and worth $2, even if they work a bit.
Once the tube is idling at 65mA, you should be able to make a change of
-10V to the grid voltage,
and record the change to cathode current.
The transconductance (gm) can then be worked out for that operating
point,
and a record placed on the box for that tube.
If you have a change of 31mA for 10V grid voltage change, then gm =
31/10 = 3.1mA/V.
The tube is most likely OK if you get 3.1mA/V according to the data.
An accurate measurement of gm requires a steady B+ supply.
If 1,100V sounds like too much trouble, then 500V might be OK and a
lower bias voltage
can be used. But get the Pda up to at least 50 watts regardless, and see
how they last.
500V Ea needs Eg1 = -50V at 90mA approx.
But let ppl know how you tested them. Maybe you get $50 each.
New ones are mainly chinese, maybe $150, but then there are the KR Audio
types for about $300 each...
http://www.kraudioproducts.com/Kr/ProductDetail.aspx?CatID=13&ProductID=6
If anyone has amps made by KR, they are not likely to want to put in
some ancient old
pulls that could give them troubles.
Check the old RCA data, its available on the Net.
Patrick Turner.