I recently picked up a somewhat working Heathkit Warrior, complete with 4
572B's. Primarily, I picked this thing up to learn about tube-type
equipment. My plans are to eventually build in T-R switching and proper
grid bias circuits. Before I do that though, I have encountered a problem
with the amp and I'd like to run the problem past you experts.
The issue I'm facing is low plate current. According to the assembly docs,
the amp should exhibit resting plate current of 90 to 100 ma (after full
warm-up of the 866A's). Plate voltage is good at 1650 volts. Grid current
is zero at idle. Resting plate current is only 50 ma.
I've connected my transmitter, dummy load, and watt meter to the amp.
After warming up and tuning it with 5 watts drive, I brought up the drive
on the transmitter. With 100 watts of drive, I can only get 250 ma of
plate current out of this amp. Grid current is 125 ma. Plate voltage
doesn't drop during transmit. As a result of the low plate current, the
amp will only produce 500 watts with 100 watts of drive.
I originally suspected the 572B's. A friendly local elmer loaned me a set
of brand new RCA 811A's. I installed them and tried again. Resting plate
current dropped to 45 ma with the 811A's installed (as compared to 50 ma
with the 572B's).
Looking through the troubleshooting section of the assembly docs, low
resting plate current could be due to:
1) Cut-off bias not shorted. I've made sure that it is. No change.
2) Improperly neutralized. I've tried adjusting the neutralization
adjustment while HV is applied. No changes.
3) Open or damaged parasitic chokes. I've checked for continuity across
the chokes and they conduct. I may build some new ones though (7 1/2 turns
of solid 16 ga on 51 ohm 2 watt "non-inductive" resistors).
4) Check bias capacitors on the tube sockets to make sure they're grounded.
Although I haven't replaced those .02 uf disc caps, they are connected to
ground and the connections are good.
5) No bias on pin 3 of the tubes. I've checked for voltage there. The
assy docs details a measurement of -4.5 volts. I measured -4.71 volts.
Folks, I'm stumped. The only other component that might be troublesome is
the 1000 uuf 5 kv tubular cap that mounts between the neutralization
bracket (at the tuning capacitor) and the copper strap going to the plates.
I'm obviously new to tube-type equipment, and I'm trying to learn as I go.
Any words of wisdom from others who have "been there...done that" are
-Mitchell / W5MEC
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