[Amps] AL-572 Negative grid current?

[Robert B. Bonner]

Scott one of your tubes might have a grid cathode short.

Remove one at a time to find the offender.

BOB DD

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Scott Manthe
Sent: Saturday, November 11, 2006 12:47 PM
To: Amps at contesting.com
Subject: [Amps] AL-572 Negative grid current?

Hi Ampers,
I bought a used Ameritron AL-572 in June and have just been able to 
check it out the last couple of weeks. (It's a long story involving 
moving into an OLD house...)

Output seemed fine, but plate current reading was high with zero drive 
(over 150 mA), BUT the grid current seemed fine. A call to Ameritron 
confirmed what I suspected, Q3 and possibly Q2 and Q1 had blown. I was 
shipped new transistors and I decided today would be a wonderful day to 
work on the amp.

I replaced Q3, since it's the easiest to replace, and on a pwer up, I 
now have erratic plate current readings and NEGATIVE grip current. (Or I 
guess I should say, the grip current meter is being pulled down) Most of 
the time, plate current reads zero with no drive, but often on power up, 
the reading wobbles a bit, between zero and 75 mA and then eventually 
settles on zero.
I suspect the tubes are OK, because I'd been getting 700 watts output 
with 25-30 watts drive before I replaced the transistor. I saw nothing 
to indicate a flashover or anything else when I powered the amp up after 
I replaced Q3.

I have not attempted to TX on the thing in this condition.

Any Ameritron experts out there with with any ideas? I'm not an 
engineer, so keep that in mind when you respond.;)

73,
Scott, N9AA
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