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[Amps] Possible Answer to Centurion 422 Tubes Look Biased on

Subject: [Amps] Possible Answer to Centurion 422 Tubes Look Biased on
From: n7wbjohn <>
Date: Thu, 19 Jul 2018 15:33:38 -0700
List-post: <>
I sent:
"In standby at power on, reading the front panel meters

Plate current reads about 50 mA should be zero

Plate voltage about 2400 or 2500V should be 3100

Ig goes negative about 40 mils

Four five minutes amp is trying for space heater status - plates get red
and exhaust is hot.

PPT/VOX to QSK switch no effect.

That was yesterday but today all normal reading and not hot so there's an
intermittent - moved the amp a few feet on a rolling bench was all the
shaking it got and it started behaving.

Where to start? (At turn on, point K on the Filament AC Board did shoot up
to 55 or 60V at turn on then dropped to 34V and there was some Ip but it
zeroed out and all was cool.)

I did look at the archives but I could of missed something."

Thanks to all that replied.

Later info revealed that the amp had been shipped  from Germany - later
word the tubes had been replaced in 2014. This was prior to the return to
USA. It ran for awhile, then suddenly exhibited the symptoms I related.

Replacement tubes were RF Parts with the graphite plates 3-500Z G ? - They
use adaptors to fit the aluminum disks original to the amp I guess to the
smaller diameter graphite plates. The adaptors were very loose on both

This was discovered following up on the group consensus that one of the
tubes could be bad. I pulled one at a time . Found the loose plate

Cleaned all the sockets, pins and plate connections with deoxit.
Reassembled and it plays fine. I tapped and banged a bit and it hangs in.
So we will put it back in service and see.

I guess the poor plate connection caused the grid(s) to collect current. I
did see some unrelated posts about neg. grid current and bad B-
connections  - could go for B+?

I guess the transportation vibration got the plate connections. (The amp
was very well packed for shipment.) However, the connections are fastened
with small Allen screws and you must take some care to get the screws
seated on the flats on the graphite plates - the factory aluminum disks
likewise fasten to the adaptors, not real robust connections in shipment.

Again thanks for the help.

Sorry - Not sure how to connect this back to the thread?

73 de John N7WB
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