From: Jim Thomson
Sent: Friday, July 20, 2018 8:40 AM
To: amps@contesting.com
Subject: Possible Answer to Centurion 422 Tubes Look Biased on
Date: Thu, 19 Jul 2018 15:33:38 -0700
From: n7wbjohn <n7wbjohn@gmail.com>
To: amps@contesting.com
Subject: [Amps] Possible Answer to Centurion 422 Tubes Look Biased on
<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
tubes.
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
connections.
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
## Typ when u see neg grid current, you have a grid to cathode short. I
worked on a
buddys centrurion, and both 3-500Z’s ran bright red on stby, with neg grid
current.
I also saw the B+ interlock had flashed over at least once. I pulled one tube
at a time,
and the cause was one bad tube had a grid to cathode short. Grid to cathode
shorts
can and do happen if the fil current is not limited on start up. IF the amp
has step start for the
B+.. it will also step start the fil. In some cases, the fil xfmr is a high
Z type, designed to
limit the inrush to no more than double the normal fil current.
## On a ONE tube GG triode amp, IF the tube loses B+ for whatever reason,
and drive is applied,
the grid will act like the anode, and attract all the current. With very
little drive, the grid meter will be pegged.
## On a 2 tube GG triode amp, with on tube losing its B+ via a lousy anode
connection, I suspect the same thing can happen.
## with only one grid meter to read both tubes, you can not read individual
grid current.
## any of the these GG triode amps need a resistor between B+ and base
of plate choke. I use a 50 ohm, 50 watt wire wound
on my 4 x Drake L4B 2 x 3-500Z amps. You can also use 2 x 25 ohm @ 25
watt wire wounds in series, if you have space
constraints.
## On the original centurions, they all drew normal idle current on stand
by. Some how they had omitted the RX cut off bias.
Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|