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

[Jim Thomson]

From: Jim Thomson 
Sent: Friday, July 20, 2018 8:40 AM
To: amps at contesting.com 
Subject: Possible Answer to Centurion 422 Tubes Look Biased on

Date: Thu, 19 Jul 2018 15:33:38 -0700
From: n7wbjohn <n7wbjohn at gmail.com>
To: amps at 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