[Amps] re compability of 3cx1200Z7 in lieu of 3cx1200D7eimac

[Jim Thomson]

Date: Wed, 17 Nov 2010 09:11:33 -0700
From: "Fern" <crc at cyberlink.bc.ca>
Subject: [Amps] re compability of 3cx1200Z7 in lieu of 3cx1200D7eimac


Can one use an eimac 3cx1200Z7 tube in lieu of the 3cx1200D7 tube? I note that the Z7 series has a flange at the bottom of the ceramic portion so will that pose a problem for a swap? Also the grid pin is directly grounded! This is to use at hf in a henry 3K Premiere amplifier.  Any comments would be greatly appreciated.   Fern

##  good question.    The Z7  version is the best   of the 1200 series.   The A7  and D7  are dogs imo.   The big advantage of the  Z7 version is it has the least  stray  C,  between anode and cathode.
The lower the anode to cathode C, the more stable the tube will be.  Also, the grid flange on the Z7 version  is the best way to directly grnd the grid to chassis.  That too, will also promote  stability. 
 I THINK  that the  Z7 version  requires  NO socket per se,  [flange bolted directly to chassis],  but still requires the 2 x directly heated fil connections.   If that's the case, the fil connections  would
have to be hardwired  to the hot side of the bifilar fil choke.   Also, with that grid flange in place, you have to see how the airflow is going to work.   IF you removed the stock D7 socket,  the grid flange
of the Z7 still has to be bolted to the chassis.   You might require a new, small AL plate,  with a hole in the middle  for the Z7 fil connections [ stem part of tube],  big enough, such that you can bolt the grid
flange to the new plate..and also have the plate big enough that it's  outer circumference  can then , in turn, be bolted to the existing chassis. It sound' like a lot, but it's  straightforward really.   You
also nned slots/holes in the new small plate, for the  airflow. 

##  It would  be very similar to using any other tube that has a built in grid flange, like a YC-156  or  YC-243.  When you really look at it,  sockets on HF are a waste of time..on any GG triode.  All  you have
is the fil/grid and anode.   The grid is bonded to chassis.   A simple built in grid flange and a pair of fil lugs is all that's  required.   The megabuck  socket  [esp on big triodes] is extra money.   I went a few go arounds with
Eimac on this subject years ago.  They did come out with a socketless 3x20,  called a 3CX-20,000C7..and also a YC-243  [socketless 3x6 /YU-148].   The 1200Z7  was another one.  They shoulda done the same thing
with the 8877, and the rest of the GG triodes.   It does take longer to change a tube out though, but not an issue, since tubes rearely get changed out anyway. 

## You will see that Eimac  sells  these emergency replacement  socket repair kits  for the bigger tubes.   A local broadcast eng buddy of mine sez he has seen  SK-300A  sockets that end up burnt to a crisp,
burnt finger stock, etc. [ those are tetrodes, and do require a socket].   The SK-1320  is the GG  socket  for the   3x10/15/20 triodes....cost huge $$...and is a prime candidate for a socketless version.

##  if your  1200Z7 tube is in good conditon, it may well be worth the 'easy' conversion  from the D7.    Dunno what the future availability of the Z7 is though. 

later........... Jim  VE7RF