[AMPS] tetrode output

Tue, 14 Mar 2000 08:16:44 -0000

Hi Lane and reflectees !

Re:   Homebrew 4CX1500B amp. spec.

1. The screen voltage has taps for 150, 250, or 350 vdc. I am using it on
>the 250vdc tap. When I go to the 350vdc tap the ZSAC rises to about 1000mA.
>>Why?

This is what you would expect - you will need to find how the control grid
is
biased and increase the negative voltage on G1 - should be about -55V dc
- make it more negative to reduce ZSAC back to 250mA or 300mA.
However +350V dc maybe a bit high - should ideally be +325V but a 1500B
will work well with +350V - the problem is the dielectric in the tube base
screen
decoupling cap., which may be rated at 400V dc, may eventually arc-through
with the combination of +350V dc + RF.

>2. I can not reduce the output out of my TS-850 any lower than 10 watts,
>even with alc. What kind of variable attenuator network can I build on the
>input side of the amp so I can vary the drive that the tube receives from
>the exciter? There are times when 400 watts is all that is needed.

The TS-850s has an internal power-level control (RF board R7)
which you can set to any value you like from 1W - 100W, however
it's not such a good idea to run such low drive levels
(sigs. not clean from 850) - your new amp. maybe has tuned input
rather than passive grid, from the sound of the drive levels involved
 - a passive-grid (50ohm) 4CX1500B needs about 40W min. drive
for 1400W output.

>3. With ten watts in and 1100 out or so is this a typical reading for a
>4cx1500B? There is zero grid current or screen current with this drive
>level. If I increase drive to 15 or 20 watts then it starts drawing grid
>current. I understand this is a no no with tetrodes. I am trying to
>determine if this tube is full out. Plate voltage under load is 3500 vdc
and
>Ip is about 550 mA with 1100 watts output.

This 4CX1500B sounds like a good one !   Maybe better to reduce the
plate voltage slightly (to around 3200V under load) and run more current
- they are quite happy up to 900mA (or more) of Ip.

73
Chris GM3WOJ/GM7V
gm3woj@talk21.com

Visit www.gm7v.com

--
FAQ on WWW:               http://www.contesting.com/ampsfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm