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
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