Hello Amp People,
I acquired a very nice home-brew amplifier recently and I am in
the process of figuring out what I need to do to get it on the
air. Unfortunately, it came with no documentation, ugh! It is a
grid-driven tetrode design using a 4CX1500B tube. It has a
filement supply and an adjustable bias supply built into the RF
deck. It is built like a battleship, uses vacuum variables, a
manly band switch, and is fully metered. It appears to only
require HV + screen voltage to operate. Unfortunately, my
limited amp experience is with grounded-grid amps and I have a
few questions for anyone on the list here with significant
experience with such a beast.
I guess the biggest question I have is in regards to the
requirements of the screen supply. There appears to be no screen
by-pass resistor in the RF deck so I'll assume the builder (have
no idea who it was) included that in his screen supply. The
Eimac data sheet states 70mA for this resistor so I guess its
value should be around 5k ohms? What is the recommended size of
this component. I would guess 5 watt would be large enough but
maybe I am not considering possible failure modes? Some
references I have seen state that HV should be applied before the
screen voltage so I'll assume it is a good idea to apply screen
voltage with the TR relay activated. Any thoughts on a
recommended screen supply circuit. I wouldn't think with the
screen bleeder that regulation is required and I was thinking of
using a choke input filter (anyone have a ~4-5 HY 350mA 400VDC
filter choke lying around?).
None of my references have any constructed amps described using a
grid-driven 4CX1500B design (or any grid-driven tetrode for that
matter). Anyone out there have a good reference of a successful
design that I could use for my reference. I will be using a 3000
VDC 1 amp supply that I already have built up for high voltage.
The HV xmfr in this supply has a 375V tap that I will use to
power the screen supply so that is no problem.
Any other words of wisdom to help me out here.
Thanks for your time reading this and hopefully you won't spend
too much time helping me out here (time is a precious
commodity).
73 de Mike
K7PI
Mercer Island, WA
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