>
>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.
See Figure 10 on my Web site.
>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.
? The 8660/4CX1500B has a 12w-rated screen but it requires very little
in the way of screen current. In fact, it generates about 14mA of screen
current in Class AB1 operation. In other words you need to have a shunt
type screen regulator that will absorb 14mA at 400v. A simple way to do
this is with a string of 20 or so c. 20v, 5w zeners.
>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 screen must be RF-grounded, either with a built-in socket bypass
C, or directly grounded. If this is not done, the amplifier can Easily
oscillate at VHF.
>The
>Eimac data sheet states 70mA for this resistor so I guess its
>value should be around 5k ohms?
? Say what?
> 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
? A shunt regulator is required.
>and I was thinking of
>using a choke input filter (anyone have a ~4-5 HY 350mA 400VDC
>filter choke lying around?).
>
? For SSB operation, the filter choke needs to be resonated at roughly
2.2 x the electric mains frequency. Otherwise, there will be unreguation
during current transients. (such transients can not be seen on an panel
meter)
>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.
>
? Using an ordinary screen supply Is a problem. The 8660 sort of has a
built in screen supply, so you need a shunt regulator to control it.
>Any other words of wisdom to help me out here.
>
? If you have a transceiver that will do around 150w, you can use a 12.5
ohm grid termination - with a 4 to 1 bifilar stepdown transformer. Using
a 12.5-ohm termination instead of a 50-ohm termination increases vhf
stability and reduces the influence of the grid/cathode capacitance on
the higher bands. .
later, Mike.
- Rich..., 805.386.3734, www.vcnet.com/measures.
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