On Tue, 30 Sep 1997 12:40:59 +0100 Mike Schelly <jmschell@fedex.com>
writes:
>Greetings to all...I'm trying to get this amp to behave but it seems
>to
>be a T.G.T.P oscillator. Without drive at about 4KV grid and plate
>current start to flow.
At 4KV those tubes should be very stable.
What socket are you using? The old military-broadcast style SK-500
sockets have caused problems for some. I suggest the SK-510 or the
Italian all metal socket that was available for several years.
Also remember that the 4-1000A metal shell MUST also be grounded with
spring clips at a minimum of 4 places. The shell is part of the
grid-plate isolation.
With slug tuned input coils a seperate shielded box is almost always
required but limit the RG-58 lead to the filaments to 10" or so. I prefer
relay switched toroid based networks such as the 160-10M assemblies sold
by Omega Electronics.
All non-RF socket leads should be shielded and go thru a L or Pi filter
thru the chassis wall. The filament xfmr should be below deck but not in
the pressurized compartment. Yes this means 25A feed thru caps for your
series string.
Try backing the Ep down to 2-3KV and check for instability. Use a GDO in
the wavemeter position to determine the problem frequencie(s).
I've been away for 9 days and my E-Mail has a limited buffer so please
excuse any duplication.
GL Carl KM1H
The amp is a 20m monoband with vacuum tune and
>load caps. Makes no difference what type or variation of parasitic
>suppressor is used...copper/silver or nichrome. I get a dip at 62Mhz
>at
>the plate blocking cap. The grids and screens are grounded with .25"
>copper strap thru the sockets directly to the chassis. The filaments
>are in series with a 15VCT transformer, bypassed with .01
>micas. Bias is
>provided by 4 series 1N5402 diodes. With any type of tuned input,
>parallel C/L, pi, or T-input I have an oscillation. Feeding the
>filament
>direct thru a .01 mica....no oscillation, no Ip/Ig without drive. Does
>any tuned input have to be in a shielded enclosure under the chassis?
>Do
>I necessary have an input/output feedback problem? Any help
>appreciated
>guys...
>
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