Hi Matt,
I don’t think you’ve got anything to worry about, even with -160V, either
with 811As or 572Bs. Both tubes are routinely run with -200V in Class C, so
since you just want them to be cut off, I don’t see any problem. Chances
are you could go to -300V!
73,
Jim w8zr
Sent from my iPhone
On Mar 31, 2018, at 1:27 PM, Matt <maflukey@gmail.com> wrote:
Hi Jim,
Thank you and also K7FM for the advice. Correct about no tuned input, the
desire to put the old girl back in service is to complement an FT102 that
just got restored by N4CL. There’s no ALC on the Gonset either, so I’ll
just have to be cautious with the drive level.
There is another question that I have… The later model GSB201s (with 572B
finals) included a provision for internal cutoff bias by adding a small
transformer to the circuit, and rearranging the 120VDC keying relay circuit
configuration to provide –?VDC voltage to the grids in series with the
un-keyed relay coil. When the relay is externally grounded for keying, the
–?VDC from the keying circuit is removed, returning the grids to TX bias at
~-4.5VDC rectified from the heater supply through a bleed resistor and
filter cap. The circuit diagram for the later model shows all the
component values except it does not identify the secondary voltage of the
keying transformer. If I assume it is 1:1 isolation, it would provide
~-120VDC to the relay when keyed via a ½ wave rectified series dropping
resistor (exact same as original design). However, if my 1:1 assumption
is correct, then the resulting cutoff bias to the plates in standby would
be something in the vicinity of -160VDC. This is quite a bit more cutoff
than the -100VDC external cutoff bias specification listed in my original
owner’s manual. Does -160VDC cutoff bias sound reasonable for an 811A? I
just don’t have enough experience to know. I would love to have internal
cutoff bias capability for this amp as otherwise it’s a real room heater.
Having internal control power for the keying relay would also simplify my
keying buffer, which currently requires an external line voltage source.
Many thanks for any advice you can offer.
73
Matt
KM5VI
*From:* Jim Garland [mailto:4cx250b@miamioh.edu <4cx250b@miamioh.edu>]
*Sent:* Saturday, March 31, 2018 10:41 AM
*To:* Matt <maflukey@gmail.com>; Amps group <amps@contesting.com>
*Subject:* Re: [Amps] Revival advice
That's a nice old amp, Matt. I wouldn't worry about the tubes. The Svetlana
tubes are well-built and should power right up. That said, it's probably
not a bad idea to cook them with filament and HV,( but no drive) for a few
hours before applying RF. That should also drive out any residual moisture
from the plate transformer.
SInce you've replaced the caps already, the only routine maintenance is to
lubricate the fan bearings, clean the bandswitch and relay contacts, and
veryify that the line cord is in good shape.
As I recall, the amp doesn't have a tuned input, so I hope you're driving
it with a vacuum tube exciter that won't worry about the input VSWR.
73,
Jim W8ZR
On 3/30/2018 9:14 PM, Matt wrote:
I am in need of some amp revival advice. I have a 1st generation Gonset
GSB201 that I want to restart. I have not put power to it since 2008 when
I replaced the power supply caps. It worked fine at that time. Since
then the amp & myself have moved cross country with the amp stored for years
in a semi-conditioned (no humidity control) environment. The tubes are the
older model Svetlana 811As (with anode heat sinks) last replaced about
2002ish and have low hours. I also have the set of well used RCAs that I
pulled in 2002. I have the amp at home now in a full conditioned
environment and thought I would let it set there for a week or so to dry out
in the event something inside soaked up some of that humidity.
Should I bring it up on the old RCAs since I understand it's hard to get
good replacement tubes?
Should I/can I use a Variac to bring it up slowly without having to do
disconnect anything inside the amp?
If I get it up to HV, how long do I need to heat the anodes to activate the
gettering process? The amp has no internal shutoff bias so it normally
flows anode current at idle.
Anything else I should be aware of?
Many thanks for advice!
73
Matt
KM5VI
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