[Amps] FW: Revival advice

[Matt]

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 at miamioh.edu] 
Sent: Saturday, March 31, 2018 10:41 AM
To: Matt <maflukey at gmail.com>; Amps group <amps at 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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