Bottles like the 4-1000A thrive on such high plate voltages, but the
internal close-spaced electrode construction of the GS35B (designed for UHF
service) with its oxide coated indirectly heated cathode is not conducive to
robustly withstanding such high plate potentials. As Ian GM3SEK mentioned
these frangible oxide particles can find their way onto the proximate grid
structure and modify tube behaviour, as well as cause potentially damaging
internal arcs.
I forgot to ask, but presume that your filament voltage is precisely 12.6V
and is stable under line and load variations when measured at the socket?
The GS35B does not tolerate either under or over filament voltage and has a
Goldilocks factor sweet-spot for its emission.
While a buck xfmr or heavy duty Variac placed on the HV xfmr primary winding
are messy, its one of the few practical ways to reduce the P.S. output
voltage if there are no primary or secondary winding taps available. Presume
the secondary feeds a traditional FWB rectifier.
Leigh
VK5KLT
_____
From: Mike [mailto:k4gmh@arrl.net]
Sent: Thursday, 31 October 2019 4:41 PM
To: Leigh Turner
Subject: Re: [Amps] GS35B Amps Common Problem
Hello Leigh,
Yes, your probably right. The HV xfmr was for a 4-1000 amp. However, this
is the only HV xfmr I have so gotta use what I got.
Leigh, do you know any way of reducing the output voltage? The only couple
of ways I know is to install a buck/boost type of xfmr in the 220 VAC lines
going to the HV xfmr, or inserting a 220 VAC 30 amp. variac in the 220 VAC
lines. Any ideas on how to reduce the HV supplies HV to 3 KV will be
appreciated.
Thanks for your help, Leigh.
73,
Mike, K4GMH
On Wed, Oct 30, 2019 at 6:42 PM Leigh Turner <invertech@frontierisp.net.au>
wrote:
Hi Mike,
In view of this experience, I'd suggest running the tubes with a lower plate
voltage circa 3 kV max if that can be easily facilitated.
I suspect the premature tube failures experienced now exhibiting unstable
emission / RF characteristics here are somehow related to and likely
instigated by excessive plate voltage....
Whatever internal structural changes that have occurred imparting the wobbly
behaviour must have been sustained cumulatively over time without leaving
any physical evidence in the glitch resistor, etc. from say individual
violent tube internal arc events.....something a fast-blow HV fuse can
mitigate the damaging effects of.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Mike
Sent: Thursday, 31 October 2019 4:54 AM
To: amps@contesting.com
Subject: Re: [Amps] GS35B Amps Common Problem
Hello,
Swapped out the GS35B in one of the amps with NOS GS35B and the amp. works
as it did. Why did the initial tube fail - don't know as HV supply, bias
system, filament voltage, etc. checked out.
I did see Ian's pictures and the same thing may have happened to the GS35Bs
in my amps. Again, all the HV supply parts in the common HV power supply
checked out okay including the glitch resistor.
Any ideas on what caused the problem?
One thing I might do is to put a fast blow fuse in the HV circuit although
not sure a grid to plate arc caused the tubes to fail or even if the fuse
can blow fast enough to "save" the tube(s).
73,
Mike, K4GMH
On Sun, Oct 27, 2019 at 11:52 PM Mike <k4gmh@arrl.net> wrote:
> Hello,
>
> My two homemade GS35B amps exhibit the same symptoms of a problem and
> can't seem to figure what is the "common" problem. This happened
recently
> after several years of operating both amps without a problem in various
> RTTY contest running 1500 Watts output (SO2R). The symptoms are seen
> initially tuning the amps up.
>
> The symptom(s) occur after the amp is warmed up, filament and blower both
> on for over 5 minutes before high voltage is applied to the amp. RF is
> applied to the input and plate tuning starts. The amp output gets to
~1200
> Watts output when the amp input impedance start changing causing the radio
> driving the amp to fold back its output which in turn decreases the amp
> plate and grid current. The radio's output decrease continues until the
> radio is putting out very little RF. The amp is correspondingly putting
> out very little RF. The amp's plate current has gone from ~500 mA to less
> than 100 mA along with the grid current going from 200 mA to less than 50
> mA. Once this condition is reached, removing the input RF from the amp
and
> immediately reapplying the input RF does not change the last condition of
> amp's input impedance.
>
> Usually a long period without the amp being keyed has to occur before the
> amp's input impedance seems to start returning to normal. However, once
> RF reapplied, the amp immediately starts to return to the high input
> impedance.
>
> The amp plate voltage goes from ~3400 Volts at 1200 Watts output to ~3600
> Volts at lowest output. Quiescent plate voltage is ~3700 Volts. Note: a
> single power supply is used by both amplifiers along with a common blower.
>
> The things check and found okay are the W4ZT bias circuit in each amp, 120
> VAC to each amps filament xfmr, power supply's start up resistors
by-passed
> after HV power supply start up, HV diode string, different radios used,
each
> amp uses a separate antenna, etc.
>
> Again, these amps have operated okay for several years without an issue.
> Then both amps start exhibiting the same symptoms of a common(?) problem
> at the same time.
>
> Appreciate any ideas on where to look next for the problem.
>
> 73,
> Mike, K4GMH
>
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