The "Gold Standard" tube is the 3CX3000A7 which has about the best IMD even
at full HV. Throttle down to a mere 3000W and it defies detection on the
bands.
A 100W rig will drive it to a loafing 1500W with 2500-3000V.
The 8877 is a fragile short life wannabee befitting an oxide cathode tube
Both tubes sell for reasonable prices from China.
Carl
----- Original Message -----
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: <amps@contesting.com>
Sent: Thursday, October 31, 2019 9:50 PM
Subject: Re: [Amps] GS35B Amps Common Problem / few more comments
I concur with your comparative assessment of the three tube types here Lou.
The 8877 is the indisputable "gold standard" for QRO ham amplifier
triodes..its a pity it is now so damn expensive :-(
To ensure robust longevity, personally I would not push a GS35B beyond circa
3 kV max on the plate...it was created with a completely different set of
design objectives to that of the 8877 / 3CX1500A7 tube.
Leigh
VK5KLT
________________________________________
From: gudguyham@aol.com [mailto:gudguyham@aol.com]
Sent: Thursday, 31 October 2019 11:35 PM
To: invertech@frontierisp.net.au; k4gmh@arrl.net
Cc: amps@contesting.com
Subject: Re: [Amps] GS35B Amps Common Problem / few more comments
Tony King has comprehensive data on this tube on his webpage. It is
interesting to note that the data sheet actually calls out a degradation of
performance of the tube running on a frequency of 60cm. I assume running
24/7. The data shows that at an anode V of 2500 volts and anode I of 0.7A a
new tube will make 800 watts output. Then after 500 hours of use it will
drop down to 650 watts. Although this is for 60cm and not HF frequencies,
does a similar degradation take place on HF? Maybe, and it may be a longer
period of time however and going above the 2500V anode voltage, it may add
less time to the Po. I know many hams pushing these tubes to a plate
voltage of 4KV and a bit higher to match the performance of an 8877.
Many years ago I was told that the GS-35b was "the poor man's 8877", indeed
the $135.00 price for a new one was attractive compared to that of an 8877
so I tried one in the same setting as an 8877 for a comparison. All things
being equal I took an Ameritron AL-82 and I retrofitted in a GS-35b a little
tweaking or the input and I had a perfectly working unit with a GS-35b.
Having the pleasure of also having an AL-1500 on hand I made comparisons on
band and power in vs power out. I was sadly disappointed when I realized to
get the same performance from the GS35b vs an 8877 I would need more than
the 100 watts I had available for test purposes. In retrospect, the GS-35b
had a bit of an edge over the pair of 3-500's and in comparison one could
say that the GS-35b had it over the 3-500's for all things being equal.
Comparing to the 8877 was no contest.
The Ameritron amps in question all run plate voltage of 3600v. Price wise
the tube is very attractive and I can see the affection for it however I
myself stayed with 8877's and never looked back.
-----Original Message-----
From: Leigh Turner <invertech@frontierisp.net.au>
To: 'Mike' <k4gmh@arrl.net>
Cc: amps <amps@contesting.com>
Sent: Thu, Oct 31, 2019 3:27 am
Subject: Re: [Amps] GS35B Amps Common Problem / few more comments
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, you're 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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