Hi John.
I don't know if this is the same fault but a few years ago, in my local
club's amp, which runs a similar but larger tube, the Russian GU84B, we
had a runaway situation. We found that the ZSAC was not staying
constant. it was rising and rising and we could not stop it. It became
worse very quickly and it did it even when the amp was not keyed. As the
tube warmed up after turn on, the plate current started rising and it
didn't stop until we turned it off.
We changed out the tube for a new one and the problem went away.
We did not have a hi pot tester. All we had was a 1000V megger and with
the tube cold, we could not find any evidence of any solid or partial
short. The fault only appeared when the tube was hot.
I still have the dead tube here and I have a socket and filament
transformer as well. I was going to warm it up and see what I could see
with the megger. I guess lack of interest has kept me from doing it. I
will have to do it one day, as I have 3 tubes here and I can't remember
which one is the dud. I/we never marked it. Yeah. Pretty dumb!
Maybe your GU91B (4CX1600B) is showing a similar fault. I have a spare
one of those here but I also have an amp that uses it and so it is not
for sale. There is a guy in ZL who has or had one or more for sale. He
may be a subscriber on this list.
Your idea of a tube transplant sounds like the cheapest option though.
Cheers, Alek
VK6APK
On 6/04/2014 7:24 AM, John Lyles wrote:
I have a Ten Tec Titan II model 416 with a supposedly bad tube,
according to the factory. I haven't high potted it yet, and with an
ohmmeter I measured clear between elements except there is 1.2 Meg ohm
between heater and cathode ring. The 1600 has crossover geometry where
the control grid contacts are completely below and inside of the
cathode ring. For a 4CX1500B or 1000A with breechlock terminals G1 is
between cathode and G2.
It was said (by factory) that this tube blows up the amp. I assume
they meant the screen regulator board or bias circuits. This may
indicate a hot fault where anode is breaking down to the grid(s).
Anyone else experienced this sort of failure with this tube/amp combo?
The Titan II used bipolar series regulator, and the later Titan III
(4CX800A) used a mosfet, both had shunt resistor of 15K only across G2
to K, for reverse screen current handling. The regulators were only
capable of working positive current, with a series diode between them
and the shunt R.
Are there any more Svetlana 4CX1600B's available?
I have both a 4CX1000A and 4CX1500B plus SK-800 socket and filament
transformer. Cin, Cout and Cgp are the same for all three tubes.
Transconductance of the 1600B is highest, followed by 1000A and 1500B.
For cooling, taking an example of 1 kW anode dissipation at > 5000
feet above sea level, estimated air from datasheets:
Eimac 4CX1000A ~32 cfm, ~0.3" water bp
Eimac 4CX1500B ~21 cfm, 0.3"
Svet. 4CX1600B 27 cfm, 0.3"
From the constant current curves, the biggest differences are noted.
This is indicated by the different cutoff characteristics, where the
4CX1500B requires considerably less negative bias on G1 to cutoff the
electron beam. The older Eimac and Svetlana are similar, ~-56 volts
for 200-250 mA of Ib with Eb at 3 kV. Eimac 4CX1500B is plotted only
with 225 volts of G2, whereas the other tubes are defined for 325 volts.
From the collective wisdome here, I welcome experience/knowledge in
converting from a Svetlana to Eimac tetrode in this amp. Or a
reasonable source for 4CX1600B. I think conversion to a better screen
regulator is needed too. I am not interested in a pair of
4CX800A/GU74b as I already have the bigger tubes and 800s are pricy
and become rare as well.
73
John
K5PRO
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