Pop in a big GG triode and be done with it. Sell the tetrodes and sockets to
someone really needing them.
----- Original Message -----
From: "Alek Petkovic" <email@example.com>
To: "AMPS" <firstname.lastname@example.org>
Sent: Saturday, April 05, 2014 8:48 PM
Subject: Re: [Amps] 4CX1600B options
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.
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.
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