[Amps] 4CX1600B options

[Leigh Turner]

John,

Had it been 10 years ago, the logical replacement for the 4CX1600B tube was
the mil-spec GU-84B with its larger 2500 Watt plate dissipation and
prodigious power output capability that would restore the Titan amplifier to
full spec, plus some. 

These excellent SSB linear tubes were not so well known outside of Europe,
but have since been popularised by ham amplifier manufacturers resulting in
supply/demand forces driving the once excellent prices up into the
stratosphere to the point where other tube options are now more cost
effective, e.g. the Chinese FU728F tube which is well worth considering.  

The rugged mil-spec FU728F is a beefier 4CX1000A/1500B on steroids that will
yield greater linear power output; otherwise these latter tubes that you
already have will be adequate substitutes in the Titan II 416 amp with the
appropriate tube operating condition changes. 

As you note the screen regulator in the 416 is nothing to write home about,
and improvements could be made there. One could consider retrofitting a
G3SEK tetrode control board to effect an excellent improvement to the Titan.

73

Leigh
VK5KLT

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of John Lyles
Sent: Sunday, 6 April 2014 8:55 AM
To: amps at contesting.com
Subject: [Amps] 4CX1600B options

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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