Hello Greg
Even if the the cathode and filament are connected,
you may still use cathode R for negative feedback
and "equalising" the tube currents, but you have to
use chokes for the filaments.
There is another way to do it, as it is done in the Harris
RF-110A amplifier. It uses screen regulator which is
separately adjustable for each of the two 4CX1500B
tubes.
The screen regulator uses a string of high voltage power
zeners (three 75V and one 62V at 10W each) and five
6.2 V low power ones (these are located at the "top"
of the zeners string).
Different voltages are accessible from taps connected to
the low voltage diodes. The screen connection use small
jacks for separate connection of the finals screens to the
taps of the top chain of low voltage zeners.
So, the screen voltage may be separate adjusted for the
two tubes between about 287 (3 x 75 + 62) to about
315 V (285 + 5 x 6.2), plus or minus.
Note that the screen voltage can't be the same for both
of the two tubes (you can't connect the two jacks to
the same tap!), and this confirm the fact that it is hard
to find two identical power tetrodes...
Anyway, this can be solved by using two sets of taps.
I have two such amps, I just finnished refurbishing them.
For both I had to use different taps for the two tubes
screen voltage.
The manual states an idling plate curent of 200 to 280
mA and a difference between the two 4CX1500B tube
idling currents of no more than 40 mA.
For both amps I could obtain less difference (about 20 mA)
using a screen voltage difference of 6.2 and 12.4 V for
each of the two amps/pairs of 4CX1500B respectively.
The control grid bias is common for the two tubes.
I scanned the RF-110A manual (and for RF-124 PS also).
You may find it on the bama.sbc.edu server, thanks to
Ken K4XL.
All the Best,
Traian
qrp73@talk21.com wrote:
> I want a build a dual 4cx1500b amplifier. I managed to obtain a batch of
> unmatched 4cx1500b's all different date codes.
>
> I was wondering which method would be best to achieve matched and balanced
> tubes? Since the cathode is tied to one side of the filament i cant use a
> cathode resistors to equalise currents. Is there a drive configuration that
> would allow me to drive these 2 tubes without worrying about matched tubes?
>
> Assuming there is no drive configuration that will achieve this, would it
> best to use seperate bias supplies to try and equalise each tube
> individually? Someone suggested that i use the Super Cathode Configuration,
> is there any merit to this configuration.
>
> I have looked at G3SEK regulator board kits, it seems these boards require
> matched tubes. I would also like to DC ground the screen and these boards
> dont appear to support grid driven DC grounded screen. I think some of these
> tubes might be down on emission so would like to use max screen voltage at
> 400 volts, however the screen capacitors on the SK800a sockets are flaky over
> 300 volts.
>
> I am not familiar with the procedures for matching tubes, can someone explain
> what the exact procedures are for matching transmitting tubes DC and RF wise.
> I assume that resting equal grid current would be the indication of a close
> match. Or is there more to this than meets the eye.
>
> Appreciate the Help
>
> Greg
>
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