Peter,
Correct! I always figured that running them just under the design-maximum was
pushing it, and as you've seen, I ran them only 90 Vdc under the maximum at 900
Vdc. The one using three for 100 watts PEP was surely conservative, and would
have lasted for a long time in my opinion. A general rule of thumb for sweep
tubes was that they would be good for 3 times the dissapation rating output
wise. So if a tubes dissapation rating was 30 watts, it would do about 90 watts
out safely. Using 4 tubes would have been good for say 360 watts output.
Running 6 of them using the X3 factor would have gave 540 watts out which would
have been about right for a 1000 watt input AB1 amp. The X3 factor was used by
about everyone who built amps using sweep tubes I know of, and was how they
figured the number of tubes needed to build a certain size in output. The
M-2057 I used had a 400 mA cathode current rating, but it would handle
temporary overloads of 1400 mA according to GE. However, they sure d
idn't want anyone to run them that way, and that value was never included in
the design-maximum ratings. It's cathode was built a good bit heavier than a
normal sweep tube, and the anode had folded back wings which acted as a
heatsink. It was almost the same as the 8908 except for the tube base, and
lower interelectrode capacitances. That was the reason for the change from an 8
pin socket to a 12 pin, and each grid then had two connections instead of one.
It was designed so more could be paralleled than the 8908 could due to
capacitance. There was a few that were built using up to 10 in parallel. At 40
watts dissapation, that would have been 120 watts each at X3, 1200 watts out,
and would have been about 2400 watts input for 10 of them. They would actually
do a shade more, but this was the recommended way to run them.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/9/06 at 8:00 AM Peter Chadwick wrote:
>I think the real problem at the end of the day was that the tube
>manufacturer wasn't going to provide any 'design maximum ratings' for
>sweep tubes except in horizontal sweep service. Realistic design maxima
>for ICAS linear service would have seen some severe derating from the
>power levels that amateurs wanted, anyway. I did read that many of these
>tubes derived originally from 6L6/807 structure: certainly, there were TV
>sets over here that used the 807 as a horizontal sweep output tube in the
>years just after WW2 when TV was starting. Additionally, in the ham world,
>the mistreatment of tubes like 6L6, 807 and 813 was legendary, and they
>would stand up to a lot. I ran, in my mispent youth, an 807 at 200 watts
>input, (Va = 1750, Vg2 = 500, Vbias about -250, driven hard by a 6L6!) and
>it lasted well - but 807s were very cheap as war surplus, and I believe,
>in the US, 1625s even more so. So a belief readily grew up that you could
>take liberties with all tubes.
>
>I have a seen a 2MHz marine radio over here that used 3 6LQ6 in the PA
>with about 600 volts on the plates: but that was rated for 100 watts PEP
>out, and, I suspect, didn't push things at all.
>
>73
>Peter G3RZP
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