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Author Topic: What should I do with these?  (Read 7930 times)
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KM1H
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« on: August 18, 2012, 04:51:07 PM »

They are Russian GU-81M pentodes mit der sockets picked up at the Friedrichshafen hamfest back in June by a friend from the UK. Then taken to his company HQ in Hamburg, mailed to my son at Ramstein AFB, and sent via USPS APO mail here. No damage!

Two of the tubes are NOS with 80's date codes and the other probably is also but was not boxed. All for 50 of that funny money they use over there these days.

I love the chassis of this one Shocked
http://f5ssk.blogspot.com/2006/04/2-fois-gu81m-2kw.html

Carl


* GU-81M.jpg (77.5 KB, 660x495 - viewed 1801 times.)

* GU-81M Sockets.jpg (82.44 KB, 660x495 - viewed 1510 times.)
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ke7trp
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« Reply #1 on: August 18, 2012, 07:17:32 PM »

Neat tubes Carl.  If I read the data sheet correctly they are 700 Watt Plate?  Or maybe it was saying 700 watt output per tube?  2KV at 450 MA max so that makes sense. 

12.6 volts at 11 amps per tube for fil. 

C
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KL7OF
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« Reply #2 on: August 18, 2012, 09:39:47 PM »

833 lookin tubes......."cept they're pentodes...sexy plates
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kb3ouk
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« Reply #3 on: August 18, 2012, 11:43:01 PM »

For some reason it seems the Russians favored pentodes more than they did beam power tetrodes. My favorite example of that is the GU13 and GK71, both are basically Russian 813s, but the GU13 is identical to the 813, while the only difference between the 813 and the GK71 is the 71 is a pentode, and they seem to be more common and durable than the GU13.
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Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
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Patrick J. / KD5OEI
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« Reply #4 on: August 19, 2012, 02:18:02 PM »

Three is an odd number. If 2KV is right, they would be great as RF amplifiers using rather common 1-2KW plate iron. Its a 450W anode. They have a lot of "output capacitance" according to the data sheet but someone has probably overcome that in one way or another.

Remember the ARRL handbook article showing 5 identical linear amps each with a pair of 813's?  They got the pair to work well on 10M despite their high output C by tapping the tuning cap off the PI coil..
So, could you run all three in parallel like that?  WTH I saw an amp with nine 3-500Z's in it before..

Those are unusual tubes, so how long do they last? Back to the the ARRL handbook article - maybe a couple of identical looking one-tube amps, one for HF-15M and one for 10 and 6. That way no need to balance the idling currents. They could share a common power supply and could be plate or cathode modulated by whatever else might be lying about.

You tell us, what do you want to do with them?

* GU-81M.pdf (290.68 KB - downloaded 211 times.)
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Radio Candelstein
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« Reply #5 on: August 20, 2012, 11:34:29 AM »

I also have a pair of these--mainly because they look cool. I am going to use one in a BC 610 conversion, instead of a pair of 813's. The logistics are pretty easy because of the way the 250TH socket is mounted. It should be an easy 160, 80, 40 conversion.

I have a schematic and link to a Russian who used a pair of these as a leenyar. If I remember correctly it was a GG unit. My guess is that these tubes are plentiful and cheap over there.

Skip
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KM1H
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« Reply #6 on: August 20, 2012, 01:36:19 PM »

I doubt they will be very efficient on 10M, similar to the 813 and the spec sheet Ive been reading derates them above 6MHz. OTOH some have claimed 1000W+ out of one in an all band amp but at 3500V. Ive also run 803's way above frequency and HV ratings and that is also a true pentode with very long internal leads.

That 120W screen rating is crazy. Must have something to do with its use as a self excited oscillator......maybe since it could take huge load variations without damage?? You could use the screen as the anode and run as a triode  Grin

The suppressor being one of the top caps plus also pin 6 of the base is another strange thing, never saw that before.

I think I'll save them for a modulator if I can locate suitable iron and choke that can handle a pair at close to full bore. Dont think the BC-1T iron would survive long Huh Roll Eyes And I havent even started to plot the P-P loads at various voltages.

Something else is to set filament voltage a hair above the point that output drops; Ive heard they can be short lived.
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Patrick J. / KD5OEI
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« Reply #7 on: August 20, 2012, 11:13:47 PM »



The suppressor being one of the top caps plus also pin 6 of the base is another strange thing, never saw that before.

use as an interlock? It is odd.
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Radio Candelstein
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