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looking for gu-81m pentode

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ubuntupokemoninc

Thu Nov 11 2010, 11:58PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220

I am looking for a gu-81m russian pentode, there are some on ebay for $35 plus $20 shipping, wondering if there legit (working) tubes, if not would some sell me 1 preferbly no more than a total of $60

Arcstarter

Fri Nov 12 2010, 02:10AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I got one of those GU-81M tubes. It's dead... I have no idea what it is... The filament is ok-ish, though sometimes when i first turn the filament voltage on (i use current limiting resistors, i switch from 2.7 ohms, to 1.45 ohms, then full voltage) i see sparks inside the tube.

I can get plate current, and enough of it to get a little color change on the plate. But then, the grids don't vary the current... Or something. I need to test it again to see who it acts, as i forgot, but i am assuming a dead grid.

I think it is damage from shipping. It may be worth trying to buy one though.

1n5ane

Fri Nov 12 2010, 01:59PM

Registered Member #3283 Joined: Wed Oct 06 2010, 11:03PM
Location: Toronto, Canada
Posts: 29

Your best bet is probably ebay... Majority of the sellers will refund you if the tube doesn't work...

Arcstarter - What voltage are you putting on the control, screen and suppressor grids? Also, are you lighting the filament before applying power to the plate?

Arcstarter

Fri Nov 12 2010, 07:14PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I run the tube as intended. I used a bit below the rated voltage on everything, and only 2kv on the plate i think.

Ubuntu, i say you should try it. The reason i could not return is because i bought it for Christmas a month ahead of time.

ubuntupokemoninc

Fri Nov 12 2010, 07:45PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220

the reason, i was asking if there real is cuz i bought 2 811a tubes on ebay, and my tc never worked, got an old rca 811a from a freind, the tc worked fine, whats up with that!?!?!?!?

just curios, how do you test to see if i tube works, do you light the filiment up and check with a multmeter between the cathode, and the anode, then apply voltage to the grid and see if current stop s flowing?

or do you have to have high voltage?

Arcstarter

Sun Nov 14 2010, 10:19PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Use a current shunt connecting the tube to ground and check current with a meter. Use whatever voltage, 2kv is good, and bring down the control grid voltage with the screen at the rated voltage and suppressor at ground. The current should get higher as the voltage on the control grid gets lower. Start with maybe 50v positive and it should start conducting heavily below 0v, the more negative the higher current.

Proud Mary

Sun Nov 14 2010, 11:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

ubuntupokemoninc wrote ...

how do you test to see if i tube works, do you light the filiment up and check with a multmeter between the cathode, and the anode, then apply voltage to the grid and see if current stop flowing?

You need to build a simple test rig to supply the valve with the design centre rating voltages stipulated in the data sheet - heater voltage, control grid voltage, screen grid voltage, and anode voltage. You then vary the control grid voltage over its design range and measure the change in anode current that this produces, and see how nearly this curve matches the graph in the datasheet.

After heater or filament failure, reduced emission is probably the next most common reason for replacing a valve, and this cannot be measured without the ability to supply the full HT voltage, and perform the very basic test I have outlined above.

Dr. Drone

Tue Nov 16 2010, 11:50PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

ubuntupokemoninc

Sat Nov 20 2010, 07:05PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220

I've decided to go with the gu-5b, and the 2 811a's that i bough awhile ago was just as i feared, i powered the filaments up, and put one of the electrodes of my multimeter on one of the filament pins and the other on the pin for the grid and it told me about 3.5volts of the 6.29 volts i was putting into the filament, is that a filament to grid short? but good news the person i bought it from had a 1 year warranty, i'm gonna try to get new tube, most people know how annoying it is to spend $100 bucks on tc and 2 weeks working on it, all for you to flip the switch and nothing to happen!

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