GU-81m tube amp schematics??? - diyAudio
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Old 14th February 2011, 06:49 PM   #1
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Default GU-81m tube amp schematics???

Hello,
Anyone have schematics for gU-81M?I have some tubes with sockets and i like to use them.I don't care if it will be hybrid or to drive it with a solid state amp to not use a lot of tubes.Just something simple to hear some watts of music from this tubes.I dont have experience with tube amps.But have the experiencfe with the high killing voltages from other projects.
Any help please?

Thank you
George
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Old 15th February 2011, 09:37 AM   #2
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Anyone?I have found some amp photos with this tube in the net and some photos on this site.But i cant find some simple schematics.
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Old 15th February 2011, 09:52 AM   #3
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I have a few ... but HF amps


Good luck Giorgos and greetings from Belgrade!
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Old 15th February 2011, 10:12 AM   #4
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Haven't found anything but a description (in Dutch) of a bloke who built an amplifier and considered using this tube, but settled for the smaller TB2.5-300.
Fishbowl TB3 Amp - Experimenten
Apparently, he used a SE parafeed design, but it's unclear what he used for a plate load. I suppose that will be the biggest challenge in any case: finding a plate load that will handle that sort of power. Maybe a push-pull design with a ginormous center-tapped power transformer acting as OPT is more realistic. Frequency response will probably horrible, but you might get some sound without mortgaging the house for the proper parts. With any kind of luck, the low frequency response will be halfway decent and you might be able to use the amp to drive a subwoofer.

However, it's going to be a challenging project in any case. You'll need >130W of heating power, anode voltage somewhere north of 1kV @ > 100mA per tube for PP (say, 200mA for SE), and to drive the gU's you'll need something like a single ended EL84 at the very least (a beefy mosfet could be used as well). -Vg of 100 to 150V, Vg2 of 500 or 600V @ 50 or 100mA.

It's one of those projects for which a schematic can be drawn up in 15 mins, but that'll take 2 years to complete.
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Old 15th February 2011, 01:47 PM   #5
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Just give me a schematic to start!!I'm horrible to this part!!
I prefer to drive the tube with a mosfet.I know that will not be a good amp in the ears.But will be a good to the eye and "i have done that!!!" amplifier.
Thats the reason i want a design with less tubes and more solid state.

Any help accepted please for the little monster!!
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Old 15th February 2011, 03:38 PM   #6
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hey-Hey!!!,
That is a nutty-Big tube. I also have finals and sockets and effectively gave up once I learned enough about output transformer limitations. I've been considering using them with g2 as the anode( that's still 120W of dissipation ).
cheers,
Douglas
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Old 15th February 2011, 04:59 PM   #7
banat is offline banat  Serbia
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Quote:
Originally Posted by giorgos View Post
Just give me a schematic to start!!I'm horrible to this part!!
I prefer to drive the tube with a mosfet.I know that will not be a good amp in the ears.But will be a good to the eye and "i have done that!!!" amplifier.
Thats the reason i want a design with less tubes and more solid state.

Any help accepted please for the little monster!!
Just forget the GU81M tube for Audio aplication !

Generaly this big RF power tube live is relative short ,common failure is usual break of extreme tiny DC heated thoriated tungsten kathodes wire .They live for around 1000 Hrs,and thats tube live expectancy is signed in the factory specs to.

BTW this GU81M tube power capacity is aproximately like 2 x 813 tube in the parallel,but 813 tube(GU13) is the right one, for AF or RF aplication !

Best Regards

Last edited by banat; 15th February 2011 at 05:02 PM.
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Old 15th February 2011, 05:44 PM   #8
db! is offline db!  Canada
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giorgos - A dynamic coupled mosfet as you described would work but a Gainclone and grid-drive transformer would probably be simpler to design.

A simple TDA2030 amplifier connected to the secondary of a 230VAC:24VAC-36VAC toroidal should work. One end of the primary would be connected to the grid while the other goes to the bias supply.

If you are insistent about using a dynamic coupled mosfet, the tubelab powerdrive might be something to look into...

I don't mean to question your expertise but are you sure you're experienced enough to play with high B+ voltages? A large tube like the GU-81 will require +1kv to get any decent power/performance...

Last edited by db!; 15th February 2011 at 05:47 PM.
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Old 15th February 2011, 08:34 PM   #9
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hey-Hey!!!,
Since you want to do it...get a big 5k a-a, U-L output trans and build a B+ of ~700V. Idle at 250-275 mA for Class A. That is a nearly 2/3 of an amp per channel... We'll build the phase splitter out of resistive loaded 7355's( or perhaps 7591's ) in LTP riding a good sized CCS.

Maybe we can work together on an output trans design...and I can build this for myself.
cheers,
Douglas
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Old 15th February 2011, 10:45 PM   #10
db! is offline db!  Canada
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There aren't too many SE OPTs that will handle 275mA... Cheapest one that comes to mind is the hammond 1642SE. IIRC it's sells for about $300-350 and it weighs 28lbs...

I think CCS or gyrator loaded parafeed would be more realistic...

Last edited by db!; 15th February 2011 at 10:49 PM.
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