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GU 50 output modification

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I'm getting a little swamped by not totally grasping all theory about output stages, so i ask foor some help.

I have my poweramp with GU50 in push pull 450V B+

It's fed by an ecc99 LTP phase splitter which is in turn fed by 1/2 6sn7

I've had it running for that 2 years, and want to make it better.

First, my opt's have kinda lowish primairy impendance (3-4 K). So maybe i'll go ppp. They are from a beard p100 .

But to reduce distortion even more, i'll probably need to get some NFB, or strap it in triode. (triodes cope better with low impendance opt's don't they?)

Or both.

Anybody care to help me a bit. I've tried to do something with curves, but i think the datasheets I have might not be complete.

Allso there seem to be 2 ways to triode strap it. UL is an option too.

Regards
 
You have an interesting experience using GU-50 tubes in 6L6 mode. I would strap G2s through 100 Ohm resistors and check distortions and screen grids first applying max available negative bias to 1'st grids then slowly decreasing it until G2 current (you may measure voltage drop on 100 Ohm resistors) is close to max by specs.
 
beamnet said:
What should i look out for? Can I just chsnge the connections and fire the thing up?

IE what's the first thing to probably fry?

I'll check the UL modes and yout 6l6 mode...

If you are willing to fry one tube for experience sake you may decrease bias observing anode and G2 currents and see if G2 is not red when anode current is as desired. Anodes are made of nickel so they will survive even when red, but I am not sure about screen grids. The toobs were designed for very linear applications so robust screens were not needed.

I went with standard approach and ordered power transformers for 750V anode and 300V screen voltages and 10K PP output transformers. If you want I may share some.
 
Hi wavebourn,

I've read around and you seem to have quite a bit of experiance on the GU50.

I read an old topic in which you participated (regarding the translation of a russian document featuring the gu50 replacing a 807)

There was a measurement of a GU 50 in triode with screen and anode at 400V. Anode current 70 ma.

What;s your guess? would 445 V B+ be suitable for Triode?

My opt's and B+ I can not change. In theory, what would be most suitable? Penthode, UL or triode? In other words, generally, what kind of mode would require the lowest Raa?

I have no power supply to go experimenting, but tubes to spare, so any experiment IN the amp that doesn't harm it is cool with me..

Bas
 
As I said, I would experiment with G2 only. Despite it is rated for 250V max I run it with 300V. Frankly speaking I don't know about 400V, however I saw and heard about 400V triode mode, but never tried that for myself. Try your 450V, but increase a G1 bias negative voltage to the available maximum before decreasing it until G2 gets red (if gets before red anode) then report what A and G2 currents did it survive. You may connect both anode and G2 to the B+ through ammeters if you are afraid to fry your output tranny.
 
This is what works well for me, but must be seen as a prototype!
 

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beamnet said:
The 7193 is 1/2 6sn7

I would recommend e1148 tubes there. These are wonderfull if you can get the non microphonic ones...

CV6 work a champ too with beefier music.


Dear Sir,

Thank you very much for your soon support. I'm very high appreciation.

I'll try to do it by myself follow your schematics.

I did make a Single End GU 50 Ampli, using 6H8C for pre tube, D.I.Y PSU & OPT, it sounds very good.

I'am very sorry with my English

Thank you very much

Best regards,
 
I also like 6H6P instead of ECC99 tube.
So far, I've been made the chassi and PSU for PP GU50. The next is OPT and choke.

I think, the steel sockets are not ugly, they make me feel safe and easy to determine heater, cathode, anode and grid point.

Thank you very much for your advice me use 6H6P

Best regards,

thomas-NGU
 
Bas, I made amp in 1986 with close schematic, shortly:
1/2ecc82(pre)-1/2(phase) - 6n6p(driver)-gu50, PP, UL,
Ua 560V, FB 10db, Diods bridge rect. Bias fixed smth near -50V as i remember. It was my first amp with self made toroids opts. I was very happy with it 5 years, later i present it to my friend. So, i think u will like gu50's. They have a bit sharpy sound, but that for taste.

imho UL mode the best, but need careful approach. OPT the most important thing. Better to listen all variants. Avoid using resistors in cathodes they spoils a lot. Put 1 ohm to measure current and finally remove.
 
The ecc82 is known to be sounding a bit sharp. I have no Fb and bandcore OPT's. It sounds good in penthode mode.

Currently too busy to try the mods, a shame...

The use of the E1148 was a "golden choice" in this amp. It's like 400% better than any 6sn7 i tried there.

The ecc99 are good sounding tubes, but i actually put in the 6n6p to clear up the highs...

Bas
 
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