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Old 6th June 2007, 11:03 PM   #1
beamnet is offline beamnet  Netherlands
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Default GU 50 output modification

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
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Old 10th June 2007, 03:48 PM   #2
beamnet is offline beamnet  Netherlands
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I guess thta was too many q-s in on message.

So i'll reduce it to one: How do I strap them in triode?

Regards

Bas
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Old 10th June 2007, 04:42 PM   #3
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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.
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Old 10th June 2007, 04:44 PM   #4
beamnet is offline beamnet  Netherlands
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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...
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Old 10th June 2007, 04:47 PM   #5
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Quote:
Originally posted by beamnet
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.

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Old 12th June 2007, 09:19 AM   #6
beamnet is offline beamnet  Netherlands
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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
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Old 12th June 2007, 04:54 PM   #7
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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.
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Old 12th June 2007, 05:44 PM   #8
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I have a spare socket laying around

I'm looking forward to blowing up some tubes next week!
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Old 12th June 2007, 06:19 PM   #9
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Quote:
Originally posted by beamnet
I have a spare socket laying around

I'm looking forward to blowing up some tubes next week!
Crossing fingers!
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Old 12th June 2007, 06:25 PM   #10
beamnet is offline beamnet  Netherlands
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why am I secretly hoping something melts or explodes?

I bought the 100R resistors today...

Just connect G2 to the anode with 100 ohm and g3 to the cathode? (just checking, haven't the faintest clue what 6l6 mode would be else, but it looks really much like triode then...)
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