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GU-50 push pull amplifier
GU-50 push pull amplifier
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Old 5th April 2015, 06:28 PM   #1
JakubKubo is offline JakubKubo  Slovakia
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Default GU-50 push pull amplifier

I'm starting a new project with GU50 tubes so I'll post as many pics as I can about it. Hope you like it and most of all I apreciate your opinions , advices and so on...
I'm already finishing wooden chassis
GU-50 push pull amplifier


Already finished and tried new output transformer R aa=4k, sec. 4/8 ohm
GU-50 push pull amplifier


GU-50 push pull amplifier

Last edited by JakubKubo; 5th April 2015 at 09:31 PM.
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Old 6th April 2015, 01:07 AM   #2
Ejam is offline Ejam  Australia
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What circuit are you using?
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Old 6th April 2015, 05:54 AM   #3
JakubKubo is offline JakubKubo  Slovakia
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It will be this one http://hlava.webpark.cz/Dvojcinne/sch2x50wGU50.GIF

Last edited by JakubKubo; 6th April 2015 at 06:00 AM.
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Old 6th April 2015, 10:09 AM   #4
artosalo is offline artosalo  Finland
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The screen grid voltage arrangement of GU50's is not optimal.
According to my tests the optimum voltage is about 350 V and the current supply needs to be up to some tens of mA's.
In your circuit there is 56k 5 W series resistor from +Ub line to form the Ug2. This is high impedance.
Also the screen grid resistors should be lowered to max. 1k. Otherwise you wont't get max. available output power.

I have used NFET regulator to form the screen voltage to GU50 PP (attached).
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File Type: gif GU50_6F12P_PP_poweri.GIF (18.0 KB, 905 views)
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Old 6th April 2015, 10:24 AM   #5
JakubKubo is offline JakubKubo  Slovakia
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yeah I was also wondering about those 7k5 grid resistors since they are big values.. ill see what will be the result after first run, but im sure I will have to change some values according to a measurments
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Old 6th April 2015, 11:13 AM   #6
Ketje is offline Ketje  Belgium
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Some variations on a theme ♫
Mona
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Old 6th April 2015, 11:54 AM   #7
Parafeed813 is online now Parafeed813  Netherlands
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Quote:
Originally Posted by artosalo View Post
The screen grid voltage arrangement of GU50's is not optimal.
According to my tests the optimum voltage is about 350 V and the current supply needs to be up to some tens of mA's.
In your circuit there is 56k 5 W series resistor from +Ub line to form the Ug2. This is high impedance.
Also the screen grid resistors should be lowered to max. 1k. Otherwise you wont't get max. available output power.

I have used NFET regulator to form the screen voltage to GU50 PP (attached).
I noticed that as well.
The difference in g2 current between idle and full power is something like a factor 5. You need a low impedance source here.
The bias pots seem to be a little high: 300k where the gridleaks are only 110k. Changing bias will change the input impedance a lot.
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Old 6th April 2015, 11:56 AM   #8
Bandersnatch is offline Bandersnatch  United States
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I agree...the final's g2 supply can be improved. Not to mention the single-C filtering for the plate supply. A simple regulator consisting of a power MOSFET with its gate hung off a voltage divider of appropriate value( say in this case 350V ) will work well. Then 1k stoppers on each grid. Use an enhancement mode, FQP2N90 and don't forget the gate clamp Zener... Shunt its source to ground with some resistance in order to drive more current through it and increase its gm( and therefore performance ).
cheers,
Douglas
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Old 6th April 2015, 12:23 PM   #9
JakubKubo is offline JakubKubo  Slovakia
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thank you for help so far, now Im taking care of chassis to be done but need to get some more things.
There will be mV meter and four position switch on the front panel to bias all output tubes easily and behind each tube will be trim pot... just have to get those with very short shaft with cutout for screwdriver , didnt find any on ebay
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Old 6th April 2015, 07:37 PM   #10
rotaspec is offline rotaspec  New Zealand
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Quote:
Originally Posted by JakubKubo View Post
thank you for help so far, now Im taking care of chassis to be done but need to get some more things.
There will be mV meter and four position switch on the front panel to bias all output tubes easily and behind each tube will be trim pot... just have to get those with very short shaft with cutout for screwdriver , didnt find any on ebay
eBay user kwtubes has small metal cased Russian trim pots that might suit your use.
The downside is that they usually come in boxes of 50, but they are very cheap regardless.

I have purchased from him and have been happy with the quality.
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