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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 1:44 am 
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I believe with my Transformer I am limited by the idle current.
This would be the spec of the transformer. At 5K, my current will be down to 68mA.

http://www.diyaudio.com/forums/attachme ... 6123hs.jpg


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 7:18 am 
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Location: Pleasant Hill, California
quikie22 wrote:
I believe with my Transformer I am limited by the idle current.
This would be the spec of the transformer. At 5K, my current will be down to 68mA.

http://www.diyaudio.com/forums/attachme ... 6123hs.jpg


What power is it rated for?

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 7:34 am 
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20W


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 8:09 am 
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quikie22 wrote:
20W


So, 120 mA / 350V is fine. You should get better bass response and damping factor than me, but output power will be lower (see graphs to calculate)

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 8:30 am 
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I think maybe not. Max current rating for my OPT is 90mA. I suspect the core will saturate if current goes above 100mA.
BTW, how do we calculate the power for the OPT? Current^2*Impedance or Current*Voltage?


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 8:34 am 
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My transformer specs:

http://tctubes.com/james-transformers-JS-6123HS.aspx

I managed to get these from a friend as used for a much cheaper price. (He upgraded to Tamura OPT).


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 9:18 am 
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Шаман, типа...

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Let's see.
20W / 5K means 316V RMS, that means 447V peak. You will have about 200V half-swing (according to graphs), that means more than twice lower value.
Let's calculate peak current for maximal flux;
447/5K=89 mA + 90 MA = 180 mA. It is the value according to specs.
In your case 200V/5K = 40 MA. That means, you can have idle current = 180 mA - 40 mA = 140 mA for the same flux.

I.e. your transformers will provide about 4W of linear output power on frequency 447/200 = more than octave below rated for full power frequency. 510 Ohm cathode bias resistor will result in about 90 mA of idle current.

For more of output power from the same tube and transformer you may consider pentode mode with parallel negative feedback, 500V B+ and 90 mA idle current. In such case 6P14P should be used as well as pentode, with resistive load.

Which option do you want to try first?

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 9:38 am 
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So basically you mean my transformer can handle the current? This is good news. Smile
The transformer has got multi-taps, 2.5k, 3.5k and 5k. Planning to use the 5K first because of better linearity, but can move to the other impedance for better performance/power ratio.

I think I will try with Triode first . Smile And as you suggested earlier, with a gyrator in driver circuit.

Maybe I will try the Pentode mode once I get the circuit working. Should not be more than a small switch to change between modes and maybe a different power supply. A question, why do we use resistive loading if 6P14P in pentode mode instead of gyrator?


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 9:43 am 
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Because of parallel feedback from GU-50 plate to 6P14P plate. With gyrator load output resistance of the stage equals to tube's internal resistance that is not stable and not linear. That means feedback ratio non stable and non-linear. When pentode is loaded on resistor feedback ratio is more stable and more linear.

No magic, just a math... And triode/pentode switches you can see in some amps look like a hoax: flipping G2 from anode to B+ does not make an optimal pentode output.

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 10:04 am 
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If I go to plate-plate feedback, then it will be very similar topology to RH84? http://www.tubeaudio.8m.com/RH84/rh84.html

Would it be optimal to drive the GU-50 in pentode with a triode 6P14P? maybe I need a 6N1P as the first stage before the 6P14P?

About the switch for pentode/triode switch, I believe I will need different grid voltage circuits made for them. Not just a simple B+/Anode switch


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 10:12 am 
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Triode driver in RH 84 is mistake. I explained above why.

Yes, for GU-50 you need higher amplification factor than for EL84.

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 10:15 am 
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I meant triode 6P14P but using a resistive loading for the 6P14P instead of gyrator?


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 10:23 am 
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quikie22 wrote:
I meant triode 6P14P but using a resistive loading for the 6P14P instead of gyrator?

No, Rp in triode is too low for my taste... But if you like sound of RH 84 you may try. Triode stage in RH 84 has unbypassed 250 Ohm cathode resistor, it helps a bit. You may try the same, instead of LED bias.

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 10:24 am 
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What is Rp? Plate resistance?


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Wed Nov 03, 2010 10:26 am 
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Yep. Plate resistance that is non-linear, so feedback ratio would be non-linear as well.

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