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 Post subject: Re: GM-70 Amplifier
PostPosted: March 20th, 2016, 10:21 pm 
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You can't see me right now, but I have a massive smile on my face. :D

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PostPosted: April 27th, 2016, 9:01 am 
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To all,

Based on the listening session at Roscoe's, I will removing the interstage transformer in the driver circuit and replacing it with a choke assisted source follower. With a cathode biased output tube, the grid is already elevated 85V with respect to ground, which means I have this to play with in the tail of the source follower. With a resistive loaded direct coupled source follower referenced to ground, the negative swing would be limited to 85V, which would be great for Class A, but the driver will clip as it goes into A2. I could add a negative supply to extend this, however it is alot of trouble to add yet another power supply to the amps. I happen to have a couple 200H/40mA chokes which I could put in the tail of the source follower that (hopefully) should allow a larger negative Ac swing, supplemented by a series resistor to drop the 85VDC at the current I want to operate the FET. I will also add a CCS to the top of my driver (triode strapped 7722) and increase the C+ from the current 190V (feeding the interstage transformer) to around 300V (easy to do, since I will be converting a choke input to a cap input supply with the addition of an small input cap). This should provide 210 Volts on the FET, with lots of positive swing headroom.

That is my plan. Any feedback on the choke assisted source follower idea would be appreciated.

David


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 Post subject: Re: GM-70 Amplifier
PostPosted: April 27th, 2016, 10:49 am 
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OR -- you could do what Dennis Had did with the Cary CAD-805C and "jump over" the inter-stage transformer with a high quality 2-ufd cap. with a smaller bypass.

It definitely works great as the sound is exceptional. When the 845 output tube's grid starts to go positive with increasing signal the transformer/cap enables the 300B to drive the 845 past that point.

I expect David Berning could explain it better -- but that is a consideration.

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 Post subject: Re: GM-70 Amplifier
PostPosted: April 27th, 2016, 11:47 am 
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SoundMods wrote:
OR -- you could do what Dennis Had did with the Cary CAD-805C and "jump over" the inter-stage transformer with a high quality 2-ufd cap. with a smaller bypass.

It definitely works great as the sound is exceptional. When the 845 output tube's grid starts to go positive with increasing signal the transformer/cap enables the 300B to drive the 845 past that point.

I expect David Berning could explain it better -- but that is a consideration.


The bifilar wound interstage I am using already does that to a certain extent, since there is close capacitive coupling between the primary and secondary windings. Not sure exactly what would happen during the transition to A2, thinking that the inductive coupling prevents the blockage effect across the capacitor when the output tube pulls grid current. So the circuit acts as a cap coupled circuit until then. However, the nice thing about using a FET source follower is the low impedance and drive power, where I have a higher impedance and more limited drive with the 7722 tube.

Still, a good though, and something I could try pretty easily.

David


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 Post subject: Re: GM-70 Amplifier
PostPosted: April 27th, 2016, 3:13 pm 
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I like to follow the I.B.M.practice by using the K.I.S.S. method. The more complex something gets -- there is that much more that can go wrong. :doh:

Besides, based on my experience with these amplifiers, it would seem to me that Dennis Had did his homework regarding what was needed to drive the 845s.

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 Post subject: Re: GM-70 Amplifier
PostPosted: April 28th, 2016, 9:01 am 
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Location: Potomac, MD
Dave,

I think your idea of a choke-assisted current sink for the source of the source follower is a good one. I can't think of any downside except more weight. But if you remove the coupling transformer you may come out even. Again, you want to use the smallest chip FET you can find. Chip size increases with higher rated voltage and higher rated current. Modern FETs avalanche harmlessly if their drain-to-source voltage capability is exceeded. But don't do that gate to source. Use a zener clamp here.

Dave


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 Post subject: Re: GM-70 Amplifier
PostPosted: April 28th, 2016, 10:26 am 
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The choke I will be adding is smaller than the transformer, so the chassis will lighten by a pound or two. Unless I just use the interstage transformer as my choke. :idea: The 5K:5K unit has 44H of inductance, and perhaps I can connect both windings in series for increased inductance (double?).

David


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 Post subject: Re: GM-70 Amplifier
PostPosted: April 28th, 2016, 10:39 am 
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I'm not an iron guy but most transformers do not do well as DC chokes due to saturation. David could give a more informed answer.

Tom


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 Post subject: Re: GM-70 Amplifier
PostPosted: April 28th, 2016, 10:58 am 
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But this will be an AC choke. The transformer is already rated for 30mA in normal service as an interstage. I am just connecting it to the source of the MOSFET, with an added resistor for the DC drop need. Not planning on running alot of idle current through the FET, maybe 20mA or so, therefore it should keep within its rating.

David


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 Post subject: Re: GM-70 Amplifier
PostPosted: May 1st, 2016, 9:03 am 
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Location: Potomac, MD
David,

You need an air gap for your choke function. Unless the interstage tranny has such, it will not do. Also, you need to cap couple to the MOSFET so that the gate can swing below ground reference or there is no point in having the source going negative. Probably easier and better to add a negative supply. As far as the source of the MOSFET is concerned it does not have to be very clean as the source-follower impedance is so low.

Dave


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