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Old 11th June 2009, 08:37 AM   #11
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Quote:
Originally posted by revintage
Ciao Emanuele,

What are the tube types in your amp?

The CVS on top of the input tube has just been discussed and found interesting as it mimics a anode choke at the same time as it has a fixed voltage. It will probably not compromise your top end. Though I wonder about the 1u cap, resistor and potentiometer from anode to ground, some kind of hum reduction?

The cap, resistor and potentiometer look like an attempt to improve the low frequency response.
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Old 11th June 2009, 02:12 PM   #12
DougL is offline DougL  United States
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Looking at the circuit, the input / driver looks like its a mu-follower with the required FET bias circuit. See "Valve Amplifiers" P 115 to P 120 by Morgan Jones. (Highly recommended book )

Without knowing the tubes (valves) involved, its impossible to comment further on circuit values.

I agree this is a good design. The design choices all point to someone who understands what he is trying to accomplish.

To me, if you like the sound, leave it alone.

Doug
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Old 11th June 2009, 04:01 PM   #13
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Hi Emanuele,

Congratulations on getting this far along!

The circuit topology has no problem I can see, and I do like the
MOSFET driver arrangement: Constant Voltage Mu-Follower.

Having built this driver circuit and tested it, I agree w/Lars that there is
no HF extension problem with the MOSFET. It's gate capacitance is
bootstrapped by the 1K resistor, i.e. when the MOSFET conduction
increases, it's own source terminal helps to drive it's gate. So the
extra capacitance seen by the driver tube is only a small fraction of
the Cgs. I still think it might be slightly improved by using a cascode
instead of a single MOSFET here but I haven't tried it yet.

I also like the cathode regulator circuit in the output. It looks like the
220K/1W feeding the 5.1V zener may be a way to compensate in the
cathode voltage for line voltage changes? I have done something
similar but without the B+ compensation. This provides the stiffness
of fixed bias using a cathode circuit.

What happens when you turn the 22K potentiometer on the driver
anode? That will definitely impact the HF response. Maybe try without
the anode shunt circuit. It looks more appropriate for a pentode, but
maybe you're using a very high Rp driver...

Cheers,

Michael

PS The circuit could easily be adapted for DC drive by increasing the
cathode voltage on the output tube and providing an adjustable
driver anode voltage control for bias setting in place if the 200V zener.
The B+ would need to increase as well. If you match the driver
anode voltage to the output grid... I'm not recommending you do this,
just pointing out the possibility... It would enable a little (or a lot)
of class A2 operation...
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Old 11th June 2009, 05:56 PM   #14
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Why the P-CH MOSFET, Bipolar, and Zenier under the Cathode?

If you used N-Channel, you could put the gate directly to the
wiper on the 10K pot. Toss those other needless parts.
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Old 11th June 2009, 06:50 PM   #15
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About the overcomplicated cathode-bias circuit:
Go fixed-bias and get rid of all the junk under the cathode. Would make everything a lot simpler.
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