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Old 12th March 2019, 09:28 PM   #11
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by stocktrader200 View Post
Hi Goatguy, I used the MJE13007 as a current booster following the tube so that the tube characteristics dominate. the schematic is here. 3 (repl \ with slash) x 3 Tube amp 30 watts
What a remarkable circuit.

After as many decades as I've been designing topologies from the mundane to the opaquely clever, I don't think I'd've come up with that little gem-in-the-finals.

Really nice idea.

I bet there is a fair bit of sensitivity choosing the resistor above the 1N4700 (or whatever it is) diode to get linear-over-signal-range transistor response. Not a place-holder that!

How say you again, regarding its dynamics? I like it.

Just saying,
GoatGuy
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Old 13th March 2019, 12:47 AM   #12
stocktrader200 is offline stocktrader200  Canada
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Thanks Goatguy. I have not seen that implementation before or since I posted the circuit.
The 2.2k ensures that the MJE13007 remains fast. Frequency response extends well past 50khz flat. The 6922 cannot drive the transistor into saturation thereby keeping it in the linear range. This amp is essentially a power 6922 with ( up to ) 1A of current and 50w plate dissapation.
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Old 13th March 2019, 09:24 AM   #13
tikiroo is offline tikiroo  New Zealand
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Attached is the output stage of my latest amp. It is similar topology to Stocktrader's (it's another Broskie idea) but biasing is more complex with the mosfet instead of BJT. The npn acts as a bias servo for the mosfet as well as a CCS for the pentode. The clipper LED allows servo operation into class AB. The pentode needs ~ -11V on its grid (allows trimming of current), or a larger value cathode resistor to get the voltages right for the mosfet. Feedback to the screen makes the pentode behave like a triode, without the plate current passing through the output transformer.

The valve/mosfet arrangement acts like a triode with gm of 150 mA/V, rp of 50 ohms and mu of 7.5. See attached spice generated curves and approximate loadline with 400:8 output transformer (a 30V mains toroid). It's good for more than 50 watts into 8 ohms. I can't fault the sound or measurements, only hassle is the heatsinks.

Sorry for the threadjacking Fenris, but you did mention "a crazy mashup of JFETs, transformers, tubes, and high voltage MOSFETs"
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File Type: png Supertriode PP output stage.png (34.2 KB, 210 views)
File Type: png ECL85+2SK3746.png (51.8 KB, 184 views)
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Old 13th March 2019, 10:04 AM   #14
kokoriantz is offline kokoriantz  Lebanon
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Originally Posted by Ketje View Post
Any voltage on the grid of the 6N1P has practically no effect on the anode voltage, the current is in the triode determined by the fet and so is the voltage drop on R6.
Only the cathode voltage = drain voltage fet is fixed by the grid voltage.
But yes, there is a very small change.As the Vag changes the -Vg changes also since the current is fixed.
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On simulator at 4ma the j177 has drain impedance of about 30k . This provokes a NFB of 6db .
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Old 13th March 2019, 11:22 AM   #15
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by stocktrader200 View Post
Thanks Goatguy. I have not seen that implementation before or since I posted the circuit.
You mean the supertriode?
SuperTriode
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Old 13th March 2019, 12:59 PM   #16
Fenris is offline Fenris  United States
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Originally Posted by tikiroo View Post
Sorry for the threadjacking Fenris, but you did mention "a crazy mashup of JFETs, transformers, tubes, and high voltage MOSFETs"
Actually tikiroo, that's almost exactly the same topology that I'm working on, except using a 6J11P (6688, EF180) and independent constant current sources for the tube and mosfet.
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Old 13th March 2019, 03:47 PM   #17
stocktrader200 is offline stocktrader200  Canada
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Tikiroo, Your circuit will behave differently as the plate is connected to B+ and the output is driven from the mosfet and the screen (upper) grid.
I hadn't seen this done with a BJT, Broskie's supertriode setup has the mosfet transistor characteristics equal to if not dominate the triode characteristics.
A mosfet is a crappy substitute in my circuit

Last edited by stocktrader200; 13th March 2019 at 03:55 PM.
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Old 13th March 2019, 04:06 PM   #18
Fenris is offline Fenris  United States
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The output isn't driven by the screen grid (it's only a tiny fraction of the total current), the output drives the voltage on the screen grid which acts as a feedback mechanism to the pentode.
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Old 15th March 2019, 07:26 PM   #19
stocktrader200 is offline stocktrader200  Canada
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Tikiroo or Fenris, what is the effect on plate current using the screen grid?
for instance, it is not possible to drive the transistor into saturation because the tube cannot pass 10ma with less then 25 volts plate to cathode. I am assuming your circuit can drive the transistor right to the rails

Last edited by stocktrader200; 15th March 2019 at 07:33 PM.
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Old 16th March 2019, 07:53 AM   #20
tikiroo is offline tikiroo  New Zealand
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Hi Stocktrader. The mosfet can't be driven to the rails, the valve/mosfet combo clips with onset of grid current, just like a normal output valve (without cathode or source follower driving it). The plate current remains constant by design. With positive going grid voltage there is a positive going cathode voltage which increases mosfet current and results in a negative going drain/screen voltage. The AC amplitude at the cathode is much less than the AC amplitude at the grid due to the feedback through the screen. See attachment which shows conditions at clipping.

Actually there is little difference in performance with a real triode or the pentode connected as a triode (plate+screen connected to mosfet drain) or with only the screen connected to mosfet drain. With a triode, the voltage feedback is through the plate rather than the screen. The advantage of using the screen for feedback is that most of the constant current through the valve is coming directly from B+ rather than going through the output transformer where it is not doing anything useful.
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