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Old 30th October 2011, 09:37 AM   #11
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by Defiant View Post
Not that great. Any uncompensated semiconductor CCS is going to drift all over with temp

Try a setup like the one I attached. Rcs sets the current through the tube, while Rb sets the bias, and hence the quiescent voltage across the tube. D1 compensates against temp changes, thus keeping the current relatively stable over temp. Do NOT use the D45H11 - I just picked it for an example. It doesnt have the voltage rating for this circuit. Pick a transistor with Vce equivalent to your supply voltage and with an hfe as high as possible (but NOT a darlington!).

You notice that I used a resistor in the cathode circuit rather than LED's. The reason is that diodes are essentially a fixed bias, which will make the circuit sensitive to the characteristics of the tube. This is a Bad Thing™. You want the circuit to be as insensitive as possible to variations between individual tubes (and semiconductors). That is why a resistor is used to set the bias, as well as in the emitter circuit of Q1 to set the current (rather than using a divider in the base circuit). Also, depending on the characteristics of the diode and transistor there may not be enough voltage across D1 to forward bias Q1. If that happens, add some resistance between D1 and the base of Q1 . Do not add another diode or a transistor with the base tied to the emitter of Q1 like you see in a lot of SS circuits as it will screw up the temp balance. You can replace D1 with another transistor identical to Q2 with it's base tied to it's collector. In that case the emitter goes to B+ and the collector to the junction of Q1's base and R2. Provided both transistors come from the same wafer lot it will give you the best thermal tracking.
Very interesting comments. The gyrator in theory tackles the variation of valves and aging by setting a fixed voltage, not like the CCS which sets a fixed current.
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Old 30th October 2011, 10:28 AM   #12
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I think that the 1uF is a good idea. The LF pole is formed from the M-ohm level gate resistors against the cap.

The 1uF is useful:

1. because the LF response can be lower, minimizing the risk of phase changes accumulating across all the stages in your system. This can be an advantage over the choke.

2. The 11M resistor is potentially too high. I know it simulates OK, but the gate leakage from a MOSFET can be as high as 1uA.... probably won't be but the high impedance can be a problem for other reasons. Anatolij used 4,3M elsewhere, and that seems safer to me.
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Old 30th October 2011, 10:41 AM   #13
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Makes sense then to reduce it. See simulation attached. I guess that you would prefer a MOSFET with lower input capacitance as this will be in parallel with the capacitor and influence the frequency response?

With a 4.7Meg resistor and 1uF capacitor the response is nearly equivalent to a 500H choke! Nice...

Thanks
Ale
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File Type: png 6j5 gyrator 3 response.png (33.6 KB, 339 views)
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Old 30th October 2011, 10:57 AM   #14
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Maybe Anatolij (Wavebourn) is on holiday - I am surprised he has not appeared, and explained (and defended) his circuit.

It's pretty neat, actually. I hope he won't mind if I try to explain.

I think it is easier to understand its workings if we look at his Губернатор-7 (Gubernator-7) design. This is a robust design for Russian tubes 6Ж5П - 4П1Л (6J5 to 4P1L) SE power amp. It tolerates very well the wide variation between samples of the 6Ж5П - a useful feature for any amp.

The 6Ж5П stage uses the Gyrator circuit to automatically find an operating point. The anode voltage programmes the gyrator current (dc-only) through R2-R3. Meanwhile the cathode circuit is fixed-bias (~1,5V) - but this does not prevent the circuit from self-stabilising. You could pull out the 6Ж5П and use another one (with VERY different Ia-Vg1 characteristics) and the Gyrator will find a suitable operating point. The same process compensates for aging in the 6Ж5П - say for instance the anode current falls over time - the gyrator will increase the dc anode voltage to compensate.

At the same time, the fixed bias in the cathode eliminates the cathode capacitor - a well-known weakness in any design.

This is a very well designed circuit that gives EXCELLENT performance, perfectly stable dc operating conditions, high stage gain - and yet all the components are dirt cheap.

The version of Gyrator you are working with is cascoded, too, which may increase the performance (and power supply rejection) even further.
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Old 30th October 2011, 11:20 AM   #15
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I guess that you would prefer a MOSFET with lower input capacitance as this will be in parallel with the capacitor and influence the frequency response?
Hi Ale,

It's worth choosing the P-FET carefully, because the drain-source capacitance [Coss] varies very much. according to the size of the FET. Low capacitance means better HF power supply rejection, and which amp could afford to turn down such an offer?

The 33V cascode diode can also help. When you choose your FET, look at the Coss Vs. Vds graph. Then make a 3-way tradeoff between FET dissipation (big FET needed), low 30V capacitance (small FET needed) and the anode-voltage swing required (low diode voltage needed).

Maybe start by looking at the Gubernator's BSP225, and compare other FETs to that.
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Old 30th October 2011, 11:31 AM   #16
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Ok, the BSP225 has a Coss of 20pF whilst the FQP3P50 has 70pF! The big difference is on the CiSS (65pF against 510pF of the latter). How does the CiSS impacts in this circuit? Is it shunt capacitance in parallel with the 1uF capacitor?

When you refered to the diode characteristics, did you mean looking for a zener with low capacitance, correct?

I guess that if I want to use the BSP225 will need to learn how to solder an SMD device!
cheers,
Ale
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Old 30th October 2011, 11:36 AM   #17
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Coss of 20pF (please check what voltage - often 25V) - that's hard to beat.

Ciss is no worry, it's in parallel to the 1uF, as you noted.

I was thinking only about changing the diode voltage - sometimes there's a sharp improvement in Coss for a small increase in Vds.

SMD soldering is a piece of cake, provided you get some solder paste (Edsyn syringe, from Farnell)
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Old 30th October 2011, 05:24 PM   #18
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Yes, above 20-25V it goes down to 20pF. With current circuit the VDS is around 25V so should be optimal from that point of view.

Will buy some soldering paste for SMD and adventure in that quest :-)
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Old 30th October 2011, 05:48 PM   #19
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Voltage Drift of the MOSFET scheme I used is not a probem....

Since it was mentioned, I tested. From stone-cold to fully hot (after 2 hours and tested 4 different CCS Stages in same amp)

--Voltage 'drift' was less than 2V at MOSFET....
--I think that was more mains supply variation than anything to do with the semi-device!

So, better than 1%

Voltage-drift isnt an issue.
--A personal thing--I DONT like Bi-Polar transistors!

Here is the idea I used, check out near bottom of paper--

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Old 30th October 2011, 08:11 PM   #20
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Quote:
Originally Posted by Rod Coleman View Post
Maybe Anatolij (Wavebourn) is on holiday - I am surprised he has not appeared, and explained (and defended) his circuit.

It's pretty neat, actually. I hope he won't mind if I try to explain.
A CCS by any other name still does the same thing And the use of a fixed bias will still make the circuit sensitive to variations between tubes.
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