Tube input and mosfet output

[Stormo]

Hello, i have been playing around with this combination - just for the fun of it - sweet tube sound and power - is it worth building ?

Do you have any comments ?

Kind regards

[anatech]

Hi Stormo,
I don't think that will perform well. The impedance faced by the driver tube is too low. I don't understand why the 220K resistor is there either, it forms a voltage divider and lowers the impedance still more. You need to swing 70 V peak to drive this. A 10uF coupling cap will be expensive and can be reduced if you get the impedance up to a reasonable level.

My feeling is that you should avoid parallel tube sections. Possibly use one 12AX7 section for each channel and a follower to drive the output stage. A classic choice for that would be a 12AT7, others will have an opinion on that. Your output stage needs work too. Your drivers are operating in class B, could be argued class C. There will be crossover distortion from the driver stage. Add gate stopper resistors to the output FET's.

It will make noise as it sits. I don't know that you would like the noise.

-Chris

[Bas Horneman]

I can't comment on the mosfet schematic.

However generally speaking :
The 12ax7 is bound to struggle driving the input capacitance of the mosfets.

If possible you should try to use something like an ECC99/6N6P/5687 or similar.

I know a guy who is helping his buddy build something like that. Maybe he could give you a couple of pointers. He posts onder 6h5c. http://www.diyaudio.com/forums/membe...o&userid=14574

Regards,
Bas

[ilimzn]

I agree with Anatech here...
12AT7 would give you approximately the right gain with littel or no feedback, and a lot less output impedance.
You can make things better by converting the first NPN into a follower configuration, terminated with a bootstrap current source (essentially, change places of BJT and bootstrap). This will increase your output stage's input impedance a LOT. You need to bias the base of this BJT to approx. 0V plus one diode drop, easily done with a pair of resistors and trim pot (although there are better ways, you may want to try using a Baxandal pair instead of a sole BJT).
The bias for the output stage is insufficient. It needs two diodes to compensate for two BE junctions, but in order to make this thermally stable, you need to include some emitter resistors. In particular, split your uotput BJT/MOSFET CFPs into lower and upper, then add two resistors in series (about 0.1-0.15 ohm should do), connect output to center point. The two diodes should be carefully chosen (do not use 1N4148 and the like as their forward drop is too high) and in good thermal contact with the driver BJTs. You may even use diode conencted BJTs of thes ame kind as the drivers, and mount them one on the other (keep in mind you need to isolate the metal tabs frome ach other!).
Your output MOSFETs are not chosen too well, I'm afraid. For starters, IRF9540 and 640 make a better pair. 540 and 9640 have well over 2x difference in gm! IRFI parts are not suitable for outputs although their isolated case may be very attractive - the thermal resistance to heatsink is too high for this application.

[Stormo]

Wow, thanks alot.....

Im already working :-)

Kind regards

[Miles Prower]

All you have there is a rather conventional SS amp with a glassFET front end. I'm not too sure that'll make a difference, and probably won't work as well as a standard SS front end.

Here is a unique approach to a hybrid VT/SS amp.

[chrisfound]

Hi
Several problems on first glance, but an initial word.

Valves are voltage drive, transistors are current drive, try not to mix and match.

Move the 220K resistor from valve anodes to ground to inbetween the 10uF cap & 1K and ground, you are dragging the anode down.

Biasing the first transistor with resistors from the + and - rails, not good, you cannot cancel the noise or ripple, find altermative method that is clean and free from nasties.

Cascode output stages are very unstable in terms of biasing and are prone to oscillate, you can use a single pair of mosfets to drive a speaker far more effectively and remember the first statement about drive.

IRF540 and 9640, bad mix, the RDS-on is totally wrong, match the fets for the same RDS or close to, also make sure the VDS is high enough.

I would be more inclined to use a mosfet to drive the output stage rather than transistors, simple and more effective, three mosfets and a single valve and you have more than enough power drive + matching the fets and you have 40W easy.

Lastly, HT, you do not need this much, 200V maximum maybe even less than this, you only need to swing 10V rms clean with an output stage working properly.

With all amplifier designs, the simpler it is the more transparent.

Have fun

Chris

[anatech]

Hi Chris,
Actually, there are a few hybrid amps out there using bipolars and tube front ends. They work fine. Most of the distortion and problems come from the tube section.

Bipolar outputs are more linear and have less distortion than mosfet outputs. I don't know if this applies to lateral mosfets. It is not very hard to make the current amp section a high enough impedance for the tube to drive.

If the gain stage = 1, then you need about 50V peak of drive (for +-25 V supplies). The high voltage on the tube section allows a more linear curve to be followed.

-Chris

[ilimzn]

Originally posted by chrisfound
IRF540 and 9640, bad mix, the RDS-on is totally wrong, match the fets for the same RDS or close to, also make sure the VDS is high enough.

Definitely a bad mix but Rdson is not t he parameter you should be looking at, although it is correlated with the one you do need to look at.
Although CFP makes the circuit less susceptible to it, the parameter you need to match is gm (Id/Vgs relationship), just take care to match by curves, not by the typical spec number as this is often taken under different conditions for different MOSFET types, so you cannot compare directly.
Matched gm (also often marked S) also, to some extent, means matched Rdson and even total Cin, but will give you very different maximum Vds, typically 2x higher for the N-type device. Not particulairly important, as long as it's well over the span between the power rails.
I do agree that CFP tends to be problematic, be especially careful with inductive 'Re' (putting this in quotes as it's reallt the emitter degeneration resistor for the equivalent transistor formed by the BJT and MOSFET CFP combo). Mosfet Cgs and lower gm makes it a bit less susceptible to oscillation, but the BJT collector resistor has to be chosen carefully.

[Stormo]

Hello everyone and thanks for your comments :-)

Some changes have been made :

* Still parallel 12ax7 tube input
* old 220 K resistor to ground from anode is removed
* 1.st bjt is now a follower at 8.5 mA, still biased from the rail
* a Vbe multiplier has been added, holding Mosfets at 120 mA
* driver stage removed
* gate stoppers added for stability
* emittor resistors added for stability

Next step is building :-))