Phase Inverters with CCS

[R.G.]

Good point. You never want to starve that CCS for voltage.

Since there are input DC blocking caps and a good 300V of plate supply, the simplest thing might be to just bias both grids up at about 50-75V.

There are variants of CCS circuits that would let you run the CCS with very, very small reference currents. Using a MOSFET for the series pass device and then using a bipolar to "steal" gate voltage from the MOSFET based on the voltage across a MOSFET source resistor comes to mind.

This setup would probably eliminate the need for a negative supply and the issues with input voltage size.

[Mike Sulzer]

Avoid fancy circuits in guitar amps. The guitarist always finds some way to make them misbehave. So if you want a current source, build a negative supply like your positive supply with a pair of diodes, and a couple of Cs and Rs. (You do not need that much current.) Then use a 100K resistor (or whatever your specific circuit requires) for the CCS. 100K is large compared to the inverse of the gm, and so the balance is good. Hard to make that circuit misbehave!

[uneumann]

Avoid fancy circuits in guitar amps. The guitarist always finds some way to make them misbehave. So if you want a current source, build a negative supply like your positive supply with a pair of diodes, and a couple of Cs and Rs. (You do not need that much current.) Then use a 100K resistor (or whatever your specific circuit requires) for the CCS. 100K is large compared to the inverse of the gm, and so the balance is good. Hard to make that circuit misbehave!

Not to be dismissive ... but building another power supply sounds "fancy" to me.
Just how would you make a CCS "misbehave" if it's properly designed and built as some of the examples show?

[Roberto]

Thanks uneumann!

Thanks E.G.,
Is this configuration better than a single 75 V zener instead of the 1n4148s, and then set the current with Re as before?
EG for 7,5 mA would be around 10k.

[Roberto]

Hi Mike,

Mesa Boogie 295 and 395 had this configuration, Coliseums as well.
Marshall 9100, 9200 and relatives had this configuration.

Tons of guitarists have enjoyed and still enjoy the CCS PI with no issues at all.
I have a Marshall 9200 and the Mesa 295 is the best sounding power amp I've ever had (except mines! ).

On top of that, the guitarist will be me.

Not to be dismissive too, but I hope you don't feel outraged if I consider your post as the answer to the question:
"you know the difference between a guitar and a piano?"

[catalin gramada]

Avoid fancy circuits in guitar amps. The guitarist always finds some way to make them misbehave. So if you want a current source, build a negative supply like your positive supply with a pair of diodes, and a couple of Cs and Rs. (You do not need that much current.) Then use a 100K resistor (or whatever your specific circuit requires) for the CCS. 100K is large compared to the inverse of the gm, and so the balance is good. Hard to make that circuit misbehave!

Add some remarks: negative supply should be dimensioned by nominal current draw by cathodes and also the cathode resistor should support the power dissipation. In respect with current draw sometime can be pretty cumbersome. Also the supply should have very low ripple to not modulate the signal. Othervise it works pretty well in some project I did.
And a question: Does it a danger to keep voltage over max. hk rating whilst the tube is heated and start to draw current, please? I don.t know how "healthy" is to keep -400v in cathode (I exagerated) expecting to drop whilst tube is heating. How much voltage should consider safe in respect with hk rating from this point ? If is a limit then tail resistor is limited by max. voltage admited by hk isolation aso. Not when running but when is cold instead. Thanks

[catalin gramada]

Hi Mike,

Mesa Boogie 295 and 395 had this configuration, Coliseums as well.

Mesa used negative power supply without regulator in 400 series as well, best amps ever heard by others..
Does it a guitar or a piano?

[Roberto]

well, the 400 series is more FORTISSIMO than piano

[Mike Sulzer]

I apologize for my rude remark.

cg: I have had no trouble with the brief over voltage, but I do not know why. I think there could be a problem; there just never has been!

un: I do not think of the negative power supply as fancy. It is an additional simple circuit with components that interact with each other, but not so much with the components in the signal path circuit. It allows making the signal path circuit simpler.

Perhaps I am misusing my experience as a designer of complicated signal processing software where you must modularize, then isolate and simplify the modules. It seems that the same rules work here.

Roberto: Yes. A guitar is simple enough so that you can redesign it to achieve specific goals in a short enough time so that you can do a lot in a life time. A piano has so many interactions that even if you just try to copy a good design, you might end up with something very different. It is difficult to achieve much directed piano redesign in a life time.

[uneumann]

I apologize for my rude remark.

cg: I have had no trouble with the brief over voltage, but I do not know why. I think there could be a problem; there just never has been!

un: I do not think of the negative power supply as fancy. It is an additional simple circuit with components that interact with each other, but not so much with the components in the signal path circuit. It allows making the signal path circuit simpler.

Perhaps I am misusing my experience as a designer of complicated signal processing software where you must modularize, then isolate and simplify the modules. It seems that the same rules work here.

Roberto: Yes. A guitar is simple enough so that you can redesign it to achieve specific goals in a short enough time so that you can do a lot in a life time. A piano has so many interactions that even if you just try to copy a good design, you might end up with something very different. It is difficult to achieve much directed piano redesign in a life time.

No problem... I think you did make a valid point - which is that there are several ways to achieve a CCS. The "best" way for a given situation is often a matter of opinion and preference. I don't see any option we've discussed here as *always* better or worse. It depends on context too.
That's part of the "art" part of circuit design -- considering all the options and arriving at an outcome that works (and sounds) satisfying.

[Gingertube]

Looking into the cathodes of the PI you see a very low impedance, the CCS represents a very high impedance. Any residual negative rail ripple and noise will be divided by those impedances so the negative rail can be very simple.
If using the negative rail for a bias supply for the output tubes then the rail needs to be more sophisticated and cleaner.
Can you have the best of both worlds?
YES.
Run mosfet source followers AC coupled off the PI with output tube grids direct coupled to the mosfet sources (just the grid stop in between). Apply the bias (with additional filtering) to the mosfet gates AND use a current source load on the mosfet source followers back to the negative rail.
Negative rail needs to be at least X3 the output tube bias voltage to guarantee drive to cutoff.
I've done this with HIFi Amps but not tried it for guitar.
Post #602 for schematic here:
EL84 Amp - Baby Huey - Page 61 - diyAudio
The feedback scheme is a bit different too.
Cheers,
Ian

[Roberto]

Thanks Gingertube,

I've seen the schematic: diyAudio
I will work on that. Maybe not for the Marshall 9200 (it would be complicated to implement it on the already crowded layout of that amp), but for the future.

I also like the idea of a EL84 HiFi amp for the future!

[catalin gramada]

Looking into the cathodes of the PI you see a very low impedance, the CCS represents a very high impedance. Any residual negative rail ripple and noise will be divided by those impedances so the negative rail can be very simple.
If using the negative rail for a bias supply for the output tubes then the rail needs to be more sophisticated and cleaner.
Can you have the best of both worlds?
YES.
Run mosfet source followers AC coupled off the PI with output tube grids direct coupled to the mosfet sources (just the grid stop in between). Apply the bias (with additional filtering) to the mosfet gates AND use a current source load on the mosfet source followers back to the negative rail.
Negative rail needs to be at least X3 the output tube bias voltage to guarantee drive to cutoff.
I've done this with HIFi Amps but not tried it for guitar.
Post #602 for schematic here:
EL84 Amp - Baby Huey - Page 61 - diyAudio
The feedback scheme is a bit different too.
Cheers,
Ian

Hey Gingertube. I used negative power suply to force a differential power stage driver into the balance. It works well till a certain signal amount when clipping put this stage into intermitent operation. Does a CCS the same thing, please ? Thanks
Edit: this was never happen for the same amount with individual bypassed cathode resistors, and also my subjective impression it sound more "open, like that

[Roberto]

Like blocking distortion?

[catalin gramada]

Like blocking distortion?

yes