Designing for AB2

[Shea]

Yes, you can DC a Schmitt to two CF's. The plates of a Schmitt splitter are inherently DC-imbalanced, so if you want both AC and DC balance (as I would) you would need to compensate one of the CF Rk values.

Then wouldn't the plates of the Schmitt have to be sitting at the negative voltage needed to bias the power tubes?

Shea

[Ray Ivers]

Shea,

If the output tube cathodes were grounded, then yes, the Schmitt-splitter plates would have to be a few volts more negative than the desired bias voltage so that the CF cathodes would be right where you wanted them. OTOH, if the output cathodes were run at a positive voltage and the PI biased hot w/large Rp values, run with a negative tail voltage and preferably a reduced B+ as well, you might be able to end up with a situation where the PI plates/output grids were somewhere in the region of ground potential, give or take. You could also drive the grids from the junction of a voltage-divider CF Rk setup, sacrificing drive for still more negative DC idle voltage.

On a side note, I've seen many references to the 'lower the water' method of reducing B+, using Zeners, bucking xfmrs, Variacs, etc., but never any references to 'raising the bridge' (so to speak) by biasing the cathodes positive in a fixed-bias amp - probably due to the added complexity of another PS voltage, although IMO it's kind of a cool 'low-voltage route to lower B+' alternative, if that makes sense.

Ray

[Satamax]

Huh, just a daft question, does AB2 cathode bias exists?

[Ray Ivers]

Max,

Good question! You could combine AB2 grid drive with a cathode resistor(s), which would be more for heavy-drive current-limiting than actual bias (although it would contribute to idle bias too). There's also cathode-drive AB2, where the control grids are held positive and the cathode driven by an active device (like in some Music Man amps) but that may not be what you meant.

If you just added ground-referenced AB2 drive to a cathode-bias-only amp, the cathode bias voltage would always be fighting the drive voltage and you would have a harder time actually driving the grids positive than with a fixed-bias design, but it could be done with enough signal swing.

Ray

[Shea]

If you just added ground-referenced AB2 drive to a cathode-bias-only amp, the cathode bias voltage would always be fighting the drive voltage and you would have a harder time actually driving the grids positive than with a fixed-bias design, but it could be done with enough signal swing.

Ray

One of our Ampage friends has told us about bypassing the cathode resistor with a zener to limit the positive travel of the cathode voltage.

And I suppose you could just replace the cathode resistor with a zener, to hold the cathode at a fixed positive voltage.

Shea

[Satamax]

Hi everybody!

Ray and others, sorry for asking questions that i'm not able to understand the replies to ! Shea, using big zenners on the power tubes, huh, that's intresting, i'd think this would be as tight as fixed bias, and easy to implement, in a normal AB1 amp. I don't know about in class AB2, as i need to reread and dig a bit more infos, to have a beter understanding.

Bye.

Max.

[Merlinb]

Simpler version of SVT?

Ok, here's my take on a much simplified version of the SVT cathode follower driver. The LED sets the CF bias, so the CF is automatically biased regardless of how you change the power valves bias.
The only drawback is that the negative bias supply needs to be able to handle the current flowing through the CF (and the other CF for push pull) which could be upto 20mA with ECC82's. If you have a big tranny then it's not a problem. If you aren't using a separate bias winding on the tranny then it would drag the HT down a bit, but not enough to shout about.


(PS, I only guessed at the 47k load on the CF. Obviously a proper loadline would be required, but I don't think I'm way out).

[Arthur B.]

Max,

Good question! You could combine AB2 grid drive with a cathode resistor(s), which would be more for heavy-drive current-limiting than actual bias (although it would contribute to idle bias too). There's also cathode-drive AB2, where the control grids are held positive and the cathode driven by an active device (like in some Music Man amps) but that may not be what you meant.

If you just added ground-referenced AB2 drive to a cathode-bias-only amp, the cathode bias voltage would always be fighting the drive voltage and you would have a harder time actually driving the grids positive than with a fixed-bias design, but it could be done with enough signal swing.

Ray

The inherent NFB created by the cathode resistor would try to correct for the change in bias caused by grid conduction. By the way, I checked out the Music Man schematics. Really interesting design, with the solid state preamp and PI and the grounded grid tube output stage.

[Ray Ivers]

AB2 - Doing the Math

I've been looking at various driver/interstage transformers lately (mostly NOS stuff) and was a bit surprised at the number of 15-or-more-watt driver transformers that had been offered by the various American xfmr makers over the years, many with substantial primary/secondary step-down ratios. I was curious just how much overkill this was for an average guitar-amp pentode or UL P-P output stage, so I did the math - and found that this kind of power really can be needed to drive the grids positive with low distortion, using a driver transformer directly coupled to the output tube grids.

Take a push-pull fixed-bias two-6L6 output stage at 450V B+, using -50V for bias voltage, with a design intent of driving the grids 10 volts positive WRT the (grounded) cathodes - and further assume that a conducting control grid will present roughly a 2K ohm load to the driving stage (my own measurements indicate something more like 3K to 4K in conducting pentode grids, but I'll use 2K as a worst-case scenario).

For low distortion, a common rule-of-thumb is to present a load 10X or more to the preceding stage, to minimize loading-related distortion.

So - the drive stage will need to swing 120V peak-to-peak (to overcome the -50V bias plus drive the grids another 10V positive), at an output impedance of 200 ohms (for low waveform distortion into a 2K load using a transformer, as mentioned above).

This is a substantial amount of power. Here's the calculations I made:

120V x .35 (.5 x 120 x .7, for RMS voltage) = 42
(42 x 42)/200 ohms (Ohm's power law, Esquared/R) = 8.82W

This surprised me - I would have thought a 2W to 3W driver stage would have been sufficient, but almost 9W? For a two-KT88 P-P output stage @ 600Vp/-75Vg/+10V grid drive, the power required is 17.7W!

Sorry to be repetitive, but this is for low distortion (i.e., so that the waveform doesn't immediately flat-top or round off when it transitions into grid conduction) using a transformer - the situation with direct-coupled cathode followers is much different. The actual 6L6-stage power requirement is about 1.8W peak - a 60V peak swing into a 2K load - which can easily be achieved by a beefy 9-pin CF like a 12BH7, etc. Even using a push-pull xfmr drive stage - which I would use anyway - there's no real way I know of to design a transformer-drive stage so that it puts out much more power on waveform peaks than on the rest of the cycle, hence the much greater drive-power requirement.

If the design objective is merely to drive the grids positive, one can design for max power transfer, with a 2K output impedance driving the @ 2K conducting grids, and the power requirement drops by a factor of ten, accompanied by a big increase in distortion during positive grid drive.

Ray

[kg]

Huh, just a daft question, does AB2 cathode bias exists?

no, it does not. grid current and cathode bias do not get along due to the degeneration across the Rk

[kg]

And I suppose you could just replace the cathode resistor with a zener, to hold the cathode at a fixed positive voltage.

ahh, but that's no longer cathode biased..

[Ray Ivers]

Is that really you, big guy...? Jeez, WELCOME BACK!!!

Ray

[Arthur B.]

I've been looking at various driver/interstage transformers lately (mostly NOS stuff) and was a bit surprised at the number of 15-or-more-watt driver transformers that had been offered by the various American xfmr makers over the years, many with substantial primary/secondary step-down ratios. I was curious just how much overkill this was for an average guitar-amp pentode or UL P-P output stage, so I did the math - and found that this kind of power really can be needed to drive the grids positive with low distortion, using a driver transformer directly coupled to the output tube grids.

Take a push-pull fixed-bias two-6L6 output stage at 450V B+, using -50V for bias voltage, with a design intent of driving the grids 10 volts positive WRT the (grounded) cathodes - and further assume that a conducting control grid will present roughly a 2K ohm load to the driving stage (my own measurements indicate something more like 3K to 4K in conducting pentode grids, but I'll use 2K as a worst-case scenario).

For low distortion, a common rule-of-thumb is to present a load 10X or more to the preceding stage, to minimize loading-related distortion.

So - the drive stage will need to swing 120V peak-to-peak (to overcome the -50V bias plus drive the grids another 10V positive), at an output impedance of 200 ohms (for low waveform distortion into a 2K load using a transformer, as mentioned above).

This is a substantial amount of power. Here's the calculations I made:

120V x .35 (.5 x 120 x .7, for RMS voltage) = 42
(42 x 42)/200 ohms (Ohm's power law, Esquared/R) = 8.82W

This surprised me - I would have thought a 2W to 3W driver stage would have been sufficient, but almost 9W? For a two-KT88 P-P output stage @ 600Vp/-75Vg/+10V grid drive, the power required is 17.7W!

Sorry to be repetitive, but this is for low distortion (i.e., so that the waveform doesn't immediately flat-top or round off when it transitions into grid conduction) using a transformer - the situation with direct-coupled cathode followers is much different. The actual 6L6-stage power requirement is about 1.8W peak - a 60V peak swing into a 2K load - which can easily be achieved by a beefy 9-pin CF like a 12BH7, etc. Even using a push-pull xfmr drive stage - which I would use anyway - there's no real way I know of to design a transformer-drive stage so that it puts out much more power on waveform peaks than on the rest of the cycle, hence the much greater drive-power requirement.

If the design objective is merely to drive the grids positive, one can design for max power transfer, with a 2K output impedance driving the @ 2K conducting grids, and the power requirement drops by a factor of ten, accompanied by a big increase in distortion during positive grid drive.

Ray

I think you may have bitten off more than you can chew. Why not start off with preamp triodes, like a 12au7? As for maintaining a high enough impedance for the driver stage, why not use an OT? Sure you'll throw away a lot of voltage gain, but gain is cheap to create.

no, it does not. grid current and cathode bias do not get along due to the degeneration across the Rk

With a cathode resistor, no. But with something stiffer in the cathode, you could do it.

ahh, but that's no longer cathode biased..

Yes it is. It doesn't matter what you stick between the cathode and (AC) ground. You could use a battery to bias (like old tube radios) and it still would be considred cathode biased.

[kg]

With a cathode resistor, no. But with something stifferin the cathode, you could do it.

how do you propose to "stifferin the cathode?"

Yes it is. It doesn't matter what you stick between the cathode and (AC) ground. You could use a battery to bias (like old tube radios) and it still would be considred cathode biased.

umm..

no.

cathode (or self) biasing REQUIRES that a resistor be used.

whether or not you are injecting a bias voltage at the cathode does not matter. if i put a positive voltage at the cathode, via a battery, or a zener, then it is NOT cathode biased any longer... it is FIXED bias. whether you put a -ve voltage on the grid, or a +ve voltage on the cathode, the tube doesn't care.

you MUST have a pure resistance across which degeneration at DC occurs. that is the absolutely key, critical point. it is precisely the reason why max g1 circuit resistance values are HIGHER for cathode biased tubes vs. fixed bias--the tendency for positive grid current, and subsequent runaway is decreased via the DC degeneration occuring across the cathode resistance.

[Ray Ivers]

Arthur,

I think you may have bitten off more than you can chew.

Funny you should use this term... it's exactly the impression I get every time I read one of your "corrections".

Ray