Haven't indulged in AB2 much, but I'm about to and have some basic questions.
It is generally advised to increase driver current capability (no surprise there), but I find no data. Where do you find the grid impedance curve in that region? How do you know if a CF is enough or you need a 3A source follower?
Is it trial-and-error or just ancient Chinese secrets passed from father to son?
It is generally advised to increase driver current capability (no surprise there), but I find no data. Where do you find the grid impedance curve in that region? How do you know if a CF is enough or you need a 3A source follower?
Is it trial-and-error or just ancient Chinese secrets passed from father to son?
Ancient secret. Will be revealed by year's end. But there's a few datasheets that are nice enough to spill the beans.
If you can pump 10 mA comfortably, you can drive 6L6GC (a really excellent tube for this service) well into AB2. I've used MOSFET source followers (IRF820) for this- if you want to be cute, the source followers don't have to run class A...
If you can pump 10 mA comfortably, you can drive 6L6GC (a really excellent tube for this service) well into AB2. I've used MOSFET source followers (IRF820) for this- if you want to be cute, the source followers don't have to run class A...
Thanks Sy. Is that December 31st or Chinese year's end? Or perhaps Rosh Hashana?
Well, it so happens that I have a nice Williamson with 6L6GCs driven by 6SN7 diff pair that might just be able to do that without any followers. Do you think?
That's the frustrating part for us neophites. Should I try this or that? We need a trade union with apprenticeships so we can pass these secrets down...
Well, it so happens that I have a nice Williamson with 6L6GCs driven by 6SN7 diff pair that might just be able to do that without any followers. Do you think?
That's the frustrating part for us neophites. Should I try this or that? We need a trade union with apprenticeships so we can pass these secrets down...
December 31.
No, you want followers to get AB2 with any kind of acceptable distortion. It could also possibly be done with a transformer at much greater expense and somewhat lower performance; ditto using drive from the plate of a power tube. Followers are easy, help prevent blocking, and can have impeccable performance. There have been some commercial designs (e.g., Fisher 50A) that used transformer-coupled followers and performed quite well.
No, you want followers to get AB2 with any kind of acceptable distortion. It could also possibly be done with a transformer at much greater expense and somewhat lower performance; ditto using drive from the plate of a power tube. Followers are easy, help prevent blocking, and can have impeccable performance. There have been some commercial designs (e.g., Fisher 50A) that used transformer-coupled followers and performed quite well.
Sadly no, because the grid current will charge up the coupling capacitor, which will throw the bias out, causing blocking distortion
It may be specified in some big transmitting tubes, but I made some measurements. A 6L6GC will draw 10 mA or more if really hammered. An 845 can hit 50 mA in normal operation, and 100 mA when the Big Dumb Blonde One plugs his guitar into it and sets it on eleven. I have seen nearly 250 mA peaks on the 833A (guitar again). Sweep tubes eat lots of grid current, hundreds of mA under the right conditions.
If you have a dual trace scope with the ability to subtract the traces, put a 100 ohm resistor in series with the grid and stick a probe on each side. Have the scope subtract the two traces and you will see the voltage drop across the 100 ohm resistor. Compute the current. Small tubes may need a 1K resistor but keep the voltage drop well below a volt. The grid current is proportional to the grid voltage, and inversely proportional to plate and screen voltage. The grid can draw big current in a triode as the plate voltage approaches zero.
In an SE amp or class A P-P amp the followers must be returned to a negative supply that is greater (more negative) that the anticipated bias range and the peak signal voltage. Example, if your bias can adjust from -25 to -50 volts and you have 100 volts peak to peak maximum drive (50 volts peak) the follower must be returned to a negative voltage of -100 volts or more. In reality you need a bit of headroom so I tend to use -150 volts.
Technically this doesn't need to be true in push pull applications since once the negative going tube is cut off you don't need to keep pulling its grid more negative.
In reality the mosfet / tube intreraction may tend to oscillate for a short burst as the mosfet goes in or out of cutoff. This can be particularly nasty to tame, and sounds bad, so I tend to keep a sand state follower in class A and even use a CCS under it sometimes.
For a simple mosfet follower supply I use a small transformer with two 120 volt secondaries. Wire them in series to make a +/- 150 volt supply and wire the mosfet's drain to +150 and the source resistor (or CCS) to the -150 volt supply. The Triad N-68X works good if you are using NA 120 volt line voltage. A small isolation transformer or toroid with 4 120 volt windings can be used world wide.
Below is a simple example of a driver suitable for AB2-class operation in class EL84 and similar tubes. I made my prototype with Russian 6P9.
So even ordinary silicon transistors are sufficient and required negative rail voltage is only 30 V. This example worked very well and especially well with triode connected output tubes. Then the power increase was huge compared without emitter followers.
So even ordinary silicon transistors are sufficient and required negative rail voltage is only 30 V. This example worked very well and especially well with triode connected output tubes. Then the power increase was huge compared without emitter followers.
An externally hosted image should be here but it was not working when we last tested it.
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