7802 twin triode

[Paul Birkeland]

To run two cathode followers, you need twice the high voltage current and twice the heater current. To do that, you need a second power transformer and another regulated B+ supply.

You can raise the current a bit, but you'll end up making compromises elsewhere that will limit power output, and you'll have to rebalance the shunt regulator for both current and thermal load, which is not a trivial process.

A transformer isn't the end of the world. A pair of 600:60 units of an appropriate size would be small and inexpensive. Four resistors for feedback will be even less expensive and easier to try.

You also should consider that you'll give up a fair amount sonically (in my opinion at least) by using paralleled cathode followers. This isn't a particularly well thought of method by those who have tried it.

If you want to make a cathode follower with no negative feedback and a single cathode flower triode per channel, the 6C33 would be a good place to start. Since you'd be running 150-200mA of high voltage current per channel, shunt regulating will be very difficult, and I would expect such an amp to not compare well to a Crack-a-two-a on higher impedance headphones.

What you would get is an otl amp with an output impedance well below 50 Ohms.

[hpjun]

Could I also bias the Cathode Follower with higher voltage to lower the output impedance? I was thinking of using a tube that runs higher voltage like an e80cc to bias the cathode follower with higher voltage, but I am not sure if its safe to do that or if it sounds good.

6c33 or 6c33c? Looking at the spec sheet and that's two triodes connected in parallel internally. So I would need a pair of 6c33c. The higher current doesn't affect the gain right? cathode followers have a gain less than 1 right? So current lowers impedance by increasing gm.

I think the sensible approach for me would be to increase the current in the cathode follower on my 7802 tube. I don't mind losing gain or power since I can barely turn the pot past 25% before it gets too loud. Not sure how to do this though, is it more complicated than changing a resistor?

[hpjun]

So I think I may be getting high frequency oscillation. Start up has a low buzz that goes away and likely there is out of band, high frequency noise. I don't hear it but I feel my ear stiffen up like my ear doesn't want to hear something an uncomfortable feeling. Also to further confirm I noticed the treble is a bit veiled probably from low level higher harmonic distortion products.

So, I think I'll put a grid stopper resistor. Is there draw backs to using a higher ohm resistor more than 1k, I assume it will only just atteunate the input to grid, so less gain and maybe some distortion if using bad resistors.

[Paul Birkeland]

Could I also bias the Cathode Follower with higher voltage to lower the output impedance? I was thinking of using a tube that runs higher voltage like an e80cc to bias the cathode follower with higher voltage, but I am not sure if its safe to do that or if it sounds good.

You'll need to study datasheets that graph transconductance.  The 7802 datasheet has such a graph on the final page.  Note the relationship of Eb (plate voltage) to tranconductance.  Also note the relationship between grid voltage and transconductance.

6c33 or 6c33c?

Same tube.

cathode followers have a gain less than 1 right?

Yes, don't worry about the gain of the cathode follower.  It's always less than unity, and it's generally very close to unity.

So current lowers impedance by increasing gm.

There is a governing relatinoship that mu=gm*rp.  In a cathode follower, we can throw 1 in for mu, so we get 1=gm*rp, or rp=1/gm (rp is output impednace in this case).  To maximize the transconductance of something like the 7802, you would ground the cathode and have the grid at ground potential, with about 100V on the plate, to get 30,000umhos of transconductance.  Unfortunately, if you do this, the tube will very likely draw a bit of grid current, and you'd have to have a very beefy circuit to drive the tube running in this manner.  (you'd need another cathode follower behind such a circuit to handle the grid current/impedance demands)

Without those helpful curves, you can look at the simple current/voltage graph with lines based on grid voltage and seek to pick an operating point where the lines are as steep as possible.

I think the sensible approach for me would be to increase the current in the cathode follower on my 7802 tube. I don't mind losing gain or power since I can barely turn the pot past 25% before it gets too loud. Not sure how to do this though, is it more complicated than changing a resistor?

Again, you can't just increase the current in the output stage of the Crack-a-two-a without a major redesign, which means starting over.  If you're going to start over, there are much better tubes than the 7802 to achieve your goal.  Also, increasing the cathode follower current will not affect gain, but you may end up inadvertently adjusting the grid bias the point where you get very early clipping of the first stage.