Better Low Z driver stage design based on our Bootstrapped Gain Stage thread

[SoulFetish]

However, i will probably experiment with class AB2 operation as well to see how it feels and sounds. So, those are place holders for the time being.

[SoulFetish]

Sorry SF, got a little sidetracked here

don’t sweat it nick.
Getting side tracked is how I ended up in this electronics rabbit hole.

[Helmholtz]

I'm wondering if the HF response nickb mentioned is actually problematic in this design.

This is the concern I was referring to.
CF output impedance doesn't matter and with 51k grid stoppers the HF response will be about the same as with a standard AC coupled PI.

I think class AB2 makes more sense with lower gain power tetrodes but let your ears decide.

Also as your 51k grid stoppers roughly emulate the typical output impedance of a LTPI , grid current "capacity" should be about the same. Of course without blocking distortion effects.

[SoulFetish]

So after a couple of Tweaks to the power supply, and lead dress. It's getting really close to awesome.
There is a couple of things which still need to be sussed out.

***1st, does anyone have a solution to grounding the pot bodies on RV4 Pots? I'm using internal toothed washers, torqued down tight to the chassis and I'm getting static and noise as if they aren't grounded. I folded up a strip of aluminum tape and ran it across the backs of the pots and it's okay for now. But I'd like to think of it as a band-aid solution. It doesn't look great.

2nd, there is some HF oscillating it sounds like. Ever so slight, I can hear it at idle. If I were to guess, it sounds like 14-16kHz, (I know, it shouldn't be audible through a guitar speaker. But I can still hear the ambient sound. Kind of like when a CRT is on in the other room, but higher pitched. It's definitely in the preamp, before the phase inverter.

[SoulFetish]

Okay!!
preliminary testing on the cathode biases/bootstrapped cathode follower output impedance:
*driven from the inverting output of the PI -
unloaded output voltage = 10.44V RMS
Loaded output voltage into a 10uF cap in series with. 1k resistor = 4.455V RMS
for an output impedance of....
1.34k !!

[Helmholtz]

Where did you take the CFoutput? As shown in your schematic or directly at the cathode as Nickb recommended?
And did you terminate the CF input with a source impedance of e.g. 50k?

[SoulFetish]

You mean the cathode follower output?
- as shown in my schematic.
but easy enough to change. Standby...

[Helmholtz]

Sorry, I meant CF output and source impedance at the CF grid. Corrected my post above.

[SoulFetish]

Output taken directly from the cathode as Nick recommended:
unloaded - 10.81V RMS
loaded - 5.129V RMS
Zout = 1.11k

[Helmholtz]

Fine. If correctly done that measurement won't lie.

Neverthelees 1.1K is only 2% of 51k (grid stoppers).

BTW, 51k grid stoppers won't allow for real class AB2 operation.

[SoulFetish]

Right,
they’re there now specifically to prevent grid current ( much like the 47k grid toppers in SVT amps).
The my amp is not currently designed for AB2.

[Helmholtz]

As long as there is no grid current (AB1), CF output impedance doesn't matter anyway.

[SoulFetish]

As long as there is no grid current (AB1), CF output impedance doesn't matter anyway.

That's a fair point. As currently constituted, a 12AX7 would probably do the job just fine.
But, again, I'm doing some experimenting. I may not settle on this configuration, and I have the option for a low z driver (should I need it).

But, I think this point buries the lead here all together. The story of today, is that we have tested and confirmed that bootstrapping, in this case, increased the output impedance by 3X what is conventionally expected!
Leaving my purposes out of it, I believe this calls for a bit of hyperbole here...
This discovery is going to shake the very foundations of audio electronics!!

[Helmholtz]

But, I think this point buries the lead here all together. The story of today, is that we have tested and confirmed that bootstrapping, in this case, increased the output impedance by 3X what is conventionally expected!

We already discovered this effect in the earlier thread.
The bootstrapped grid leak resistor feeds back an in-phase output signal to the grid. That's positive feedback. Positive feedback increases output impedance.
The amount of feedback voltage at the grid depends on the grid circuit/source impedance (voltage divider).
In typical circuits the grid leak resistance is much larger than the source impedance, so the moderately increased output impedance is likely to go unnoticed.
Anyway, with tube circuits bootstrapping is rarely necessary.

[SoulFetish]

We already discovered this effect in the earlier thread.
The bootstrapped grid leak resistor feeds back an in-phase output signal to the grid. That's positive feedback. Positive feedback increases output impedance.

I made that point in my first thread. I know that this is the result you and I expected. We just spent the last page talking about testing this to confirm it.
But there was some discrepancy about this.
For instance, in post #3, nick projected that the impedance taken off the cathode would be 370Ω. It was not. In fact it was 3X that amount. I thought it would be helpful to test this, and confirm the result and clear it up.
I could have been wrong in my understanding about our previous discussion.That's all