Bootstrapped Gain Stages?

[Mr. Lime]

Hi guys,

well I guess a lot of us came across Merlin Blencowe's suggestion of a bootsrapped cathode follower which increases the gain of the previous stage while omitting it's cathode bypass cap.
Here's the link to the explanation: http://www.valvewizard.co.uk/dccf.html

My question is, wouldn't a bootstrapped gain stage work as well by taking the feedback from the cathode of a cold biased gain stage and lead it back to the anode of the previous stage just like in my drawing?

bootsrapped gain stage.png

I could also imagine to wire it in front of a cathodyne phase inverter and bootstrap the master volume and previous gain stage as well.
Any thoughts or ideas on this?

bootsrapped gain stage cathodyne.png

[pdf64]

I suspect that bootstrapping to a cold biased cathode may give poor results, as the cathode will run out of signal swing a lot earlier than a proper CF.
And also as it's cold biased, it's likely to get pushed into cut off a lowish signal levels, and the Vac at the cathode may disappear for that section of the wave, so the bootstrapping effect would kick in and out.

The cathodyne method proposed only looks to feed ~1/3 of the cathode Vac back, so the bootstrapping effect may be reduced significantly.

[gingertube]

You need to consider the impedance being driven and the driving impedance. The bootstrap relies on providing a signal voltage the same (phase and voltage level) as at the anode to the top of the anode load resistor. The driver must be able to drive the required signal current into the resistor above that which ties back to B+.

In the cold bias stage example that 39K load will be approximately 39K parallel 47K = 21K which will change it's sound somewhat.

The cathodyne arrangement you show is severely compromised with respect to that, in addition to pdf64s comments above.

Also:
The bootstrapping disappears when the circuit is overdriven, this results in very hard and harsh clipping.
You will note that the description of Merlins favourite 14.7 High Gain Preamp design he specifically states that bootstrapping of the 3rd stage was tried and found to make the distortion harder and less pleasing and so was deleted from the design. See page 259 of the 2nd edition of Designing Tube Preamps for Guitar and Bass.

This has been my experience too. I just haven't liked the overdrive sound of any of the bootstrapped gain stages I have tried.
Cheers,
Ian

[Colossal]

This has been my experience too. I just haven't liked the overdrive sound of any of the bootstrapped gain stages I have tried.
Cheers,
Ian

I agree. I tried a bootstrapped cathode follower and found it harsh

[tubeswell]

In Merlin's bootstrapping examples, the cathode of the CF stage (or cathodyne stage) has unity gain (in terms of being equivalent to the input signal on the grid). Therefore, the signal being fed back to the split plate load point on the driving stage's plate load, is the same as the signal at the driving stage's plate, which is why the same signal voltage appears at both ends of the driving stage's split plate load resistor, which results in that resistor appearing to have infinite resistance, resulting the gain of the driving stage being equal to the amplification factor of the driving tube.

If the source of the feedback from the following stage is not at unity gain, then this condition cannot assert itself.