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Bootstrapped Mullard 5-20-style front end

thorpej

thorpej

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Spent some time tonight fiddling around with my idea for a bootstrapped Mullard 5-20-style front end (AF gain + inverter stages) for a 6550 U-L amp. The simulation behaved more or less exactly as I expected it to (modulo my triode models not liking a CCS in the inverter tail for whatever reason), giving an open loop gain of ~70 for the gain stage, and ~6 for the inverter using 6SN7 and ~5 using 12AU7 (with no additional circuit changes). With -20dB of feedback, using the 6SN7, that would allow me to drive the amp to full power with ~750mVrms.

(Note in this drawing, there are a couple of extra nodes to make probing the simulation more convenient.)
 

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Why do you need so much gain for the front end? I would think that it could be a source of unwanted noise or sensitive to microphonics.
Never was sure what the term boot strapped means. What is your understanding of that term?
 
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20 dB feedback. Direct line level inputs.

I used basically the same front end (not quite as much feedback but also no bootstrapping cathode follower) on a 6L6 amp I just finished. Had to pay careful attention to grounding but that sucker was quiet.
 
I'm playing around with ideas, mostly. 6550s take a lot of voltage swing to drive in an ultra-linear topology (96Vpp), so the additional gain is going to have to come from somewhere. I could use a higher gain tube in the LTP, but that would require making changes to the gain stage as well (because it's all direct-coupled). The bootstrapping cathode follower was essentially drop-in for the extra gain without having to make any other changes.
 
kward said:
20 dB feedback. Direct line level inputs.

I used basically the same front end (not quite as much feedback but also no bootstrapping cathode follower) on a 6L6 amp I just finished. Had to pay careful attention to grounding but that sucker was quiet.
Click to expand...

And maybe the best part about using the bootstrap method? If it does, in fact, end up being too much gain in the front-end, you can "fix" the amp by simply removing the bootstrap capacitor and adjusting the feedback loop to compensate for the reduced open loop gain (at the cost of input sensitivity).
 
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