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In Reply to: No longer... posted by Allen Wright on February 15, 2006 at 04:18:27:
I'm sorry that you don't like mu follower amps. I built one in the mid 80's when Chris Paul's article was first published in 'Audio Amateur'. I built it up into an old Heathkit integrated amp chassis that looked like the dashboard of a mid 50's cadillac. Rather than following his original circuit, I used parallel sections of 6DJ8 tubes, 4 in all for two channels. I used a regulated V+ at around 350V, and two regulated heaters for the upper and lower triodes. I connected to my rebuilt Hafler amps and , Eureka!, it sounded like crap. Hissy and spitty and terrible.
So, unwilling to give up on the thing, afterall I had two linear Knobpots as volume controls at the input of the preamp and those things were expensive, I rebuilt the exact same circuit around parallel sections of a 12AU7 type tube, a 6189, relying on my RCA tube manual to put the 6189's within the design max parameters. And the results were damned near perfect. The voltage divided just the way I hoped it would. 115V across each triode and approximately 115V in between. And the sound was unbelievable. Quiet, musical and really dynamic. Cool!
Now I was happy with my mu follower incarnation, but I don't think everyone else was as lucky. That circuit became so political so fast that it was amazing, particularly because it was issued, effectively, into the public domain instantly. All these 'different' designs, put into production within so short a time and all of them tauting superiority in one characteristic or another. Ridiculous. And it seems to me that the one great advantage of it is it's simplicity. Although there is a little NFB in the way the two triodes are coupled, it really does not admit feedback easily, readily or well. Whenever you try to limit the gain of the lower triode you have to bolt something external on that always looks silly.But the political nonsense that erupted in the late 80's was the worst problem of all. As soon as people in the US started working on the circuit, the Europeans popped up shouting, "No,No,No, this isn't an original circuit. Leonardo da Vinci had designs for it in his notebooks and Hegel dismissed the circuit as derivative." Then they came up with this SRPP that you and most everyone else assume is the same thing as a mu follower, and it simply isn't. The plate of the lower triode is AC coupled to the grid of the upper triode and that turns the upper triode into a big, dumb resistor. That's it. When you have said that about a mu-follower, you are done. There isn't anything else. And when you don't AC couple the two triodes, as in the SRPP and others, it's not a mu-follower, and it doesn't work very well.
You were absolutely right about the boot strapping of the grid in the pentode. They did exactly that same sort of thing in the second generation of mu followers published in 'Audio Amateur' in the late 80's. And though you are correct about the volume pot being in the wrong location, can you see why the designer put it there? You've got a cathode output pentode pumped up to half the V+ which is going to enable maximum voltage swing, AC coupled to a 12AX7 which is entirely ham-strung by low voltage and limited gain. If the builder of this circuit just wiggles the ECL 86 tube in it's socket with the power amp on, it's going to blow the woofers right out of their enclosures without that volume pot installed, even though it is in the worst place possible. Well, I guess we have trashed this poor French guy's design pretty well, and I'm entirely ranted out. Thanks for your response.
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Follow Ups
- Re: No longer... - corerosin 13:49:29 02/15/06 (1)
- Well, I guess we have trashed this poor French guy's design pretty well - Allen Wright 15:41:20 02/15/06 (0)
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