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I'm trying many different designs in a plug in board test preamp and I would like to try a mu stage with the top tube a pentode. Any design using the 6gw8, 6bm8, 11ms8 or similar would be helpful. Thanks
...I rather like the bootstrapping of the pentode's screen.
But why oh why a pot on the output? Why go to all this bother to get a low output Z with high current drive capability, and destroy it all with the resistance of a pot???
There should NEVER be a pot at the output of a stage before a cable - only at the input of the next stage AFETER the cable and it's attendant capacitances and inductances
Really? You like that? I always thought that the benefit, the reason why the output followed the mu of the bottom triode was that the top triode, or in this case the pentode, was floating. That's not the case with this circuit. Also, the triode's plate resistor is 220K. That seems enormous to me. So large is it that I wonder whether one could simply connect the top of the 220K resistor to V+. And if you look at the circuit modified in that way, you might see that this is just a ho-hum common cathode triode voltage amp connected to a pentode cathode follower with a large voltage swing. It is drawn like a mu follower, but I really wonder whether there is that benefit when the cathode of the top pentode is grounded.
> I always thought that the benefit, the reason why the output followed the mu of the bottom triode was that the top triode, or in this case the pentode, was floating. That's not the case with this circuit <
Looks "floating" to me?
> Also, the triode's plate resistor is 220K. That seems enormous to me <
Agreed - crazy high - I'd didn't look at the component values. Hate it when you have to go to a list to get them as it's impossible toget a one glance concept . I imagined it would be running 5 - 10 mA at least... But the tube choice is then incorrect - as that triode is prety much 1/2 a 12AX7 yes?
> but I really wonder whether there is that benefit when the cathode of the top pentode is grounded. <
I must be missing something here - I don't see the pentode's cathode grounded...where/how?
I am having a problem with the ECL 86 circuit. It may just be my problem, but I think it has to do with mating a pentode with a triode in a mu follower ciruit.
The whole point... the reason why the circuit is called a mu follower is because the entire voltage output of the lower triode is across the lower triode's plate resistance and the upper triode's plate resistance times the mu of the upper triode. That makes the upper triode appear to the lower triode as if it were an enormous resistor from a very high voltage source. Because the lower triode's plate resistance is so much smaller than that mu x upper triode plate resistance, all the voltage gain goes across to the upper triode. The output of the circuit follows the mu of the lower triode. Simple, right? Yeah, I know, not the way I tell it, and I apologise for that, but the ECL 86 circuit posted in the above thread fails as a mu follower because, although the lower triode is AC coupled to the grid of the upper pentode and the plate resistance of the pentode would be enormous compared to the 62K plate resistance of the triode, unfortunately the pentode's plate resistance is in parallel with the 15K/2 watt cathode resistor to ground. The 15K resistor effectively murders the mu of the lower triode and the structure of the gain is the pentode resistance in parallel with the 15K resistor, which is effectively 15K, plus the 220K plate resistor at the plate of the lower triode. So, the net circuit is a common cathode lower triode with a 235K resistor connected to a V+ which is half the voltage potential of the pentode portion of the circuit. Like I said above, one would achieve greater voltage gain by simply connecting the 220K resistor to the 200V V+ directly.
Bottom line - I dislike SRPP and muF's - and hate that they use some form of NFB to gain the qualities that they are used for.
The upper tube is meant to be a CCS - but it's not really - and the OPZ is meant to be low - but it's also not really because it can't drive curreent without affecting other parameters.
I only commented on that circuit because I saw they were bootstraping the pentodes's screen off the output to make it work like a double triode cascode - which I do like.. And seeing that gace me an AhHuh about something completely different.
Had I looked into it as deep as you have - I would have kept my mounth shut!
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.
, and I'm entirely ranted out. Thanks for your response. <
Great rant - I've enjoyed it! Particularly the hit about Hegel - my girl friend did her Masters thesus in & around some of his work - and it's going to be wonderful to drop on her he had an audiophile connection...not!
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