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Old 24th August 2012, 05:45 PM   #21
SY is offline SY  United States
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Note the use of grid stoppers on the low gm tubes and their absence on the high gm tubes...
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Old 24th August 2012, 06:00 PM   #22
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Originally Posted by kevinkr View Post
WRT to the 150pF cathode cap I believe this was intended to compensate for the considerable miller capacitance of the 12AX7. Cathode current feedback [large cathode resistor] in a mu-follower has no effect on gain, but it does raise rp significantly which in conjunction with miller capacitance does result in really poor HF response. (The effective rp in this circuit is not constant, decreasing slightly with frequency due to that cap.)
Υes , it has to do only with the miller capacitance of the 12AX7 but the raise of the rp
and the outuput impedance of this stage will not affect the HF gain because the next stage is a DC cathode follower with very high input impedance .

Last edited by Dimitris AR; 24th August 2012 at 06:04 PM.
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Old 24th August 2012, 08:09 PM   #23
DF96 is offline DF96  England
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Kevinkr:
Thanks for shedding some light on this circuit. As the 150pF only has an effect from around 1MHz it is clearly there to aid stability. The centre stage is not a mu-follower, but an active load, even with a 1M grid resistor (unless the signal take-off from this is also wrong in the circuit diagram).
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Old 24th August 2012, 08:21 PM   #24
kevinkr is offline kevinkr  United States
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Originally Posted by DF96 View Post
Kevinkr:
Thanks for shedding some light on this circuit. As the 150pF only has an effect from around 1MHz it is clearly there to aid stability. The centre stage is not a mu-follower, but an active load, even with a 1M grid resistor (unless the signal take-off from this is also wrong in the circuit diagram).
No it's not utilized as a mu-follower but as a common cathode with active load, but it's convenient to (incorrectly) call it such. The signal was indeed tapped off of the lower plate in the one I worked on.

The 150pF IMO compensates for HF phase shift introduced by the combination of very high rp due to the cathode feedback and the miller capacitance of that stage. I don't know whether or not it would oscillate without it, but I suspect that might be the case with the very capacitive interconnect cables popular at the time. (And which frankly still are)

There were a few revisions of this very long lived design over time - most I believe to be very subtle, but I have not seen one in decades.
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Old 24th August 2012, 08:52 PM   #25
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Interesting that so many of the diagrams were wrong. Is it everyone copying a poor reverse-engineered one, or a deliberate spoiler? In either case it is a warning: several diagrams all agreeing doesn't mean they are right. Did nobody ask any questions?

The use of the 150pF to give some phase advance in the forward path is unusual. I don't think I have seen that before.
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Old 25th August 2012, 01:38 AM   #26
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Originally Posted by DF96 View Post
Interesting that so many of the diagrams were wrong. Is it everyone copying a poor reverse-engineered one, or a deliberate spoiler? In either case it is a warning: several diagrams all agreeing doesn't mean they are right. Did nobody ask any questions?

The use of the 150pF to give some phase advance in the forward path is unusual. I don't think I have seen that before.
I'm pretty sure the misinformation is deliberate and intended to foil slavish copying of the design. Anyone who understands good tube design will figure this out quickly and fix the obvious spoilers.

During the original time frame this was designed companies like VTL, ARC (SP-3/6), and MFA were making "improved" versions of what was essentially the Marantz 7 topology. This represented a significant improvement in performance IMO over those designs. And NO I would not build anything inspired by a design based on any of these today.
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Old 25th August 2012, 03:42 PM   #27
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I bought one of the blank circuit boards on ebay a few years ago. This is the circuit that I used on it. It worked well and sounds fine, but I agree it is very complicated.

I really didn't think the original circuit was biased optimally, so I made several changes. I used the 6DT8 since they are similar to the 12AT7 but very avaialble and very inexpensive. I didn't need all the gain of the 12AX7.

The board required a trace to be cut or you'll but too much voltage on the heater-cathode. I used two heater supplies and boosted the top tube heaters.

I'd really only recommend this board if you want to experiment with the mu follower in a feedback loop.
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Old 25th August 2012, 07:33 PM   #28
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Originally Posted by jonusgrumby View Post
I bought one of the blank circuit boards on ebay a few years ago. This is the circuit that I used on it. It worked well and sounds fine, but I agree it is very complicated.

I really didn't think the original circuit was biased optimally, so I made several changes. I used the 6DT8 since they are similar to the 12AT7 but very avaialble and very inexpensive. I didn't need all the gain of the 12AX7.

The board required a trace to be cut or you'll but too much voltage on the heater-cathode. I used two heater supplies and boosted the top tube heaters.

I'd really only recommend this board if you want to experiment with the mu follower in a feedback loop.
Interesting. What is the different in terms of sound performance after those the changed that you made...

Have you ever try to maintain the ecc82 input drive?
Should I change the value of the components if I used the 12at7 instead of the 6dt8?
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Old 25th August 2012, 08:43 PM   #29
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R6 should be 1M, although with such heavy feedback it probably won't make much difference.
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Old 25th August 2012, 10:07 PM   #30
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You can use a 12AT7 directly, just make sure you have the heater wired correctly.

I don't understand what you mean by "Have you ever try to maintain the ecc82 input drive?"

I originally tried 1M at R6, I don't remember why I went to the lower value. The board showed 1K there but I made lots of changes to it. I'll see if I can find my notes.
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