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Old 7th January 2012, 02:54 PM   #81
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Interesting point Sy, although I've never found that to be particularly true. But for the sake of the conversation, if it were (or is) true, theoretically, the capacitive feedback loop from the screen tap can't address the issue you raise any better than the main NFB loop can. In the real world however, where transformers can and do have different response characteristics on different windings and taps, I suppose the screen tap loop could be more effective in dealing with the issue. But since all windings produce a composite output of the push-pull signal, the idea that feedback taken from only one output tube of a push-pull design corrects only issues on that side of the push-pull circuit seems quite implausible in this case, given that the loop in question is returned to the same input that the main NFB loop is returned to. In my experience with this design, I have found that the capacitive loop acts to tailor the response of the active amplification circuits first, so that the main NFB loop applied around the active elements and the OPT remains stable under a wide range of possible loading conditions. Hafler was at the forefront of the whole NFB stability issue, which peaked about the time his original MK II amplifier was introduced in 1955. The all too typical Williamson offerings of the day were infamous for breathing woofers and blown tweeters, where as the Dynaco design (at the time) represented a real advancement in stability in FB amplifiers over the more traditional offerings that were available. It is my experience that his approach to tailoring the response of the basic gain model before application of GNFB, in addition to the wide response his transformers offered, contributed significantly to his success in this area. Others may have different experiences regarding this, so I appreciate any and all input offered.

Dave
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Old 7th January 2012, 03:29 PM   #82
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The idea is to balance the open loop frequency response for both polarities before applying the overall loop. The cap doesn't work perfectly, but it does work. Pull the cap out of any of the Dyna amps (this was a trick they used in almost anything they made). In some cases, the amp will still work fine, but in many others, you'll see ringing on one side of square waves. In some, you'll get bursts of oscillation, especially when driving speakers.
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Old 7th January 2012, 04:04 PM   #83
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Absolutely agree -- although I've found that simplying adjusting the components of the typical transitional phase shift network so often seen connected to the plate of an input tube will also allow you to either eliminate the ring, produce exaggerated ring on both sides of a square wave, or shove it all to the top or bottom of the wave. I have always found these effects to be from an overall response that was not tailored appropriately (appropriate tailoring always resolves them) -- regardless of the type phase inverter used -- rather than being due to significantly different response on one side. I appreciate your input. It give me more to investigate!

Dave
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Old 7th January 2012, 06:46 PM   #84
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Quote:
Originally Posted by dcgillespie View Post
Pixworld -- Thanks for the kind words about my EFB modification.
Where can one find out more?

dave
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Old 7th January 2012, 06:59 PM   #85
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Dave -- You can read my article introducing the EFB modification here:

http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf

It is posted in the Dave's Lab portion of the Tronola site which is operated by my good friend Steve Lafferty. He also has numerous other articles and modifications posted there you might find interesting.

Thanks for the interest!

Dave
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Old 7th January 2012, 07:27 PM   #86
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Thanx. I have a dual mono set of SCA iron, and a (big) chassis so far, and am brainstorming a quite mutable amplifer.

dave
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Old 8th January 2012, 12:21 AM   #87
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mr2racer:

Here's a post on another forum describing how to implement the PAS phono on the SCA35 board. supposedly better. You should be able to bypass the selector switch mounted components completely, if you eliminate tape head equalization. I'm not a big fan of the pas though, I'd put a RCA phono circuit with buffer, or Koren phono (haven't heard that one) if I was doing it.

Dynaco-Doctor Forum: SCA-35 Phono Board Modification by Flyquail56
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Old 8th January 2012, 04:55 PM   #88
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JGF,
Thank you, that is EXACTLY what I was looking for! I've heard Joe's upgraded PAS and for this project I think its fine. (For myself I'm waiting for SY's design.) I looked at converting this board to a PAS since its layout is the same except for the tape head. But I wasn't sure about what to do with those tape head lug connections on the board.

But still I have a question. I did away with the Dyncao selector switch entirely so none of the connections are still there. The instructions say to replace R11 with a jumper and connect, as in the original, the phono input to lug #6. Which, I assume, is connected to lug #8 via the selector switch. Wouldn't it be easier to connect the phono input directly to lug #8 like the PAS. And only connect the output from lug #11 to the selector switch?

Kevin

Last edited by mr2racer; 8th January 2012 at 05:00 PM.
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Old 8th January 2012, 05:41 PM   #89
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Kevin,

I haven't done this mod on the SCA35, only worked on the PAS. I wouldn't want to try to reverse engineer those switch connections... If you want to put a pas phono circuit in, I think your best bet is to draw up that schematic, then work it into the SCA phono board. Yes, if you're only using mag. phono, the rca jack input should go directly to the board, the phono circuit's only connections to the selector switch will be its outputs.

Good luck!
John
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Old 8th January 2012, 06:15 PM   #90
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JGF,
As I was writing the last post I realized I had figured it out. I think what they were trying to do was keep the changes to a minimum. But there schematic means two connections for phono on the selector switch. If the input is connected to #6 it has to arrive at #8 eventually. That much I know. And the output is from #11. The rest of the connections are accounted for. In fact I think a better way to do it is a direct input to the board at #8. Then instead of replacing R-11 with a jumper I can put the first half of the feedback network there. Then connect #7 to #3 and put the second half of the feedback in place of the jumper for R-9. And Then I can leave the coupling cap on the board where it belongs.

Kevin
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