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Compensation networks
Compensation networks
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Old 10th October 2017, 03:33 PM   #11
mskl99 is offline mskl99  United States
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Compensation networks
Are you looking at the right picture? The uncompensated picture shows a large overshoot. The set of 3 pics show compensation at 1,3, and 10 kHz.
Mike
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Old 10th October 2017, 03:35 PM   #12
jazbo8 is offline jazbo8
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Compensation networks
You are right, I looked at the wrong picture... So never mind.

rosanne-rosannadanna.jpg
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Old 10th October 2017, 04:32 PM   #13
anatech is online now anatech  Canada
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Compensation networks
Hi mskl99,
That looks like something I'm designing! Of course, I like what you have done there. That goes without saying.

Why did you design the output bias circuits as you have?

-Chris
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Old 12th October 2017, 02:02 AM   #14
mskl99 is offline mskl99  United States
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Compensation networks
Quote:
Why did you design the output bias circuits as you have?
I didn't design them. I lifted them directly from Merlin Blencowe's book "Designing Power Supplies for Valve Amplifiers," which he took out of print because he didn't like it, but I like it a lot and recommend it.
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Old 12th October 2017, 02:09 AM   #15
anatech is online now anatech  Canada
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Compensation networks
Hi mskl99,
Didn't he explain this in his book? I have one of his other books, but have not seen anything like this in there. I might be wrong, but it looks to apply negative bias quickly, but discharge more slowly. Maybe that makes turning off the amp doesn't make a thump noise. Otherwise, I'm at a loss on it. I don't like it when there is something I don't completely understand.

-Chris
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Old 12th October 2017, 04:17 AM   #16
kward is online now kward  United States
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The first attachment is an example of what the square wave response looks like for a well tuned amp with really good output transformers. This particular example uses late model Fisher 400 outputs and 18 dB feedback. Small signal HF response is flat to 40 KHz with these, and -1 dB down at 50 KHz.

Hammond output transformers are not bad I guess...not as well wound as the Fishers. I don't have any experience with their newer "-A" versions with the "easy wire" secondary. All my experience is with their earlier versions that have multiple separate secondaries that you connect in various configurations to achieve the secondary impedances you want. With those models, my experience tells me it's best to pull the feedback signal from the 4 ohm tap--you get the most stability headroom that way.

The second attachment shows the best square wave response I was able to get with Hammond 1650H's. This example pulls feedback from the 4 ohm tap, and utilizes 18 dB feedback. Small signal HF response is flat to just barely 30 KHz, and -1 dB down at 35 KHz. The several amps I've built with Hammond iron present an interesting compromise in tuning. To get the amp unconditionally stable, I needed a quite aggressive step network--so much so that feedback (18 dB at 1 KHz) was partially negated at 15 KHz and totally negated at 20 KHz. But on the other hand, LF response was strong clear down to 30 Hertz at full power output.

Both amps I'm describing here are unconditionally stable (meaning, stable under any type of output load, including no load).
Attached Images
File Type: jpg Square wave, 8KHz, Right Channel.JPG (247.9 KB, 95 views)
File Type: jpg 8 KHz.JPG (301.5 KB, 90 views)

Last edited by kward; 12th October 2017 at 04:27 AM.
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Old 12th October 2017, 09:24 AM   #17
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by mskl99 View Post
I'm just surprised that I need such a big cap. Is there any problem with such a big cap.
While the optimum values vary with output transformer, most valve amps do seem to end up with a time constant in the region of 1.5 to 1.8 microseconds for the feedback RC. You are spot on 1.8us.
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Old 12th October 2017, 12:36 PM   #18
anatech is online now anatech  Canada
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Compensation networks
Hi kward,
The current Hammond transformers are pretty good, but Hammond is in the process of redesigning the audio transformers in their line. Check out their new signal transformers they recently released.

The easy wire output transformers simply return to their really old wiring before those terrible windings you just used. I expect to see their engineer within a month and we will be talking about the new transformers (tube output stage types).

-Chris
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Old 12th October 2017, 04:03 PM   #19
kward is online now kward  United States
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Very cool anatech. Can't wait to see the new offerings from Hammond for tube output stages. I just feel there is a dearth of good quality, well wound, reasonably priced output iron, especially in that 6K to 8K PP range. I know people discuss Edcor as another option. I think Edcor's offerings are par. But I have had consistently better results with Hammond than I have had with Edcor.
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Old 12th October 2017, 11:38 PM   #20
Shef is offline Shef
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What is "terrible" in that wiring "he just used"? To me it makes absolute sense as the secondary's copper is being used more efficiently. With the "old type of wiring" if e.g. only 4R is being used - the stretch from 4R -> 8R -> 16R becomes a waste. What s the point of it, cheaper production? du-h.
In my PP amp with Hammonds I have zero compensation networks, it is absolutely stable. I do connect 4-8R speakers (they are 8 but have the impedance dive at LF) to the 4R-commutated secondaries with awesome outcome.
I have same feeling that the return to "easy wire" manifests the end of quality. All OPTs will generally suite to the guitar combos, forget the Hi-Fi.
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