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Old 30th April 2012, 07:57 PM   #11
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Originally Posted by SpreadSpectrum View Post
If you missed it, it is all of the stuff after "A Modern Approach to Bias Balance." All of those approaches will give essentially zero imbalance as opposed to the 8.5% imbalance of the Blumlein circuit in the example.
That (two parallel tubes) was on a rather extreme example, no? With a typical imbalance I'd expect that the error would be very much smaller and so the advantage to the SS circuits reduced in that regard.

After reading the second (followup) article, I came to better appreciate the advantage that would apply when adding this circuit to an existing design. Avoiding the loss of 50 volts of plate voltage seems almost a necessity in a retrofit.
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Old 30th April 2012, 08:01 PM   #12
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The example given is an extreme example - which illustrates how effective it really is. In normal operation, with the same triode on each half, expect the imbalance to be less than 1%. As I said I use it in exactly the scenario you intend to and it works excellently.

I would look at the second implementation of the transistor garter as a compromise which offers the best of both worlds without undue complexity.

Shoog
Are you saying you use it with a toroidal OPT or that you use it in an amp with cathode feedback windings and a DC coupled driver? I'd be interested to see how you did it if you have a schematic.

Sure, I agree garter bias is an excellent solution but I was just pointing out that it is unfair to say that gain doesn't improve the performance.

I am still not liking the solid-state garter bias since it is still cathode bias, and a high-powered class AB power stage seems like the worst place for cathode bias. This amp will play loud electronic music with long sustained bass notes, etc. I don't just listen to female jazz vocals, etc. I think that bias voltage will fluctuate significantly with signal and since I am planning on running output tubes as pentodes, tubes will be very sensitive to bias voltage fluctuations due to the high gain.

This is a link to the thread for the amp I am building.
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Old 30th April 2012, 08:10 PM   #13
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I have used it with toroidal outputs but not cathode feedback, though I do not see why that would be an issue.
Unfortunately you have just pointed out that it is DC coupled which makes garter an impossible option. I have been building direct coupled amps for a while now and have thought about how garter might be used direct coupled - can't be done as far as I can see.

As for AB - it all depends on what size of cathode bypass you are prepared to tollerate really. I use cathode to cathode bypassing which is slightly different in terms of overload behaviour - I cannot say I have encountered problems and get good response down to 10hz.

Shoog
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Old 30th April 2012, 08:10 PM   #14
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Quote:
Originally Posted by Captn Dave View Post
That (two parallel tubes) was on a rather extreme example, no? With a typical imbalance I'd expect that the error would be very much smaller and so the advantage to the SS circuits reduced in that regard.
You seemed to be implying that there was no advantage when there was in fact a tiny advantage. Sorry, I'm an engineer.
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Old 30th April 2012, 08:13 PM   #15
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I think he was implying that there was no practical advantage - which is been a realist.

Shoog
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Old 30th April 2012, 08:20 PM   #16
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Just a side observation.
Gary Pimm has some interesting observations about the benefits of screen regulation as opposed to simply tying the screens to B+, he measured better results with the screen tied straight to B+.

Shoog
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Old 30th April 2012, 08:25 PM   #17
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Me too, but they don't let me blow the horn anymore.

As an engineer, it is important to evaluate the cost to benefit ratio, no? At what cost to complexity and reliability comes a benefit that may (or may not) be significant. It is that aspect I was questioning.

Shoog, you are correct in that observation, but also the practical advantage may be found when adding the Garter to an existing design. For that it is significant.
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Old 30th April 2012, 08:28 PM   #18
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Originally Posted by Shoog View Post
Just a side observation.
Gary Pimm has some interesting observations about the benefits of screen regulation as opposed to simply tying the screens to B+, he measured better results with the screen tied straight to B+.

Shoog
Could I trouble you for a link?
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Old 30th April 2012, 08:35 PM   #19
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Quote:
Originally Posted by Shoog View Post
I have used it with toroidal outputs but not cathode feedback, though I do not see why that would be an issue.
Unfortunately you have just pointed out that it is DC coupled which makes garter an impossible option. I have been building direct coupled amps for a while now and have thought about how garter might be used direct coupled - can't be done as far as I can see.

As for AB - it all depends on what size of cathode bypass you are prepared to tollerate really. I use cathode to cathode bypassing which is slightly different in terms of overload behaviour - I cannot say I have encountered problems and get good response down to 10hz.

Shoog
Actually, mine would be easy since the DC coupled driver is a follower which is AC coupled to the previous VA stage. Just send the bias to the gate of the follower instead of the power tube grid.
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Old 30th April 2012, 08:40 PM   #20
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Originally Posted by Captn Dave View Post
Could I trouble you for a link?
Sorry - its not on his pages so I can't. I think infact that it came up in a discussion I had with him when I was building my Tabor clone and it came up in one of these threads. He was quite insistent that it measured better and I trusted his judgment. Try a search on my name and Tabor.

Shoog
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