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Old 30th November 2010, 03:10 AM   #81
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"special" - but I'm sure my wife would agree with a least one interpretation of that
I was just pulling your chain.

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While I will admit that some of the cheaper tubes of Chinese origin that I've heard were "sure guano", by all accounts the latest Black Treasure series by Shuguang are not to be sniffed at - for $170/pr for 6CA7, I certainly hope not.
Actually, the EL34's weren't that bad, but the small signal tubes sure stunk. Ok, who's going to shell out $340 for a quad of Black Treasures?

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Old 30th November 2010, 09:58 AM   #82
Ian444 is offline Ian444  Australia
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That LM337 circuit seems quite original, I have never seen that before, thanks for the link Randy. Its very simple and makes me wonder why no-one thought of it before, surely someone out there has. I had a bit of trouble understanding it due to it being a negative reg which I am not very familiar with. As an example, this reg might normally operate with a -20V input, a -8.75V output and have -7.5V on the adjust pin. Now, if we float this reg +20V higher we get 0V (ground) as the input, 11.25V output (for the cathodes) and 12.5V on the adjust pin.

It needs an external voltage a bit higher than the desired cathode voltage in order to function, so the author uses the B+ and divides it down. On the Simple PP there is a low bias voltage (around 30V or so?) applied to the heaters, this could be used instead of B+ and the adjust resistor network changed to suit. It will still reflect B+ but save using a 2W 360K resistor, something much smaller rated at 1/2W could be used instead. Or a zener reference could be used if you just wanted a solid regulated cathode voltage. Or the cct could be referenced from the screen voltage of each tube (pentode mode) if you built four of them. Need to think about it more...

Lets take this rabbit and run with it. I would prefer the old-fashioned fixed bias because I could do with an extra 11V added to the B+. But it would be interesting for me to hear how this cct compares to LED bias which I am currently running. This cct is adjustable but LED's are fixed, that's the main advantage. Below is the basic schem as David Gillespie designed it.
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Old 30th November 2010, 03:22 PM   #83
chrisb is offline chrisb  Canada
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Ian, FWIW, I think that much of Dave's approach for this particular application results from what he describes early in the document as a "deep affection" for the SCA35, and his desire to keep the mods to the minimum cosmetically and conceptually.

I'll admit never having heard one of these, and my own "first big audio love" was of a different vintage and type ( Quad 33,303 circa '71?) , but I can certainly understand it.

I doubt that I'm the only one here interested in your findings; I'll likely keep mods to my SimplePP to a minimum, but have certainly been inspired to get creative with my next rebuild of the "El Cheapo" with same Scott OPTs.
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Old 11th February 2011, 09:52 PM   #84
Knarf is offline Knarf  Denmark
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That LM337 circuit seems quite original, I have never seen that before, thanks for the link Randy. Its very simple and makes me wonder why no-one thought of it before, surely someone out there has.
It has been thought off before. The problem with using stabilized cathode voltage in a power stage is the lack of sufficient resistance in the anode- and screen grid circuits. This allows for tube thermal runaway if things start to go poorly.

In a normal configuration the cathode resistor would of course increase the bias if one or both tubes starts to overheat. However increasing tube temperature excessively due to overload or external factors, or if a power tube becomes gassy, may drastically increase the cathode current at a given bias point, resulting in further heating. End result is likely to be tube thermal runaway, smoke and destruction.

Using a simply CCS in the common cathode lead of a PP or paralleled tube output stage may lead to the same problem. A CCS doesn't guarantee that two or more tubes share the total current equally. So if one tube becomes a bit hotter than the other, then there exist a point, where it will increase its cathode current due to self heating even against an increasing grid bias from the CCS. So one tube turns off, and the other takes the full current for both tubes. Usually not good.

Using a CCS in a long tailed pair phase splitter is fine, of course, due to the current limits imposed by the anode resistor voltage drops.

- Frank.
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Old 12th February 2011, 07:08 AM   #85
Knarf is offline Knarf  Denmark
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Adding to my previous post: No, I cannot immediately see either why the constant cathode voltage situation is any different than having simple fixed grid bias in an amp. However I have been led to believe there is a difference, and I have seen the CCS problem demonstrated. Would be interested in hearing if people have corrections to this subject.

Note that large, high mu 'zero bias' transmitting triodes are frequently operated in grounded grid configuration with the cathode raised a few volts via a high wattage zener diode. However those tubes are not nearly as sensitive to thermal runaway as are their smaller all-glass cousins with oxide coated cathodes. The TX tubes have either a Thoriated Tungsten directly heated filament, or are of ceramic/metal construction with external anodes requiring forced air cooling at all times.
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Old 14th February 2011, 07:09 AM   #86
Ian444 is offline Ian444  Australia
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Thanks for your thoughts Frank. I can see where there could be a problem if two tubes shared a CCS and especially if there was no bypass cap. I experimented a bit in the past with CCS but have not tried the constant voltage cct with the LM337. With CCS's I had consistent results, not too bad but not fantastic either. I usually had one CCS and bypass cap per tube. LED's as cathode loads sound better (to me) and so does fixed bias. In many amps the cathode load is just a resistor and bypass cap and many people like this too, so each to their own I guess. Using LED's seems to give great clarity and definition, whether it is a true reproduction of the music or not I don't know, but I like it.
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Old 15th February 2011, 10:25 PM   #87
chrisb is offline chrisb  Canada
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Default back to original thread topic

I've been running my SimplePP in pentode with stock JJ's for output and NOS (Sylvanias, I think) for a few weeks now, and am quite happy with the results.

Due to fortuitous circumstances, (let's say a combination of Valentine's and early anniversary gift from my wife, and less painful to her than the alternative "celebration" ) I was able to order a set of Psvane 12AT7 and EL84 for the P/P amp, and the Shuguang Treasure 6CA7 for one of my SE amps. With any luck they should arrive by the end of the week, then we'll see what the fuss is about.
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Old 15th February 2011, 10:43 PM   #88
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Due to fortuitous circumstances, (let's say a combination of Valentine's and early anniversary gift from my wife, and less painful to her than the alternative "celebration" )
I was able to order a set of Psvane 12AT7 and EL84 for the P/P amp, and the Shuguang Treasure 6CA7 for one of my SE amps. With any luck they should arrive by the end of the week, then we'll see what the fuss is about.
Wow, you got tubes for Valentine's day, not too shabby. All I got was steak.

You'll have to hump your amp over here when the new tubes are broken in.

jeff
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Old 17th February 2011, 04:31 PM   #89
chrisb is offline chrisb  Canada
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Wow, you got tubes for Valentine's day, not too shabby. All I got was steak.

You'll have to hump your amp over here when the new tubes are broken in.

jeff
you got steak, and probably some baked goods as well? - now that's not shabby

Well as I said, combination of Valentines, this year's anniversary (#38), and for yet another round of home renos that will include at least one full day of crawling around in the attic (hopefully for the last time)

actually, now that I think about it as the amp resides in the living room, she got the better part of the deal (even if she doesn't fully appreciate the sonic difference)

burning in on background simmer as we speak - should be ready for serious listening in a couple of weeks.
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Last edited by chrisb; 17th February 2011 at 04:34 PM.
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Old 17th February 2011, 06:48 PM   #90
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you got steak, and probably some baked goods as well? - now that's not shabby
Some really amazing chocolate lava cake, but had to go "out" for that thou.

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actually, now that I think about it as the amp resides in the living room, she got the better part of the deal (even if she doesn't fully appreciate the sonic difference)
No comment.

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burning in on background simmer as we speak - should be ready for serious listening in a couple of weeks.


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