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Old 21st September 2010, 01:55 PM   #1
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Default Pentode to Triode Operation

Hi.

I'd like to try operating my Cary SLA 70A Signature amp in triode mode and have found the following instructions:

"To triode connect output : 1) disconnect ultra-linear output tap from screen grid G2 (pin 4) and carefully insulate the ends of the wires 2) connect a 100 ohm power resistor between the anode A (pin 3) and pin 4 . I used a 5 watt sandblock type. 3) set bias on output tubes."

The amp is self biasing btw, if that matters.

Will this work for me and will this automatically reduce the NFB or do I need to perform any further modifications? Thanks as usual.
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Old 21st September 2010, 02:19 PM   #2
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Triode mode greatly INCREASES local NFB in the output stage.
Might decrease need for a global NFB, but won't automagically
do that other mod for you.

Last edited by kenpeter; 21st September 2010 at 02:22 PM.
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Old 21st September 2010, 02:36 PM   #3
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Thanks Ken. So this mod is not really complete without reduceing GNFB? I'm doing this more to assess the difference between pentode and triode configurations to see if it works for me.

Also, would 100 ohm 1/2 watt resistors work or do I need a higher wattage resistor?
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Old 21st September 2010, 02:37 PM   #4
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1/2 watt resistors are fine.
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Old 21st September 2010, 03:13 PM   #5
ilimzn is offline ilimzn  Croatia
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Triode mode operation significantly reduces open loop gain. Since the feedback is derived by dividing the output by a constant, reducing gain means less output for given input, and also means less feedback signal. In other words, GNFB is automatically reduced.
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Old 21st September 2010, 03:59 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by ilimzn View Post
Triode mode operation significantly reduces open loop gain. Since the feedback is derived by dividing the output by a constant, reducing gain means less output for given input, and also means less feedback signal. In other words, GNFB is automatically reduced.
It is as ilimzn says, and generally you will not need to make any other changes to maintain loop stability.

GFB is significantly reduced, however the effective rp of the output tubes is substantially reduced as well so damping factor is usually not too adversely effected by the reduction in feedback.

Note that power output is typically reduced by about 3dB.. (half)
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Old 21st September 2010, 05:53 PM   #7
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Thanks for the replies.

I'm listening to triode mode as I type as the mod was quick and simple.

First off, understand that the Cary amp is crossed over to a pair of Kinergetics subs so deep bass reproduction is not an issue. In fact, the overall bass sounds slightly more articulate and musical if that's possible. I also noticed that the 3db power loss is not noticeable so far. I'm getting the same overall volume level at the same pot rotation on the preamp. And at the loudest levels I listen to, I don't perceive any premature clipping of the signal.

There also seems to be a slight grain that has disappeared but I won't be able to ascertain true differences until the system is properly warmed and I can run through some familiar software.

I'll let you know what I think in the long run. I just hope this mod doesn't quickly burn out a set of Siemans EL34's I just paid good bucks for!

Thanks again gents.
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Old 21st September 2010, 09:57 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Monjul View Post
Thanks for the replies.

I'm listening to triode mode as I type as the mod was quick and simple.

First off, understand that the Cary amp is crossed over to a pair of Kinergetics subs so deep bass reproduction is not an issue. In fact, the overall bass sounds slightly more articulate and musical if that's possible. I also noticed that the 3db power loss is not noticeable so far. I'm getting the same overall volume level at the same pot rotation on the preamp. And at the loudest levels I listen to, I don't perceive any premature clipping of the signal.

There also seems to be a slight grain that has disappeared but I won't be able to ascertain true differences until the system is properly warmed and I can run through some familiar software.

I'll let you know what I think in the long run. I just hope this mod doesn't quickly burn out a set of Siemans EL34's I just paid good bucks for!

Thanks again gents.
Long term I think your impressions will be confirmed, this is what got me started designing and building triode pushpull amps in the first place, and the natural progression over time was to PP DHT triode amps and then finally DHT SE amps. (Where I am today with the exception of the newly finished little SEP I am currently listening to.)

Provided that there was a fair feedback margin to start with the switch to triode mode will result in a barely perceptible (if at all) change in the closed loop gain - anything less than a dB or so is not really going to be noticed.

As long as the overall plate current was the same as before the modification your tubes will last as long as in UL or pentode mode. The EL34 has very good linearity in triode mode IMO..
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Old 21st September 2010, 10:00 PM   #9
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Quote:
Originally Posted by kevinkr View Post
The EL34 has very good linearity in triode mode IMO..
In any mode it has better linearity than any clone of venerable RCA 6L6.
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Old 21st September 2010, 10:56 PM   #10
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There's no doubt that this was a good move. In fact, I can't believe I listened to this amp in pentode mode for over 20 years although it's still a great sounding amp in that configuration.

The slight grain and glare is gone, it's less forward sounding and the soundstage is tighter although it doesn't sound as wide with images outside the speakers which might have been a distortion to begin with because it always had a slight "phasey" character to it. But recorded ambience sounds more correct. Less electronic sounding. The bass is definitely better defined and musical. It sounds more like a part of the soundstage than previously. The power loss strapping the amp in triode mode is a non-issue and I hear very little difference even when playing loud rock.

The only negative I can put my finger on is a slight roll off on top. Treble instruments are not quite as bell-like and move back somewhat in the soundstage. Is there a mod for this or am I finally hearing recordings the way they were meant to be?
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