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Pentode to Triode Operation

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21st September 2010, 01:55 PM

#1

Monjul

diyAudio Member

Join Date: Jan 2010

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.

Monjul

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21st September 2010, 02:19 PM

#2

kenpeter

diyAudio Member

Join Date: Nov 2007

Location: Dallas

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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21st September 2010, 02:36 PM

#3

Monjul

diyAudio Member

Join Date: Jan 2010

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?

Monjul

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21st September 2010, 02:37 PM

#4

Frank Berry

diyAudio Member

Join Date: Sep 2003

Location: Midland, Michigan

1/2 watt resistors are fine.

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21st September 2010, 03:13 PM

#5

ilimzn

diyAudio Member

Join Date: Feb 2005

Location: Zagreb

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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21st September 2010, 03:59 PM

#6

kevinkr

diyAudio Moderator

Join Date: Sep 2004

Location: Boston, Massachusetts

Blog Entries: 6

Quote:

Originally Posted by ilimzn

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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21st September 2010, 05:53 PM

#7

Monjul

diyAudio Member

Join Date: Jan 2010

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.

Monjul

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21st September 2010, 09:57 PM

#8

kevinkr

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Location: Boston, Massachusetts

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Quote:

Originally Posted by Monjul

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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21st September 2010, 10:00 PM

#9

Wavebourn

diyAudio Member

Join Date: Sep 2006

Location: Pleasant Hill, CA

Quote:

Originally Posted by kevinkr

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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21st September 2010, 10:56 PM

#10

Monjul

diyAudio Member

Join Date: Jan 2010

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?

Monjul

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