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Triode jumping ???

PostPosted: Mon Dec 30, 2013 3:53 pm
by hilldweller
Hi all,
Hope everyone had a great holiday!
I recently heard of jumping triodes together in dual triode tubes where only one triode is used in the circuit. I never heard of this. Is this something viable? Pro's and Con's? Or is this just BS (???)
Iv'e been around for a little while but I never heard of that one. Any input would be appreciated.

Re: Triode jumping ???

PostPosted: Mon Dec 30, 2013 5:37 pm
by TomMcNally
Parallelling tubes works. The 6SL7's in the JE Labs EL-34 SE which I've build a dozen or so of does that.

Re: Triode jumping ???

PostPosted: Mon Dec 30, 2013 5:58 pm
by hilldweller
Ok. so what is the purpose of doing that?

Re: Triode jumping ???

PostPosted: Mon Dec 30, 2013 6:11 pm
by kheper
Generally, to reduce noise.

http://tubes.nekhbet.com/math.shtml#para

Re: Triode jumping ???

PostPosted: Mon Dec 30, 2013 6:43 pm
by hilldweller
kheper wrote:Generally, to reduce noise.

http://tubes.nekhbet.com/math.shtml#para

WOW! Many thanks! HAPPY NEW YEAR (wine)

Re: Triode jumping ???

PostPosted: Tue Dec 31, 2013 7:30 pm
by hilldweller
TomMcNally wrote:Parallelling tubes works. The 6SL7's in the JE Labs EL-34 SE which I've build a dozen or so of does that.

Is it true than that you would cut the cathode and plate resistor values in half?

Re: Triode jumping ???

PostPosted: Tue Dec 31, 2013 10:44 pm
by Geek
hilldweller wrote:
TomMcNally wrote:Parallelling tubes works. The 6SL7's in the JE Labs EL-34 SE which I've build a dozen or so of does that.

Is it true than that you would cut the grid and plate resistor values in half?

Nope! The plate curves remain the same, hence cathode and anode resistors.

Re: Triode jumping ???

PostPosted: Wed Jan 01, 2014 5:15 am
by hilldweller
So this is incorrect?

Parallel triodes
nekhbet
In the diagram above - with the 12au7, two sections of a dual triode can be paralleled by strapping their grids and plates together. Paralleling triodes doubles the transconductance and halves the (internal) plate resistance - relative to a single triode section, but the mu remains the same. The noise performance improves significantly (though it does not double in improvement over a single triode). The cathode and plate resistor values used in parallel triode circuits are - approximately - one half of those in a single triode section.

Re: Triode jumping ???

PostPosted: Wed Jan 01, 2014 5:29 am
by Geek
No. it's not.

What they are doing is maintaining the current-per-section by halving the resistors. If you do what I suggest, you half the current. The DC operating point remains the same either way. With a 12AU7 you have an advantage because it likes current and has a lowish Rp. With a 12AX7, it doesn't matter too much as the Rp is relatively high compared to a 12AU7.

When I parallel sections, I do it for AC rather than AC and DC:

Image

The advantage here is you can use a tube with mkismatched sections and one section won't hog current.

Re: Triode jumping ???

PostPosted: Wed Jan 01, 2014 5:46 am
by kheper
hilldweller wrote:So this is incorrect?

Parallel triodes
nekhbet
In the diagram above - with the 12au7, two sections of a dual triode can be paralleled by strapping their grids and plates together. Paralleling triodes doubles the transconductance and halves the (internal) plate resistance - relative to a single triode section, but the mu remains the same. The noise performance improves significantly (though it does not double in improvement over a single triode). The cathode and plate resistor values used in parallel triode circuits are - approximately - one half of those in a single triode section.


No. Check out a circuit with one volt-amp and one that substituted for it with parallel triodes. The cathode and plate resistances are halved (the effective resistances are halved) - because they share the same resistors with two triode sections, instead of one.

viewtopic.php?f=4&t=601&p=3072&hilit=parallel+triodes#p3072

http://www.el34world.com/Forum/index.php?topic=14595.0

Re: Triode jumping ???

PostPosted: Sat Jan 04, 2014 5:33 pm
by hilldweller
Geek wrote:No. it's not.

What they are doing is maintaining the current-per-section by halving the resistors. If you do what I suggest, you half the current. The DC operating point remains the same either way. With a 12AU7 you have an advantage because it likes current and has a lowish Rp. With a 12AX7, it doesn't matter too much as the Rp is relatively high compared to a 12AU7.

When I parallel sections, I do it for AC rather than AC and DC:

Image

The advantage here is you can use a tube with mkismatched sections and one section won't hog current.


Thank you so much, but I am still not clear as to how to implement this. I am getting ready to build some rev.b eiclones with fixed bias and will be using 6SL7 instead of 12AX7.
The plate resistor is 100k and the cathode resistor is 1k. Would I just cut these values in half or do I cut the value in half and use that half value resistor twice (one on each section)? One 50k for both plates and one 500 ohm for both cathodes, or One 50k for each plate section (2) and one 500 ohm for each cathode section (2)? I really would appreciate anyone's help on this. (???)

Re: Triode jumping ???

PostPosted: Sat Jan 04, 2014 7:35 pm
by kheper
hilldweller wrote:The plate resistor is 100k and the cathode resistor is 1k. Would I just cut these values in half or do I cut the value in half and use that half value resistor twice (one on each section)? One 50k for both plates and one 500 ohm for both cathodes, or One 50k for each plate section (2) and one 500 ohm for each cathode section (2)? I really would appreciate anyone's help on this. (???)


If you want to share resistors (1 resistor for 2 plates and 1 resistor for 2 cathodes = 2 resistors), leave them at their original values (100K and 1k - respectively). If you are going for 1 resistor per plate and 1 resistor per cathode, halve the original values (50k per plate and 500ohm per cathode = 4 resistors). If you have (2) 50k and (2) 500ohm resistors on hand, go with the latter.

Re: Triode jumping ???

PostPosted: Sat Jan 04, 2014 7:45 pm
by hilldweller
Thank You for clearing up my brain fog (lol)
I'm a little rusty on this subject, have to clear out the cob webs and reboot (lol) What a drag it is getting old!

Re: Triode jumping ???

PostPosted: Mon Jan 13, 2014 5:52 pm
by Robert Cham
Conrad Johnson has been using what they call composite triodes for decades. They do line level preamps using them in a single gain stage with no feedback. The low output impedance (500 -800 ohms) allows them to do away with the cathode follower that they would normally use to get the output impedance down. They use between one and two 6922s or 6N30s per channel with their sections paralleled.

I've wanted for some time to emulate their ACT2 preamp, without such silliness as the remote controlled volume and input selection functions. Most of their capacitors are custom $240 each, 2 microfarad teflon units, even in the power supply. The unit costs something like $16K in it's present incarnation, but I think that it should be possible to duplicate it's basic design for a whole lot less, especially if one uses good, but lesser capacitors.

I'd want to keep the separate regulators for each channel as SS experiments back in the '70s convinced me that reducing the co-channel crosstalk using separate regulators dramatically improved the stereo image. As I'l be using 2 6N30s per channel and each tube draws 830 ma of filament current, it probably makes sense to use separate filament transformers for each channel as well.

Part of my reason for doing this is my old but great Tympani 1Cs. I'd like to bi-amp them with Dyna stereo 70s, one for each channel. I want to use Nelson Pass's phase coherent passive line level crossovers to drive the Dynas. The crossovers require a preamp that can drive a 5K load, The paralleled 6N30s should do it. If they don't then I'll have something interesting to sell!

While I've installed and maintained tube FM transmitters as a part of my job for the last 30 years, I can't claim to be a tube design engineer. SS is so much easier, but I don't like to listen to it at home! So, any help in design would be most welcome.

BTW, reversing the normal path, I've moved North from Texas to central Virginia to retire, build my geeky engineer's house, raise grapes and orchards, and produce various potables.

TIA

Bob Cham