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Zen triode/pentode switch? (Read 2436 times)
Chris_G
Ex Member



Zen triode/pentode switch?
01/04/07 at 17:46:13
 
Is there any easy way for me to take my old Zen triode and add a switch to put it into pentode mode for more oomph?

Would I just put a switch linking the screen grids to the cathodes? My SE84C is otherwise stock, from whatever version of the kit was being sold in 02-03'.

Thanks for any advice y'all can give.

-Chris
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Eli Duttman
Ex Member



Re: Zen triode/pentode switch?
Reply #1 - 01/05/07 at 00:26:01
 
IMO, you should abandon the pentode switch idea.

Pentode mode requires the screen grid be connected directly to B+.  For best fidelity, the screen grid B+ should be regulated at approx 0.5X of the anode supply.  Also, connecting SV83 screen grids to the anode supply is out because of dissipation limits.

The O/P impedance of pentode mode is HIGH.  NFB of some kind is essential to lower that O/P impedance and obtain a tolerable damping factor.  Are you up to the engineering challenge?

THE way to get more "sock" out of a SE84 is the use of 2 Ohm speakers.
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Chris_G
Ex Member



Re: Zen triode/pentode switch?
Reply #2 - 01/05/07 at 12:20:52
 
I was just wondering if there was a way to insert a DPDT switch in the circuit somewhere to make it like the first Zens -- the ones Steve abandoned, because nobody used the pentode option -- so that I could easily switch between the two modes without redoing the whole amp.

The old schematic is not on the Decware site, and the schematic for the pentode amp he makes has a different PS. Is it kosher to ask here whether that original schematic is floating around?

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Lon
Ex Member



Re: Zen triode/pentode switch?
Reply #3 - 01/05/07 at 12:52:44
 
Well, it's a doable thing, Steve did it.  I used to have an amp that way. . . but I think he abandoned it when he made Rev. A possibly because of the change to SV83, probably because few preferred this setting.

I have an amp that was first made before Rev. A (#27 of Zen amps) and then revised. When I had the triode/pentode switch I really preferred the triode by a wide margin.  The amp went from laidback to forward, and I'm just NOT a fan of forward sound.

In your shoes, I would sound out Steve about doing a mod or allowing you to have the information to do the mod. . . if it's feasable now with SV83s.
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« Last Edit: 01/05/07 at 12:53:49 by Lon »  
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chrisby
Ex Member



Re: Zen triode/pentode switch?
Reply #4 - 01/05/07 at 17:39:37
 
Lon / Chris, I think Eli hit the nail on the head - the SV83 was not originally designed as an audio output device, and has some definite limitations in pentode or U/L mode.

IINM, as the model designation suggests, Steve's original SE84 circuit was designed for the EL84 not the SV83.

Suggested reading :

http://www.boiaudioworks.com/TubeData/SV83/Technical%20Bulletin%2029.htm


Mr Barbour is quite familiar with the SV83
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« Last Edit: 01/05/07 at 17:42:27 by chrisby »  
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Chris_G
Ex Member



Re: Zen triode/pentode switch?
Reply #5 - 01/05/07 at 17:48:41
 
Sorry, I should have mentioned I was planning to pull the sv83s and use EL84s.
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veryoldcat
Ex Member



Re: Zen triode/pentode switch?
Reply #6 - 01/05/07 at 18:35:09
 
I'm not sure about the electrical particulars of what happens to the sv83 connected in pentode mode, but geesh, it sure sounds EXTREMELY GOOD connected as a pentode in the Taboo.

Couldn't help but say... Kiss

Karl

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chrisby
Ex Member



Re: Zen triode/pentode switch?
Reply #7 - 01/05/07 at 19:21:40
 
[quote author=veryoldcat  link=1167936373/0#6 date=1168022109]I'm not sure about the electrical particulars of what happens to the sv83 connected in pentode mode, but geesh, it sure sounds EXTREMELY GOOD connected as a pentode in the Taboo.

Couldn't help but say... Kiss

Karl

[/quote]


Karl, FWIW the operating voltages noted on Steve's original assembly instructions for the kit version of SE84 series, even after converting to SV83 device and eliminating the pentode switch, are more than 100V in excess of Svetlana's recommended maximum for the screen.


http://www.tubezone.net/pdf/sv83.pdf


I have no argument that both the SE84 and Taboo are great sounding amps. While not owning either, I've heard both in my personal and other very familiar systems.

Assuming that much of the character of the earlier amps and Taboo etc, are derived from the selected operating points, it might be opined that long term service life could be shortened when operating these tubes beyond max ratings.  
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Chris_G
Ex Member



Re: Zen triode/pentode switch?
Reply #8 - 01/05/07 at 19:29:48
 
Thanks. I have the datasheets for both the EL84 and the SV83, so I have all the max ratings in front of me.

Does anybody have any information on adding a triode-pentode switch?
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chrisby
Ex Member



Re: Zen triode/pentode switch?
Reply #9 - 01/05/07 at 19:53:37
 
[quote author=Chris_G  link=1167936373/0#8 date=1168025388]Thanks. I have the datasheets for both the EL84 and the SV83, so I have all the max ratings in front of me.

Does anybody have any information on adding a triode-pentode switch? [/quote]


contact Steve  -  he'll certainly have the required circuitry and his own observations to make.

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