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    A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

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    Anyone else have interest in trioding EL84 amp?

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    [ 2 ]
    33% [33%] 

    Total Votes: 6

    Tazsmonn

    Posts : 18
    Join date : 2009-01-18
    Age : 54
    Location : Richfield (S. Minneapolis) Minnesota

    A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by Tazsmonn on Sat Jun 13, 2009 11:15 pm

    For an El84 would you remove the green or green white wire from pin #9, insulate.

    Then put 100 ohm resistor from pin #9 to pin #7? Suspect

    If this is correct then to add a ultralinear to triode switch you would.

    Remove green or green white wire from pin#9 of each tube and solder to separate poles of DPDT switch on the same end.

    Solder wire from pin #9 to center pole of DPDT switch.

    Solder 100 ohm from pin #7 to DPDT switch opposite end from green or green white wire.

    All wiring to and from switch side must be to same tube.

    Also I once had a MKIV which I triode, did not work. Was wired correct. Asked a guy about it, he thought it may be osculating at radio frequency, suggested 220 ohms. It worked fine. Is this common?

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by Bob Latino on Sun Jun 14, 2009 11:00 am

    Tazsmonn,

    What you have said is correct. On an EL84 based amp the plate lead is pin #7 and the ultralinear screen tap is pin #9. On Dynaco amps the plate leads are blue or blue/white and the ultralinear leads are green or green/white.

    Your explanation of what to do is also correct on hard wiring the ST-35 or SCA-35 and also the DPDT swich wiring is correct the way you have mentioned it.

    Two things on the ST-35. It you want to use the switches you will have to drill two 1/4 inch holes somewhere in the front face of the amp to allow switching. If you put them on the top surface you will have to remove the tube cage to make the switch each time. On the SCA-35 you have more room on the back to drill two holes. A couple of caveats. Anytime you drill into an original Dynaco amp you usually devalue the amp somewhat even if your drilling results in an "improvement" in the amp's function or features. The other thing is that neither of these amps has much power to begin with and your 17.5 WPC in pentode will become about 10 watts in triode. If you have a smaller room and fairly efficient speakers though, this may not be an issue for you.

    Your issue with the Mark IV seems strange in that a 100 ohm resistor didn't work between pins 3 and 4 (pin 3 is the plate lead and pin 4 is the ultralinear screen tap on Dynaco amps with octal sockets) but a 220 ohm resistor did? I have always used a 100 ohm 1 watt carbon film resistor here and never had an issue. Strange that a 100 ohm resistor wouldn't work in the Mark IV ??

    Bob

    Tazsmonn

    Posts : 18
    Join date : 2009-01-18
    Age : 54
    Location : Richfield (S. Minneapolis) Minnesota

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by Tazsmonn on Sun Jun 14, 2009 1:32 pm

    Thanks Bob!!!

    I have very efficient speakers that can do with only 10w. Also I have been designing speakers for 30 years & would be Bi or t
    Triamping with a triode strapped EL84 amp.

    Anyone else if you do this tell us your results.

    HF9

    Posts : 4
    Join date : 2011-07-20
    Location : East Coast

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by HF9 on Wed Jul 20, 2011 10:49 am

    Sorry to bump a two year old thread, but I'll be trying this soon with the ST35 I'm currently building: http://www.diyaudioblog.com/2011/06/customized-dynaco-st35-kit.html

    Has anyone actually added this functionality to their ST35 yet? I almost always prefer my ST-70 in triode mode, seems a little more warm with perhaps a bit more midrange bloom.

    HF9

    Posts : 4
    Join date : 2011-07-20
    Location : East Coast

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by HF9 on Tue Aug 09, 2011 4:09 pm



    Here's a photo of my Dynaco ST35 with Triode switches. Thanks for the insight Bob, this old post came in handy Wink The rest of the amp and build notes are here: http://www.diyaudioblog.com/2011/06/customized-dynaco-st35-kit.html

    I actually found that the speakers that I have connected, a very small pair of Tektons with 4" full-range Fostex drivers, sounded better in ultra-linear mode, perhaps they needed that extra little bit of drive. But at least I'll always have the option now Very Happy

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by Bob Latino on Tue Aug 09, 2011 4:42 pm

    I did see the photos of your ST-35 build on some of the other forums. Probably the nicest custom ST-35 build I have ever seen. You can tell a lot of time, effort and care went into your build. Thanks for the link to the "diyaudioblog" and the other photos and the explanations on that web site ...

    Bob

    HF9

    Posts : 4
    Join date : 2011-07-20
    Location : East Coast

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by HF9 on Wed Aug 10, 2011 4:25 pm

    Thanks very much for the kind words Bob. I hope one day when $$ permits I'll be able to put together a set of those hot looking M125 monoblocks you've just rolled out Wink Keep up the great work!

    tubes4hifi
    Admin

    Posts : 1291
    Join date : 2008-11-30

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by tubes4hifi on Wed Aug 10, 2011 9:24 pm

    Bryan,
    just curious where you got those special sockets you used on your ST35 build?

    HF9

    Posts : 4
    Join date : 2011-07-20
    Location : East Coast

    Re: A question for Bob or anyone, ST35, SCA35 or any EL84 based amp to triode mode.

    Post by HF9 on Thu Aug 11, 2011 11:19 am

    The Teflon sockets I found on that auction site. You can also find similar ones at PartsConnexion and WooAudio, those two vendors also selling the corresponding PCBs.

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