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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

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    Post by Guest Sat Jan 07, 2012 8:46 pm

    Ideally, if you had an audio version of heavy duty jumpers, similar to non-fused HVAC service disconnects, you could do it that way. It still would not be advisable to change modes it with the amps on, but you would not have to worry about voltage passing through the thing.

    Ask far as the sound of anything who can say what's right. A friend of mine bought a bunch of Mark Levinson gear (ML-2s ML-7, plus a pair of Quad 63s) from a guy named Peter McGrath. Michael said that McGrath always said that with any gear, it was a case of choose the distortion you like best. It's the old: Ford or Chevy, Martin or Gibson, etc. I honestly feel that the pursuit of audio perfection can be a very long road with no end. I like to sit and listen every once in a while, so I don't what to let tweaking become an end in itself. I can't remember exactly who, but I think it was Picasso that said that, paintings are never finished, they are merely abandoned.
    Bigron865
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    Post by Bigron865 Thu Apr 04, 2013 8:13 pm

    Bob I know this post is kind of old but its new to me. Does all of this still pertain to the newest low gain or classic vta mods that I just did? I'd love a sweeter sound. I have the PIO caps and an upgraded quad cap on order. Should I leave it in pentode till I break those in and then try the triode mode?

    Ron
    Bob Latino
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    Post by Bob Latino Thu Apr 04, 2013 8:19 pm

    Hi Ron,

     Yes - The triode mode also applys to the lower gain CCS VTA driver board. In triode mode you only have about 60% of the power of ultralinear but the amp but the amp sounds a little "smoother" with more mid range projection. This is nice for vocals and acoustic jazz.

    I don't think it makes any difference which mode of operation you use to break the amp in ...

    Bob


    Last edited by Bob Latino on Sat Mar 02, 2019 9:16 pm; edited 1 time in total
    Bigron865
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    Post by Bigron865 Fri Apr 12, 2013 3:44 pm

    Bob I bought 2 switches today from a local electronics supplier. They look like the blue ones in your pictures but these are red. The rating was 10a 250v ac. Will these work for triode/pentode switches?
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    cheehpogi

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    Post by cheehpogi Tue Mar 10, 2015 10:25 pm

    Hello from KY, my first post here and I would need some help. I did the resistor mod on a newly acquired st 70,well before the mod for me I think the amp sounded great but after the mod and adjusting the bias it seem flat so I revert it back to the original pentode mode and bias was still at 1.30 but to me it still sounded flat, is there anything that I did wrong? maybe I biased it wrong. What I did was the black probe on ground and the red probe I stuck it to where it would read 1.5++ and then adjusted the bias screw until the numbers went down to 1.30. Did I miss or messed up something? Newb here need some guidance. Thanks.
    peterh
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    Post by peterh Wed Mar 11, 2015 2:39 am

    If it is a "dynaco" the bias should be 1.5V
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    cheehpogi

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    Post by cheehpogi Wed Mar 11, 2015 9:59 am

    peterh wrote:If it is a "dynaco" the bias should be 1.5V  
    Reading on Bob's previous post, he stated that stock dynaco tubes should run at around 1.25 since we are not using the original mallard tube.
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    rovano

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    Post by rovano Sat Mar 02, 2019 8:51 pm

    Bob Latino wrote: You can also install DPDT (dual pole dual throw) triode/pentode switches and make the move from triode to pentode easily switchable. The only problem is where to put the switches. Bob

    When you still use the bias pots and you don’t want to compromise the chassis, you may want to consider this possibility.
    I created a sub frame that I mounted on extended screws of the tube sockets so that the switch handle sticks through the ventilation slot. This is how I did it:

      Top view:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Triode16

      Bottom view:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Triode15

     Sub frame:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Triode14

      Left channel:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 P1040910

     Right channel:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 P1040911

     Top view:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 P1100710

    See my complete point to point wired rebuilt:

    https://dynacotubeaudio.forumotion.com/t973-st70-rebuild-hard-wired

    Rovano
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    Dogstar

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    Post by Dogstar Tue Sep 03, 2019 3:37 am

    Last night I had my amp cranked listening to ZZ Top and I noticed what to me sounded like harshness when listening to higher pitched notes. So....I switched to Triode mode and the music was much smoother. I’m using Gold Lion KT88’s with maybe 500 hours on them, a Webber rectifier and a 6SN7 tube in the center socket on the board. Speakers are Klipsche with Bob Crites crossovers and custom cabinets.

    I thought the tubes might be going bad but the system really did sound good in Triode mode.

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    tubonos

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    Post by tubonos Tue Mar 03, 2020 3:40 pm

    I think that there's differences between this post, and the possibly newer versions of the instructions and the diagrams that come with the ST70. I'm comparing against these documents, and the diagram for the output transformers, and it seems that in the recent diagrams, the amp is shown with pins 3 and 4 having one wire each to the output transformers with no resistor.
    What mode would this be: pentode? Triode?
    Bob Latino
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    Post by Bob Latino Tue Mar 03, 2020 3:59 pm

    tubonos wrote:I think that there's differences between this post, and the possibly newer versions of the instructions and the diagrams that come with the ST70.  I'm comparing against these documents, and the diagram for the output transformers, and it seems that in the recent diagrams, the amp is shown with pins 3 and 4 having one wire each to the output transformers with no resistor.
    What mode would this be: pentode?  Triode?  

    In order to switch between pentode ultralinear mode and triode mode you need a pair of switches > one for each channel. If the amp has a single wire connected to pins 3 and 4 of each output tube, then the amp is hard wired into pentode/ultralinear mode. See below ..

    Bob

    Below > VTA ST-70 wired to switch between pentode ultralinear mode and triode mode.

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 ST-70-photograph-new-K42-Y

    Below > Original Dynaco ST-70 pictorial hard wired into pentode ultralinear mode.

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 ST-70-pictorial-with-stock-driver-board
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    tubonos

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    Post by tubonos Tue Mar 03, 2020 4:24 pm

    Thanks bob, you're awesome as always. This shows as a new account but you've been helping me now for 5+ years (not that I often need it) Smile
    Pillo69
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    Post by Pillo69 Fri Mar 06, 2020 4:41 pm

    A few years ago, when the forum was not exclusive to VTA, there were schemes and ideas for some modifications on the original ST70.
    There was the option of placing the Bias potentiometers on the front, using the location of the chassis for Pentode / triode switches.
    (I hope the attached photograph is not illegal).

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Bias-frontal
    Bob Latino
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    Post by Bob Latino Fri Mar 06, 2020 5:14 pm

    Pillo69 wrote:A few years ago, when the forum was not exclusive to VTA, there were schemes and ideas for some modifications on the original ST70.
    There was the option of placing the Bias potentiometers on the front, using the location of the chassis for Pentode / triode switches.
    (I hope the attached photograph is not illegal).

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Bias-frontal

    The forum is not exclusive to VTA amps. The forum is for all types of Dynaco TYPE tube gear. Your photograph shows an option for bias from Dynakitparts that you may use on original Dynaco type amps or the amps from Dynakitparts.

    Bob
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    Post by jrethorst Sun Jul 04, 2021 12:23 am

    Bob Latino wrote:If you use EL34's in a Mark III they will probably have a short life. KT88 or 6550 tubes are best in a Mark III.

    I've seen advice elsewhere to bias a Mark III down to .9 if running EL34s. Would that still be good advice after the mod to triode mode?

    Thank you
    John
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    Post by Bob Latino Sun Jul 04, 2021 8:50 am

    jrethorst wrote:
    Bob Latino wrote:If you use EL34's in a Mark III they will probably have a short life. KT88 or 6550 tubes are best in a Mark III.

    I've seen advice elsewhere to bias a Mark III down to .9 if running EL34s. Would that still be good advice after the mod to triode mode?

    Thank you
    John

    People like to run EL34's in a Mark III because EL34's are a relatively inexpensive output tube. An EL34 is not make to look at 500 volts DC on the plates of the tubes. An EL34 is supposed to run about 435 - 450 volts on the plates. If you run the amp in triode mode, that does not change the amount of voltage on the plate of the tube (pin # 3). Running the amp in triode mode just removes the ultralinear connection and allows the output tube to run in a simpler, less efficient mode (with less power).

    Bob

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    jrethorst

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    Post by jrethorst Mon Jul 05, 2021 11:59 am

    Bob Latino wrote:
    jrethorst wrote:
    Bob Latino wrote:If you use EL34's in a Mark III they will probably have a short life. KT88 or 6550 tubes are best in a Mark III.

    I've seen advice elsewhere to bias a Mark III down to .9 if running EL34s. Would that still be good advice after the mod to triode mode?

    Thank you
    John

    People like to run EL34's in a Mark III because EL34's are a relatively inexpensive output tube. An EL34 is not make to look at 500 volts DC on the plates of the tubes. An EL34 is supposed to run about 435 - 450 volts on the plates. If you run the amp in triode mode, that does not change the amount of voltage on the plate of the tube (pin # 3). Running the amp in triode mode just removes the ultralinear connection and allows the output tube to run in a simpler, less efficient mode (with less power).

    Bob

    Point well taken, but I like the sound of the EL34 as compared to KT88 or 6550. Would setting a lower bias reduce the voltage presented to the tube?

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