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

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

    Post by Bob Latino on Sat Jan 17, 2009 7:17 am

    Any Dynaco ST-70, Mark II, Mark III or Mark IV may be run in TRIODE mode as well as the normal pentode ultralinear mode of operation. You can hard wire these amps in the triode mode by two simple modifications.

    1. Disconnect the GREEN or GREEN/WHITE wire from pin 4 of each output tube and insulate the bare end so it doesn't short out against the chassis.
    2. Solder in a 100 ohm 1 watt resistor between pins 3 and 4 of each output tube.

    You are now in triode mode. The amp will now have only about 60% of the power it had before but you may like the subtle change in sound. The amp will have a little more mid range projection. Singers will move slightly forward in the sound stage. The extreme top and bottom end of the frequency spectrum will be rolled off very slightly from what it was in pentode mode but you probably won't notice any change in frequency response. The amp will sound a little "smoother".

    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. If you replace your stock driver board with the VTA driver board the choice is easy. You don't need the stock bias potentiometer with the VTA so just pull it out and you now have a nice 3/8 inch hole which will be suitable for installing your DPDT switch. If you have a stock driver board then you may be able to find a place for the switch(es) inside the chassis. You could also drill a couple of extra holes in the chassis somewhere. Get a high quality switch - one that can deal with 400 + volts DC. The diagram below shows a switch installed for one channel of an ST-70 which would also be applicable for the mono Mark II, III and IV.

    Bob


    jrethorst

    Posts: 18
    Join date: 2009-02-01

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

    Post by jrethorst on Sun Feb 01, 2009 4:17 pm

    For the Mk III in triode, what differences are there between the KT88 and 6550 and EL34 output tubes (the EL34 requires another small circuit modification)? I'm using the Triode Electronics upgrade board, if that matters.

    BTW nice forum! Just found it.

    Thanks, John

    Bob Latino
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    Re: Triode/pentode on a Mark III

    Post by Bob Latino on Sun Feb 01, 2009 5:21 pm

    John,

    When you go from pentode to triode the sonic changes are pretty much the same in all Dynaco amps whether you are using a KT88, 6550 or EL34 tubes. Your power drops to about 60% of what it was but the amp seems to sound a little "smoother". Which mode you like best depends somewhat on your musical tastes and your associated equipment. Triode seems to sound better with small group acoustic jazz combos and vocalists. If you are a rock fan you may favor the pentode mode ? You will get a little more bottom end smack with pentode.

    As for the three tubes that you mention the "tubiest" sounding is the EL34 with a little more mid bass output than the KT88. An EL34 has a little less extreme bottom and extreme top end response than a KT88. The 6550 is somewhere in between the EL34 and the KT88. In a Mark III I really wouldn't use an EL34 because it wasn't designed for 475 - 500 volts DC on pins 3 and 4. 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. A relatively inexpensive and good sounding KT88 is the Sovtek KT88. About $100 for a matched quad. These have proven to be durable and have that interesting "coke bottle" shape. The Sovtek's also have a tri-alloy plate structure which is said to give a higher minimum plate current for better transconductance.

    Yep - we are all friendly here. No one has any axe to grind ...

    Bob

    jrethorst

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    Join date: 2009-02-01

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

    Post by jrethorst on Mon Feb 02, 2009 12:10 am

    Bob Latino wrote:Triode seems to sound better with small group acoustic jazz combos
    Just what I'm looking for, along with classical, which would also probably benefit from the increased smoothness, and not miss the enhanced high and low ends that rock listeners might prefer.

    I have not installed the Triode Electronics boards yet, but that's my next project, after which I'll look for tubes. Any thoughts on KT88 vs. 6550? Thanks!

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

    Post by Bob Latino on Mon Feb 02, 2009 4:49 am

    jrethorst wrote:
    Bob Latino wrote:Triode seems to sound better with small group acoustic jazz combos
    Just what I'm looking for, along with classical, which would also probably benefit from the increased smoothness, and not miss the enhanced high and low ends that rock listeners might prefer.

    I have not installed the Triode Electronics boards yet, but that's my next project, after which I'll look for tubes. Any thoughts on KT88 vs. 6550? Thanks!

    John,

    KT88's can take higher plate voltage and dissipate more wattage, so they have the potential with the right power supply to give a few more watts than a 6550. Some say a KT88 is more of a "hi-fi" tube and a 6550 is better in guitar amps but I don't find that to be necessarily true. KT88's tend to be a little more expensive than 6550 tubes. The best production KT88 out there now is the Genalex Gold Lion KT88 reissue but it is expensive > $45 - $50 a tube. A great 6550 tube is the Svetlana Flying =C=. I bought a pair of Mark III's from someone and they came along with the two amps. These are very nice sounding tubes also.

    Bob

    Bugs

    Posts: 91
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    Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Bugs on Fri Apr 17, 2009 1:13 pm

    Bob, how does this work with the VTA board if you want to switch between triode and
    pentode? I believe the VTA is in triode to start with.

    Thanks,
    Bugs

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

    Post by Bob Latino on Fri Apr 17, 2009 2:16 pm

    Hi Bugs,

    The DRIVER CIRCUIT on the VTA driver board is all triode circuit but that doesn't make the amp run in triode. To run the ST-70, Mark III etc. in triode you must disconnect the ultralinear screen tap at pin #4 ON THE OUTPUT TUBES and then connect a 100 ohm 1 watt resistor between pins 3 and 4. You can do this easily with a switch on any ST-70, Mark III etc. with a VTA driver board because the VTA driver board has its own bias system doesn't use the original bias system. You can then just pull the original bias potentiometer and now you have a nice 3/8 inch hole where you can install a DPDT switch. On a mono amp like the Mark II, Mark III or Mark IV you need just one DPDT switch. On an ST-70 you need two DPDT switches. The diagram for the switches is above for the left channel of an ST-70. You would just do the same thing for the right channel on an ST-70.

    Bob


    Last edited by Bob Latino on Thu May 28, 2009 2:59 pm; edited 2 times in total

    Bugs

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

    Post by Bugs on Fri Apr 17, 2009 2:45 pm

    Thanks Bob.

    I'm probably showing how new I am to all of this, but what is the difference between a triode circuit and running in triode mode?

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

    Post by Bob Latino on Fri Apr 17, 2009 3:44 pm

    Bugs,

    An all triode driver circuit is a circuit that uses only triode tubes. Triode tubes have THREE parts > an anode, a cathode and one screen. Triode driver circuits are simpler and if properly designed are usually superior to driver circuits that use pentode tubes. Pentode tubes have five parts - an anode, a cathode and three screens, are more complex and are difficult to use in a driver circuit because they are not as linear as triode tubes at all drive levels. Dynaco used pentode/triode driver circuits because it let them get by with only ONE driver tube (a 6AN8 or a 7199 tube) in each channel. This allowed Dynaco to save ONE tube when you really need "a tube and a half" per channel if you use an all triode design. In most upgraded Dynaco ST-70 driver boards like the VTA driver board you have THREE dual triode tubes. The center tube is the initial voltage amplifier for BOTH channels while the two side tubes do the phase splitting/phase inverting for each channel.

    To run the amp in "triode mode" you take a tube which is a pentode (5 section tube) like an EL34 or KT88 and you tie one of the screens to anode with a resistor making it a "quasi triode" - in other words you make the pentode output tube act like a triode output tube. The only downside is that power output is now only about 60% of what it was as a pentode BUT the amp will sound a little different. In triode mode a pentode tube will sound a little "smoother" and the midrange will be projected forward a bit. Depending on the type of music you listen to and your system components you may or may not like the sound. I like the triode mode for acoustic jazz and vocals. If you are into classical music or are a rock fan you will probably like the pentode mode since it has more power and a little bit more output at the extreme top and extreme bottom of the frequency range.

    Bob

    Bugs

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

    Post by Bugs on Fri Apr 17, 2009 11:15 pm

    Thanks Bob.

    SFORZANDO0

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    Join date: 2009-07-05

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

    Post by SFORZANDO0 on Sun Jul 05, 2009 6:07 am

    Hello Bob,

    I have two sleeping dynaco st70 II (1992 version)different of the original one.

    Do you thing it is possible to modify this version in triode mode without many problems?

    Nice site!

    Regards from Belgium,

    Marc

    Bob Latino
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    Posts: 1881
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    Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Bob Latino on Sun Jul 05, 2009 6:16 am

    Hi Marc,

    Although the driver circuit on the '92 version of the ST-70 is different from the older original ST-70, the output tube pin outs are the same. Pin 3 is the plate lead and pin 4 is the ultralinear screen tap. Alteration of this ST-70 to be set up for the amp to work in triode (with or without a switch) will work exactly the same way.

    Bob


    Last edited by Bob Latino on Tue Jul 07, 2009 4:10 am; edited 1 time in total

    SFORZANDO0

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    Join date: 2009-07-05

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

    Post by SFORZANDO0 on Sun Jul 05, 2009 6:24 am

    Hi Bob,

    Good to know, thank's for this very quick response!
    I 'll go this way and let you know,

    Regards,

    Marc

    PerryC
    Guest

    Triode Convert

    Post by PerryC on Sun Aug 15, 2010 4:07 am

    I love the way it sounds. That's what you call SWEET!!!

    Guest
    Guest

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

    Post by Guest on Wed Sep 15, 2010 6:08 pm

    Would there be a problem if you use a 110 ohm resistor, rather than the 100? I have some 1 watt 110 ohm on hand that are 1% tolerance. I am not sure if you need a 1% in this application. Thanks!

      Current date/time is Mon Jul 28, 2014 11:36 pm