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

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    Post by Bob Latino Sat Jan 17, 2009 9:17 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
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo Triode-PentodediagramforST-70_corre
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    Post by jrethorst Sun Feb 01, 2009 6:17 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
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    Post by Bob Latino Sun Feb 01, 2009 7:21 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. (NOTE - In late 2016 Sovtek discontinued the production of KT88 output tubes. We have found that the Sovtek 6550 works well in these amps and is also modestly priced) About $130 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


    Last edited by Bob Latino on Sat Mar 02, 2019 9:19 pm; edited 2 times in total

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    Post by jrethorst Mon Feb 02, 2009 2:10 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!
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    Post by Bob Latino Mon Feb 02, 2009 6:49 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,

     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. 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 that reasonably priced is the Sovtek. I bought a pair of Mark III's from someone and the Sovteks came along with the two amps. These are very nice sounding tubes also.

    Bob


    Last edited by Bob Latino on Sun Oct 08, 2017 6:25 pm; edited 1 time in total
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    Post by Bugs Fri Apr 17, 2009 2:13 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
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    Post by Bob Latino Fri Apr 17, 2009 3:16 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 3:59 pm; edited 2 times in total
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    Post by Bugs Fri Apr 17, 2009 3:45 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?
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    Post by Bob Latino Fri Apr 17, 2009 4:44 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
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    Post by Bugs Sat Apr 18, 2009 12:15 am

    Thanks Bob.
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    Post by SFORZANDO0 Sun Jul 05, 2009 7:07 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
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    Post by Bob Latino Sun Jul 05, 2009 7:16 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 5:10 am; edited 1 time in total
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    Post by SFORZANDO0 Sun Jul 05, 2009 7:24 am

    Hi Bob,

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

    Regards,

    Marc
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    Post by PerryC Sun Aug 15, 2010 5:07 am

    I love the way it sounds. That's what you call SWEET!!!
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    Post by Guest Wed Sep 15, 2010 7:08 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!
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    Post by Bob Latino Wed Sep 15, 2010 7:18 pm

    Michael Noroña wrote: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!

    Hi,

    No - I am sure that a 110 ohm resistor would work also. I have used a 50 ohm resistor in there and the amp sounds the same. The 110 ohm resistor will work just fine ..

    Bob
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    Post by Guest Wed Sep 15, 2010 7:38 pm

    Bob: Thanks for the prompt reply with the info. I am thankful that you have this explanation posted here. The following are directions I received from a major supplier of Dynaco parts, on how to covert to triode. They just didn't make sense so I checked the web and found your instructions.

    Find the green (not green/white) wire (one from the output transformer) hooked to pin 4 on an EL34, there is also a wire leading from that pin to pad 4 on the circuit board.
    Disconnect the green wire from pin 4 and run directly to pad 4 of the circuit board.
    Remove the wire that previously ran from pad 4 to the EL34/6550 sockets.
    Disconnect the screen tap lead (green/white) from pin 4 of the other output tube, then insulate the end of the wire so that the bare leads are not exposed.
    Install a 100 ohm (1/2 watt minimum) resistor between pin 3 and pin 4 of each of the output tube sockets.
    Reinstall the output tubes, and recheck bias (BIASET on front panel). This will result in a 10 to 20 watt per channel.

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    Post by Guest Wed Sep 15, 2010 7:48 pm

    Is it a good idea to increase the size of the 50uF bias supply caps ?
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    Post by Bob Latino Wed Sep 15, 2010 7:54 pm

    Hi Michael,

    Yes - those instructions that he gave you for converting your amp ARE confusing.

    One other thing that I didn't mention at the beginning explanation is that the bias will drop slightly when you convert to triode mode. If you were at the standard 1.56 VDC when you convert to triode the bias will drop very slightly (and insignificantly) to maybe 1.53 VDC. If it bothers you then just turn the bias back up again.

    A 1.56 VDC bias on a stock ST-70 gives a bias current of 50 milliamps per each output tube. This is a little high by todays standards. Dynaco really pushed those EL34's hard but those original Mullard EL34's were a lot tougher than today's EL34 tubes. Conventional wisdom is to run a stock ST-70 at about 1.25 VDC which will give about 40 milliamps per each output tube. The amp will sound the same and your output tubes will last longer.

    Yes - you can increase the size of the 50 uF bias supply caps to 100 uF with no problem. All you need is a 100 uF @ 100 volt cap because those caps only have to deal with about -70 VDC. There is no need to increase those two caps beyond 100 uF and no real need to go above a 100 volt cap.

    Bob
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    Post by Guest Wed Sep 15, 2010 11:17 pm

    Bob: It turns out that the 100 Ohm resistors I have are only 1/2 watt. Will they work, or should I get some 1 watt?

    Finally I was reading H. L. Eisenson's Tu-be, or not tu-be, and he proposes some pretty serious mods for the ST-70, which I assume could be applied to the MK IV's. Do you know of any one who has done the mods he proposes, and whether or not they felt the changes were good? Thanks again for your help!
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    Post by Bob Latino Thu Sep 16, 2010 7:27 am

    Michael Noroña wrote:Bob: It turns out that the 100 Ohm resistors I have are only 1/2 watt. Will they work, or should I get some 1 watt?

    Finally I was reading H. L. Eisenson's Tu-be, or not tu-be, and he proposes some pretty serious mods for the ST-70, which I assume could be applied to the MK IV's. Do you know of any one who has done the mods he proposes, and whether or not they felt the changes were good? Thanks again for your help!

    Michael,

    A 1/2 watt resistor should work fine in there. You probably don't really need a 1 watt resistor.

    If you want to upgrade your Mark IV's then get in touch with Roy Mottram on the tubes4hifi web site > www.tubes4hifi.com He offers an upgrade board for the Mark IV that would give you a significant increase in sound quality from your Mark IV's.

    Bob
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    Post by oatstao Sun Oct 23, 2011 3:59 am

    thanks for sharing this fabulous circuit.
    Very handy.

    I'm currently using my DYN-stereo 70
    it can run Stereo fine but I'm augmenting it
    with a stereo Yamaha Natural Sound on my desktop
    to run a single JBL 4311 on the floor for the lowend.
    I'm running the JBL in bridged mode (connecting one speaker
    to the two positive terminals on the DYNaco 70)

    Is there a benefit to adding the triode scheme to that setup?
    Sorry if it's a redundant query.

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    Post by Roy Mottram Sun Oct 23, 2011 8:52 pm

    triode mode is normally used more to enhance midrange than for bass, and it would provide less power for bass,
    so I'd suggest you continue to run it as is
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    Post by Bob Latino Sat Jan 07, 2012 11:25 am

    Re: > "I modified my first stereo 70 with a triode/ult switch probably 10 yrs ago. I used a single switch with 12 connections so that I didn't have to switch two switches."

    The purpose of the two switches is to allow each channel to be switches separately AND use the two original bias pot holes for the switches. These holes are not used on the VTA version of the ST-70. There are some audiophiles that use TWO ST-70's and vertically biamp with the woofers in ultralinear and the tweeters in triode - and as such, there is an advantage to have separate switches for each channel. Yes - I know there is less output in triode but a number of audiophiles have told me that they like the way their system sounds like with the woofers in ultralinear and the tweeters in triode.

    Re: >"I do not however believe in throwing the switch on the fly because any switch you use in that location is NOT rated for the voltage it sees so my recommendation as well as others on other forums is to not throw the switch on the fly for safety sake."

    There is negligible voltage difference from ultralinear mode to triode mode on Dynaco amps and, as such, there is no harm to switching the amp while the amp is running as long as you don't do it at higher volume levels. You should turn the volume all the way down when making the switch. With the switches used on the VTA kits all you hear is a tiny "blip" when the switch is made. There is no harmful voltage spike when the switch is made because the voltage gradient is very low.

    Bob
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    Post by Roy Mottram Sat Jan 07, 2012 3:39 pm

    I'm not so concerned with the switch itself, a good quality switch doesn't have to have a high voltage or high current rating here (use a toggle, not a rotary)(the voltage difference across the switch is less than 10v),
    my concern is that during that millisecond of switching, that little glitch or pop could take out $100+ worth of tubes (or worse, your speakers). For that I can wait a minute or two to shut the amp down, let it cool off slightly (don't want to pop the HV rectifier when you turn it back on hot) make the switch, then power up.
    It's been said a thousand times, you'll have more bass and treble (and more power) in ultra-linear,
    but you'll have a better midrange in triode. The difference is very similar to the difference between
    the sound of KT88s and EL34s.
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