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    MKIV : Ultra LInear or Triode ?

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    MKIV : Ultra LInear or Triode ?

    Post by Guest on Sun Oct 23, 2011 3:09 am

    Is there any consensus on using the MKIV's in triode or ultra linear mode? Is this a case of the modes being different, rather than one or the other being "better"? In triode, clearly you lose power, are there any drawbacks for either mode beyond this one? There is a good bit written about this subject, but I never get the feeling that any one wants to commit to saying that one or the other is clearly better. The amp is designed as an ultra-linear, so maybe that's an answer in itself. Thanks for any thoughts you have on this.

    Bob Latino
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    Re: MKIV : Ultra LInear or Triode ?

    Post by Bob Latino on Sun Oct 23, 2011 11:00 am

    There is really no "best mode" of operation for the Dynaco/VTA amps ... Depending on you musical tastes and your associated equipment, you may prefer one over the other. In general, rock music is probably better in pentode/ultralinear mode. You have a little more power and more energy in the bass region. Triode is nice for vocals and acoustic jazz. Sometimes certain speakers take to one of the modes better than the other. Many times triode mode will "tame" somewhat slight top end harshness found in some speakers. Pentode ultralinear may make up for, to a degree, slightly lowered bass response found in bookshelf speakers. The only thing to do in any amp with a triode/ultralinear switch is to try it both ways and see which way sounds better to you ..

    Bob

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    Tiode a different way?

    Post by Guest on Sun Oct 23, 2011 12:36 pm

    Bob: Some time ago I was talking to another guy about switching the MKIV's to triode, and I was confused about the process because if was different from your's. He got back to me to explain the difference, and I never passed it on. So here it is many months later.

    What you are describing utilizes the absence of Feedback in triode mode, our method incorporates the use of feedback, which I prefer.

    For Running your MkIV in Triode Mode:

    1. Remove the Green Wire from the OT that goes to Pin 4 on one of the power tubes. (Do not disconnect the Green/White wire going to pin 4 on the other power tube.)

    2. Remove the wire that went from pin 4 on that socket to pad #7 on driver board.

    3. Run the disconnected Green wire directly to pad #7 on the driver board.

    4. Install a 100 ohm 1/2W or better resistor between pins 3 & 4 on each power tube socket.

    You may try both ours and Bob Latino's method to see which yields the best sound to your ears.

    My question is, will this work with the VTA boards, because I suspect this is for the stock Dynaco board?

    Thanks! Michael

    Bob Latino
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    Re: MKIV : Ultra LInear or Triode ?

    Post by Bob Latino on Sun Oct 23, 2011 1:14 pm

    Michael,

    If you remove the GREEN wire from pin #4 and don't remove the GREEN/WHITE wire from the other tube, then the tube running with the GREEN/WHITE wire still attached is still running in pentode ultralinear. Pin # 4 on Dynaco ST-70/ST-120's/ Mark III's and Mark IV's is the ultralinear screen tap.

    What you are describing is some type of "hybrid" mode of operation with one tube of the pair in triode (the one with the GREEN wire removed from pin #4) and one tube still in ultralinear mode (the one with the GREEN/WHITE wire attached to pin #4). I am not saying that it won't work, I am just saying that the channel being utilized in this mode of operation is not being run in true triode mode.

    As to your statement > "What you are describing utilizes the absence of Feedback in triode mode". The feedback is still being utilized when you disconnect BOTH the GREEN and GREEN/WHITE wires and run a 100 ohm resistor between Pins 3 and 4 because you are only altering the wiring on the PRIMARY side of the output transformer. The feedback on a VTA or stock Dynaco amp is tapped off the SECONDARY side of the output transformer. This wiring does not change when you switch from ultralinear to triode. The feedback line still remains connected and the amp still has feedback in both triode and pentode ultralinear mode.

    Bob

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    His words, not mine

    Post by Guest on Sun Oct 23, 2011 8:31 pm

    Bob: Regarding the: "As to your statement > "What you are describing utilizes the absence of Feedback in triode mode".". Those are his words, not mine. I am not knowledgeable enough with regards to the amps to make that statement. I was just running the info by you to see what you thought. I was trying to avoid setting the amp on fire. Thanks!

    tubes4hifi
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    Re: MKIV : Ultra LInear or Triode ?

    Post by tubes4hifi on Sun Oct 23, 2011 9:00 pm

    Michael,
    I believe what your other advisor is trying to tell you concerns the STOCK Dyna ST70, where the green wire on one channel
    is used as an additional feedback (to pin 11) on the STOCK PCB. That design was a band-aid (IMHO) due to the poor frequency response
    of the STOCK circuit using those old 7199 tubes. If you have a VTA driver PCB in your Dyna ST70, that change is non-applicable.

    Guest
    Guest

    Thanks!

    Post by Guest on Sun Oct 23, 2011 10:45 pm

    That's what I was thinking. It recently occurred to me that he was talking about the stock boards because when checking back through my e-mails with him, I realize I had never mentioned having replaced the OEM boards with the VTA'S. Had he known this I am fairly certain he would have waved me off. Thanks for clearing this up

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