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    Alternative to Cathode Bias on ST-35

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    corndog71

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    Join date : 2013-03-19
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    Re: Alternative to Cathode Bias on ST-35

    Post by corndog71 on Tue Jun 25, 2013 12:55 pm

    sailor wrote:Interesting that Carver chose the 12AX7 as a phase splitter. The impedance miss match between the split outputs is many times worse with a 12AX7 than with a 12AU7. Also there is no amplification advantage because the phase splitter will always be about .9 gain no matter what triode you use. The third problem is the 12AX7 has much less available amperage and more prone to microphonics. I think it was a bad choice all the way around.
    Which brings up another point I have been harping on for quite some time. The ST and SCA both have positive feedback. This feedback comes from one side of the phase splitter. I don't like the use of positive feedback in this type of amp to begin with and Dynaco only put it in to increase the output of the driver stage. But forget that, the main problem it creates is no matter how matched your EL84s are the split driver signal is miss matched because part of the output from the cathode is being pulled off to run the positive feedback loop. The result is distortion which gives the amps a very pleasant dispersed tuby sound. Most people actually like this distortion and call it to detail.

    So, could the positive feedback part of the circuit be eliminated?  Looking at the stock circuit of the ST35, it looks like the .22uF cap and 150K resistor.  If I'm understanding you correctly doing this would reduce output of the driver but would also eliminate some distortion?

    tubes4hifi
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    Re: Alternative to Cathode Bias on ST-35

    Post by tubes4hifi on Tue Jun 25, 2013 1:12 pm

    I'm sure it's primarily there for circuit stability

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Alternative to Cathode Bias on ST-35

    Post by sailor on Tue Jun 25, 2013 3:14 pm

    I have removed it on my SCA and documented the change on another thread on this board. It actually is more complicated for the SCA because of the 7199 being a pentode. Removing it actually increased stability in this amp. http://dynacotubeaudio.forumotion.com/t1503-sca-35-can-you-replace-preamp-section-with-vta-preamp-section
    You will loose about 6db of gain if you remove the positive feedback, but should not be a problem if you use a preamp. I have never owned or changed it on an ST. However the change would be simple and easily reversible. I would change only one channel and plug your source into your preamp so you can AB the change. You will also need to raise the volume when AB on the changed channel because of the 6 db loss.
    Using a Dynaco schematic as a reference. On the ST you would remove R4 and C3 [positive feedback loop]. R6 and R7 must also be changed to a tightly matched pair of 27K/1 watt resistors. They don't need to be exactly 27K they just need to match each other. I usually buy 6, 5% resistors and can usually find 2 that match. The closer they match the more accurate the split.
    I can only write about the SCA but the change was a little less low bass, but a lot more front row and detailed with the mid bass, midrange, and highs.

    sailor

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    Join date : 2011-04-04

    Re: Alternative to Cathode Bias on ST-35

    Post by sailor on Tue Jun 25, 2013 3:43 pm

    Sorry corndog, I should have explained one other thing. The imbalance happens at the split. R6 [27K] and R7 [33K] resistors on the plate and cathode of the second triode should be equal [27K in this case] but can not because part of signal at the cathode must be used for the positive feedback loop. But it still must equal 27K at the cathode. It just so happens if you put a 150K [feedback] resistor in parallel with a 33K [cathode] resistor you get the necessary 27K resistor. But as can easily seen the plate and cathode are balanced but part of the cathode signal is being sent back to the first tube as positive feedback hence the imbalance. This loss of signal at the cathode will cause distortion which happens to be pleasant to the ear and tends to give the sound a dispersed non direction quality. Kind of like a Bose 901 without the front speaker. But it is still distortion.

    corndog71

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    Join date : 2013-03-19
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    Re: Alternative to Cathode Bias on ST-35

    Post by corndog71 on Tue Jun 25, 2013 4:05 pm

    Hmmm...  Something to consider I suppose.

    My ST35 is a bit of a frankenstein.  I started with Shannon Parks' driver circuit (for easier tube rolling) but swapped the individual tube bias scheme for Dave Gillespie's EFB.  I went for a very different layout at first just to be different but now am looking to go back to the classic tubes and trannys on top layout for easier measuring and tweaking.  (learning some things the hard way)



    I don't get a diffuse or tubey sound though.  It's marvelously clear and dynamic with very good bass.  Despite the low power it's one of my best-sounding amps.

    sailor

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    Join date : 2011-04-04

    Re: Alternative to Cathode Bias on ST-35

    Post by sailor on Tue Jun 25, 2013 7:34 pm

    I have owned every Dynaco tube amp except the ST so as stated my comments were based on the SCA. I still think it would improve the sound of the ST but no way to actually test it without buying an amp. I started to buy a Dynakit ST35 on Audiogon a few months back but the seller had done such a poor job of wiring I let it pass.
    By the way, do you wear high voltage gloves when you adjust that pot next to those big caps?

    corndog71

    Posts : 451
    Join date : 2013-03-19
    Location : It can get windy here

    Re: Alternative to Cathode Bias on ST-35

    Post by corndog71 on Tue Jun 25, 2013 10:18 pm

    sailor wrote:
    By the way, do you wear high voltage gloves when you adjust that pot next to those big caps?

    Yeah, that's another reason why I'm rebuilding it.Embarassed  Thankfully, the bias holds pretty steady.

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