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    MK3 Resistor question and a couple other questions

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    bubbasweet

    Posts : 53
    Join date : 2016-03-14

    MK3 Resistor question and a couple other questions

    Post by bubbasweet on Mon Jul 18, 2016 9:20 pm

    I just got a couple mark3 amps and thus the sudden lot of questions. I wwant to do the minimum amount of modification but want to do the most practical things to make the amps run safe and consistent.

    I have replaced the Selenium with an IN4007 and the 2 caps were switched with 100uF 100V caps that look fairly new. No other resistors or caps were added. I heard you are supposed to use a 4.7K in place of the 1K in the bias inside the chassis but so far I see no reason as they bias fine. Let me know if I should do anything else.

    I did the .27s and the .1 on the circuitboard.

    I am going to do the can cap as soon as it arives. going to so the 80,40, 30 ,20. And use the 30 after the GZ34.

    I am posting the circuitboard. When I got it the resistors that look to be 1K circled in red were replace by a 1 Watt 47ohm. This was done before I got them. I am planning on switching them back but have no idea what it is for or if this value effects anything. Please feel free to chime in.


    PeterCapo

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    Re: MK3 Resistor question and a couple other questions

    Post by PeterCapo on Mon Jul 18, 2016 9:43 pm

    If you are using the quad cap with the higher voltage ratings, the 30uF section has the lowest voltage rating of the four sections.  I would not put the 30uF section right after the 5AR4, as this is the highest voltage node in the B+, which would kind of defeat the purpose of the higher rated sections of the quad cap.  IMO, the new quad cap should be wired in this order: 40uf before the choke, 80uf after the choke and the other two sections after that for the 6AN8.

    As far as the resistor value, I’d suggest taking a holistic view of the amp.  Who knows why it was replaced with a value different from the original.  Maybe it accompanied other changes to the amp, or maybe it was a mistake.  I’d compare the amp in its entirety against the original manual and pictorial diagram to see if you can find any other changes.

    bubbasweet

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    Join date : 2016-03-14

    Re: MK3 Resistor question and a couple other questions

    Post by bubbasweet on Mon Jul 18, 2016 10:03 pm

    THX. On the schem which cap is the circled 1K.. Amp is pretty much stock except the changes I listed and the 1K circled is a 47ohm.. THX for your input on the Can.. will do.

    GP49

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    Re: MK3 Resistor question and a couple other questions

    Post by GP49 on Tue Jul 19, 2016 12:48 am

    The original resistor value in the factory Dynaco Mark III was 1000Ω, 1W.  It has been replaced on your board with a 47Ω, 1W.

    This resistor is in the negative feedback loop.  The decrease in resistor value will INCREASE negative feedback which theoretically will reduce distortion.  However it will also reduce the closed-loop gain of the amplifier circuit; you may not be able to drive it to full output at maximum rated input signal.  More importantly, I would worry about the stability of the amplifier with too much negative feedback; also, since the frequency-shaping capacitor in parallel with the resistor has not been changed.

    This is based on the circuitry in the assembly manual for the Mark III.  It is identical to the circuit in the Mark II; that one also has a 1000Ω resistor in that location.  The Stereo 70 and Mark IV also have a 1000Ω resistor but their frequency-shaping capacitor differs, as would be expected due to a different output transformer.

    Even the Dynaco-recommended circuit for a low-power amp using the Dyna A-410 output transformer and 6V6 or 6BQ5 output tubes uses a 1000Ω resistor in that position.

    Without seeing the rest of the amplifier circuit I cannot be sure...but I have a feeling the resistor substitute is a mistake. I would contact the supplier and ask why the substitution has been made.

    bubbasweet

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    Join date : 2016-03-14

    Re: MK3 Resistor question and a couple other questions

    Post by bubbasweet on Tue Jul 19, 2016 9:53 am

    Thx for that. I got these amps from a strabger. The rest of the amps parts are pretty much stock 1959 and 60 Mk3s. Transformers are original. That was basically the only value someone had changed. The rest of the resistors are original values. Thx for explanation. I am aware of negative feedback. Thx for confirmation. I will put them back to 1K 1Watt THX..

    GP49

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    Re: MK3 Resistor question and a couple other questions

    Post by GP49 on Tue Jul 19, 2016 10:08 am

    A thought...check to see where the feedback loop is connected to the secondary of the output transformer. A prior modification might have moved it from the factory location on the 16 ohm tap to one of the others, which would necessitate a change in resistor value, but from 1000 to 47 ohms "feels" like too much. If it has been changed I'd restore it to factory configuration if it were my amp, or do a careful and thorough check for stability throughout and well above the audio band with varying loads, including capacitive.

    bubbasweet

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    Join date : 2016-03-14

    Re: MK3 Resistor question and a couple other questions

    Post by bubbasweet on Tue Jul 19, 2016 11:58 am

    Do I have to remove the output transformer to see this or do you mean what Ohm lug the 47Ohm goes to on the speaker ohm terminals? Sorry but not to familiar with there it might have been modded underneath. I will look..

    Also re: filament bias circuit. Do you think I should I leave the 1K in the bias circuit with the IN4007? Some say change it to a 4.7K. I do not find any issues setting the bias though with the 1K so far. All I changed is the diode and caps with 100uF and seems to work fine.

    GP49

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    Re: MK3 Resistor question and a couple other questions

    Post by GP49 on Tue Jul 19, 2016 4:23 pm

    You should see where the feedback line is connected, inside the chassis on the loudspeaker terminal strip.

    The change of resistor in the bias string is to compensate for the lower voltage drop of the new silicon diode, compared to the old selenium rectifier.

    bubbasweet

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    Re: MK3 Resistor question and a couple other questions

    Post by bubbasweet on Tue Jul 19, 2016 5:27 pm

    Should I ad the 4.7K then in your opinion? For the Bias

    GP49

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    Re: MK3 Resistor question and a couple other questions

    Post by GP49 on Tue Jul 19, 2016 8:24 pm

    Change it if you can't bias the output tubes within range of the bias pot. An indication that you need more resistance would be that you cannot get the measured DC voltage at the cathodes...what is colloquially but incorrectly referred to as the "bias voltage"... hign enough.

    bubbasweet

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    Join date : 2016-03-14

    Re: MK3 Resistor question and a couple other questions

    Post by bubbasweet on Tue Jul 19, 2016 9:59 pm

    Ok I can bias tubes fine the way it is with th 1K and my bias pots are fine with room. I am getting -57 and -60 on the 100K resistors on circuit board though Dynaco Manual says -55 so not too far off.


    Also the 47ohm is directly to the 16 ohm tap. 0.1 ohms to it from one side of that resistor and higher resistance to the other taps. So I guess I will just change it back to a 1K. Must have been a mistake when switching the others on the circuitboard out..

    daveshel

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    Re: MK3 Resistor question and a couple other questions

    Post by daveshel on Wed Jul 20, 2016 12:36 am

    I didn't do the rebuilds on my MK-IIIs, but the guy who did said he considered it necessary to replace all of the caps - the most critical being the quad can and the the big ones in the bias circuit belowdecks. So the circuit configuration is original, but the caps are of better quality than the originals. And this pair sounds way better than any ST-70 I have ever heard, including some more costly mods.

    bubbasweet

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    Join date : 2016-03-14

    Re: MK3 Resistor question and a couple other questions

    Post by bubbasweet on Wed Jul 20, 2016 9:20 am

    I agree. I think these sound amazing. I do not feel the need to mod them. There sure seems to be a lot of interest with all these other types of circuit boards out there. I do not see a reason so far..

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