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    Mark III 680 ohm resistor on output terminals



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    Join date : 2011-07-31

    Mark III 680 ohm resistor on output terminals

    Post by SoundBound on Thu Aug 04, 2011 8:16 pm

    During the installation of the VTA board on my old Mark IIIs, I noticed the original design has a 680 ohm resistor on the speaker terminals going from the 16 ohm tap to ground. I looked at the schematic provided with the VTA driver board and saw that this resistor was not present. So, I removed the resistor.

    Looking at some pics of other peoples Mark IIIs with the VTA on the web I noticed that the resistor was still in place.

    So, should this resistor stay in or come out?

    Thanks for your suggestions.


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    Re: Mark III 680 ohm resistor on output terminals

    Post by tubes4hifi on Thu Aug 04, 2011 11:32 pm

    I'm not 100% sure but I think it was there as protection in case anyone ever powered up the amp without a load on the outputs, so it's not absolutely needed but has almost zero effect when left in place since it's nearly 100X as large as an ordinary speaker load.
    Amplifiers should never be powered up without a load on the outputs.

    Bob Latino

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    Re: Mark III 680 ohm resistor on output terminals

    Post by Bob Latino on Fri Aug 05, 2011 7:26 am

    I agree with Roy. That 680 ohm resistor keeps a load on the output transformer at all times even with no speaker attached. Whether it is there or not probably doesn't matter as long as you always have speakers attached to your amp when you turn the amp on. Turning the amp on without speakers usually won't harm the amp as long as you don't run a signal through the amp and turn the volume up on your preamp.

    I once had someone tell me that his amp would not bias and that the amp had a "cycling bias" on both channels. Cycling bias is when the bias goes above and below your bias set point in a regular manner in maybe a 2 or 3 second cycle. His bias was supposed to be 1.56 VDC as on a stock ST-70 but the amp's bias kept moving between .5 volt and about 3 volts in a regular rhythmic manner. It turns out that he didn't have speakers connected. When he connected speakers, the bias held steady. You need a "load" on the secondaries of the output transformer to keep the circuit stable.

    I had someone build a VTA ST-70 amp and tried it out with his new home made speakers. He had all new tubes in his new amp but the bias wouldn't hold steady no matter what. It turns out his home made speakers presented a load that the amp did not like. He then tried his new amp out on his home theater speakers and the bias was steady as a rock. Something in the crossover design of his home made speakers created an improper load for the amp.

    I remember talking to someone who was into tube guitar amps and happened to mention to me that Marshall guitar amps are particularly sensitive to a "no speaker" situation and are more likely to blow an output transformer than other guitar amps.

    My recommendation is to always have speakers connected to your Dynaco tube amp when you turn the amp on.



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    Re: Mark III 680 ohm resistor on output terminals

    Post by GP49 on Fri Aug 05, 2011 11:07 am

    Other than Bob, are there any real old-time amplifier builders out there? Not kit builders putting together a predesigned unit according to a specific circuit and layout...but those who bought a couple of transformers, some resistors, capacitors, tube sockets and a breadboard, and wired up an amplifier from scratch? You would remember watching an oscilloscope while trimming an amplifier's feedback loop for optimum, extended frequency response with stability, and would recognize what can happen when there isn't a load across the output transformer's secondary.

    The amplifier oscillates due to phase shift at the extreme high frequencies, above the audible range. That resistor is chosen to present a load to the secondary that reduces or eliminates that oscillation so that it doesn't have the potential of burning out something.


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    Re: Mark III 680 ohm resistor on output terminals

    Post by SoundBound on Fri Aug 05, 2011 11:36 am

    I put the 680 ohm resistor back in. Just for safety's sake. I never power up a tube amp without attaching speakers, but you never know if something happens to a speaker wire connection that you don't notice !

    Perhaps in the VTA Mark III schematic it might be a good idea to insert this resistor. Better to play it safe.


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    Post by zx on Sat Aug 06, 2011 7:46 am

    On my Mk3s,With stock frontend. I run the output in pentoiad not UL. I put the feedback on the 8ohm tap an moved the 680ohm over two the 8ohm ,sounds better to me.
    I took the 680ohm out an put a Zobel in it place,10ohm 1w an a .47cap It has a good sound all so,use a good cap!

    I would like to thank Bob for the site.


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    Re: Mark III 680 ohm resistor on output terminals

    Post by SoundBound on Sat Aug 06, 2011 12:00 pm

    I subscribe to the less is more camp, so I try not to add any components in a system that truly do not have a major affect on the performance/sound.

    So, as a safety mechanism only, the 680 resistor should do it's duty in case of no load and not sonically impact the low impedance outputs.

    As for distortion, I am also of the idea that this is not a tell-all indicator of sound quality. So long as distortion is within reason, I will not want to go to extensive measures to try and reduce it as introducing more components or adding feedback may affect subtle qualities of music playback that can be heard (imaging, speed, localization, etc.). As they say, the best amplifier is a wire with gain. The VTA triode design already has lowered distortion compared to the original Dynaco design and the fact that I built the low gain version also results in reduced distortion, so I feel no need to aggressively purse that demon. Coming from the world of class A zero feedback amp designs, I prefer low as possible feedback. That is one reason why I chose the VTA design.

    IMHO Smile

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