bias wiring MKIII

for Dynaco Mark II/III/IV and DIY PP monoblocks

bias wiring MKIII

Postby vintage » Thu Apr 28, 2016 9:48 am

Hello folks, it has been quite awhile since I decided to build another one of the amplifiers with the poseidon boards. My first set of MKIII amplifiers were a great success however this was years back and I unfortunately sold them $) being a bonehead and now it is time to build some more. The back story till now, these amplifiers came to me freshly rebuilt from a very popular online re-builder who also has a presence on many forums. In my opinion the builder has a reputation that is completely unjustified. I found so many problems with this sloppy build, to name a few speaker binding posts not even soldered just wrapped around, plenty of solder bridges, severely overheated resistors to name a few but hey the amps came with a fancy brass nameplate stating who did the work.

I plan of running the amps with the new kt120 tubes, not for any power increase simply a aesthetic reason.

The problem I encountered with the first build is a bias lack of range as far as the below 0.68 vdc range. With the controls at the most minimum setting I come in at 0.68 vdc after a brief warm up with kt88 tubes. The amplifier has decent sound, without distortion but I was not pushing it at all in the slightest. Now after this checked out I dropped in the kt120 tubes and the amplifier quickly began to oscillate, the bias voltage was on a steady and enormous climb, needless to say I quickly removed power.

It is obvious right from the start I had a issue with the bias circuit due to the lack of range, Now where the manual confuses me is the talk of biasing the tubes individually. To me individual tube biasing means independent bias pots. We do not have that here, we have one pot and a balance pot on the PCB. I have read the various explanations of how this works online and in this forum but brothers, I just don't compute. I must also state I am using one bias resistor with both tubes tied into it exactly like the dynaco design. I have made no change to the dynaco bias circuit other than upgrading the old parts, is this my problem?

Now I have read on the tubezone manual some resistor value changes that I am going to make, like changing resistors R10 and 11 to 47K from 68k.

Just some quick notes. I will be ordering new 10k bias pots too as I cannot verify if these were changed when these amps were rebuilt by the vendor who will remain nameless. I guess if you fine folks could think, where would you start with a problem like this?

Some more quick notes, all new caps. I am using the original design power supply with the can cap just a new high voltage production. I am using all very good quality parts under the hood. I used one central point for all ground connections. I made sure not to make a mess with the wiring, wires are routed and kept separate for the most part.

I know I must have made a mistake with the wiring of the bias circuit, no values were changed from the original design. I am hoping it is a simple issue like using separate bias resistors like the schematic indicates. I will say that all the pictures I have seen online only show one resistor being used.

If one of you fantastic audiophiles could do me the honors of taking some real nice pictures of your amplifier under the hood I would be very grateful indeed. What would be real nice is if one of you folks decided to retain the original power supply design with the new production cap. I considered the board but opted out as my last amps many years back had the boards so wanted to try this.

Thank-you gentlemen, cheers.
vintage
 
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Re: bias wiring MKIII

Postby vintage » Tue May 03, 2016 9:19 pm

nobody here in these forums has a dynaco mk3 done up with these great boards that is willing to take a few real nice pics of the guts for a fellow diy'r? I refuse to believe that (???) Please, if you are willing to help a version with the multi section cap is idea however beggars can't be choosers now can they.
vintage
 
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Re: bias wiring MKIII

Postby WA4SWJ » Thu May 05, 2016 6:30 am

Hi,

I have some amps like you're looking for. I also have some pictures but I'll have to find them. My avatar at right shows one of them. I'll see if I can find them this evening and maybe post some.
Ed Long
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Re: bias wiring MKIII

Postby vintage » Fri May 06, 2016 6:35 pm

Thank-you, how did the search go?
vintage
 
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Re: bias wiring MKIII

Postby WA4SWJ » Sat May 07, 2016 7:35 am

I finally found some pictures this morning. I don't have an account where I can post a link for pictures. If you like you can send me a private message with your e-mail address and I'll be glad to send you a few pictures.
Ed Long
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Re: bias wiring MKIII

Postby vintage » Sat May 07, 2016 7:41 am

Absolutely, please do send. I hope they get real close in on the bias wiring area.
vintage
 
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Re: bias wiring MKIII

Postby WA4SWJ » Sat May 07, 2016 8:41 am

Check for a private message for you.
Ed Long
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Re: bias wiring MKIII

Postby vintage » Sat May 07, 2016 9:55 am

Thank-you for the PM, I replied back. These amps are amazing already, these boards only take them to a new level.
vintage
 
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Re: bias wiring MKIII

Postby vintage » Sat May 07, 2016 12:04 pm

Nothing yet in the email, I am heading out.....doing some car shopping, will check in later and thanks again.
vintage
 
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Re: bias wiring MKIII

Postby WA4SWJ » Sun May 08, 2016 7:39 am

Joe,

Glad the pictures helped. Once you get yours finished it would be good to see some pictures.

Good luck!
Ed Long
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Re: bias wiring MKIII

Postby vintage » Mon May 09, 2016 1:15 pm

sure thing, stay tuned lol.
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