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Dynaco mk3 help


McMiRA

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Been a long while since I've been in here , see things have changed a lot. Looks like this is the right place to ask so here it goes

im long winded so skip to the 4th paragraph below this one if you are not interested in the particulars

Picked up some mk3s over the weekend to run the la scalas with. finally got rid of the ugly duckling, my old '58 Lafayette stereo 236a. Hooked up the dynacos and let them warm up, then brought the marantz up to play some tunes. First thing I noticed was the amazing separation of instruments. Second thing I noticed is wtf did all my low end go. The old ugly ducking was an integrated so I had the bass bumped up just a tad on it and it would rip Sheetrock off the walls. The new dynacos.... Let's just say my tone death wife said they sound like shhhh...gave them the weekend and yesterday to convince me they had something hidden that I was over looking. With over 10 hours of listening in i finally gave in and started bumping up the bass on my preamp to get some bottom back until I ran out of adjustment. Something's not right. on the old stereo 236 i would generally listen betwee -30 to -12. with these amps -60 is loud- 44 in uncomfortable. after i reset the tubes in the sockets it calmed down. Did some research and made some phone calls.

Amps have RCA tubes on asc rev9 input boards with the dynakit parts differential bias kit. sovtek 6550s on the power., rubys on the back end with dynakit parts 30/20/20/20 multi section cap and it looks like a good amount has been replaced inside along with new binding posts and rca input.

from what i have gathered even though the 6550s are more detailed with less low end i should not be completely lacking in bass.. I was told by a few people to swap to kt88s and ditch the rubys for something better but to first reflow all the solder and reset the bias so i did.

after i reflowed the solder i reset both bias pots( factory and differential) on both amps per bbcu1 directions and plugged the amps in to bring them up. within a minute they started arcing and sparking so i quickly shut them down and took them back to the bench and pulled the bottom plates, one amp blew the fuse but the other didnt. set them back up on the workbench, plugged in a load and shorted the input as directed. set the amp up so i could see the guts and flipped the power on. armed with a Flir thermal imaging camera i sat back and watched. it was very uneventful for the the first minute then the socket started arching across 3/4/5 on one of the power tubes. cut the amp off immediately. Set up the other amp and the same thing though it was arcing across the socket over the center of the tube and poped the fuse that i pulled out of the first amp to power this one up.

What gives? what did i unleash? the only thing i can think of is i put a dab of dielectric on the pins to keep the corrosion away. would that have caused this or did reflowing the solder "fix" something that been hidden. I checked for shorts and dont see anything. pretty disappointed. been wanting these guys for years and now within three days i let the smoke out. It would be one thing if i had no business turning a screwdriver but im pretty sure i can make it through i can make it through the day without the urge to lick windows.

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I have the impression that you didn't audition the Dynacos before purchasing.  The fact that both amps had arcing across the same socket suggests that something was not properly arranged.  At this point the best course of action is to go through the boards component by component to find the mistake.  Assuming you put a little Deoxit on the tube pins, that shouldn't have caused the problem.  More info and lots of pics may be helpful on here in terms of helping you find the problem.  And, I hope you didn't get rid of the 236a!  That's a delightful little amp which always enjoyed a good reputation.

Maynard

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Auditioned the amps for about an hour. The sounded great. I don't know much about brands but he had a tube cd transport ran into an emotive xda1 into the amps playing through pioneer andrew jones bookshelf speakers. I put 10 hours or more on them over the next three days. They sounded amazing, just no bottom.

Unfortunately the ugly duckling was used as a partial trade for the dynacos. The recipient really didn't want it. When I found the amps I sent him a message and didn't hear from him for a week, then we spoke over the course of a couple days, I made my offer and a few days later he got back to me and said he really doesn't want any more vintage gear, but he also doesn't want the dynacos to sit so he will accept my offer if it means they are getting used. He didn't even want to hear it when I brought it with me. It was really hard for me to get rid of the lil amp but it was the only bargaining chip I had.

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I spent some more time tonight researching and looking up output transformer issues and replacement sockets. I'm wondering if they sockets weren't worn out to start with and by cleaning everything I removed the corrosion that was making the connection to the tube. Either way the sockets now need to be replaced and they are inexpensive. I'll order enough to do both amps and have a couple spares and start with the first amp and see if this does it. Have some more researching to do.

I would post pictures on here but there is an issues with the tapatalk app. It keeps giving me a login error so it will not let me add photos. Looks like the clowns at tapatalk are blaming everything on everyone having outdated forum software when even their own forum has the same issue.

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  • 2 months later...

Just giving a quick update. Didn't have to order any sockets, just swapped the front socket out for the bad top socket. Put in some new Mullard kt88s and we are up and running. Still not getting much bottom end out of them but I believe it's a source issue that I will be correcting shortly.

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