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Old 20th January 2007, 09:46 PM   #1
Duo is offline Duo  Canada
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Default Not a typical MKIII Dynaco.

Here I present one of my projects, the re-design of a dynaco MK-III amplifier.

This amplifier sat around my place for ages until I decided to bring it back to life. Basically all that was good was the transformers, chassis, driver board, and the 6AN8 tube. I had a pair of dead KT88 Genalex but oh well, can't dwell on that.

The filter inductor was shorting to ground and had to go, and the PSU cap had been boiled out. A lot of work needed to be done.

I re-did the power supply in some strange way; I forget exactly how I set it up as it's back home and I'm 500km away for university. I set up screen regulation with a gas tube and, yes, transistors to follow it to allow enough current.

What tubes you ask? I have a mess of nice tubes around, just no KT88 or 6550 variants. I pulled a pair of 6164A tubes out of the trunk and gave them a real looking at.

After some circuit modification they were in place and the amplifier operates beautifully.

Being that I have modified the power supply to also use SS rectification, and it no longer has a choke, the operating voltage is a lot higher.

It deals out around 112 watts, instead of the 60 it was designed for; so I worry about the output transformer, though I don't operate it that far open unless for testing.

It sounds fantastic and looks rather interesting; though with KT88s in place it also looks absolutely gorgeous.

Click the image to open in full size.
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Old 21st January 2007, 08:51 AM   #2
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No comments? It may not look sexy, but I at least thought it looked interesting.
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Old 21st January 2007, 11:23 AM   #3
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Can't see the pic.
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Old 21st January 2007, 11:45 AM   #4
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Nor can I.
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Old 21st January 2007, 05:11 PM   #5
Duo is offline Duo  Canada
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Darn IP problems.

It should work soon now.
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Old 21st January 2007, 05:24 PM   #6
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Default 6146a?

6146a or 6164a?
rt
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Old 21st January 2007, 05:47 PM   #7
Duo is offline Duo  Canada
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6146A Is the tube. It is a VHF beam pentode.
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Old 21st January 2007, 09:34 PM   #8
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For me, Dynaco mk3 is well known. I had single dynaco mk3 couple of years ago. It was fully original, no replacement parts at all. So there was some problems: Anodes went red hot (coupling caps shorted), choke boiled (shorted to ground?)..

Anyway, nice amp as long as I had it. I have some 6146A tubes in stock..You said that over 100watts can be achieved? That is pretty much. Sounds interesting..maybe I will do something with those tubes too

Dynaco has really big output transformer that should handle 100watts easily so I can't see any problems with that.
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Old 21st January 2007, 09:41 PM   #9
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Well, when I go back to home on vacation, I can get you the information on the exact changes I made if you like.

You don't have to do the screen regulator just the way I did it, there are numerous ways including zener diodes.

As with yours, my choke boiled out and shorted to the chassis too. That hurt a bit, but it pushed me enough to decide to put it away until I was ready to either restore it to original, or re-design it.

The drive board didn't have any changes to its design if I recall correctly. The 6146 tubes had plenty of gain to make this work in the original setup.

You do have to re-wire the sockets of course, and add anode connectors to come through the slots in the chassis and up to the tubes like I did. I didn't have any connectors, so I simply wound a nice little coil of copper and fit it to the tubes by friction. It works very well, though a few years of heat might change that.

Being that I've used a system to regulate the screen voltage, it is obvious that I didn't maintain the ultra-linear aspect of the output stage. The taps on the output transformer were far too weak for the 6146 tubes. Regardless of that change, the distortion is still low and it sounds excellent.
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Old 21st January 2007, 11:42 PM   #10
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Hi Duo, do you recall the operating conditions? Plate and screen voltages primarily. I was given a pile of thes things and though the triode curves look appealing the screen voltage limit makes it impractical.
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