Dynaco Mk IVs rebuild

I finally rebuilt my Dynaco Mk IVs. The quad cap and bias capacitors were replaced two years ago. I used Kiwame resistors through out and with Russian K42Y2 PIO capacitors. The only exception was the screen grid resistor of 1.5M ohm which is not available in Kiwames. The feedback caps are polystyrene. The amps were in very nice condition to start with and the boards were clean and nice to work on.

This is a similar rebuild to my MK IIIs that I posted here March 5, 2018. I finished off the clean up work using Herman’s Simply clean. It is a fine silver polish and works great on the plating. It is not abrasive so it will not remove the lettering on the back panel but produces a wonderful finish. Obviously the plating must be in very good condition to start with. I usually start with a light application of Maas or Simichrome to get the worst of the grunge off. Then the Hermans. These came out almost as nice as the MK IIIs. Playing now and they sound great.

WoW!! Those are stellar! I assume you stuck with the 7199 driver tube? Did you give any consideration to doing a 're-wire' of the driver socket so as to be able to use a 6GH8A in there? I don't know if I still have a 'printed' copy of an article written by Van Alstein some years back, but he did some research and testing where-by he compared the results of the 7199 tube to use of a 6GH8A. According to his article, the 6GH8A actually performed better in most all respects as I recall it. Since then I've not felt so bad about getting out my dremmel tool and cutting traces under the circuit board, and 're-wiring' old Dynaco ST-70 boards so as to be able to simply 'plug in' a 6GH8A without the use of 'adapters'. Of course, prices of adapters have come down now.....so it's probably not worth the effort.

So I have to ask: Do you perceive any great change in audio performance in having done all this work? I actually have 2 pairs of Mk IVs.....one set is 'original', and the other set is really more of a 'rebuild' using Kevin Devany's boards and chassis. So, one of my sets is virtually all 'old' parts, while the other is virtually all 'new' parts. And, I'll be damned if I can really hear any difference...though I consider the 'new' set to be more 'reliable'....and they certainly 'look' better. But, your's are truly beautiful! I doubt you'll want to use the cages to cover those up!! Eye Candy!!....lol

Tom

Tom - The boards are stock OEM with no mods. I tried the 7199 first. What is not shown in the photos is the adapters I use to switch to 6GH8s. I have several sets of these Chinese adapters from Ebay. Some from 6AN8 to 6GH8 (MK II and MK III) and the others from 7199 to 6GH8 (MK IV, VI, ST70, & SCA 35). The 6GH8 tubes are what are in the amps now. You are right - no need to rewire the board if you use the adapters. Makes it easy to go back to 7199 if you want. I have a large collection of NOS 6GH8 and 6U8 tubes to roll. Over time the low cost, well made 6GH8 are the way to go as the 7199 are getting expensive.

The sound of this rebuild is very nice. The tight tolerance carbon film resistors and the PIO and polystyrene caps all work together well. One thing I think is important. Both channels need to have all resistors and caps matched to each other and to the schematic as closely as possible. These are all less than 2% from each other and the schematic values. Half are exact matches with another 30% being less than 1% deviation. Then both channels work the same (or as close as possible). The stock circuit is simple and works well. When you optimize the components it brings out the best the circuit has to offer. I am sure other boards will work OK but there is really nothing wrong with the original layout if optimized.

I do use the covers although these are in truly beautiful condition. My Mk IIIs and St70 are in at least as good a condition. If it looks good it must sound good. Right?

If you refreshed your “old” set with the same parts I did I would love to hear what you thought of the difference between your “new” pair vs the “old”. It is a lot of work but I have the time which many people do not have for this type of effort. I suspect you would be impressed with the sound quality of the rebuild.

Nice!!

WLT, I'm actually in the process of cleaning house over here. I officially 'retired' as of last Friday, but in terms of 'time for projects'..... I've got a million of them!! I believe I've been rather a 'horder' over the past decade or so......not just with 'electronic' stuff, but with motorcycles and parts there of, guitars and guitar amps as well as 'support' stuff for rebuilding, and TONS of 'fishing' stuff, plus 3 boats, down from 4. Probably a dozen or so 'outboard motors' around here as well.....so more projects there too. Add to all this the fact that I have MASSIVE issues with my house from a structural perspective. I have floors that are sinking away from the walls due to erosion, wood around windows and door ways that is rotten and needs replacing, leaky skylights that have damaged the ceiling internally, a family room floor that was badly damaged by termites and now needs some significant repair if not full replacement, approach walkway problems from flooding, 'grading' issues that are part of my erosion problems....and the list goes on. Honestly, I'm not sure where to start all of this stuff.....so I'm leaving in about any hour and taking a 3 week vacation..... if I can handle being gone that long. I need to 'think'!! Then, I need to come home and 'act'!!

Look for a LOT of my stuff coming up in BT in the not-too-distant future!!

Again, nice job on your amps! Glad the work paid off with the performance of them meeting your expectations.

Tom