DIY ST 35 Built with Heathkit Transformers

[scottfixer]

I recently completed construction of my DIY ST 35 amplifier using a leftover custom chassis and a set of power and output transformers removed from a Heathkit AA-151 amplifier that was a cosmetic wreck. The resulting amplifier is a real beauty and a sonic sweetheart.

The Heath transformers are the same used in the highly touted UA-1/2 mono blocks and they provide 4 ohm output taps which I grounded. For four ohm loads, I connect the speakers between the 4 and 16 ohm taps (same impedence & voltage as the 0 to 4 ohm tap) so the feedback sees the load directly. The Heath transformers have slightly different roll off characteristic than the Dynas so I found I couldn't use, and really didn't want the feedback loop from the output screen tap. I did some tests and settled on 9 db of overall feedback, which is obtained with a 47k resistor in place of the 100k pot. I tested various paralleling caps based on square wave rise time and found 18pf was about the best.

Since the whole amp fits inside the chassis, I had to locate the larger electrolytics on the top of the pc board along with the tubes and in-rush current limiter. I was thinking about using a relay to bypass in-rush current limiter, but then found a neat three position, two pole toggle switch I bought many years ago from either All Electronics or MP Jones. In the bottom position, the wipers connect to the upper connections, in my case nothing or off; in the middle position, only one of the wipers connects to the lower connection which I provide power thru the in-rush limiter. In the upper position, both wipers connect to the lower connections which I use to bypass the current limiter. Just have to remember to move the switch to the middle, hold it for about 10 seconds then, flip it all the way up.

To allow direct connection of either tuners or CD players, I decided to add volume control pots, in this case I had some nice 250K units that I installed on the rear just above input RCA jacks.

I used teflon insulated, silver plated wire, 1 and 2 percent metal film and metal oxide resistors and Xicon polypropylene coupling capacitors. I thought about going to much more expensive caps, but I've had very good luck with the Xicon's on earlier Heathkit, Dyna and Scott restorations and modifications.

I've been listening to the little amp for about a week now through my Magneplanar 3.6's. Yes they will drive the big Maggies and quite well I might add. Compared to modified Heath UA-2's, they are definitely cleaner and significantly lower noise. One of the problems with many old amps using pentode/triode drivers like 6U8, 6AN8, 7199, or 6BL8 is that the pentode can be noisy and its operating point can vary all over creation from tube to tube. The DIY ST-35 circuit solves that with easily available good triodes. I'm using some nice vintage Telefunkens for my drivers and a good set of Mullards for the outputs. I tried some new ElectroHarmonix EL-84's but with the standard DIY cathode resistors/pots, I couldn't get the current down below 40 ma per tube. The Heath power transformer puts out a little more voltage than the Dyna, especially with the current limiter bypassed, probably causing this problem.

This may start a run on Heathkit AA-151's or SA-2's. They are readily available on ebay, generally are a little cheaper than the Dynas and provide a 4 ohm tap.

I'll try to include or post the photos I took of my ST 35 project.[/img]

[Shannon Parks]

http://www.diytube.com/gallery/NewST35ampfront.jpg
http://www.diytube.com/gallery/NewST35ampbottom.jpg
http://www.diytube.com/gallery/NewST35amprear.jpg
http://www.diytube.com/gallery/NewST35a ... bottom.jpg
http://www.diytube.com/gallery/NewST35amptoprear.jpg

Nice work!! I'll be posting these on the 'front page' later tonight.

Shannon

[Shannon Parks]

First let me say, great DIY amp. The chassis has a very modern, futuristic look. Impressive job of improvising the PCB mounting - looks like cutting that hole took forever. Hey - I want one!

Couple notes:
- I used Xicon coupling caps for the original stereo 35 prototype and do concur that they sound very nice and are cheap to boot.
- The original ST-35 design had DC blocking caps on the inputs (.1uF, C18 & C19) but many tube power amplifiers did not have DC blocking caps on the input, though. Scottfixer has jumpered these out. I left them in, but some bandwidth benefits may be gained by jumpering them. The Dynaco engineer may have possibly used this in conjunction with the stability network immediately after it, but I honestly doubt know.
- Back in late April, I changed the main cathode resistors on the parts list to 560 ohms after some folks have problems containing the current draw on some strong EL84s (namely JJ tubes). If anyone using 470 ohm resistors would like to upgrade to the 560 ohm ones, just email me and I'll mail you some free of charge.

Shannon

[Shannon Parks]

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... gory=14974

Shannon