Yup, I’m a tube nut! It got me about 3 yrs ago when my wife dragged me to a church rummage sale. $10.00 for a Scott 299B and 350 tuner. After trying the tuner in my basement and picking up every station in the city and county. Something I’ve never done with any sand tuner, there must be something there. Well since, I’ve built a number of amps mostly single-end DHT. My first amp a 2A3 strap together using P-P trannys. Then I built a stereo 300B, 2 other 2A3’s mono-blocks and the stereo 2A3, all on old P.A. chassis. Then came the (2) 845 SE mono-blocks and one EL-84 P-P to duplicate a amp I had. But SEDHT is the way  for me!

So, what, why,…. Where another amp! Well it all started as a project to upgrade the old 300B stereo amp. First order of business was to get another 19 X 12 X 1/8 inch aluminum sheet cut to accommodate the new power transformer I was planning. Just after I got the sheet, I was trolling thru e-bay and picked up 4 used Sylvania VT-52’s at a reasonable price. I sometimes try to get triodes to play with and after hearing about VT-52’s from R.A.T. I had my interest peaked already. Looking around around the surplus electronics store, I find a double 6.3 Volt 3amp filament transformer.

Now it’s starting to get serious, but do I need another stereo SEDHT amp?  Well maybe I do if this one could do all the things I thought about upgrading on the  other amps. Like having a milliamp meter to read plate current. Having an adjustable bias voltage on a DHT amp like my P-P Scott 340B receiver. Having a copper-ground  buss to help reduce noise and of course parafeed outputs. Next I needed iron, how do I go I needed OPT’s in the 5K impedance range and a power transformer to 400 VDC and between 80 and 150 ma . That would let me optimize the VT-52 output load and let me run 45’s and 50’s too. From discussions on RAT I had rationalized using a nice vintage P-P OPT and not to worry about core saturation. Because I plan to use parafeed on the outputs anyway (Parafeeding reduces the amount of DC voltage the OPT sees).

A 7591 type stereo integrate would suite all the power and OPT requirements I had. Which one Scott, Fisher, Sansui, Heath all had 7591 amps. One other consideration was rectification, for me only tube. So no Sansui, Fisher, Okay Heath A100. These I’ve seen a lot on e-bay at below $100, 3 vintage transformers not bad. Now to gather all the parts together : Solen Fast caps for power supply and China made oil-caps for the signals all from Angela.com : Ceramic sockets for VT-52s and 6SN7’s from SND ( right here in town): Resistors, wire, switches, copper plate lamp all from Gateway Electronics here in town too: Red Oak frame I built an stained wood from Home Depot: The oil-caps for the parafeeds were purchased from The Electronics Exchange here in town : The Electra-Print Plate Load chokes were the last item to be added, until they got in I used small 6BQ5 PP trannys.

One of the first things I did once I got the power and OPT transformers I made a paper layout of the aluminum chassis plate. On that I planned the layout of the amp in lead pencil, so any changes could be made easily. Then came the job of getting all the holes for transformers and sockets cut and punched out. This is never an easy job although using aluminum as the chassis top made the job easier than if I used steel. After the basic layout was complete and cut outs made, I started on the copper ground buss. This started as a 4"x8" 18ga. Copper sheet. I wanted ground buss to encompass all grounded components of both channels from input to output. I used a "U" shaped design that had tabs solder points for each component. By using the paper layout sheet I was able to keep track of locations of components and their connections with ease. From Vol #5 of Dr. Bottleneck’s Valve magazine, the article Buddhafied 2A3 Paraglow amp Buddha described how to build a fast copper ground buss. I followed his instructions and used Fiberglass tape backing and I found some small Nylon bolts and nuts to attach the buss to my aluminum chassis plate.

With ground buss in place it was time to construct the red oak frame for the chassis plate. With a radial arm saw I made the cuts for the top plate and 45 degree cuts for the corners. Some small braids and wood glue, add a little Cherry stain let over from my house window project and the frame was ready. Now the ritual burning of the solder begins, for this amp I used a lead-free high silver content solder from Radio Shack. Does it make a difference, I don’t know but when you lay down a joint with this stuff It ain’t moving at all. I started with all the signal stuff first then did the power supply last.

Okay here’s the horror story part, after doing what I thought was a careful inspection of all the wiring. I wanted to apply power to the chassis to bring everything up slowly with a variac. So with only a 5U4 in place, and a variac on power line I flipped the switch …….. then the fuse blew Oh No! Try again and the same thing happens. I search the whole layout for a short, but could find none. I change 5U4’s same thing. Go to bed try again tomorrow night. The same thing happens could my Heath A100 power transformer be bad I did test it to check all AC voltages, it was okay. Pull the transformer and try another power transformer I had in an old Wurlitzer amp. That would be a shame to have to use that one after I had painted the 3 Heathkit transformers a nice shiny enamel black. Well in went the Wurlitzer tranny and the fuse blows again. What the Heck is going on, flip the chassis right side up and I notice my rectifier socket key slot is facing 180 degrees off different from the paper layout…….. oooops. Well another lesson in checking your wiring even after you’ve checked it. Back in goes the Heath power tranny.

After 4 days of frustration with that rectifier, I finally get a chance to fire this puppy up, just in case I try only one side of the amp first with a 6A3, yes it works then the other side it works too. Now to install 2 of the Sylvania VT-52 ….. ah what a sweet sound. Even with the small 6BQ5 P-P transformers as plate load chokes it sounded really good. Once the Electra-Print plate load chokes were installed, the difference in the bass was really magnificent. This amp with its Milliamp meter, adjustable bias, switchable B+ and filament supply, makes it really easy to try all kinds of DHT and tube rolling becomes a breeze. Did I really need another Single-Ended Directly Heated Triode stereo amp ? Yup!

download power supply schematic (81kb)

download signal section schematic (66kb)

Thanks for your time.

Willie J Rivers Jr