VALVE Magazine Online!

Joining The Singles Club
Article By Dan Schmalle
VALVE Volume 2 Number 1 January 1995

  DHT, 60 years young, still full of life, tired of push-pull relationships, seeks single ended romance. Jazz and classical music lover who can rock in the right environment, particularly fond of female vocalists.

I believe in strong emotional involvement, attention to detail, and efficiency. I admit to favoring quiet play, but if you are slightly horny, amply loaded, and interested in coupling in oil, I am sure you would find me still quite hard. Won't someone transform this quiet type into an efficient speaker?

I'd had in mind a SE project ever since Mike raved about a pair of Eico HF 20's he had modified to use a 665 in SE configuration, and Fred sent us his design for an amp using a 6SL7 driving a 45, with much enthusiasm for its realism with a JBL speaker. I've collected a pretty good assortment of #10 Directly Heated Triodes over the last couple of years, and had my pair of old Stancor transformers, rated 35 Watt PP and 9000 Ohms primary impedance, that get used in all manner of projects. They aren't SE transformers. No air gap, probably wound less than optimal, but they weigh about 4.5 pounds, so I thought they might be OK for one watt. Besides, they were free.

So I decided to read up in Sound Practices, the journal of artistic experimental audio, and my 1940 RCA tube manual. Put together a simple RC coupled amp circuit using a combination of opinions on tube sound quality, most notably by J.C. Morrison (try some of his ideas, they're good) in Sound Practices, and component values from the tube manual. See the centerfold for the current circuit. I wanted to use a cool old triode for the driver and Fred mentioned a system using 56's a few months ago, so that was my first choice.

I have a funky bench top rack containing a Lambda regulated B+ power supply and a couple of AC/DC filament supplies, so power was as easy as adding a couple of 40,000 mfd @ 50V caps across the outputs of the filament supplies to cut hum. My single channel prototype was literally nailed to the workbench in about 20 minutes. Even in this crude state it was obviously special sounding over my JBL LE175 / Stretchorn, and it actually made one of Eric's A7's sound great for the first time!

Initially the driver was coupled to the output with two parallel Mouser 0.1 mfd @ 630V polypropylenes. After a little play I remembered that I had a Vitamin Q of the same value. The difference between the virgin poly and the oil cap was obvious. If I hadn't heard the oil cap I would still be raving about the sound of the amp, but the oil cap made you Shut Up And Listen, my gauge for truth in reproduction. Next step was to build a second channel so the whole magilla got screwed to a piece of 0.75" particle board. Breadboards live!

The Radiotron Desgner's handbook suggested that the cathode resistor of the output tube might benefit from a bypass cap of about 50 mfd. I thought the sound a bit thin without it, so I tried it. Bingada bangada boom -- there was the difference is there for sure. I thought oil caps might be an improvement over the Mylar bypasses, so I shunted the Mylar with 10 mfd @ 660V oil filled caps. I couldn't hear a difference when AB'ing one channel with and one channel without, so I pulled them off.

The next experiment was subbing a 27 tube for the 56. The 27 is similar, but draws more filament current, a supposed way to better sound. I liked the 56 better. I thought maybe it sounded better because it had a little more gain thon the 27, so the next step was subbing a high mu triode in for the driver. I had a 6SF5 around, so that went in the circuit. Not as good as the 56, so it came back out. The one tube I still want to try is the 26. It is a DHT, while the other tubes tried are all cathode/ heater types. Maybe it will make o difference.

I started the B+ at 350 VDC, but later boosted it to 425VDC. This forced me to put a dropping resistor in the driver plate supply leg, but boosted output from about 0.9W to a whopping 1.6W. Believe me, you take any scrap of power you can get at these levels! My most recent mod has been the temporary addition of a pair of humongous Audio Note SE transformers rated 10k primary impedance @ 50W, for 211s and 845s. Dave brought these over for some experimentation with higher power tubes. Actually, he brought 'em over because he thought I might get a 211 SE amp built before he would. If this damn computer will behave and I can get this newsletter done in one pass instead of three, he may get his wish.

The better transformers made their presence known in the high end, not the bass, as I had assumed they would. Highs were cleaner, plain and simple. The change sounded like the clarity improvement you get from a good recap, but more subtle. I don't know that it would be worth 450 bones to get a permanent pair for this wimpy amp, but the difference is there for sure.

I tried a few different types of 10's. I have a couple of globe style, one of which has a mesh It sounded very nice, but I only had one, so it was not a contender for the final stereo amp. A solid plate globe style sounded good too, but not quite as detailed as the mesh version. I have a really awesome looking pair of Sylvania type 210, globe style, ceramic base, and graphite plates. They looked stunning in the amp, with super bright thoriated tungsten filaments glowing like light bulbs, but they sounded too soft. I finally settled on a pair of Sylvania 10Y's, heavy duty versions of the plain ol' 10, with shouldered envelopes, hot filaments, and solid metal plates.

When the amp only has 12 parts per channel, every component can make a difference. This is part of the intrigue of these fabulous amps. You can play for hours, making a five minute change and listening, again and again. Try that with a Citation Ill. Of course you're wondering what speakers to use to get any buzz off 1.6 Watts. Well seeing as I would like to come out with an inexpensive SE amp kit and a matching speaker, I've been experimenting a bit. Firstly, you need 105 dBM sensitivity for a watt and a half. I think the 10 is a great sounding tube, but it probably makes sense to go to 2A3, at least. Besides you can't get a small SE transformer in the OK impedance range. The 2A3 type transformers are available, and 2A3's give more like three watts, and you can find new Chinese 2A3 tubes. I'm scheming on a Sway system at 105dB, even though I don't like three ways. In the meantime I followed Sam Tellig's advice in November Stereophile and bought some Radio Shack Optimus 990's. They're not bad, slightly rolled off, at both ends, but decent, and about 90 to 95 dB. Working on an upgrade, but that's a whole 'nother story.

-- Dan Schmalle