Tube hi-fi advice?

[bob p]

i dunno how i missed this thread, but i guess this is a case of better late than never.

i think that the story of how UL came about is pretty interesting, as it caused a lot of transatlantic fighting among famous designers when it was first introduced. i think that the story is so interesting that i'm going to yap about it for a while. please indulge me.

the original UL design was by Hafler & Keroes. if you google for their names and the word ultralinear, you can find their original papers on ultralinear designs. IIRC the original UL paper that H&K published in the 11/51 issue of Audio Engineering used a 6L6. The UL designs that H&K made them rich were the Dynaco designs, the EL34 based ST-70 and the 6550-based Mark series amps are the most famous.

from a historical perspective, the first real "HiFi" amp was the Williamson circuit, originally published in the UK hobbyist magazine Wireless World in 1947 by DTN Williamson. He worked for MO Valve Company, and as I recall his job was to design a good circuit to help sell their KT66 to the DIY HiFi crowd. The original "Williamson circuit" was KT66-based. It set the standard for HiFi for a number of years.

Things got interesting in 11/51 when Hafler & Keroes published their original Ultralinear Paper in the USA journal Audio Engineering. The design was 6L6-based, and it was sort of a frontal attack on the Williamson design, and Williamson didnt' like it.

Williamson published a scathing response in Wireless World in September 1952, in which he tried to top the H&K-coined term "ultralinear" with his new term, "super-ultra-linear." The feud sold a lot of magazines.

The coup de grace ( as I see it), and the biggest insult to Williamson was when Hafler published an article entitled "Modernize Your Williamson" in Audiocraft magazine in January 1956. The article included a step by step set of instructions for ripping apart your Williamson design to convert it to Hafler's better performing design. Dynco even reprinted the article and gave it away in order to enhance the sales of their Williamson upgrade kits!

[bob p]

I guess before I choose an OT I need to decide whether I'm running in Class-AB or Class-A, and whether I'm using two or four tubes. Am I right in thinking the Stereo 70 was Class-AB but biased pretty hot?

Steve, as I recall the original Dynaco ST-70 had a pretty inexpensive / lame / saggy PSU. Hafler designed the PSU to be inexpensive to produce for the kit, and he biased the amp closer to Class A in his design to make the current demand more constant to minimize the sag problem. Lots of people try to run them as a hotter Class AB amp, but the tend to be saggy, and I don't think that's in keeping with the original specification. But you should double check on this to be sure.

Back to the subject of UL for an EL34, the EL34 is a highly linear tube in pentode mode, and there were quite a few HiFi manufactuers in the era that prefered to eschew the H&K UL design in favor of conventional pentode mode designs that produced more power. The EICO 50-60W kit amps come to mind. So as far as the EL34 goes, you're going to trade off some power if you go with an UL, and many people think that the EL34 tube is one that doesn't benefit from this sort of thing as much as some of the other tubes will benefit. What would I do personally? I'd try hooking it up both ways.

[bob p]

A lot of people feel the 6L6 and 6V6 family of tubes sound bland for hi-fi, especially in the mids. Thats not to say that they aren't/weren't used as the Mcintosh MC240 used 6L6's among others. EL34's and EL84's are very sweet sounding with hi-fi stuff. Check out Triode Electronics site and the Dynaco stuff they offer.

I have set of old 15W EL84-based Heathkit monoblock amps that I have restored. The EL84 is a truly wonderful sounding tube for the midrange, and it really shines on things like strings and vocals. Its a very liquid sound for HiFi. The only real problem, IMO, is that the tube is starved for power. Bass reproduction isn't authoritative, and you have to use efficient speakers with them. For an "intimate" setup in an apartement or a den, an EL84 based amp could be just the ticket. But forget about it if you've got inefficient speakers or if your addicted to SPL.

With respect to the mids of the EL34 compared to the 6L6 and 6550, I was recently surprised by my ears. I spent the day at a shop in Chicago comparing the rather pricey Hovland EL34-based amp running Telefunken preamp tubes and IIRC Amperex EL34 to a new production Audio Research 6550 amp that used new production Sovtek 6N1P preamp tubes and EH6550 in the output section. The speakers were Sonus Faber's Stradivarius. What really amazed me was how similar the midranges were in both cases. Go figure.

[TD_Madden]

man, I'd LOVE a pair of those monoblocks for my Klipsch Heresy IIs......

[bob p]

man, I'd LOVE a pair of those monoblocks for my Klipsch Heresy IIs......

You know, those are one of the few speakers that are efficient enough to work well with a little amp like that.

The actual Heathkit amps that I'm using are the Model AA-161. Its a mono integrated HiFi amp. There's also a stereo version, the Model AA-151. Those tend to sell for $100 to $200 on eBay. The stereo version is essentially a pair of the mono amps hooked up to a beefier power supply. They're surprisingly good sounding little amps.

Here's the funny thing about my history with those amps -- I bought the first mono amp on eBay for all of $20 ($40 shipped), with the idea that if there was something wrong with it, I'd just gut it and use the chassis and iron to build a Marshall 18-watter. When I got it, the only major problem was a bad cathode bypass cap in the output section -- just like you find in the guitar amp circuits. Somebody who built the kit long ago had mounted the cap right up against the cathode resistor and heat and time took their toll on the poor little cap.

It took minimal repairs to get it up and running, so I hooked it up to my stereo as a lark. It sounded so darned good that I had to buy another one. So now I've got two of them. If I had to do it over again, I'd just pay a little extra to get the Stereo AA-151. They tend to cost more because most people want stereo amps for HiFi use.

The moral of the story? The amps are CHEAP! Get one on eBay and fix it up. I don't have the assembly manual, but I have schematics if you need them.