February 18, 2004

Revisions since August 5, 2002

I've toned down the description of the theft of copyrighted intellectual property from this website by the offender mentioned below. I've seen loose use of this information on other sites as well, including the Yahoo EICO eGroup, but I've decided to allow them a pass since they are true enthusiasts and not interested in generating commercial revenue. However, in doing so, be assured that I am not surrendering copyright protection nor am I allowing eBay auctioners to freely use this information in their ads without attribution. In other words, to mention the Loudness Mod as it appears on this page in an eBay ad means you have to provide a link to this page. You can use this one:

http://home.netcarrier.com/~rstevens/st-40.html

Thanks for your cooperation. It helps transplant candidates everywhere, believe it or not.

August 5, 2002

Revisions since July 4, 2002

It has recently come to my attention that the owner of a commercial website in Vancouver is currently displaying the EICO ST-40/70 Loudness Switch Mod that appears here as his own which, by the way, is copyrighted information as shown at the bottom of this page. I have first-hand testimony that the gentleman in question was brought a customer's ST-40 along with the stock schematic and a printout of the information on this site, and subsequently agreed that the original circuit was flawed, applied my fix to the customer's amp, and then adopted it as his own and published it on his website. When I asked him about this, he stubbornly refused to acknowledge this page as his source, citing the long-time existence of Loudness circuit designs in general, which is completely irrelevant. The fact is, this site was the first to present this information as it applies to vintage EICO integrated amplifiers, as a published DIY modification, and has been copyrighted since its inception. That the offender refuses to acknowledge these facts after being notified of same has no bearing on the truth but only serves to show not only an ignorance of copyright law but a purposeful and callous disregard for the plight of terminally ill patients in need of a life-saving transplant.

Why?  As I explained to the offender in question, I create and publish these mods for the benefit of the owners of vintage tube equipment, not for the promotion of commercial services or for any other for-profit enterprise. Also, and more importantly, it is my intent to gently urge readers that may not have previously considered tissue and organ donation to do so, so that people who are dying from a terminal illness might one day live through a life-saving transplant, as indeed I have.

Therefore, if you, the reader, are in need of any of the information contained herein, please take it and use it freely. However, if you wish to reference it in print or publish it in any way you must have my agreement in writing -OR- provide a link to this site before you can do so (an email will suffice). By the way, that includes eBay auction ads as well as inclusions on other DIY websites. All Rights Reserved--I think that covers it. The notice at the front of this site is even more comprehensive. Am I not correct in believing that the US and Canada have reciprocity in these matters?

In any case, these are my goals, and that is my intent--period, point, blank. People are dying, and while this is but a small part of my total effort, I can and will defend it aggressively.

Anyway...

July 4, 2002

Revisions since July 23, 1999

I received an email from a gentleman with background on the original--and apparently purposely flawed--stock Loudness Switch circuit design. Frankly, it's fascinating, so I present it to you here (thanks to Eric L.--you know who you are):
 

Marketing--giving America what it wants. Engineering--giving America what it wants it to have. I can say this because I was once a Product Manager, and I had to mitigate the damage from the continuous battle between them both on a daily basis.

July 23, 1999

Revisions since June 25, 1999

Nothing much.  The Loudness Switch circuit revision continues to impress in both the ST-40 and the ST-70! Retitled the page to better include the ST-70, to which I'll devote more space in the future.  Until then this is still primarily an ST-40 modpage.

June 25, 1999

Revisions since May 7, 1999

BIG-TIME BREAKTHROUGH!!! Here's the fix (finally!) for the Loudness switch that works for both the ST-40 and the ST-70 and completely changes their sonic character, emphasizing the obvious fact that, as implemented, it was a clear design and/or documentation error. First of all, get a schematic, and see for yourself--it just cannot work as designed. I finally got fed up with two problems that both these amps exhibit (especially with high-sensitivity speakers)--one, all their volume is dialed in within about the first 20% of the volume control's rotation, which exacerbates a problem inherent in the part itself--an imbalance favoring the one side at low settings, and two, the stock Loudness switch performs more like a bad Mute switch which cuts the volume but doesn't add any real bass. The ST-70 is rather short of bass anyway, and this just makes that worse. Well, guess what, it ain't the guts--it's the stupid-ass Loudness switch design. Here's how you fix it:

1. We have to reverse the positions of the resistors and capacitors on the Loudness switch, and change their values, as well as add two jumpers to the switch with this change, so bear with me. The .2mfd @ 10vdc capacitors are becoming .022mfd @ 400vdc 716P Orange Drops, and the 2.7K resistors are becoming 22K. And their positions on the four-gang switch are reversed--the resistors go from the switch to the ground points on the chassis-mounted terminal strip (where the caps were), and the caps are mounted on the switch body itself (where the resistors were).

2. Now we add two jumpers that, looking at the switch from the rear, connect the bottom left tab to the middle right tab of the left-most DPDT switch section and another jumper in the same position on the right-most DPDT half.  Think of the four gang switch that EICO used as though it were two DPDT dual-gang switches side-by-side (although this function could be performed correctly by a single DPDT switch, as in any Scott 299 circuit.)

So, from the rear the Loudness Switch looks like this:

The new jumpers go from 3-to-5 and 9-to-11.

3. And while you're at it, replace the 68pf ceramic caps with silver micas.  Keep all stock wiring intact!

That's it. The volume control now behaves normally, the Loudness switch works like a normal Loudness switch, and the amps have much better overall sound and better channel-to-channel balance at normal listening levels. And in the case of the ST-70, the amp goes from being completely unusable with high-sensitivity speakers--like my E-V Marquis--to an absolute jaw-dropper. I always knew it had it in it (well, I hoped it did).

This mod is BIG NEWS, my audio brothers and sisters--I almost hate to publish it because now the resale value of these things is going to skyrocket. Gee, I hate when that happens!

May 7, 1999

Welcome to my mod page for the EICO ST-40 integrated amplifier. (That's INTEGRATED, not INTERGRATED, you eBay sellers, you.) The ST-40 is a semi-popular 20 watt-per-channel integrated amp manufactured by the EICO Company of Long Island City, NY. It's basically the lower-powered brother of the ST-70, with a similar tube line-up and nearly identical mechanical design from a construction, operating, and visual point of view. It was available either factory-wired or as a kit (I believe mine was factory-wired). Once refurbished, it represents perhaps the best price/performance value out there on the used market for an integrated tube amplifier with built-in MM phono stage. (I find it sonically superior to its larger brother, and it lends itself well to modern tube substitution in the output tube area.) Basically, the circuit consists of a 12AX7-based phono stage driving a 12DW7/7247-based line amp into another 12DW7/7247 driving a pair of 7591A's in push-pull using cathode bias. The power supply uses a 5AR4/GZ-34 rectifier tube in a full-wave configuration and output taps are provided for 4, 8, and 16-ohm speaker impedances, along with a center channel speaker output. It has an aluminum chassis, extruded aluminum side panels and a painted steel cage.

The transformers are fairly beefy for the power level, and everything is well laid-out in a fairly large, easy-to-work-on space, as it is built on the same chassis as the more complicated ST-70, except that it's slightly shorter front-to-back. It was originally supplied with EICO-labelled Mullard tubes (except for the outputs--duh), and is wired point-to-point, which makes it either easy or hard to work on, depending on your point of view. It does make it more durable, however, and it generally sounds better than PC board construction.

As it is self-biased, it doesn't have any bias adjustments, but it does have two hum balance pots that work well. One nice touch is that it has two MM phono inputs, one terminated with 47K and one with 100K, which I find sounds better with most of the MM cartridges I've used it with. It takes quite a bit of work to get the case open, as there are screws a-plenty, so you better charge up your Black-and-Decker electric screwdriver (the most-used tool I own--no question).

Of course, a schematic is always helpful. Unfortunately, while there is one up on the Web, it's a partial, and it may help to get one for the ST-70, as much of the circuitry is shared. In any case, both are available at Fiddler's EICO site, but beyond that, good luck--I haven't found any manual copies anywhere that I've looked, which is pretty unusual. No matter, as it's fairly straightforward.

My Personal ST-40 Story

Not unlike my excuse to buy my AA-100, I originally looked at the EICO ST-40 as a cheap way to get some spare 7591's for my Fisher 800C, but then I got all involved in rebuilding it, after which it ended up taking on a (wonderful) life of its own--it's got that sound! I quickly found out a few things about the ST-40, however--it runs hot, being self-biased (almost into Class A), and as such the black octal output tube sockets can be brittle and difficult to work on, so be careful. Another nice realization--this particular example was apparently not used very much, as it had ALL of the original tubes, and every one of the Mullards was very fresh, as tested onmy TV-7D/U tube tester. This alone can be counted as a big plus. Mullard 7247's alone are somewhat rare to say the least, and I might venture to say that it's not uncommon for these amps to be pretty unused as often as not--they are big and ungainly, and probably weren't the favorite of every suburban home decorator (wife). Let's face it, they're almost ugly, although I love them--so they might have been replaced quickly with smaller, cooler-running gear when the silicon revolution came in.

As it turned out, though, one of the 7591's in mine turned up weak--the one closest to the rectifier tube, which is the hottest area of the chassis. This amp requires some serious ventilation--it gets hotter than any other 20 watt-per-channel amp I know. One thing to keep in mind about the ST-40, however--lots of room and easy to work on. Also, cathode bias--can you say Sovtek 7591XYZ's? I can.

Anyway, this first treatment explores the potential of the ST-40 with its basically stock circuit configuration and fully exploits its ability to accept Sovtek's 7591XYZ, which ended up taking more than a simple cathode resistor change in my case, but I'm getting ahead of myself. Rest assured, however, that what results is maybe one of the top two amps in the WA3FLE house--absolutely a killer machine for a shade over a hundred bucks or so. Is that a kick in the ass, or what? I'm telling you. Prepare to be amazed.

Replace All of the Caps

Here we go, and I'll say it again--just assume that all the caps in the amp are bad and replace them. You'll be a lot happier about going out of the house to get the mail, for example, with the amp still playing and coming back to an intact house. Consider the age here--we're talking on the order of 30 years. For coupling caps, I used less of what I had, and more of what I ordered specially for the job--almost entirely 716P 600vdc Orange Drops from AES. The can caps in mine are still OK (knock on wood). Just replace all of the caps with original values, with some exceptions--some of the cathode bypasses are those uniquely EICO .009 ceramic caps, so to be honest, I just left them alone. EVERY other cap in there I replaced, and it shows in the sound. By the way, I NEVER throw out any parts I remove from an amp I'm rebuilding. I store them in a separate plastic bag and label it with the amp name, and that simple act has saved me a few times--you never know when you'll need to refer back, or whatever.

While you're at it, check all critical resistor values--you know, like the output tube grid resistors and the phase inverter plate and cathode resistors. I generally now use Radio Shack carbon films--they're fine as far as I can tell. Measure every component before you put it in--a good rule to follow, whether the part is new or used. Take my word for it.

You also want to clean all the pots, switch sections and tube sockets with DeOxit5 deoxidation spray solution (another must-have). Small point here--don't spray the sockets with the amp plugged in and turned on. Trust me on this one.

Replace the Line Cord

This is so important, I'm going to give it a section all its own (I ALWAYS do this and nobody ever mentions it). Replace the line cord with a 3-wire, 16-gauge line cord (or larger--I used Radio Shack), bypassed with .01@2KV (line-rated) caps from each side of the line to chassis ground. You can use Panasonic X-Y Series (best) or Radio Shack ceramic caps (also fine). This is a must-do, in my opinion. The IEC says green is chassis ground, so attach that to the chassis (e.g., transformer mounting bolts).  White is neutral, and goes to the unswitched side of the power transformer primary winding (in this case, the fuse holder). Black is HOT, so this goes to the AC power (on/off) switch. NOTE--more optimum would be to have the fuse in line with the switched side of the power transformer primary, but I'm lazy and usually just use the configuration that's in the amp at the time. Connect a .01@2KV from black to the chassis and another .01mfd@2KV from white to the chassis at the point of entry of the line cord, and you're done.  No more shocks, no more hum, and no more electrical code violations.The chassis cord clamp can be a challenge, and I haven't mentioned it before--I remove it with Vise-Grips, and then open up the inner hole witha Dremel Tool and a cone-shaped grinding bit, then stick the new (fatter) line cord in and re-insert it. Then, just to be sure, I put a honking big wire wrap tie around the line cord on the INSIDE of the chassis, to help relieve the strain. Works for me.
 
 

Replacing the Output Tubes With 7591XYZ's

OK, here's really the best feature of this entire rebuild. With this change, I get the extra real 7591's I need for my Fisher 800C (and EICO ST-70 and Scott 299D, etc., etc.), and I have to say, contrary to some reports and my own expectations, it sounds terrific in the bargain. Sovtek, in their own brand of pseudo-wisdom, repinned their 5881 and sells it as a (cheap--big feature) 7591, so it can drop right into an amp that uses them. One detail you have to be mindful of is that it's not the same tube, so you have to rebias it, or in the case of the ST-40, change the cathode bias resistor value. Sounds simple, right? So simple, in fact, that they even include a 250-ohm 10 watt Xicon wire-wound resistor--I got one with my quad from Ned, but I have a feeling you're supposed to get one per pair of tubes in the package.

Well, it wasn't that simple for me, but because I have an Allessandro dual analog plug-in bias meter, I can see real-time what's happening with the output tubes while they're running, and with the 250-ohm resistor, they just drew too much plate current--58mA per tube! So I started ordering up new values from DigiKey, I think it was, and thinking that maybe 275 ohms would do it, I got 5 of them and five 300 ohm jobs too. No dice--even with a 300 ohm cathode resistor, each tube in a pair was drawing 52mA. So back to DigiKey and this time I got in some 330's and some 360's (I may even have gone for the 390's)--it got out of hand, but I'm prepared for anything now. Answer? A single 330-ohm 10-watt resistor per output tube pair, bypassed--in my case--with a 220mfd@100vdc electrolytic cap (from AES) and you're done. This yields approximately 49-50mA plate idling current per tube, and it sounds great. This is at about 420vdc with my line voltage and a fresh 5AR4 rectifier tube, by the way.

Now, what to do about the negative feedback? I worked on this issue quite a bit, and even emailed Ned about it, mistakenly thinking he had conflicting advice on his website. It (of course) turned out that he was right, but in my own case, while theoretically since the gain of the output stage is now less you should probably increase the negative feedback to preserve the original ratio of negative feedback, when I did it, it actually ended up sounding worse. So the good news here is, DON'T TOUCH THE NEGATIVE FEEDBACK! Seriously, it ain't worth it, even though it's as simple as paralleling an existing 15K 1-watt resistor with another, but take my word for it (or don't), it isn't necessary. Here's another view of the ST-40 with the Sovtek 7591XYZ tubes (not my amp, but typical):

Tubes--What To Use?

This is the last decision point of the whole deal. As far as preamp tubes go, Mullards were original, so if you've got 'em, use 'em. Use quality 12AX7's, as the phono stage is pretty decent, especially when using Phono B--the 100K input. As for driver tubes, I use the stock Mullard 7247's, but I'm going to try some EI 12DW7/7247's in there as soon as I can (they're getting scarce quickly, you know), and see if I can't bank the Mullards for a rainy day. One thing I would advise is when you test whatever output tubes you're going to use on a tube tester, do what I've been doing lately--measure the plate current drawn by each tube as it is tested and write that down along with the tube tester rating number and try to match up those numbers in each channel--they're actually more important than the tester rating. I use the Allessandro dual analog bias meter with one plug-in plugged into the TV-7D/U octal socket into which I plug the tube under test, which works well. Then I move the tubes around until I get two per channel that draw equal plate current at idle, so at least they're somewhat matched, although it doesn't account for loading conditions all that well. Again, my advice? Break down--buy a bias meter.

Sit Back and Let 'er Rip (first on a Variac):

Bring it up on a Variac, if you have one. Otherwise, pull the rectifier, and check for smoke, observing all the other first-time turn-on precautions and then successively plug in the driver tubes, then the rectifier and then the output tubes (I think I have that right--just get a Variac and you don't have to worry about the sequence all that much). When you think you're ready, bring the amp up and play some music. Just watch how you connect the speakers--the back panel looks lke some kid's science fair project.

Have fun and let me know how you make out.

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