May 6, 2005

Revisions since May 2, 2000

Mainly html-related changes. Added an Official Introduction header. Fixed my email address as it was incorrectly incorporated in the MailTo icons at the bottom of the page (didn't even realize it was a problem until I saw the source code in Coffee Cup). Also fixed some spelling errors and removed the discrete (although correct) email address at the bottom of the page and replaced it with a link. Lastly, updated the copyright statement and date.

May 2, 2000

Revisions since August 30, 1999

Getting back into the HF-81, I fixed a few niggling things today.

1. I ordered and received from AES four (correct value) silver mica caps for the 12AX7 driver stages, namely 220pf 500V jobs. The stock ceramic caps were labeled 225pf, but you can't get 225pf silver micas, and besides these measured out at 230pf anyway. These replaced the 180pf caps I HAD used in there for want of the correct value (see step 4. from August 30, 1999 below). It makes a noticeable difference, as the driver stage is pretty finely tuned in terms of frequency response and balance. Comparison with the HF-86, which uses 125pf caps in these locations (across the plate and cathode resistors of the phase inverter stages), but with lower value resistors (82K vs. 100K in the HF-81), show that it's a purposeful and important choice of value. It sounds it--gone is the somewhat nasal high end and relatively weak low end.

2. I then pulled out the Radio Shack 470mfd@35vdc cathode bypass caps in the output stage and put in Sprague Atom 100mfd@100vdc caps, which I am partial to lately. I now believe that the practice of lumping a huge value cathode bypass cap across the cathode bias resistor in amp designs such as this, the Dynaco SCA-35, and the EICO ST-40 is both foolish and sonicly stupid. It slows down the amp, making the sound sluggish, seems to aggravate overheating, and throws off the overall character of the amp. The stock value is 50mfd (per channel), by the way. Of course, perhaps a better approach would be to use a ferocious cap such as a BlackGate (if I could financially justify it), but for now I am very happy with the sound of the Sprague Atoms (also available at AES).

3. I pulled out the somewhat soft Brimar 12AX7's I had in the phono stage and put the Telefunken ribbed-plate 12AX7's I had been using in the driver stage in their place, using new JJ/Tesla 12AX7/ECC83's (which are excellent new-production tubes) as drivers. I got great phono-stage hum balancing as a side-benefit as well as a clean, sweet sound.I continue to use the Ei 12AU7/ECC82's in the line driver section, but have to select them for freedom from microphonics.

So now Mike Stern and Robben Ford CD's on a CAL Audio DX-2 sound incredible through the HF-81 driving 3-way Electro-Voice Marquis speakers. Alnico magnets rule!

August 30, 1999

Revisions since June 26, 1999

I think I've finally solved the mysterious hum problem, or at least mitigated it as much as probably the amplifier layout and component age allows.

1. First, I down-sized those coupling caps described below in the June 26 update from .1 to .047mfd@400vdc 716P Orange Drops. I happened to remember that the originals were .025 ceramics, so I think .1's are a bit of overkill, and I was trying to reduce the hum anyway. Fortunately, I like the .047's better anyway.

2. Next, I had experienced a newer, stranger thing prior to the above change--not just residual hum in one channel that I originally attributed to an AC balance problem in one of the driver stages, but real hum when I connected associated equipment. I can't remember ever hearing it before, so I put a ground lift (a 3-pin to 2-pin AC adapter) on the power cord and it went away. But the amp seemed to be vibrating strangely a LOT more than normal. So after a few days of getting more and more worried, I took it back downstairs to the shop and opened it up and saw wax that had run out of the power tranny! Yikes! NOW I know what was causing it--I had a switching power supply-based nicad battery charger on the same power strip that is notoriously famous for generating hash and RFI (it's a Yaesu NC-29 I think), and it was feeding noise back onto the AC line and when I pulled the ground, the remaining line bypass filter caps inside the HF-81 from hot-to-ground and from neutral-to-ground--now floating at the common point due to the ground lift--were apparently giving the power tranny heartburn to a major extent. So I decided to do several things:

3. First up was remove the two AC line bypass caps--I did this on the PAS-3X when it developed hum when used with the Stereo 70, which I think is more of a Stereo 70 leaky power transformer-induced phenomenon, but the power tranny in the PAS could be leaky, too. It's seriously undersized and old--let's face it. But I left the green wire of the three-wire power cord connected to ground instead of applying it to a SPST slide switch like I did in the PAS-3X. Instead, I put an DPDT slide switch in the position formerly occupied by the accessory AC socket on the HF-81, and wired it up with both sections in parallel and used it to switch a .047mfd@600vdc 716P Orange Drop from one side of the power transformer primary winding to the other, as in any common guitar amp--a typical "ground switch", if you will.

4.Then I bit the bullet and COMPLETELY rewired the 12AX7 driver stage--every plate resistor, every cathode resistor, and every small value compensation cap. The original caps were 225pf ceramic jobs--I used 180pf silver micas, both because I had four of them, and because the HF-86 uses 125pf caps in the same circuit but with 82K splitter resistors instead of the 100K's in the HF-81, and bottom line they sound pretty nice. The original parts weren't THAT out of tolerance--the worst were a couple of the compensation caps--but I had to do it. I think I left the driver stage grid resistors alone, but I also replaced all four of the output stage 330K grid resistors as well.

5. I also rearranged the grounding of the amp. In a former abortive attempt to eliminate hum, I had tied all chassis grounds together with a ground wire "string" (which could also be looked at as a "loop", and we all know what a ground loop can do). So with the driver stage rebuild I separated each channel's grounding circuits from each other and from the power supply, leaving all of that to the chassis.

6. I also redesigned the output stage cathode circuit. I changed the value of the bias resistors from 165 ohms to 180 ohms--the amp was running too hot--and rearranged the cathode circuit grounding scheme, eliminating all tube socket mounting ring grounds in doing so. Now both cathode resistors and bypass caps (470mfd@35vdc electrolytics) connect to a common chassis ground point between the output stages to which no other grounds are attached.

7. Last, but not necessarily in order--I'm recalling this from memory--I redesigned the filament wiring circuit so that each channel has its own dedicated branch running off the hum balance pot, instead as in the original, where one channel's output tube filaments were on the filament circuit of the other channel. It's all one circuit with one hum balance pot, but I decided that I'd divide and (hopefully) conquer the hum in any way I could.

That's about it. I could probably do more--I've thought of putting .022mfd@400vdc 716P Orange Drops from the hum balance pot center tap to either side as in the Pilot 602M I just rebuilt, but haven't tried that one yet. Bottom line again is that the amp is stable, doesn't hum when other components are attached, allows for selecting a position of the new "ground switch" that produces the least amount of noise--particularly noticeable on an AM tuner--and runs cooler with no noticeable hum in the speaker. If you look at the scope, the right side STILL has about 20 millivolts of 120Hz residual AC hum, but the power tranny isn't buzzing anymore or leaking wax, and the amp sounds terrific. So fuggit, I say.

Oh, yeah--the late but sometimes great Ei 12AU7's sound terrific in the HF-81, as long as you can get ones that aren't microphonic. Mullards are just too thin. Telefunkens are, of course, the best.

June 26, 1999

Revisions since May 6, 1999

I upgraded the coupling caps to the output stages from AES-sourced yellow ECI .1mfd @ 400vdc polyester film types--chosen primarily for their small size--to 716P Orange Drops of the same value. I figured out a way to lay them over and fit them in there, and the amp sounds a whole lot better--less strident on high sensitivity (horn-type) speakers. Looking around while I was in there, I noticed that I used a lot of PS-series Orange Drops for interstage coupling--also chosen for their small size--and now wish I had used 716P's. They are a substantial upgrade over standard PS and even 715P Orange Drop caps. I know I still haven't done quite as good a job on this page as on some of the others, but it's coming. One thing I also notice is that this amp could probably use AC-balance controls (a la Fisher circuits), as there is some residual hum in one channel that's not in the other and I suspect it's due to splitter imbalance, although most of the values measure out the same. I have this same problem, more or less, in another EL84 cathode-biased integrated amp--the Dynaco SCA-35. More to come on this subject, I suspect.

May 6, 1999

Revisions since this was written (date?).

1. I've ripped out the voltage doubler off the filament supply I put in (see below) to drive an external 12vdc fan. I read that unless the voltage divider is effectively decoupled from the power transformer properly it can overheat it, and admittedly not knowing how to do this, I erred on the safe side and out it came. Seems to have more bottom end now(?!).

2. I've gone to EI 12AU7's after having excellent results with them on my Stereo 70 project as a driver tube in Ned's driver board for that amp (although I'm now using an EI 12BH7A in that position). They sound excellent in the HF-81. I used to have first clear-top RCA's and then EICO-labelled Mullard's, and I actually prefer the EI's (too bad about the war).

3. I promise to update this page soon, as there is much more detail I can add, especially about how to rewire the internal slide switch to select between tape outputs and preamp outputs at the rear panel (very useful).

Welcome to my modpage for the venerable and deceptively excellent EICO HF-81. That's it over there on the right. It's an unlikely looking beast, but has some very nice attributes. Consider these: point-to-point wiring, dual rectifier tube-based power supply, cathode-biased push-pull EL-84 outputs, all-triode (non-Ultralinear) Williamson circuit design, excellent transformers, and as it turns out, great sound. Truth is, there are few amps that sing so well. And, might I add, it has balls in abundance.

Quirks, it has a few: as a steel-chassis-based kit, albeit with a continuous ground bus above the chassis (unlike some other EICO kits I could mention--HFT-90), it can exhibit unreliable grounding although often it's just a result of overloaded and over-soldered ground solder lugs. However, I seemed to constantly run into bad solder joints as I ran through it replacing nearly every cap and quite a few resistors in the thing, so be on the lookout. Your experience may vary.

Of course, a schematic is essential. You can get a schematic off the Fiddler's excellent Unofficial EICO Page (the URL of which has recently changed) by clicking here. (This is now a PDF file--the old one was Zipped; if you have problems, email me and I'll post it here zipped.) Then, if you need one, you can order a full-blown Xerox of the whole manual from A.G. Tannenbaum. That should get you started.

From a replacement point of view, it's a good idea to replace all of the coupling and bypass caps--I used Antique Electronic Supply-sourced 716P Orange Drop 630 vdc values where I could fit them, and 400vdc values where I couldn't, especially in the final output stage coupling cap area (where space is ridiculously limited), where I used yellow ECI polyester film 400v (may have been 630v) caps, also from AES, since they are a lot smaller. The electrolytic cathode bypass caps in the preamp were substituted with as-close-as- possible new (I think it was) 50vdc jobs, also from AES--33mfd for the 25's and 47mfd for the 50's, or thereabouts.

Which brings me to the meat of it--change-wise, the biggies to me are "blueprinting" some key circuit values, and changing relatively few others, and one primary (maybe two) redesigns. The circuit values you should measure and seek to make right with new parts are the normal ones, namely output stage cathode and grid resistor values, which affect balance in the push-pull output--very important, and while you're at it, check all the carbon composition resistors, as they tend to drift high over time. I did change the cathode bypass electrolytic caps on the output stage(s) to 470 mfd@50vdc electrolytics from AES, a ten-times increase over stock, which I'm currently debating the advisability of in the larger ST-40, but it seems to be a benefit in the HF-81. And designwise, I changed the slide switch from a SPST "Common-Separate" function to a "Tape Monitor-Preamp Output" function using a Radio Shack DPDT slide switch, but if you don't want to do this you can start as I did by hardwiring it in the "Separate" position, which is the same as a Preamp Output function as it stands. At least then a dirty switch won't be living in your signal path.

The other change I made was to build a voltage-doubler off the 6.3vac filament supply and feed 13.8vdc to a rear-mounted side-exhausting fan so I might one day put the amp in a rack, or put a tuner on top of it. It became more of a "see if I can do it" thing before long, and really isn't necessary. Note--this is history (see above.)

Another factor I feel is equally important to the overall sound is tube selection--hope that the original Mullard rectifiers are good, since they are the best around. The output tubes need to be matched pairs, side to side, so test the ones you have--as Mullards, the originals are of course great, but chances are if they are like mine, they are all vastly different from each other. I finally settled on a quad of Tesla's, but Sovtek's are good, too.

The preamp tubes will determine your background hum and noise levels, so choose wisely. The 12AU7 drivers are particularly noise-prone and largely determine the sound--I'm partial to Mullards here, but RCA cleartops aren't bad. The 12AX7 line stages I feel should be Mullards though for that sweet high end, but some prefer the stoic sound (I think it is, anyway) of Telefunkens. And the phono stage 12AX7's actually like to be Sovtek 12AX7WXT+'s, as they null out the hum real well and aren't bad at all.

That's about it, except that be sure to clean all pots and switches with DeOxit 5. There is probably a lot I am forgetting but see how that works--it's still a fair bit of work, as there's a lot going on under there. Oh, the can-type electrolytic caps have always been OK in every piece of old gear I've had except one--a Dyna FM-3 tuner (alas, this is no longer true, but I digress). But there are two 30mfd/450vdc paper electrolytic caps in the HF-81 that I changed to 47mfd/450vdc jobs, also from AES. And as with all larger bypass, coupling and power supply filtering caps that I replace, I stick them to the chassis with double-sided foam tape--this is an important detail, or at least I think it is, although I can't support it with anything other than a gut feeling. Oh, and I ALWAYS replace the two-wire line cord with a UL-approved (in this case) 16-gauge line cord from Radio Shack, which I bypass from White (neutral) to Green and Black (hot) to Green with .01mfd@2KV disk ceramic caps or better yet, Panasonic X-Y Series Line-Rated bypass caps. Much safer and quieter that way.

I run this amp into a pair of PSB 800i's, and it absolutely kicks ass. I think you'll love it. Have fun, and watch this page for updates.

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