May 15, 2000

Changes since May 5,1999:

Not bad--a year without any major changes.  During this time I experienced a couple of shaky filament connections on a couple of the (Ruby) EL-34's, which fortunately I was able to fix.  Moral?  Don't throw out "dead" tubes (tubes that don't light).  Check them with an ohmmeter and try resoldering pins 2 and 7--the filament pins--before giving up altogether.  I got lucky.  One was the tube and the other was a wierd socket problem with one of the ceramic sockets I installed--it made contact until the tube was fully seated, then the filament connection on one side opened up.  Solution?  Work the socket contact with a pointy tweezer and then unseat the tube a sixteenth of an inch.  It works and the tube is tight so I'm not too worried about it.  Go figure.

The only parts change was one I made was this last week.--I replaced the 1N4004 bias rectifier with a "superfast" 400V 2.0A Rectron DO-15 diode (Mouser p/n 583-sf26, $.54).  I still bypassed it with a .1mfd@500vdc ceramic cap (Radio Shack), as I do with all silicon diodes.  Result?  Maybe noticeably smoother reponse, it's hard to say--more listening is required.  I've performed this change on an EICO ST-70 as well, and generally my impression is that it's a positive one.  Carry on.

Oh, one last thing, I'm back to a Telefunken 12AU7 driver tube.  The Ei 12BH7A is a little raucous in the final analysis--it would probably be happy driving something beefier like a pair of 6B4G's.  For the EL-34's there is no better driver tube than the Telefunken 12AU7 (talking Ned's driver board here, of course--see below).

May 5, 1999

Changes since May 1, 1999:

1. Replaced driver tube on Ned's board with new EI 12BH7A, which is a killer--probably the best driver tube I've tried yet. Except for a Telefunken ECC82, that is.

May 1, 1999

Changes since April 21, 1999:

1. Installed two DPDT toggle switches on the top of the chassis midway between the output tubes and midway between the edge of the driver board and the output tube heat vent slots, to allow switching between triode-mode and Ultralinear mode operation. I used 100 ohm 2 watt Xicon resistors between the screen and the plate of each output tube for the triode connection, leaving the negative feedback from the OPT primary screen taps (that use the green wires) intact, per Ned's driver board Notes section. I did some rudimentary testing and came to the (to me, anyway) rather obvious conclusion that, quite unlike the same test with my Mark III's, standard Ultralinear mode operation sounds a LOT better in a Stereo 70. Better highs, cleaner sound with less distortion, more impactful bass, and a much more palpable presence. Triode mode was congested, weak, and really just totally underwhelming. Go figure. In short--don't waste your time with triode-connecting your Stereo 70--it's a waste of time. If it sounds better, check your speaker connections, as chances are you are connected to the wrong impedance taps. That is all.

Introduction

April 21, 1999

Welcome to my mod page for the Dynaco Stereo 70 amplifier. The Stereo 70 is a popular 35-watt per channel amp manufactured over many years by the Philadelphia- (and then Blackwood, NJ-) based Dynaco company. Some say it's closer to 25 wpc (I'll measure it in a few days), but suffice to say it's got enough juice to drive any reasonably efficient speakers to room-filling levels. Once refurbished, it sounds marvelous on Dynaco's popular A-25's. It consists of a printed-circuit driver board with a pair of 7199's driving a pair of EL-34's in a push-pull Williamson Ultra-Linear arrangement (screens of the EL-34's connected to taps on the primaries of the output transformers). The power supply is tube-based, using a 5AR4/GZ-34 full-wave rectifier, and a multi-section 525vdc can-type electrolytic capacitor and choke in a capacitive-input configuration. Output taps are provided for 4, 8, and 16-ohm speaker impedances. It has a chrome chassis and painted steel cage.

Of course, a schematic is always helpful.  Fortunately, there is a complete schematic for the Stereo 70 up on Curcio Audio's website, which you can download or read in Adobe Acrobat by clicking here. For a full manual copy of this and other Dynaco products, Sound Valves sells--and occasionally auctions off at eBay , which is where I grabbed them--two different collections in binders, one for Preamps/Tuners and one for Amps for $49 each.  I got them both, and I'm glad I did.

My Personal Stereo 70 Story

I never had a Stereo 70. I always wanted one, but I already had a pair of Mark III's, and sort of looked down my nose at the Stereo 70 for many years--back when I could actually afford one, and then later on when they became almost unaffordably expensive used. While readily available, their used condition is always suspect. Then I received one from a nice guy (thanks, Paul--again), and decided I would jump in and see what all the fuss is about. Of course, like you, I read everything up on the Net I could about modifying them and then planned my attack carefully before jumping in and of course learned a few things in the process, which I would like to share with you here.

The unit I received had Triode Electronics' (Ned's) driver board installed in it, and was a victim of bad soldering, damp environment (rust), and some poor decisions with respect to power supply mods. I decided I would be stripping the thing down to bare metal and doing a "frame-up" restoration, not unlike a car. I strongly advise you to do this, as you discover all manner of things that will haunt you later if you don't. This is a 30-plus year old unit, remember.

So I never even listened to the unit I got before starting to dismember it--I didn't even know if the transformers were any good, which was an interesting way to begin. Your experience may vary. The thing arrived with some shipping damage as well--one of the two EF-86 driver tubes (alas, a  Mullard) was broken, but I had one, so everything looked to be in order from a supply point of view. I then started unsoldering everything, saving all the wire and parts. I managed to use a good bit of the original wire on the heavier components, but used Belden #22 stranded to connect to the driver board (a long way off from the beginning). The chassis (and the cage--even worse) had a good bit of rust on it, but I didn't care--I hit it with a heavy application of EXTREME Metal Polish, and it came up shiny, although minus most of the lettering and still with pits and rust spots. Whatever, I said.

This first treatment explores the potential of the Stereo 70 with it's stock circuit configuration, but with Ned's board with some modifications to suit NOS driver tubes. Future updates will explore modifications such as triode-connection, various feedback configurations, and other tweaks. But this first iteration will be a tube-rectified--this is a must which I'll explain, and it isn't a sonic consideration--and keeps the Ultra-Linear configuration, which after all, the EL-34 tubes were designed for and is a hallmark of the Stereo 70 sound.

Replace the Sockets

Yup--we be replacing the sockets. Fortunately I had four good ceramic octal sockets and a Micanol octal socket I used for the rectifier, but any good ceramic sockets will work better than the admittedly decent original Cinch sockets, which are old and you can assume brittle. I salvaged all the hardware, but a lot of it was rusted and I had a devil of a time rustling up enough 4-40 hardware to do everything from my existing junk parts. But I digress. After stripping everything off the chassis and properly cleaning it, mount the sockets and terminal strips, as well as the other miscellaneous stuff (minus the transformers--what, you didn't remove the transformers?). Here's the beast untamed, by the way (typical amp, not the one I worked on):

Replace All of the Caps:

Well, if you replaced the sockets, you'd be a jackass to stick the original bias filter caps back in there, woudn't you? Use 100mfd@100vdc electrolytics--I ordered mine from AES. And of course, the selenium rectifier has to go--I used a 1N4004, bypassed by a .01@500vdc ceramic cap (from Radio Shack). Remember to orient the diode properly--band end to the transfomer wire--this is a negative voltage supply. I bypass all my silicon diodes with .01mfd ceramic caps--keeps down the digital nastiness.

Also going is the original can-type cap. I used an ultimate replacement--AES has new 525vdc caps in an 80/40/30/20mfd configuration, which is PERFECT. They're blue, which I like, but not LCR jobs, just FP-type can caps with a blue sleeve, and I hooked it up like this:

Decent, huh?  Looks good, has enough balls after the choke to support some major bass, and is of a high-enough voltage rating to handle stupidity. What it can't handle, even at the 525vdc rating, is a solid-state rectifier. Don't even think about it. The power supply is so loose (my term), that before the output tubes draw any significant current, the voltage seen by the first section of the cap is WAY over 525vdc, so it's a pattern for failure if you're trying it. The unit I received had a 20/20/20/20 at 475vdc and silicon rectifiers, and THAT'S really why I didn't even turn it on before starting.

I also cleaned all the pots, switch sections and tube sockets with DeOxit5 deoxidation spray solution (another must-have).

Rebuild the power supply:

One thing I did that I ALWAYS do and nobody ever mentions is 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 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.  Connect a .01@2KV from black to chassis and another from white to chassis at the point of entry of the line cord, and you're done.  Not more shocks, no more hum, no more code violations.

Assemble the Fine Bits

Actually, as I had to order the can cap from AES, it was the last part I assembled into the amp. I put everything else back together first, using a small terminal strip in the original selenium recitifer mounting hole to support the 1N4004 diode replacement, and remounted the transformers, re-routing all their leads, starting with the power transformer. A word about the power transformer--I understand from reading around that there were a raft of different iterations of the thing over time. In fact, I've heard that the lamination stack height can vary a good bit from amp to amp, and while mine is dead quiet, it is known to hum and buzz in quite a few instances. Mine is an old one--as a matter of fact the whole amp is an old one, with all cloth leads, and the power transformer is coated with potting compound and has an ugly flat-black, dipped appearance. And like all the others, it can run hot to the touch. Not too hot to handle, but noticeably hot. This doesn't worry me, but as there are replacements out there (at Triode Electronics, for example), it might be a good time to put one in now if yours is a bugger.

Also, check the condition of the leads on the output transformers as you remount them--if they're old with cloth leads, chances are they're frayed, so cover the bad ones with shrink tubing where they come through the chassis, and be careful when tightening down the transformers that they're centered about their wire access holes. And check the manual--it has good pictures that I constantly referred to for wire routing, and you don't want to buck the "twist memory" of the wires after they've taken a semi-permanent shape after all those years. You may notice that the book has the speaker connection strips shown so that the tabs are pointing down, but I reversed them so they're pointing up , so spade lugs can come in from below. A small point, but one you'll think about too late if you care at all and get it wrong.

Remember--don't cut any lead lengths--you'll need them all as long as you can keep them. When disassembling, unsolder--don't cut, or you'll regret it. And when resoldering, redress and re-tin the lead ends before soldering, and twist the leads around socket and terminal strip tabs--don't just stick them in and solder them, like so many bad kit (and factory) wiring jobs.

As for cathode resistors on the output tubes, I used a 10-ohm 5-watt wirewound (Radio Shack again) on each pair of output tubes, but as I usually set up all my amps with the Allessandro dual-analog octal plug-in plate current-reading bias adjustment meter, they're only there for easy front panel reference--I set up the amp so it reads an even volt across the resistors, corresponding to 50mA cathode current per tube, or about 45mA plate current per tube.

Fine-Tuning Ned's Driver Board

Here's where a lot of time was spent and what drove me to write up this treatment. I don't know what the stock driver board sounds like, but I don't even care, for as long as Ned's board is available at the price, what with the cost of new old stock RCA 7199's, it's a no-brainer. it does bear comment, however, for a few things aren't so easily implemented. I don't know if the board that came with the amp I got was bought already wired or was built up as a kit, but it already had a few pad lifts and other examples of bad soldering. I carefully unsoldered all its original connections when I removed it and hit it with solder wick and a 35-watt pencil iron to clean up the pads I would have to connect to. I have to say (sorry, Ned), that I've seen better quality PC boards. No plated-through holes, no silk-screening, and the pads lift easily if mine is typical, but if you're careful, it doesn't matter all that much, as the parts are of high enough quality and the design is close--not right on, but close. It looks to me as though Ned adapted rather too closely the original 7199 pentode section design instead of putting together a real EF-86 stage--in his defense, however, he may have just optimized it for current production tubes--but I had all kinds of problems with too-high plate voltage on my old original stock EF-86's (Mullard Holland, Mullard Gt. Britain, and Mullard--I think--W. German). Here's what I finally settled on for each EF-86 driver stage--replace the 1.5M ohm screen resistor with a 1M. Replace the 332K "bleeder resistor" with a 470K. And replace the 820 ohm cathode resistor with a 1.4K (I used a pair of 680 ohms in series). This will give you between 90 and 95 vdc on the plate of the EF-86 when warmed up and swings perfectly into the 12AU7 (or 12BH7) driver tube. I used Radio Shack half-watt Carbon Film 5% resistors, and hand-picked them (they're pretty good anyway, but I'm careful). I also replaced the original film caps Ned supplies (pretty decent Xicon 400vdc jobs) with 716P 400v Orange Drops--I just have a hard-on for 716P's--and upped the coupling caps to .47mfd. This last change was done after the following mod.

Install Frank Van Alstine's Input Bandpass Filter

OK, here's a controversial move. I read Frank's Stereo 70 treatment and what he claims made sense to me, and I figured the rolloff he suggests would be almost unnoticeable anyway, and you know what--it is. So I figured if it would unburden the feedback loop and the rest of the amp with passing frequencies it couldn't handle, all the better,and it allows an upping of the coupling caps to the output tubes and doesn't adversely affect the sound. At all--in fact, it tightens it up in the bass a small bit. I liked it so much, I did it to my Mark III's. It consists of a .02mfd@50vdc film cap in series with a 10K resistor to the input grid, which has a 475K (I used a 470K) resistor in parallel with a 1000pf (.001mfd to you) 50v film cap to ground. Neat. As I eliminated the Stereo/Mono switch, a future mod might be a switch to select the input filter or bypass it, but on the other hand, perhaps not.

Tubes--What To Use?

This is the final nut of the whole deal. I swapped a raft of different tubes in and out of Ned's board, and what I ended up with was a pair of 1961 (I think they say) Mullard West German (can you say Telelfunkens?) EF-86's,which have solid gray plates (very unusual), with a Telefunken 12AU7/ECC82 flat-plate driver tube. Second place went to Mullard short mesh-plate EF-86's as driver tubes, and a Mullard 12AU7/ECC82 driver. Also good as drivers were an RCA tall-plate 12BH7A and an RCA black plate 12AU7. What I didn't like so much was a clear-top RCA 12AU7. A GE high-emission narrow plate 12AU7 was pretty decent, too, but nothing comes close to the Telefunken 12AU7 (wish I had more than one). I want to try an EI 12AU7 and an EI 12BH7 as well as some EI EF-86's as well (I think they make them). Hope the current conflict doesn't kill off their tube supply.

As for output tubes, what came with the unit I got was a quad of matched Ruby EL-34's, which I think are marked something like EL-34SVT or something. Obviously some Russian version,and they have dimple tops, so somebody out there knows what they are. I just haven't researched it enough to know--probably Svetlana's, but who knows? Maybe they're Chinese, but they sound great if you ask me, and they are matched pretty close.

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, warm up the amp with the bias settings down, and then bias the amp for 50mA cathode current per tube and try some program material through it. Check the plate voltage on the EF-86's and see that they're sitting at around 95 vdc or so, and you should be good to go. My amp runs sweet as can be, and is getting better every day (Orange Drops take some time to run in I've noticed). With 80mfd after the choke, it has pretty decent low end, even with the Van Alstine input filter. I'm driving it with a rebuilt PAS preamp, and they sound terrific together through A-25's, as I said above. Again, Telefunkens in the PAS make it sing. Not Dynaco Telefunkens, but real Telefunkens--if you haven't noticed, there is a sixteenth-inch of plate height difference between the Dyna-labelled jobs and real flat-plate Tele's, with the real jobs being shorter and better-sounding. Go figure.

So get out there and mine this gold before the newbies mangle them all. Hope you like it--I do. Makes a great little system with the rebuilt PAS, a Rotel 930 budget CD player, and a pair of Dynaco A-25 speakers. Have fun and let me know how you make out.

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