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The Venerable Dynaco Stereo 70
Article By Dan Schmalle From VALVE Issue 3, March 1994

  Said to be the most popular tube amplifier ever made, the Dynaco  Stereo 70 has been around for over thirty-five years. Its early popularity was due to a reasonable price coupled with good quality iron and a sound seemingly liked by all.

A simple circuit consisting of a then new RCA 7199 sharp cutoff pentode medium mu triode driving a pair of 6CA7 / EL34's per channel, with a GZ34 / 5AR4 for B+ , made for an easily built, easily adjusted kit It has also been a platform for more modifications than any other piece of gear made.

The mods have ranged from simple recap jobs to full redesigns ala Joe Curcio and Audio Research. Some examples of kit built mods are shown on this page.

All the mods are naturally done to correct perceived deficiencies in the original circuit and so tend to group around said deficiencies. The two most common areas of attack seem to be the power supply and the driver stage.

One look at the power transformer will make clear the motivation for power supply mods. It's not very big. Neither is the filter capacitor, with only 90 mfd total capacity. A clue to the effect this has on  output power is the 40 Wpc rating of the Mark IV, a monoblock cousin of the 70 with a separate power transformer and 90 mfd filter for each channel.

Using the voltage to the output transformer center tap suggested in the assembly manual (415VDC) total energy storage for the Stereo 70's power supply is roughly 1/2CV2 = 7.75 joules, considerably lower than some other amps of the same era. A Heathkit W-5M for example, used 110 mfd of capacity for one channel, and was rated at 25W output instead of the Stereo 70's 35W (Note that actual storage is somewhat less as two stages of the filter operate at lower voltages for the driver circuit).

Common power supply mods are increased filter capacitance and regulation, as well as solid-state rectification of B+ .

My own humble opinion is that the slight gain in B+ voltage from solid-state rectification is not worth the stress placed on the power transformer during start-up surge of the filters, or the mysterious cathode stripping which occurs when the output tube plates are subjected to high voltage before heater warms the cathode.

Try an experiment substituting a couple of PTC205 diodes for a 5AR4 rectifier in an amp and put a voltmeter on the filter inputs. Start the thing up and watch the meter. Whoa!

There are enough Stereo 70's with a blown power transformer out there. Why make more. That 5AR4 is one sweet tube. I put 2110 mfd of extra capacitance in the filter circuit on my Stereo 70, which runs about 410VDC at the output transformer center tap (about 185 joules of storage). It has a big beefy Eico ST-40 transformer 'cause it was one of those victims of stress to which I referred. It never exceeds 415V during start-up. The slow warm-up of the 5AR4 really reduces stress on the power transformer and the filter caps.  The usual comments on increased definition and extension of bass seem to apply to my filter mod. It really made the amp sound different -- louder, much more bass and mid bass.

But there was also a change in the treble. It may be smoother, or it may be depressed, I haven't decided.

The amp is much brighter without the extra capacitance, a sound I  don't particularly like. This maybe due to the fact that the 70 has very little feedback in the high end which becomes a problem when changing to polypropylene caps.

The poly caps seem to bring out the treble in many amps I've recapped. Why greater filter capacitance counters this is something I'm still working out. Regulation is another area of some controversy. I put a string of ten 39V Zener diodes across the filter at the centertap of the output transformer after hearing a Curcio Stereo 70, which relies on heavy regulation of plate and screen voltage of the EL34's.

Then I read an article by Tom Holman (JAES Jul/Aug 1981, in our library) claiming that the loss of headroom caused by the lower instantaneous voltage available in the regulated supply outweighs any gain in steady state power due to the regulator.

Ironically, the Curcio Stereo 70 I heard and liked, was measured at 11W steady state output per channel. It should be noted here that in later years Dynaco used a different method of rating their amps and lowered the Stereo 70's Wpc from 35 to 20.

So the zeners will come out and I will listen again.

I mentioned before that I replaced the coupling caps. I do this as much for protection of the output tubes as for sonic improvement. Some capacitors used by Dynaco were pretty good leakage-wise, the Pyramids come to mind, and some were not so good. I use polypropylene caps from Mouser Electronics (nice folks) because they are affordable, seem very reliable, and have made definite  changes in the clarity of the treble in many amps I've done. Cap changes are pretty standard for all mods.

The driver mods vary from a simple changing of the traces on the driver circuit board to accommodate the 7199's cousin 6GH8A to all new topology using different tubes or solid-state driver circuits.

I had intended to report on a solid-state driver conversion that was really simple, but I couldn't locate a current cross reference for the FET's that were listed in the 1966 vintage article, and the FETs I used distorted with any more than soft volume.

One commercial rework of the 70 which uses a FET front end is the Nobis Cantabile (Stereophile 15/9). Curcio's mod uses 6DJ8's in cascode for the driver, and ARC used a really complex cross coupled circuit with so many tubes they had to use 6L6GC's as output tubes to limit heater current draw.

My impressions of 7199's in other amps have been favorable, but I am constantly frustrated at how insensitive the Stereo 70 is. The 35W max output was reached at 1.3V input instead of the standard 1V used by other designs. This makes the amp hard to use in bi-amp and surround situations. A very interesting analysis of the 7199's performance in the 70 and mods to correct it's weaknesses done by Norman L. Koren are to be found in Glass Audio, 1/92. I'll give this one a try when I get the courage to punch holes in the chassis.

Well, the Dynaco Stereo 70 may be old, and it may not sound like a new $3500 tube amp, but it sure is a fun amp to dink around with. If you have one, bring it to the next meeting, March 6th, 1994 at high noon.