Upgrade Old Dynacos with Van Alstine Kit

Radio World
Reprinted from September 21, 1994

Manufacturer: Audio by Van Alstine, 2665 Brittany Lane, Woodbury, MN 55125; 651/330-9871 http://www.avahifi.com/ info@avahifi.com

by Doug Fearn

POCOPSON, Pa. Back in the 1960s and 1970s, Dynaco manufactured thousands of good-quality vacuum tube consumer preamps and power amps.

With the renewed interest in vacuum tube equipment by professionals and consumers, vintage Dynaco equipment is sought after by audiophiles, particularly those who want "tube sound" but cannot or will not pay the thousands of dollars for current production vacuum tube gear.

Several years ago, audio designer Frank Van Alstine, president of Audio by Van Alstine, recognized the potential for upgrading several of the Dynaco products. In addition to manufacturing new vacuum tube, solid state and hybrid pre- and power amps, Van Alstine offers rebuild kits for the Dynaco PAS-3, Pat-4 and Pat-5 stereo preamps; the St-series solid state power amps and the all-tube, 35 watts-per channel St-70 power amp. The upgrades can be performed at the Van Alstine factory (with your working amp), or you can get the necessary parts and instructions in kit form.

Radio World Managing Editor John Gatski found me a well-worn PAS-3 and a St-70, which were in pretty good shape cosmetically, but in need of help inside. They were perfect for the rebuilds.

Pre-upgrade listening

When I received the stock equipment, I decided to listen to them and make some measurements for later comparisons. For source material, I had twenty-five years of studio masters to choose from, plus CDs. My monitors were a pair of UREI 811s (15 inch with coaxial horn in a bass reflex cabinet - a very efficient system).

After connecting the system and turning it on, there initially was a low level clicking sound that disappeared. There also was significant 120 Hz hum and ground loop buzz that did not go away. The stereo balance was off several dB in favor of the right channel. And the old corroded RCA connectors on both units were an occasional source of extreme hum.

The stock Dynaco system lacked deep bass, and the recordings seemed to be missing the top octave. I also noticed a lack of musical detail. But the sound of the old gear was still pleasant-in a rounded off kind of way.

The kits

I obtained two separate rebuild kits: one for the PAS and the other for the St-70. The PAS kit, called the Super Pas Three, was extensive ($279 retail) and included line, phono and power supply cards, precision parts, new volume and balance controls and an AC cord. Ordered options included gold jack set ($60 retail) and a new faceplate and knobs ($75 retail), which were to be installed by the factory. The St-70 kit ($199 retail) consisted of a new main board with components, new B+ supply, bias supply and driver tubes. The only options I utilized were new input and output jacks ($49).

The Van Alstine approach should benefit those who enjoyed building Heathkits years ago, and who also want to save some money. But don't expect Heathkit-style instructions with these kits. The upgrade kits are for experienced electronics technicians or advanced hobbyists. Any broadcast engineer who performs troubleshooting and repairs at component level, however, will have absolutely no trouble.

Rebuild kits mean you have to deal with old equipment with sometimes fragile components and wiring. Fortunately, Dynaco originally offered both of these units as kits, so there are no complicated assemblies or tight spots inside. Because the unit you choose to upgrade may have been built from a kit, you could run into some pretty sloppy construction or poor soldering, depending on the skill level of the builder. In my case, the preamp was factory-wired and the power amp was kit built. The factory-wired unit was easier to work on because most of the internal wires were simply stuck through the connection hole and soldered. However, the factory used riveted-in connectors and printed circuit boards, and removing these posed some problems. A carbide burr on an electric drill, as recommended in Van Alstine's instructions, did the job, but it was slow going. The power amp wiring was not very good. In fact, I was amazed that it worked with all those cold solder joints.

The preamp

The instructions first call for loading the two PCBs, which is a good way to find out if you are up to the challenge. The components used are quality grade. Nothing is esoteric (no special capacitors or oxygen-free wire), just good, solid, quality parts such as film coupling capacitors, Noble-detented pots, Dale metal film resistors and fiberglass circuit boards.

Most of the parts are not marked with component values or color codes, just manufacturers' codes. These codes make sense if you're used to them, but they prevented me from finding the right part without several referrals to the parts list.

After the new circuit boards were finished and cleaned, the disassembly of the preamp began. And it is a total disassembly. Basically you have an empty box, the transformer (which is moved to the outside back panel), rectifier tube and some controls that get used again.

Nearly everything replaced

The upgrade replaces just about everything in the audio path and eliminates the tone controls, filter switch and loudness switch. The input selector switch is rewired to eliminate obsolete sources (e.g., "Tape Head") and adds additional CD and Spare inputs. The volume and balance controls are replaced with the wonderfully smooth Noble detented pots.

The new jack set should be considered mandatory, unless they have already been replaced. The jacks posed my only real problem with the project. The instructions tell you to tighten the nuts on the jacks "firmly." I broke the first two before recalibrating myself as to what "firmly" meant. Fortunately, there were two extra jacks in the kit.

It took me about twenty hours to complete the preamp rebuild. The unit was sent back to the factory before it was tuned on. The factory techs inspected the work and fired it up to see if it performed properly. They also installed the new black faceplate and knobs (if you chose that option) and painted the chassis a matching black.

The factory found one error in my work. One circuit board was supposed to be mounted on the under side of the chassis and I mounted it on top. The error was corrected at no charge. I checked the instructions afterwards, and they were clear.

Van Alstine evidently believes, as I do, that the simplest circuit that will do the .job will probably sound the best. Careful attention to coupling capacitor values, feedback parameters, grounding and power supply decoupling can make all the difference in the sound of an amplifier. A single 12AX7A dual triode is used for each channel, and another pair of 12AX7As for the phono preamp. These tubes are readily available. The new power supply utilizes the 12X4 rectifier of the original, but uses silicon diodes for the filament supply. All the filter capacitors are replaced in the rebuild, and the filter circuit is improved.

Power amp kit

In the St-70 rebuild (actually it is a modification), there is less disassembly involved. Total kit time was about ten hours. The new circuit board is wired into the existing circuit with a plethora of wires that make it difficult to do a neat job. The wires used are solid, not stranded, which initially caused me some concern about durability during transport. But the trip to the factory in Minnesota and back to the East Coast caused no problems, so I guess it will last a while.

The optional new input and output connectors should be considered mandatory if your amp still has the poor-quality factory parts. The new connectors are five-way binding posts capable of handling much larger gauge wire. I did not opt for the optional power transformer/rectifier diode bridge option that eliminates the 5AR4 rectifier tube.

Power amp circuit

The Van Alstine general circuit scheme appears to be similar to the original St-70 design: a pentode input stage followed by a triode phase inverter that feeds a push-pull output stage. The sonic improvement is evidently gained from improved components and better circuit design parameters. The output stage is essentially unchanged. (Van Alstine said the amp has always been a 25 watt-per channel (RMS) amp not 35.)

The original circuit used 7199 tubes for the input. These are becoming increasingly difficult to find, so the Van Alstine version uses 6GH8A tubes, which are easier to obtain.

Upon completion and a thorough double-check, the amp was sent back, rechecked and sent back to me - approved by the factory.

Listening tests

I actually built the preamp, sent it back to the factory and got it back before I upgraded the St-70. Therefore, I paired the completed PAS-3 with the stock St-70 to see if there were any sonic differences from the stock listening sessions. There were. Almost all the noise was gone. At full gain there was just a slight hiss and a hint of a ground loop buzz in the speakers, which came from the stock St-70. The balance was perfect with the "Balance" control centered. But overall, the pairing was still significantly bass-deficient and dull. The sound was quite pleasant, however, and for noncritical monitoring or background use, it was the kind of audio you could listen to all day without fatigue.

After the St-70 rebuild, however, there was a world of difference. The bass was much improved. Now I could get stuff on the shelves in my listening room to rattle at moderate levels. The highs were better too, although they seemed a bit harsher than they did before the upgrade. That harshness, however, may be attributable to the improved amp's ability to reveal defects in the recordings that weren't noticeable before.

If something different in the monitoring system reveals new details in a recording I am familiar with, it generally indicates an improvement. The upgraded PAS-3 and St-70 surprised me by revealing some nuances that I had never noticed before such as reverb.

The only deficit I noticed while using the upgraded amps was that they did seem to lose some of that "I could listen to this forever" quality. This is probably due to the improved high frequency response.

For a second opinion, I took the upgraded gear to the home of my sales manager. He has Vandersteen Model 2 speakers, normally fed with a Crown D-75 amp. We listened to some CDs with the Crown, fed directly by the CD player, and then switched to the Dynaco Super PAS-3 and St-70

The bass did not seem quite as full on the Dynaco setup, and my sales manager said it did not reveal as much detail as the Crown. He preferred the solid state amp overall, but agreed with me that the Dynaco amp made his speakers easier to listen to for long periods. Except for the loss of some low end, I preferred the tube setup. To my ear, the solid state amp was more analytical, but the tube amp was more musical.

Conclusion

The Dynaco rebuild kits from Audio by Van Alstine provided a dramatic improvement in the sound of the units I upgraded. Low- and high-end frequency response were improved. Clarity and detail were enhanced. Noise of all types was reduced. The Super Pas Three preamp looks great with the new front panel and knobs. I especially like the feel of the new volume and balance controls. The gold input/output jacks and binding post speaker connectors should improve reliability and reduce noise. Overall, these kits are a practical way to achieve modern vacuum tube performance levels at a reasonable price.

Audio by Van Alstine also offers factory installed versions, which are a bit more expensive. Bare PC boards and plans are also available. For more information, contact Audio by Van Alstine at 651-330-9871.

Products: Super Pas Three Preamplifer, Super 70i Amplifier

Please refer the the price list for current prices and options.