Thursday, January 8, 2009

Dynaco Mark III Resurrection

In an audio trade I received a pair of vintage Dynaco Mark III monoblocks. The only problem is they were missing just about everything except the output and power transformers. I read a lot and did some talking with a few friends before finally deciding to take the steps to rebuild these old amps from scratch.

Here is how they looked underneath before the operation:
I went with the SDS Power Supply boards available from Triode Electronics. In my case, I didn't use the 50uF in the first section in fear of shortening the life of a NOS 5AR4. Line voltages are higher than the old days so I used a smaller value 20uF/630V Solen in the first section and after the 1.5H choke I paralleled the first two sections together on the SDS board.

For the input boards, I decided on the Poseidon driver boards. I used 5751s instead of the 12AX7 and 12BH7s instead of the 12AU7. The neat part about the long-tail phase splitter is the current sink using a LM334Z - in this case when the tied cathodes sees a large impedance it reduces phase imbalance. There is also a trick method they use to balance unmatched power tubes using the original 10k bias pot and a trimming pot.

To finish the job I had to buy all new sockets, new chokes, new bias pots, bias resistors, RCA jacks and capacitors. I did a flurry of wiring and everything worked on the first try without any troubleshooting required. Excellent products and careful wiring paid off!

Final results on one of the monoblocks:

For final tweaks I went with some MIT RTX and Dynamicap capacitors from Michael Percy. RTX capacitors used to seem very high priced, but these days they are a bargain and well worth the money. Rectifiers are the under the radar Hitachi GZ34s. I've also found some Raytheon 'Windmill getter' 5751s and some NOS RCA blackplate 12BH7s. For output tubes I went with the rugged SED 6550c.

Mark IIIs with the Poseidon and SDS boards, good coupling caps, and the right tubes make for an excellent budget tube amplifier. They have rock steady imaging, deep soundstaging, tight bass for a tube amplifier, and a sparkling treble. With these amplifiers I can really hear 'into' the recording. Many vintage tube amplifiers I have heard simply sound compressed in comparison.