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Article By Doug Grove

From VALVE Volume 2 Number 5 May 1995

 

  How Doug Grove performs his miraculous restorations of Dynaco amps... The restoration of tube power amps such as the Dynaco Mark Ill and Stereo 70 can result in great sounds and good looks for a moderate investment of time and money. Recently I was rewarded for my efforts when a client of mine (an experienced audio recordist) exclaimed "Wow, these amplifiers have unbelievable detail - I can hear the bow rosin on the string before the note starts!" This was from a pair of carefully restored Dynaco Mark Ill's rescued from his friend's attic, running on Genelex Golden Monarch KT-88's.

These results require an almost fanatic commitment to detail, and a certain understanding of electron flow. You don't have to be an engineer, but a good working knowledge of electronics, rnechanics and finish carpentry is very helpful.

Number one, be sure to obtain a circuit diagram (which the VALVE library can provide for copying costs). A complete restoration will require total project teardown, removal of all wire, and de-soldering every component. If you're really worried about getting it back together right, a few photos and sketches will help.

A digital multi-meter is also a necessity. Once you have disassembled the unit test every electrical and hardware component  to make sure it works and meets specs. If it does not, replace it. It's generally good practice to renew the main filter capacitor and other electrolytics, and coupling/bypass capacitors. De-soldering things like tube sockets is a pain, but the reward making those new connections will be better signal path and the resulting improved sound quality.

Dynaco products use a circuit board assembly for the driver amp components. Remove all wire, coupling/bypass caps and clean the board with non-flammable solvent and a toothbrush. Unless there is physical damage, this cleaning will go a long way towards correcting high frequency drop out and distortion commonly attributed to the board.

Only when you've tom it all apart, will you be able to work on the nickel/chrome chassis. Be careful of the lettering! Finish your work off with auto paste polish to ward off fingerprints and corrosion. Transformer cans look real sharp when repainted, but have to be disassembled far best results. Reassemble and rewire with care, using high quality wire. Soldering is an art. lt is also the key to good electrical conduction and the resulting quality sound reproduction.

I consistently incorporate a few modifications to improve amplifier performance, the first being additional Bt filtration. About 200 mfd will provide some added reserve power to improve low end punch without high-end frequency loss. Do not compromise on the voltage rating -- 500WVDC is a minimum. Highs are greatly improved by the replacement of coupling and bypass caps. Another improvement is the replacement of selenium rectifiers (in power supply and /or bias circuits) with high quality diodes. This results in less voltage drift and cleaner power.

A component used commonly in solid-state rectified power supplies can preserve  your tube amp in two ways: Soft starting and voltage regulation. Use a thermistor to limit the inrush voltage an the primary side of your power transformer, resulting in a slower voltage rise at the filaments and B+ current. When warm, the right thermistor value will trim secondary voltage by about 5%, saving tubes and components from today's higher line voltage, with little sacrifice in amplifier power output. Choosing (and finding) the right thermistor requires patience. Use negative temperature coefficient (NTC) types which decrease in resistance as current heats them. They're not too expensive, so experiment with a few values for best results.

And finally, the tubes. A tube tester is really helpful. For tube testing equipment cruise the garage sales or swap meets and pick yourself up a "dynamic mutual conductance" type if possible (we hove testers for damn near everything here at the shop -- Dan). Tube testing can only identify major defects and estimate tube performance. Nothing can substitute actual testing in the circuit. I do preliminary matching using the tester, then road test, switching them around as required for equal bias, current draw, etc.

It takes dedication, patience and hard work to restore a vintage amp. One Mark Ill is easily worth 16 hours. But its well worth it. When you're done with yours, bring it to the next meeting to show it off!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     
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