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Hi!I just acquired a Dynaco ST70 II amp.
Though similar to the original ST70, which at this point I like a lot better, there are few differences. The main one is the way the bias is adjusted. One the original you simply hooked up a voltmeter to a point in the outgoing power supply, got your voltage and adjusted if necessary with knobs located between the tubes.
The new one has a set of red led lights per channel that are supposed to glow at the same rate when the bias is correct; adjustment being made w/a screw adjustment between said lights. In the unit I have the only lights that glow are the low bias ones, regardless of adjustment.
The unit has been used no more than 30 hours in its 11 year life.
Panor used inferior chinese (National) tubes as well.
So far I have hooked up the unit to a variac for over 24 hours, but
I still can't adjust the bias.
I have ordered all new tubes as well.
When I first hooked up the amp it worked, but once in a while I'd get a "crackling" noise, which sounded to me like the power supply was trying to kick in to high gear.
Anybody have any suggestions? comments?Cheers from Mid Coast Maine!
Follow Ups:
I have an ST70 II and it has been rock solid dependable. Are the output tubes chineese? that could be the problem. Mine came with E. German tubes but I tried chineese EL34's and they were garbage. Also if you change the coupling caps the unit will sound better, I used MIT's. It is well worth fixing if it's not the tubes. I'm using Mullard EL34 reissues and they are a big step up from Sovteks.
Thamks!I figured the chinese tubes were crap and I had already ordered just that, Mullard EL34 reissues. In fact, the person I bought the amp from is re-embursing me for the new tubes, which is better than I had hoped for. Which exactly are the coupling caps???
Thanks!
Also the bias lights can vary greatly depending upon the line voltage. On hot days when everyone has AC on my lights are never in balance. you're just looking for an overall average from day to day. On mine the original coupling caps were redish/brown "chicklets" about an inch X a half inch, There are six of them. Did you switch tubes around? it is unlikely that more than one are bad. They should all be removed and reinserted a few times to establish a good contact. Is yours chrome or black? There are a few other parts I may also upgrade on mine.
Hi!I'm waiting on my new tubes to arrive to work on the bias (thinking on also setting it up so I can check bias manually); did find the coupling caps, thanks! I'll shop around for replacement...
The tube recepticles on my amp are black.I have three original ST70s; two I use currently as mono blocks for my system in the house, the other is a mod ST70 that I use in the studio/workshop above the garage, I also have an SCA35 that I use in my office which is also at home...
Thanks for the help!
You may get a response here, but cosider posting your question on the Vintage and Dynaco Doctor Asylum.
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