Intermittent beeping in the left channel

knowledge base for the classic Dynaco ST70

Postby kheper » Tue Apr 07, 2009 9:55 am

I think I got to the bottom of this mystery. Tightening up the socket jacks helped. Using thicker gauge wire on the filaments helped.

There remains a slightly diminished output in the right channel. I suspect a cold solder joint - somewhere.

Since I had not removed the power tubes for 2 years, I do not know how the sockets jacks lost tension (or contact). Heat? Vibration? The sockets jacks and the pins of the tubes are not at all corroded.
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Postby TomMcNally » Tue Apr 07, 2009 1:12 pm

The pins on some of the newer tubes, and probably the JJ's
are too thin, and the sockets don't grip them very well to
begin with. A set of new sockets would be a worthwhile
fix, or just get in there with a tiny jewelers screwdriver and
tighten them all up.
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Postby kheper » Fri Apr 10, 2009 1:08 am

This amp is truely possessed.

The right hand filaments have a 3.07V per side reading.

The left hand have a 3.30V reading per side.

Cold solder joint? Right hand filament supply crapping out?
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Postby Writer Frog » Fri Apr 10, 2009 7:30 am

kheper wrote:This amp is truely possessed.

The right hand filaments have a 3.07V per side reading.

The left hand have a 3.30V reading per side.

Cold solder joint? Right hand filament supply crapping out?


The left side filaments have 6.6V end-to-end.
The output power is probably at 66W.

Call it "ST-66". (sick) ;)

This may be a good time to replace the power tubes/sockets (re-pent-ode), move away from those evil radio broadcast transmitters and Three Mile Island, and make offerings to Saint Jude. :))

Matt
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Postby EWBrown » Fri Apr 10, 2009 7:42 am

Probably have to perform a few animal sacrifices, too... (666) ;) O:) :))

TMI and UFO's can cause some really nasty troubleshooting issues >:o ;)


/ed B
Real Radios Glow in the Dark
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Postby kheper » Mon Apr 20, 2009 4:44 pm

I rebuilt the bias supply with 2 6.6k resistors and 2 Nichicon caps.

I put a quad of new-production Tung-sol KT66s into the amp.

The KT66s have a nice bottom end, but they seem to lack "something".
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