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In Reply to: RE: back in the saddle again...... posted by vinnie2 on August 10, 2015 at 17:13:59
Ladies and gents I found the problem. By accident sort of, and a good thing too because I don't think I would have thought of checking it. In retrospect though, it does seem obvious. Ain't hindsight grand?
I was checking the cables going to the 91a clone, and I noticed that when my hand was near the wires going to the caps of the EF37a tubes the noise disappered even with both channels playing. Now those wires are supposed to be shielded, and I thought they were, but apparently not well enough. Just for yucks I took a metal spring clamp about an inch long and put both wires inside the jaws and then connected a clip lead from the spring clamp to ground. How about that Mabel, no more noise, period.
That is why each channel was fine playing with the other turned down on the 91a; there was interference between the two cables that disappeared when the volume of one channel was all the way off.
I will now have to change the wires on the 91a most likely, or build some grounded sheaths from soldier wick to put around the existing ones.
The good news is the preamp sounds absolutely fantastic now that the noise is gone. I listened to all kinds of music for about an hour this morning after finding the problem, and it just blew me away with it's detail and clarity. The midrange is outstanding and the bass is also very good. I don't know why I am surprised, a dht pre driving a dht amp, one should expect good results, yes?
Still got some tweaking to do. I will try a bunch of different coupling caps since I have a good stock of different types and values of the ruskies. Also will have fun rolling 26's as I have a good stock of those as well, both globe and st.
If anyone is looking for a good preamp that is easy to build and has great sound you ought to give this one a try. It is a lovely, simple circuit with great sound.
Thank you all very very much for all the help on this one. By the time I found the problem you all had given me enough suggestions that it is all but fine tuned.
Thanks!!
Edits: 08/11/15Follow Ups:
Who woulda thought, glad you sorted it out.
Thanks for the update, I was curious as to what was causing your woes.
Infamous sockpuppet
"build some grounded sheaths from soldier wick"
That would be a real mess. What you want is the tinned, braided shield from coax cable, such as RG-58U.
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Buy Chinese. Bury freedom.
They have tons of shielded/braid for this purpose. Have used it under chassis as well with good results.
When I lived in Jacksonville, FL, 1988-90, I LOVED going to Skycraft Surplus, NICE store for sure !!! Fun.... Miss it.
Jeff Medwin
Sometimes, the shielding in pre-made cable is foil, not braid. :> ( Why take a risk? McMaster-Carr carries copper braid shielding. ;> )
Eli D.
I've never seen RG58/U with foil shield. It's always braid because that's required in order to solder it to RF connectors like PL259. That's not to say I enjoy stripping coax in order to have braided shielding. Is the McMaster braid tinned?
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Buy Chinese. Bury freedom.
"Is the McMaster braid tinned?"
IIRC, it is, but don't hold me to that. It's been several years since I last worked with the stuff.
Eli D.
I will look into that.
Thanks
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