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Re: Hiss in Point-to-Point Wired Tube Preamp

Victor,

I would never have guessed a cap, either... but darn it---if I've gone through stage after stage of resistors (which is still a strong candidate for hiss, in this person's pre-amp)---& finally changed the filter caps. And the hiss went away.

It could have been the soldering of the caps. But, I tend to rule them out, along with resistors.

Your idea is what I forgot. Pull each tube one-by-one and see if that changes the hiss. Pull one. No change. Replace it and pull another. Continue, until you've hopefully find the tube that makes a difference in the hiss level.

Now, check the components attached to the socket of that tube. While unit if off---check resistor's value. Prod it with a chopstick and see if the resistance changes. Could be a bad resistor and/or solder. Check the wire leads into the socket lugs, for poor solder.

It may be a good idea to replace any bypass electolytic cap that parallel the cathode to ground bias resistor of that tube.

These caps often get noisy from heat damage, due to the resistor. I like to go to larger voltage rated electrolytic caps for the cathode bypass. For instance, replace a 25 mfd/25 volt cap with a Sprague ATOM 25 mfd/50 volt cap. These tend to keep their value under heat, better. KEEP the capacitance value the same, though.

Also, all resistors should have their bodies butted up against the socket lugs. With the lead extending from the body to the breadboard or the terminal strip.

Try to keep the capacitor leads from the body of the cap to the socket, as short as possible, also. Both short cap and resistor leads reduces the chance of instability of the tube, due to the tiny capacitance of the lead----believe it or not.

steve


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  • Re: Hiss in Point-to-Point Wired Tube Preamp - steven oda 03/14/0208:24:35 03/14/02 (0)


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