No hum... Just static

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No hum... Just static

Postby ioginy » Sun Feb 03, 2013 7:40 pm

Good day brilliant wonderful people!

I have come across a problem that I don't have any idea where to even start looking.

I recently built and amp... Input > Gain > Vol > Gain > Bax Tonestack > PI > PA

Upon completion of the amp I was overjoyed that my time and energy to ensure that it did not hum had finally come to fruition. THEN! I turned up the volume and noticed what I thought was static. I turned it up more and the static increased. Good to know, what ever it is, is coming from the first gain stage. I listened really really carefully for voices in the static but could hear none. I figured maybe I was tuning into a radio station... that wasn't a station.

Here is a photo of the guts.


Any ideas and help would be very much appreciated.

As a side note, I noticed that my old Fender Twin is significantly louder than this one. I was wondering if putting another gain stage after the Tonestack would cure this problem. I'. not really sure if there is enough juice getting through to push the output as hard as I would like.
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Re: No hum... Just static

Postby Geek » Sun Feb 03, 2013 8:49 pm

Static can be caused by:

- Noisy tube.
- Noisy electrolytic on the cathode resistor.
- Noisy resistor.
- Noisy coupling cap.

Diagnosis procedure:

- Try a different tube. Recheck.
- Lift the cathode bypss caps. Recheck.
- I see an old carbon composition resistor in there. Replace with carbon film and recheck.
- If you used any NOS coupling caps, replace with modern ones.

Still does it? Is the input grid snubber large enough? (typically 68K)

-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: No hum... Just static

Postby EWBrown » Tue Feb 05, 2013 1:20 pm

I see more than one soldered-to-chassis terminal strip ciruit ground, that's a sure way to have noise / hum issues, a single point (star) ground shoulld clear those up.
Using the chassis for ground return is OK for RF, but not for audio circuitry.

Also check all your solder joints, especially on the older terminal strips, they can become oxidized and cause a flakey connection and a case of" the cracklies".

I've found, over the years, that cold / corroded solder joints are the cause of about 95% of tube amp problems.


/ed B
Real Radios Glow in the Dark
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Re: No hum... Just static

Postby nigelwright7557 » Tue Jun 25, 2013 3:30 pm

I was getting a lot of hiss out my new amp build.
When I looked with a scope the amp was oscillating depending on where the first volume control was set.
Removing the first 2 stages cathode capacitors fixed it still leaving enough gain to get full power.
When I worked it out I had far too much gain and this was causing positive feedback.
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