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Noise on one side of SSE

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SonicFreq

SonicFreq

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Joined 2015
sonicfreq.com
#1
I built 2 Tube Lab SSE amps last year and have been very pleased with both of them. I built a SSE 6L6 and then another SSE, KT88. Both of them operate perfectly and sound amazing except when I use a potentiometer inline with ether amp. At first I assumed the pot I was using was at fault but came to find out that it may be something related to the amp.
The symptoms are as follows. Regardless of whether or not anything is plugged in the amp sounds really clean. But when turning the pot down anywhere below max, an intermittent, crackling, static (sometimes squealing) sound starts to come from only one of the speakers. This follows when switching speakers and appears to be affecting both of my amps. I thought it was a bad pot and replaced the one in my KT88 amp, but it only made things worse. I tried a solid state amp (Gainclone) and have no issues at all (except it isnt a class A tube amp)... Originally I discovered this issue when making a little source switcher with a pot installed but it turns out this occurs in the amp directly. Please see the post below that I originally put online which led me to this discovery. A lot of people suggested RF filtering, but I want to be sure there isn't anything wrong with my amps. If requested I can post a recording of the noise.

POSTED in Analog Line Level:

http://www.diyaudio.com/forums/anal...t-static-one-side-system-pot-around-half.html
I decided to build a little 2 way source switcher and passive preamp using 100k Pot and a DPDT switch. Everything is great except when the pot is around the half way position. I start hearing what sounds like faint intermittent static on the left channel only. The right side is completely clean. Any suggestions?

My signal path

-Raspberry Pi with HiFiberry DAC + Pro (Input 1)
-Turntable built in preamp out, line level (Input 2)
-Passive 2 way source selector/preamp (2 Inputs, switch, 100k Stereo Pot - Output)
-Tubelab SSE with KT88s + Tannoy SYSTEM 800 Speakers

Note: This happens when either source is plugged in to either input. It also occurs when nothing is plugged in to any input.
I have not swapped any tubes yet or tried switching the speakers yet and will do this asap, but thought Id see if anyone had any ideas.

Thanks!​
 
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#4
w5jag said:
It's oscillating.

Win W5JAG
Click to expand...

Thank you. At least I now know what to call it now 🙂. It's usually not that bad, normally its just kinda crackle and static'ey. This was a particularly harsh example, but accurate none the less. That's one of the bizarre things about it, the inconsistency. Any suggestions on how to fix that? Should I take the pot out entirely? What causes it to oscillate on one side but not the other?
Ive tried swapping out everything, including every component (using the 2nd amp technically). The only thing that seems to work is using the digital hardware attenuation in my DAC (limiting me to just digital music) or use a small Mackie mixer between my sources and amp (It uses op amps). Right now that's what I am doing. LP & DAC -> Mackie 402 -> SSE -> Speakers. I will continue with this setup if necessary, it just seems like something is wrong.
 
#8
SonicFreq said:
Any suggestions on how to fix that?
Should I take the pot out entirely?
What causes it to oscillate on one side but not the other?
Click to expand...

I would say the big difference between an amplifier and an oscillator is positive feedback, so you likely have a positive feedback loop around that side of the 12AT7, which is a pretty high gain device, and often used as an oscillator.

I don't think the SSE is prone to oscillate, so you probably just have an unlucky combination of all the component tolerances going in the wrong direction, and then causing this. Replacing the 12AT7 would probably fix it. You might have to increase the value of the resistance in series with the grid.

Win W5JAG
 
#9
evanc said:
I looked at your pics. Nice looking amps. I can't see how the volume pot is wired in...it looks like the input wires go right to the board. Maybe a hand drawn wiring diagram would help us help you.
Evan
Click to expand...

Thanks! I actually added the pot(s) after finishing the amp(s) hence the lack of pictures. I figured I would just use a preamp, but decided to save my speakers and wire in the pot after all. I used a 100k ohm wired up just as specified in George's diagram. I did use 100k instead of 50k as George suggests, and made some adjustments to the filter and coupling cap values according to an amp designer friend of mine. I'm starting to second guess those decisions now.

tonepot.JPG
 
#11
I'm guessing the feedback path is up R11 and through R12 into the grid, so the series resistance of the pot is irrelevant to that path.

The parallel part of it may be having some interaction with R11, however.

You may have several things going on. Hiss can be oscillation or a bad tube socket. Snaps and crackles can be a bad tube or socket. Squeal is always oscillation in my experience.

I would change the 12AT7 before I did anything else.

Win W5JAG
 
#12
w5jag said:
I would change the 12AT7 before I did anything else.
Click to expand...

I swapped out the Telefunken ECC81 (12AT7) and the problem cleared up tremendously. I can still hear a faint static sound when the pot is at about 12:00 but its about 10 times quieter than with the Telefunken tube. Hmmm I wonder if I got some bad tubes off E-B@Y? Anyways, that definitely helped. Im using the same tubes in both of my amps so that might explain it.
 
#13
Okay so it seems to be a bad tube. Actually 2 bad tubes. Both are NOS telefunken ECC81 that appear to be genuine. At least they have the molded glass stamp on the bottom. I don't think they are fake but I frequently hear about this problem with Chinese made tubes. What else is weird is that the tubes went bad in both amps. With the same symptoms. I'm wondering now if I did something to fry the tubes maybe? This whole thing started with a passive preamp I made with an A/B switcher + potentiometer + 10db pad. Any chance something like that could have killed the tubes, or was it just bad luck?
 
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