Tinkering With My SLO (Clone) 50: Background Hiss & FX Loop Concerns

Amp Mad Scientist said:
But 70% of the hiss comes from 1 resistor.
rehisstor68K.png
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I changed that 68k grid-stopper resistor to a 33k (1 watt, metal film) . The change did slightly reduce the hiss. But I did an interesting comparison with my SLO-50 clone and my modded 50-watt JCM800 (2204): The SLO-50 on the Normal channel set to Crunch, & the Marshall on the High channel, with both amp's gain and master volumes set at the same levels... have the same amount of hiss! I believe there's nothing wrong with my SLO-50... Nature of the Beast! I think my OCD is making me worry for nothing.

Amp Mad Scientist said:
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These methods should be easy to try. Thanks!
 
Also, I followed jtcnj and AMS suggestion to test the amp with all the preamp tubes pulled and to leave the power tubes and the V5 phase inverter tube installed. The amp was ultra quiet in this mode. It was still quiet with V4 & V3 installed. When V2 (Overdrive channel) was installed, here comes the raging Niagara Falls sound in the background. When V1 (Normal channel) was installed, here comes a very mellow Niagara Falls effect.
 
syscokid said:
Also, I followed jtcnj and AMS suggestion to test the amp with all the preamp tubes pulled and to leave the power tubes and the V5 phase inverter tube installed. The amp was ultra quiet in this mode. It was still quiet with V4 & V3 installed. When V2 (Overdrive channel) was installed, here comes the raging Niagara Falls sound in the background. When V1 (Normal channel) was installed, here comes a very mellow Niagara Falls effect.
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And this noise is considered normal??? Even with all the gain the Ivanberg Modded Origin produces, it has no such noise.
 
Robert Herndon said:
And this noise is considered normal??? Even with all the gain the Ivanberg Modded Origin produces, it has no such noise.
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I vaguely remember that there was a very slight tranny hum that was coming through even at low to medium volumes. But that very slight hum was already there before any of the modding of your Orgy 50. I considered that normal in tube amps. What I definitely don't remember is what does your amp's background noise sound like when, with nothing plugged into the input, the preamp gain around noon and the master volume around 1 o'clock? At these settings the volume will be blistering loud if you try to play a guitar through it. I like testing like this because it helps to determine whether the amp's circuit is vulnerable to oscillation, and whether there are microphonic tubes.
 
syscokid said:
I vaguely remember that there was a very slight tranny hum that was coming through even at low to medium volumes. But that very slight hum was already there before any of the modding of your Orgy 50. I considered that normal in tube amps. What I definitely don't remember is what does your amp's background noise sound like when, with nothing plugged into the input, the preamp gain around noon and the master volume around 1 o'clock? At these settings the volume will be blistering loud if you try to play a guitar through it. I like testing like this because it helps to determine whether the amp's circuit is vulnerable to oscillation, and whether there are microphonic tubes.
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Correct. The tranny hum is coming from the cabinet and not through the speakers on the Origin.
 
syscokid said:
Also, I followed jtcnj and AMS suggestion to test the amp with all the preamp tubes pulled and to leave the power tubes and the V5 phase inverter tube installed. The amp was ultra quiet in this mode. It was still quiet with V4 & V3 installed. When V2 (Overdrive channel) was installed, here comes the raging Niagara Falls sound in the background. When V1 (Normal channel) was installed, here comes a very mellow Niagara Falls effect.
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You are talking about the hiss increase when V2 installed ?
The hiss (or most of it) is really from the V1 resistors, and V2 is just amplifying /increasing what is coming from V1.
 
Amp Mad Scientist said:
You are talking about the hiss increase when V2 installed ?
The hiss (or most of it) is really from the V1 resistors, and V2 is just amplifying /increasing what is coming from V1.
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I started with the power tubes and the V5 phase inverter installed only: Amp is very quiet! Then I added V4 (fx loop recovery & cathode follower into the tonestack): Amp is still quiet. Then I added V3 (another cathode follower & fx loop buffer): Amp is still quiet. Then I added V2 (Strictly for the overdrive channel): NOISE... High-gain artifact noise with a slight hiss! Finally, I added V1 (2-stage Clean & Crunch channel. AKA: Normal channel): Pure low-level white-noise hiss. AKA: Niagra Falls! Amp is cranked with masters at 2 o'clock!

EDIT: V1's first stage is shared by the Normal & the Overdrive channel. V1's second triode is the Normal channel's 2nd stage.
 
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syscokid said:
I started with the power tubes and the V5 phase inverter installed only: Amp is very quiet! Then I added V4 (fx loop recovery & cathode follower into the tonestack): Amp is still quiet. Then I added V3 (another cathode follower & fx loop buffer): Amp is still quiet. Then I added V2 (Strictly for the overdrive channel): NOISE... High-gain artifact noise with a slight hiss! Finally, I added V1 (2-stage Clean & Crunch channel. AKA: Normal channel): Pure low-level white-noise hiss. AKA: Niagra Falls! Amp is cranked with masters at 2 o'clock!
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What does this mean?
 
Don O said:
An oscilloscope is your friend :dood:
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Robert Herndon said:
Yes! And the brains to use it!
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CoyotesGator said:
If I can use one, anyone can!

Just watch your probe ratings and be careful.
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I'm scope-less.

I believe this amp is as quiet as it's going to get. It's not any noisier than any of my other amps when the amp is driven hard and a fair amount of preamp gain is dialed in. All the preamp & phase inverter tubes are JJ's. Perhaps there are other modern tubes that are consistently quieter than the JJ's in a high gain amp. I'll look into that, eventually.

For now, I'm about to move on from trying to reduce the amp's background noise, to the next phase: The amp's fx loop. Lot of drama out there about the original SLO fx loop... :rolleyes2:
 
CoyotesGator said:
@syscokid

What amps are comparing noise floor with?
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I'm comparing with my two amps based on the Marshall 2204 circuit. Both are handwired with turrets boards, and with multiple ground points ala "Larry Grounding" system.
Also comparing with my Fryette Memphis 30. Not as high gain as a Fryette Pitbull, Deliverance, & Sig X, but still plenty there.
Also comparing with a Rivera Quiana, which I don't have anymore.
 
syscokid said:
I'm comparing with my two amps based on the Marshall 2204 circuit. Both are handwired with turrets boards, and with multiple ground points ala "Larry Grounding" system.
Also comparing with my Fryette Memphis 30. Not as high gain as a Fryette Pitbull, Deliverance, & Sig X, but still plenty there.
Also comparing with a Rivera Quiana, which I don't have anymore.
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Have you tried this experiment with a different guitar cable?

I have some cables that are much quieter than others, leading me to feel as though the noise was coming from the amp itself.
 
Robert Herndon said:
Have you tried this experiment with a different guitar cable?

I have some cables that are much quieter than others, leading me to feel as though the noise was coming from the amp itself.
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I'm testing the noise floor of the amps without anything plugged into the input. Obviously there is a speaker cable involved, and I always make sure that the speaker cable, plugs, & jacks are not compromised.
 
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