Hum and Buzz shoot out

[peteb]

There have been discussions lately regarding filter cap size and location and how it affects hum and buzz.


I decided to test hum and buzz, a champ with 20-20-20 filter caps against a champ with 40-20-20.

I considered them the same and really didn’t know what to expect.

I thought about AB ing them but realized you can’t switch hum and buzz in and out.


I plugged them both in and set VTB all to 10 with nothing plugged in. The 40-20-20 won hands down. Then I remembered the shorting Jack does not function on the input of the 20-20-20 so that showed the affect of the shorting Jack, but nothing else.

then I repeated the test by plugging the guitar in. The 20-20-20 with a guitar plugged in is just the same as the 40-20-20 with no guitar plugged in. It made no difference on the 4-20-20 if the guitar is plugged in or not.



I know it is not a definitive test. The 40 on the OT / plate sounds like a good idea. I think the amp is a little tighter, but I heard no affect to the hum or buzz.

 

[tubedude]

In static mode it might not be a revelation. But as you strum hard. a difference in feel and sound might become apparent as current demand depletes the stored energy in the 20uF, and the current pulses try to keep up. Where the 40uF moves forward gracefully, never depleting the filtered DC in the cap. Any hum though is less of an issue at full tilt volume. So hum may end up less of an issue than dynamic response. Let us know what you find.

 

[Phrygian77]

I may have to do something similar to this at some point. I could run the test bed into my reactive load plus the CAB M, and record some clips for a real A/B comparison to share.

 

[Phrygian77]

Of course, I guess I'll need to make sure nothing else on the mains in the house changes when I do it. I just thought about how the fancy washer in our new house makes the LED lighting go nuts when it's on spin.

 

[wabashslim]

OT - "Hum & Buzz" is a good name for a duo...like Itchy & Scratchy.

 

[bebopbrain]

A larger capacitor can result in more buzz. The B+ ripple improves, but peak current (and return current which causes ground noise) gets worse.

 

[James Knox]

There have been discussions lately regarding filter cap size and location and how it affects hum and buzz.


I decided to test hum and buzz, a champ with 20-20-20 filter caps against a champ with 40-20-20.

I considered them the same and really didn’t know what to expect.

I thought about AB ing them but realized you can’t switch hum and buzz in and out.


I plugged them both in and set VTB all to 10 with nothing plugged in. The 40-20-20 won hands down. Then I remembered the shorting Jack does not function on the input of the 20-20-20 so that showed the affect of the shorting Jack, but nothing else.

then I repeated the test by plugging the guitar in. The 20-20-20 with a guitar plugged in is just the same as the 40-20-20 with no guitar plugged in. It made no difference on the 4-20-20 if the guitar is plugged in or not.



I know it is not a definitive test. The 40 on the OT / plate sounds like a good idea. I think the amp is a little tighter, but I heard no affect to the hum or buzz.

Cool experiment!

Are you using a 5Y3 for a Rectifier? Any concerns regarding exceeding or running close to the max uf rating for a 5Y3. Ive been using 30uf for the first Filter and debating stretching up to 40uf myself.

 

[NTC]

What was the 6V6 plate and cathode voltage in the two cases?

 

[peteb]

But as you strum hard. a difference in feel and sound might become apparent as current demand depletes the stored energy in the 20uF, and the current pulses try to keep up. Where the 40uF moves forward gracefully, never depleting the filtered DC in the cap.

i do think I hear a difference when playing, but it is subtle.
One is not better than the other.

 

[peteb]

I may have to do something similar to this at some point. I could run the test bed into my reactive load plus the CAB M, and record some clips for a real A/B comparison to share.

That would be cool. I look forward to hearing that if it is done.

 

[peteb]

A larger capacitor can result in more buzz. The B+ ripple improves, but peak current (and return current which causes ground noise) gets worse.

interesting. I have never heard that. Most discussion in the about filter cap size has been more about headroom and volume. Only recently did it occur to me that the 40-20-20 might have different hum and buzz than the 20-20-20.

when people build amps, it seems like there is more concerns about hum and buzz than the actual sound of the amp.

 

[peteb]

Are you using a 5Y3 for a Rectifier? Any concerns regarding exceeding or running close to the max uf rating for a 5Y3. Ive been using 30uf for the first Filter and debating stretching up to 40uf myself.

Yes, both amps use a 5Y3 rectifier. I don’t think there is a problem with 40uF. Probably, most 1964-1980 champs and Vchamps were built that way, and a good 5Y3 will last a long time. Both are old (not NOS) RCA tubes from the mid 60s.

 

[peteb]

What was the 6V6 plate and cathode voltage in the two cases?

I wasn’t focused on that aspect, but I have wondered how the 40 uF cap affects the plate voltage and the current draw.

I think the increased capacitance could boost the plate voltage, or maybe the increased current requirement of filling the larger cap might draw down the plate.

I have seen both amps take the lead on plate voltage, current draw and plate dissipation. I always thought the rectifier tube and power tube had more affect than anything else.


However, I believe they are now running similar tubes.

old RCA 5Y3, newish electro harmonic 6V6, modern production ARS 12AX7.


The last time I documented the bias they were like this


40-20-20 (1965 champ)
Plate voltage 381
Cathode voltage 23
Plate current 46 mA
Plate dissipation 16.5 W


20-20-20 (1966 champ)
Plate voltage 402
Cathode voltage 21
Plate current 39 mA
Plate dissipation 14.9 W

Edit: I just noticed on my notes, the bias info for the 20-20-20 champ was with a weak Peavee tube. The 66 was stronger, so I placed the weaker tube in there on purpose to balance the amps out. Now, with similar tubes in both, they sound equally loud and the 40-20-20 champ has a slightly stiffer feel to it.

 

[tubedude]

interesting. I have never heard that. Most discussion in the about filter cap size has been more about headroom and volume. Only recently did it occur to me that the 40-20-20 might have different hum and buzz than the 20-20-20.

when people build amps, it seems like there is more concerns about hum and buzz than the actual sound of the amp.

Worsening noise in the return path indicates the ground path isn't as low a resistance as it should be.

 

[peteb]

Worsening noise in the return path indicates the ground path isn't as low a resistance as it should be.

Could you please explain?

 

[tubedude]

For a noise voltage to develop in the ground return, a resistance has to be there for the current to form a voltage. For instance, circuit board amps can have a higher resistance in the return paths to ground if they rely on skinny traces. Ground paths should be sized much larger so the currents they will carry will keep them from developing a voltage differential. Use large wires/traces all the way back to the source/chassis ground.

 

[peteb]

For a noise voltage to develop in the ground return, a resistance has to be there for the current to form a voltage. For instance, circuit board amps can have a higher resistance in the return paths to ground if they rely on skinny traces. Ground paths should be sized much larger so the currents they will carry will keep them from developing a voltage differential. Use large wires/traces all the way back to the source/chassis ground.

Thanks

I was uncertain of your starting point. That is my fault.


You are saying the source of idle hum and buzz is a resistance in a ground path. Which ground path? Any ground path. What causes the resistance? My first thought was the resistance of a bad connection could cause noise, which is true. However, single ended amps, with or without bad grounding connections are going to have some hum, especially with the volume and the treble turned all of the way up.

I think of the noise in single ended amps as resulting from ripple in the DC power supply, resulting from a less than perfect filtering, and not getting canceled in in the output section because of no push pull.

I could be wrong about the role of being single ended. My push pull Princeton is dead quiet, where as my bassman head is noisy like my champs. I think the bassman is noisier just because it is bigger. However, I could picture a quiet 2x6L6 amp.

 

[andrewRneumann]

But as you strum hard. a difference in feel and sound might become apparent as current demand depletes the stored energy in the 20uF, and the current pulses try to keep up. Where the 40uF moves forward gracefully, never depleting the filtered DC in the cap.

@tubedude , what current demand is causing the sag in the first filter cap for a Class A Champ? I have only seen (on a voltmeter) and heard sag on a Class A single-ended amp when the screen grid draws increased average current down from the second filter cap (when cranked).

As a fun experiment, I hooked up two 22uF electrolytics in reverse series (11uF effective capacitance) and ran them through my light bulb limiter. I was able to get 560mA of current directly from the mains (60Hz) and light a 75W bulb (although dim--the capacitive reactance of the 2 caps is somewhere 200-250Ω). Surely if 11uF can pass 560mA, 20uF can pass whatever a Champ is asking of it? At least that's my thinking... IMHO, I don't believe you can "starve the plates" or anything like that with 20uF at audio frequencies, but I may be thinking about it all wrong. If the OT is reflecting 7.5kΩ and the first filter cap is only adding in another 250Ω at the lowest frequencies--that's not much additional impedance at all. There's probably some phase vectors that change things around, so my understanding is probably crude at best.

@peteb An interesting experiment would be reducing the second filter cap to 8uF and seeing if you notice a difference in dynamics (when cranked) compared to 20uF there. My experience is that the second filter cap can make a much bigger difference, but it depends on what the dropping resistance is and if screen grid resistors have been used. Increasing the first filter cap can reduce buzz, but you quickly reach a point of diminishing returns on buzz killing as the resistance of the 5Y3 limits the charging current.

 

[tubedude]

Most singleended amps are underfiltered. They can be made not to hum. I just built a stereo KT66 single ended power amp that is dead quiet. But it had a 10H choke a 4H choke and 190uf in each channel. Manufacturers are not willing to do that as they have to sell at a price point.
Again, aside from under filtering, small conductors/traces in the ground path will develop a noise voltage. The magnitude of the voltage is dependent on the resistance of the path and the current flowing through it.

 

[andrewRneumann]

Most singleended amps are underfiltered. They can be made not to hum. I just built a stereo KT66 single ended power amp that is dead quiet. But it had a 10H choke a 4H choke and 190uf in each channel. Manufacturers are not willing to do that as they have to sell at a price point.
Again, aside from under filtering, small conductors/traces in the ground path will develop a noise voltage. The magnitude of the voltage is dependent on the resistance of the path and the current flowing through it.

Yes, my limited experience has shown that for power tubes that draw relatively high current at idle (low plate voltage or parallel power tubes), the way to eliminate buzz is install a choke. 100uF will still buzz a little with a solid state rectifier because the resistance of the PT secondary also contributes to ripple.