12AX7 HUM

[soundguruman]

I have never understood the failure mechanism in old film caps.
The ones that are noisy when tapped "leak" Vdc when tapped.
Maybe the film does break down.

Yes, the insulation breaks down.
But in between, they reach a point where they leak just a little tiny bit. The leakage occurs just when a note is played.
This is when the capacitor is "ripe" and the amp sounds killer. The tiny leakage makes the sustain much better for guitar.
The effect does not last, the cap eventually starts to leak more, and spoils the sound.
This is why people think that NOS caps sound better. They do temporarily, in that "ripe" stage.

[Alex R]

Yes, the insulation breaks down.
But in between, they reach a point where they leak just a little tiny bit. The leakage occurs just when a note is played.
This is when the capacitor is "ripe" and the amp sounds killer. The tiny leakage makes the sustain much better for guitar.
The effect does not last, the cap eventually starts to leak more, and spoils the sound.
This is why people think that NOS caps sound better. They do temporarily, in that "ripe" stage.

That's a new one on me, and I thought I'd heard em all.

[Jazz P Bass]

Quote"1/2 mv of hum in the first stage is quite loud in the speakers, although it's almost too small to measure with your test equipment."
Why can the hum voltage not be tested.
If it is loud, then you can measure it.
My "ballpark" criteria is 4 mVac hum on the output.
Measured with a Fluke meter.
Anything more & something is wrong.

[big_teee]

Quote"1/2 mv of hum in the first stage is quite loud in the speakers, although it's almost too small to measure with your test equipment."
Why can the hum voltage not be tested.
If it is loud, then you can measure it.
My "ballpark" criteria is 4 mVac hum on the output.
Measured with a Fluke meter.
Anything more & something is wrong.

Can you elaborate where and how you measure this on the output with a VOM?
B_T

[km6xz]

I think voo-doo has taking over from engineering.....

People think NOS caps sound better because they are told that and pass on the rumor. The "fact" becomes critical mass before anyone tests the claims. By that time, it is so well "known" that testing is not needed or results accepted. Read how some people on this forum wax poetically about subjective descriptions of various vacuum tubes as having characteristics that tubes can't have.
Again only true believers can understand or appreciate the invisible qualities.
It is not surprising that few objective tests are done, believers do not need proof. It is more than convenient for the sellers of magic that listening tests are subjective with no controls, and worst, not continuity. The flimflam artists know a little about human perception, one being that the believers do not know about human perception. For example most subjective opinions about a phenomena is memory of the conclusion, not memory of the experience. For example different senses have different memory methods. Smell has very long term, permanent memory yet tastes does not. Professional smell testers for foods and perfumes do not require clear references compare against. Smells can be remembered in their original emotional context years or decades later.....ever walk down the street and catch a whiff of aunt Mabel's peach pie cooling in the window and it all comes back vividly....yet Aunt Mabel died 30 years ago. Taste testers have to cleanse the mouth with water and refer to a taste standard within 1-2 seconds of sampling an unknown or their accuracy drops to unusable. Melody and rhythm have similar memory differences. A melody can be altered in every way, instrumentation, key, tempo and it will be recognized almost for ever regardless... as proven by millions of elevators playing background much that is recognized as Beatles tunes instantly but no component part is the same. A rhythm can be altered slightly and not recognized and a much shorter period. Our memory of, say a meal, is not of how it tastes...we can't remember that, but we do remember our impression of it and give reviews based on our memory of our impressions but not the actual tastes. Sound system testing falls in this latter category, we remember impressions but do not remember the sound. When swapping tubes, the delay in the heater coming up to full emission makes any comparison unreliable in the extreme. What makes guitar players most unreliable testers is that they are part of the feedback loop, where minute differences in pressure on strings and timing can swamp any minor differences in tubes. Removing any of those variables results in much more ambiguous results. The salesmen and flimflam artists know this but the users only want to believe so are quite sure of themselves after uncontrolled subjective tests. Ask a speaker salesman in a shop, what sells better, the more accurate speaker or louder speaker. They know they can sell a customer any model on the floor if they audition it 1 db louder than the rest. That sort of misleading approach apparently has entered pro audio and MI much to many our regrets.

But its a living.

[Jazz P Bass]

Can you elaborate where and how you measure this on the output with a VOM?
B_T

Across the speaker leads is where I measure it.
When the amp is on the bench, hooked up to a proper dummy load.
No signal, grounded input.
Measure Vac.

[Trace]

I like the reissue Mullard and Tung-Sol 12AX7s but I also have had what I think is an abnormally large number of them being hummy... I don't know why.
They still sound pretty good but the hum us objectionable to some people... until they play on stage or with other musicians...

Hey Bruce! How have you been?

In my experience; you will have a harder time with combating hum when using Russian tubes. Some of it has to do with the quality of the metals (or lack there of) in other cases it is related to the filaments. Properly burning in the tubes can help resolve it but not always, though burning in the tubes does make the tube(s) sound and feel better. For what it’s worth you will have more hum if you are running the plate voltage on the 1st and 2nd gain stages around 200VDC or lower on the pin side verses sitting around 250VDC. It goes without saying that the hum will increase as the over all amount of overdrive/distortion increase.

If you really love the way the tubes sound and don’t want to deal with the hum then I would recommend referencing the heaters to the B+ supply. In my humble opinion it sounds and feels better than running the heaters on DC. As always mileage varies but I hope this helps.


Have a great day!
Trace

[big_teee]

Hey Bruce! How have you been?

In my experience; you will have a harder time with combating hum when using Russian tubes. Some of it has to do with the quality of the metals (or lack there of) in other cases it is related to the filaments. Properly burning in the tubes can help resolve it but not always, though burning in the tubes does make the tube(s) sound and feel better. For what it’s worth you will have more hum if you are running the plate voltage on the 1st and 2nd gain stages around 200VDC or lower on the pin side verses sitting around 250VDC. It goes without saying that the hum will increase as the over all amount of overdrive/distortion increase.

If you really love the way the tubes sound and don’t want to deal with the hum then I would recommend referencing the heaters to the B+ supply. In my humble opinion it sounds and feels better than running the heaters on DC. As always mileage varies but I hope this helps.


Have a great day!
Trace

Just a builder here.
Can you explain and give an example how to reference the Heaters to the B+ ?
Can You use a Marshall 50 Watt Amp as an example?
I have my heates floating to ground with 2- 100 ohm resistors right now.
That seemed quiter than straight to ground.
Would still like to make my 2204 Clone Quieter.
Thanks,
Terry

[kleuck]

The Valve Wizard

[Zipslack]

More ceramic cap stuff

I don't claim to have golden ears or extremely in-depth analytical knowledge of ceramic caps, but they can and do make a difference. Those little round plates on each side will act like small antennas. Back when I used to work with a company designing EMI/RFI filters, we would run into things like unexplained spikes during testing for frequency response. I still remember the first time I ran into this problem, one of the older engineers looked at it and said, "See those caps that are going to ground? Try flipping them around so the ground lead is going the opposite way." Seemed silly, but we did it and the spike went away. How did he know to do that? Years of experience - you can't find that knowledge in textbooks.

Overall, ceramic caps can be tempremental beasts just like any other component. If all else fails, try adjusting the orientation of the capacitor - remember, electromagnetic fields have an orientation/direction and having the caps at right-angles could make a difference in hum levels by reducing/eliminating the amount of induced signal on the caps.

[jmaf]

Can you elaborate where and how you measure this on the output with a VOM?
B_T

Set the VOM to mV AC on where there should only be DC. The result will be approximately the VOM's idea of the RMS of the ripple or AC there. The precise way is through a scope, but my Fluke rarely gives me too off results when compared to the measurement on the scope.

Edit: if you don't have mv AC scale, which some flukes have and it's really sensible, then you can use the regular lowest AC scale, any significant AC or ripple will probably show up, provided the VOM isn't a xingling like some I have here which acuse 700 VAC on my 220 VAC line.

[soundguruman]

The outside foil of a capacitor used to be oriented, and nowdays nobody pays attention to it at all, although it does make a difference in the noise and sound quality. So orientation can be a big deal after all.

[Chuck H]

The question I see skirted or at least most debated is how the outer foil should be oriented.?. What IS the most shield worthy orientation for a coupling capacitor? Should the outer foil be on the plate or the grid. The grid is often nearer ground than the typical 68k approx plate impedance. And are we dealing with impedance or DC resistance? When does it matter. What about amps that have switches that change parameters so the outer foil might be better inverse to how it's oriented?

I've intentionally built identical amps with the foil oriented one way on one, another on the next and then random on the next. I never heard a difference. And I wanted to or I wouldn't have tried it.

[Steve Conner]

It doesn't matter. It's a coupling capacitor, so irrespective of which plate the hum gets coupled onto, the cap itself will couple it onto the other one.

If the coupling capacitor were deliberately undersized, and didn't couple that well at 60 or 120Hz, then you might think the outer foil should go to the plate of the preceding stage. But I bet it doesn't make much of a difference even then. (Most of the annoyance factor of "hum" comes from the sharp buzz of higher harmonics, not the 60Hz fundamental.)

[soundguruman]

The outside foil is marked with a dash line on the case of the capacitor, the outside foil faces the source (the outside is the input).
The outer foil should be on the plate.
I didn't invent it, I just follow the tradition.