Hiss in new build

[Chuck H]

C72 (if the line out is part of the circuit) and C47 both remove very high frequencies (like the frequencies of hiss) from the feedback loop. Frequencies that high do nothing for the feel or tone of the amp so I can only guess why they're used (well, the C72 attenuates very high frequencies on the line out). Try removing them. That will allow more of the hiss frequencies into the feedback loop and therefore reduce their volume. I don't think you'll hear much if any difference in the actual tone of the amp either.

[tubeswell]

Or you could just get rid of the lineout and have that 470R (R71) as a 5Wer across the OT secondary. That should snub HF oscillation from the NFB loop.

[jmaf]

I cleaned an amp with isopropyl alcohol, carbon comp resistors. It hissed like an angry cat for 2 hours.

Moral of story: Humidity can do that, especially to vintage components like CC resistors.

[überfuzz]

...Since you're mucking with NFB, there is a fair chance it's the ultrasonic oscillation...

Short note: Actually, when I heard my amp I was reminded of the last noise, or whatever you what to call it, on Sgt Pepper lonely hearts club band. There's maybe 3 seconds with a very high pitched tone.

[tubeswell]

Short note: Actually, when I heard my amp I was reminded of the last noise, or whatever you what to call it, on Sgt Pepper lonely hearts club band. There's maybe 3 seconds with a very high pitched tone.

There you go. It nails the beatles tones.

[überfuzz]

There you go. It nails the beatles tones.

You're saying, I should get a nfb switch to get the amp screaming. But I like Beatles pre sgt pepper...

[überfuzz]

Darnit!

I took some measurements. Started at the rectifier, b+ plate or what ever it's called coming from the rectifier, only to see 100 VAC, DC was ok though. I checked the power section to see what the AC did...

Power section EL84
Tube 1-2
Pin2 -30V
Pin3 0V (Ground)
Pin4-5 filament
Pin7 312V and 575 to 600 VAC :-/
Pin9 265V

Tube 3 and 4 shows vertically the same voltages. Pin7 318V and about 460 VAC.

So, I guess I'm back at square... 2 or something. Any hints on what to look for..? (Thought I'd have a look at the diodes in the rectifier and the filter caps.)

[mooreamps]

Smells like some kind of a wiring problem or a missing ground somewhere when I see strange reading like this.. Still could be a high frequency oscillation from the power tube, perhaps implying a missing plate stop resistor or you people just don't explain how to null the plate on a pentode.

In the past I've tried to explain the true root causes of hiss in a tube amp, which was met with a great deal of sarcasm. Now it no longer matters. I just turned over my new design for a parametric amplifier gain stage to my patent attorney which was officially filed yesterday. Now you people will never know how to mitigate the hiss noise in a tube amp....

-g

[Chuck H]

I guess "we people" missed out. I for one am gnashing my teeth. I knew I was ignorant but my pride just wouldn't allow me to beg you for enlightenment. Oh the vanity I suffer. To relagate my people to a life of darkness while you alone sail foreward on our sea of ignorance. We people will well and deservedly fall before you in homage and be glad to lick scraps from your ample plate copying your superior circuits only to lie about our deveous intentions afore.

[R.G.]

I took some measurements. Started at the rectifier, b+ plate or what ever it's called coming from the rectifier, only to see 100 VAC, DC was ok though.

DC OK, but a meter says massive AC voltage? I'd be hugely surprised if it was not oscillating. Unfortuntely, oscillations like this are hard enough to track down when you can see them on an oscilloscope. Sorry - it's likely to be tough to troubleshoot.

In the past I've tried to explain the true root causes of hiss in a tube amp, which was met with a great deal of sarcasm. Now it no longer matters. I just turned over my new design for a parametric amplifier gain stage to my patent attorney which was officially filed yesterday. Now you people will never know how to mitigate the hiss noise in a tube amp....

Still out standing in your own field, I see.

You know, I've seen a lot of late night movies where one of the characters rants on and on bitterly about how they all laughed at him and now he'll show them. 8-) These guys were legends in their own minds.

Actually, patents become public documents and anyone can build one for themselves from the patent documentation. It's part of the deal - the government makes you tell everyone how to do it, in return for letting you pay for any defense of the patent. You oughta read up on that. Patents are only worth whatever you can spend defending them. You of all people ought to know that you can never really own anything you can't defend. The ugly thing about patents is that unless you have a legal staff to do the defense, any large company can simply start making the stuff and let you take them to court. The big companies nearly always win. The race is not always to swift nor the contest to the strong, but that's the way to bet.

I guess we really just missed it, never to know the One True Way to ...er, mitigate??... hiss in a tube amp. 8-)

hmm... what was that acronym? Oh, yeah. ROFLMAO

[Chuck H]

Back to the orininal post for a sec... R.G. has a line on oscillation. To Gary's credit he is angling a miswire. Either could be a problem. And checking the diodes seems a good thing to do but since most rectifiers use a couple of diodes in series for either polarity I doubt your likely to have two failing diodes in series. I'd look into the above possibilities.

[Chuck H]

OK... Gary's line on hiss was the cathode resistor. The lower the better for hiss. If you get the cathode impedance to 0 that will be as little hiss as is attainable.?.

I can only speak from a practiced standpoint, but IMHE cathode resistance is not a significant contributor to hiss. What exactly is happening in the tube relative to cathode resistance that may increase hiss is a mystery to me. High resistances in the AC path and poor resistor quality in the plate supply seem to affect hiss in my very real world of building amps. I recently adjusted a cathode resitor value in one of my designs. I temporarily used a 25k pot in place of the cathode resistor on one of the preamp stages (with corresponding adjustments to overall gain for the preamp) and in my very real world experience there was no increase in hiss with elevated cathode resistance even when overall gain was adjusted "." (period) I have to believe what I actually experience. Hum on the other hand did increase. So now I need to elevate the filament supply to compensate. Different problem. Unless Gary is talking about cathode resistances below ordinary I have to say that in real world experiments done by myself that cathode resistance makes no significant difference in hiss in typical guitar amp circuits "." (period)

[überfuzz]

I to shall wonder in darkness, like a Kafka castle the holy mitigate I'll never reach... Ops sorry I got carried away.

I will look for gnarlys in the wiring. That's sorta my main concern at the moment. Like Greg said, grounds are somewhat the main suspect. If I can't find any I'll make a scheme with the circuit I aimed for...

[Steve Conner]

I've seen Gary's patent, it involves a variable capacitor and a hamster wheel. Parametric amps are kind of retro, I think everyone uses cryo-cooled HEMTs now.

The high AC voltages show that the amp is oscillating like crazy. Does it have grid stopper resistors mounted just at the power tube grid pins?

[Jazz P Bass]

Gnash, Gnash.
Oh, the vanity I suffer.
Chuck you are a true poet