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'distortion' knob in schematic
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Old 20th September 2014, 08:47 PM   #11
cyclecamper is offline cyclecamper  United States
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Could replace the supply's dropping resistor with a choke; I probably have one that would work.
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Old 20th September 2014, 08:55 PM   #12
cyclecamper is offline cyclecamper  United States
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The background noise is in the power amp section, as it's still there with the master volume off. I could pull pairs of tubes, but I think it's the resistors. My dad's Harman Kardon Citation II always had the same background noises, as did everything of the era. Yet I hear modern boutique amps with more gain and they're completely silent; of course they don't put out 280 watts, so by comparison even this signal to noise ratio would be quieter at that low a volume. It would be much more versatile if the noise was less. I could burn off some output with an Aiken Reactive Load to practice at home, but I already have practice amps.
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Old 22nd September 2014, 05:00 PM   #13
teemuk is offline teemuk  Finland
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Cathode follower clipping. With the different bias the stage "starves" in current and goes to cutoff on negative halfwaves. Result is HARD clipping of the negative halfwave and nothing else. Plenty of asymmetric clipping distortion and those "musical even order harmonics". ;-)

...That is, if you believe any of the remarks concerning musicality of different types of harmonics. In that regard, this amp is a good eye opener.
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Old 23rd September 2014, 02:50 AM   #14
cyclecamper is offline cyclecamper  United States
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But why does the power section make noise when there is no signal? What's that got to do with cathode follower clipping? I don't think anything in this power amp section is likely to clip hard before your ears bleed.
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Old 23rd September 2014, 05:47 AM   #15
djgibson51 is offline djgibson51  New Zealand
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I would check v4 and v5 for noise as well as the power tubes.
V4 has those 220k load resistors as possible suspects.

So... As you said...tubes and resistors...
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Old 23rd September 2014, 06:34 AM   #16
cyclecamper is offline cyclecamper  United States
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There are just SO MANY tubes and resistors! I definitely got my money's worth just in transformers. Or dollar per pound, best deal in a hand-wired tube amp. I'll see how the new amp sounds.

Good thinking though, if I pull pairs of tubes and move 'em around in different sockets that should tell me a lot about the tubes and the resistors local to them, and be a lot easier than trying to tell what's going on in a complete circuit via an oscilloscope. I might even try the tubes in my champ if I just want to find the ones that are noisy at idle. Yessir, now I've got a logical approach to start with.
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Old 4th October 2014, 01:17 PM   #17
shanx is offline shanx  Canada
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Quote:
But why does the power section make noise when there is no signal?
I am going to purely guess here, but I would suspect the biasing circuit for the power tubes. Any noise on that DC will show up at the grids of power tubes. Check electrolytic and suggest replacement, and resistors on that circuit replace with lower noise metal films, you can up the wattage on them. Use a high quality pot if it's going to be an adjustable bias circuit. Have a look at the screen resistors for possible replacement too.
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Old 4th October 2014, 05:07 PM   #18
Enzo is offline Enzo  United States
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What "Noise"? 60Hz hum? 120Hz hum? Hiss? Crackle/rumble? Every one of those is from a different thing, and so would have not only a different source but also a different cure. You mentioned several in passing, but which are you trying to solve at this moment?

Isolate the problem. The noise is power amp, so you mean after the master volume? OK, that means it is in the V4 stage, the phase inverter, or in the V5 stage, the drivers, or in the power tube stages. Could be the tubes themselves or something else in one of the stages.

So pull V4. Noise stop? If yes, then that stage is central. If no change, then pull V5. If that stops it, then something about the V5 stage is key, and if neither stops the noise, then the power tubes are suspect, or their circuits. Sounds simplistic, but it narrows down your search.

First thing for any suspect stage is a different tube.

V4. The two 220k plate resistors are most likely for noise like hiss and crackle.

One powerful technique is to ground the signal path. Just a clip lead to chassis will do that. If it stops the noise, you have at least found part of its path, the noise is before what you grounded. But what if the point you want to ground is at 200v or 50v or anything not ground level? Use a cap. I have a lot of 0.047uf/630v film caps around, so I grab one, but the value is not critical. Ground one end, and have a free clip wire on the other. Now we can "ground" a point with this cap. A cap to ground will shunt off a signal, even a noise signal.

And you can shunt a resistor with a cap in similar fashion.

V5. again possible noisy cathode resistors.

Power tubes. You can power up the amp with just one of them in at a time to check them individually. And you can take a tube you know is good and move it socket to socket to check them. Surely ripple on the bias supply will make hum on the power tubes.


But start with tube pulling. Pull V4 and then V5, and you ought to have a pretty clear idea where the noise is coming from. Then by "grounding" point to point along the signal path, you can narrow it down further.

As to all those resistors, oh please... The phase inverter has only seven, the driver stages each have three. Each power tube has two. There may be 100 resistors in the preamp, but they are not making our power amp noise, so...?
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Old 9th October 2014, 09:00 PM   #19
cyclecamper is offline cyclecamper  United States
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Very practical advice.
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