Preamp hiss Volume II - the Master Volume saga...

[dehughes]

Well, I FINALLY FINISHED MY BUILD (yay) after much tinkering and tinkering and, did I mention tinkering? I'll attatch a picture of the preamp and power section. I think the layout is just dandy, and that there aren't any particular oscillation issues to speak of. I've re-wired all the pots and jacks and they are much cleaner and way more solid than the first go.

However, when I turn up the (pre-phase inverter) master volume on the 12AX7 channel, I'm still getting lots of HISSSS. I thought this was an input jack grounding issue, but I re-worked those and still am suffering from hiss. Cleaning up the wiring and moving some grounds helped with the clarity of the channel, but I'm still getting a lot of preamp hiss with the master up. With the gain up and the master down there is surprisingly less hiss than with the master up and the gain down....odd. Again, killing the EQ kills the hiss, as does killing the gain and master volumes...so I'm lead to believe the problem indeed lies in the preamp section.

I'm at a loss at this point.... Would an oscilloscope reveal where the noise is coming from? I've done all the solder re-flowing and carbon comp resistor replacing that I can, and am stuck as to what else might be the problem. I know I'm beating a dead horse here, but any input is greatly appreciated (so long as it doesn't produce more hiss). Ah, that was bad....but I'm sleepy.

If I can't figure out what's up in the next week or so I'll have to take it to a tech, or someone with a scope...I've reached the end of my meager abilities and knowledge. Other than the hiss, it sounds GREAT, so I'm itching to get this totally wrapped up.

THANKS!

[mhuss]

What is it? I probably missed your earlier threads.

If you're using CC resistors in even a medium-gain design, you're going to have hiss. This is not avoidable by resistor selection, it's just the way they are. Unless you're completely dead set against it, I would try metal film. or at least carbon film, in the first stage, particularly the 1M input resistor and the first plate resistor.

--mark

[dehughes]

What is it? I probably missed your earlier threads.

If you're using CC resistors in even a medium-gain design, you're going to have hiss. This is not avoidable by resistor selection, it's just the way they are. Unless you're completely dead set against it, I would try metal film. or at least carbon film, in the first stage, particularly the 1M input resistor and the first plate resistor.

--mark

Sorry...it is a two channel amp based on both the AC30/6 and AC30/4. Two channels: one EF86 with a 6-position plate cap selector a la the DC30/London, the other 12AX7 but with a cathode follower EQ network. Each channel has its own PI side where it connects. A quad of EL84s and a 5AR4, plus Mercury Magnetics Woden clone trannies. Two inputs per channel, and a switch for reducing overall voltages (adds a 121ohm resistor after the recto) and increasing both preamp filtering (adds 16uf to the first 12AX7 stage) and power amp filtering (adds 16uf to the 1st cap off the recto).

The reason I find it dubious that it is a carbon comp issue is that I've played many similar amps that have used carbon comps throughout and have nowhere near the level of hiss I'm hearing. This is definitely something unusual, and I cannot seem to locate the source. Hence my inquiry as to if an osciloscope would pinpoint where the noise is being created, or if there is something obvious that I've not tried. And I've tried a LOT of things so far....

[lastwinj]

try just using one plate cap, without the switch. if the hiss goes down with that, then...

germ

[dehughes]

try just using one plate cap, without the switch. if the hiss goes down with that, then...

germ

Hey lastwinij,

I'm not sure what you mean...exactly... The switches either 1) add more capacitance a right off the recto, 2) add more capacitance on the 1st 12AX7 triode, or 3) add in a 121ohm 10w resistor right off the recto to drop the overall voltages. Engaging or disengaging the switches does not effect the hiss in any way (unfortunately...). I guess I'm not understanding your reply...sorry...

I called a local tech today to see about having him scope out the amp, but he's off until the 7th of March. Dang....

However, I'm thinking that not only does this "hissue" have to be located BEFORE the PI, but it has to be located either IN or BEFORE the EQ network. I say this because when I adjust the EQ knobs I can adjust the tonality of the hiss. That is, if I cut the bass the hiss looses low end, and if I cut the treble, it looses high end. Cutting the Bass, Treble, and Midrange all the way down kills all noise. SO, my inclination is to say that the hiss is being generated even before the EQ network. Is this valid reasoning?

[mhuss]

SO, my inclination is to say that the hiss is being generated even before the EQ network. Is this valid reasoning?

Yes.

--mark

[dehughes]

SO, my inclination is to say that the hiss is being generated even before the EQ network. Is this valid reasoning?

Yes.

--mark

Okay, fair enough. Thanks. That REALLY limits the components that could be causing this. It SOUNDS like either a crackly-hissy resistor, a tube socket issue, or as some have suggested, a ground issue. I've replaced all the resistors from the tube sockets to the EQ network, and have re-flowed all ground connections and solder joints, both on the pins and on the turrets. The socket joints have been re-flowed, and the pins are all clean and tight. The ground layout, while not being the star ground scheme that many seem to prefer, is a very solid layout patterend after a killer Top Hat King Royale that was as quiet as a mouse.

So, I guess there are just two questions I have: 1) Would an oscilloscope point to the troubled area/component? 2) Is there any other thing that I could check before I take it to a tech? I'd really like to fix this myself, but am at the end of my knowledge and diagnostic abilities.

THANKS SO MUCH!

[smzinno]

dehughes, for what it may be worth, I recently completed a home-brew amp that had a LOT of hiss. I was able to completely eliminate it by replacing ONE carbon comp cathode resistor in my second preamp stage. I also made a smaller improvement by replacing a leaky capacitor. maybe try checking your caps for excessive DC on the output side. hope this helps.

[dehughes]

dehughes, for what it may be worth, I recently completed a home-brew amp that had a LOT of hiss. I was able to completely eliminate it by replacing ONE carbon comp cathode resistor in my second preamp stage. I also made a smaller improvement by replacing a leaky capacitor. maybe try checking your caps for excessive DC on the output side. hope this helps.

Okay...now we're talking. Thanks. I don't have a way to test for leaky caps, or a bad resistor....the only "tool" I have is a digital multi-meter. Would that work for the caps? How would one test if a resistor is "bad" or noisy, and how would one test if a cap is bad if their DMM can't test them? Will I need to get a cap tester? Oscilloscope?

THANKS SO MUCH! I really appreciate your help...

[ampdoc1]

Go get a can of "air" used to clean computer keyboards. Turn it upside down (the propellant is cold) and spray the resistors and caps in the area you suspect, one at a time. If it is a hissy component a lot of the time this will make it easy to identify. A scope will tell you nothing.

[dehughes]

Go get a can of "air" used to clean computer keyboards. Turn it upside down (the propellant is cold) and spray the resistors and caps in the area you suspect, one at a time. If it is a hissy component a lot of the time this will make it easy to identify. A scope will tell you nothing.

Very interesting... I assume I do this with the amp on? Where should I set the controls on the amp? What will be the indication that I have "found" the problem component?

Also, how would I test for a bad cap, using only a DMM? My DMM doesn't let me test caps in the circuit....I'd have to take them out, and even then it would only tell me their value, not if they are "good" or "bad".

THANKS!!!

[smzinno]

I only have a DMM too, which doesn't have a capacitance test. What I did was to measure for DC voltage on the output side of the caps, let's say the interstage coupling caps for example. These caps should block DC, so if there's more than a couple millivolts on the output side there's likely to be a leakage problem. A couple of mine (recycled stuff) showed a lot of DC here, and the new ones I used to replace them show virtually none.

I don't truly know how technically correct my test was, but there was certainly a detectable change afterwards. But I still have to say, that single carbon comp cathode resistor was the money shot in my case.

[ampdoc1]

A lot of noisy resistors react to thermal differences. Most good resistors will not react when sprayed with a coolant. But a lot of times resistors that are noisy have a thermal problem, and will go silent when sprayed directly. I'd set the amp up to a good volume, cause it's the difference in noise you'll be looking for.
This seems to work best on plate resistors.

[rfgordon]

Carbon comp resistors can vary by extremes in the noise they produce. If you went to an engineering school library and looked up IEEE proceedings from the 1930s thru the 1950s you would find tons of articles on this issue. Carbon comps can suffer from thermal noise, shot noise and other forms of instantaneous or short-duration noise. Brute force replacement is the best way to troubleshoot this.

I know some folks live and die by CC resistors, but I find them problematic. Most amps using 1 watt CF I build are very quiet. For the jazz amps I use Kiwame Japanese CF (they come in 2 and 5 watt ratings). They are very warm and dead silent. BUT, they cost over a buck a piece. Rikens are nice, but even more expensive, now that they're out of production. A good average is Precision Resistive Products 1/2 watt audio CF resistors. They are "noise checked" according the factory rep I spoke to. Even though they are 1/2 watt, they're rated for 500 VDC continuous, so I don't worry about using them. They sound great. They're available at www.soniccraft.com. The nice folks at PRP will also send you a sample assortment--I got enough to build one amp! I did the same from Reidon for their 0.6 watt MF, but I'm not wild about the sound, plus ya gotta order 100 each value.

[dehughes]

Hmmm....okay. Makes sense. I KNOW I just replaced the plate resistors on V1, so those should be good, but then there's no way of knowing unless you swap them out, OR do the coolant trick (pretty neat, BTW).

Thanks. I'll look into this again this weekend. I've had others on the 18w Forum say that their amps gave them this EXACT same problem, and it was a bad 56k carbon comp on the cathode in the cathode follower circuit. I don't know if I've swapped that one out, so we'll see.

Also, what about 2w metal film power resistors, like up near the filter caps in the preamp picture I posted? Can they go bad, and cause noise like what I'm describing? Hissy, spitty noise? I figure they are pretty hardcore, but who knows....

THANKS!