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

[dehughes]

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.

Nice. Thanks Steve. I APPRECIATE YOUR ADVICE, and will let you know what comes of this....

Had you done any moving around/re-soldering of that resistor before you put it in place, or did you just wire in a brand new resistor that happened to be noisy?

[smzinno]

The CC resistor was basically new-old-stock, but first time use. How old ? No clue, was in a grab bag of hundreds. I can tell you it had drifted pretty high - was originally a 750, measured nearly 1K so I used it as such. I ended up replacing it with a 1.5K (not certain of its makeup, looks like a large mouse turd), and the leaky caps were replaced with new Xicon MPP. No more hiss on the clean channel, even when wide open going thru 2 gain stages, no tone stack or master volume.

[dehughes]

The CC resistor was basically new-old-stock, but first time use. How old ? No clue, was in a grab bag of hundreds. I can tell you it had drifted pretty high - was originally a 750, measured nearly 1K so I used it as such. I ended up replacing it with a 1.5K (not certain of its makeup, looks like a large mouse turd), and the leaky caps were replaced with new Xicon MPP. No more hiss on the clean channel, even when wide open going thru 2 gain stages, no tone stack or master volume.

Very cool. THANKS MAN. I'll test the caps and swap out the carbon comp 56k and go from there...

[dehughes]

I checked all the coupling caps in the preamp section, and none had more than about 1.5ma of DC on them. Most read far less. This is about normal, yes? I think that rules out DC getting into places where it shouldn't.

Hey, I just measured the 56k cathode follower resistor and noticed that it's value was shifting as I turned the amp from off to standby to on. Is that normal? It went from 66k down to zero then back up to 13k where it stayed whilst the amp was running. Would a "healthy" resistor really have its resistence shift so dramatically when in use? The 1k5 cathode resistor on the cathode follower (the 1st triode of the CF circuit...) exhibited this same effect, going from 1k5ish to about .813k and hovered around there while the amp was on...

Is this normal?

[oldhousescott]

You can't check the resistance in circuit with the power on. You can measure the voltage drop across a resistor with the power on, or you can remove one leg from the circuit with the power off and measure resistance.

[dehughes]

You can't check the resistance in circuit with the power on. You can measure the voltage drop across a resistor with the power on, or you can remove one leg from the circuit with the power off and measure resistance.

Ahh, right. Makes sense. So, is there a way to tell using a DMM if a resistor is "bad"?

[dehughes]

One other idea:

Is there a possibility that the plate wires from the first channel are inducing noise into the 2nd channel? Check the pic....the plate wires (red) from the EF86 channel have been run right under the Rk and 1st coupling cap on the noisy 12AX7 channel....look at the area circled in white. Should I change that wiring, too, or is that most likely not an issue?

THANKS SO MUCH!

[dehughes]

Well, I did some re-wiring and swapping out of resistors today and noticed that the plate voltage on V1a was about 200v...way higher than the 150ish that it should have been. I replaced that resistor with another 220k and now the voltage is back towards what I'd consider normal. That helped a LITTLE bit with the hiss, but the crackly poppy stuff is now gone. Good deal.

I also replaced the 56k cathode follower resistor with another one of slightly higher value (59k-ish) and that did nothing to remove the hiss.

There is no sign of excessive DC on the coupling caps.

I even replaced other plate resistors (pretty much all resistors in general...) and nothing really changed.

The grounds all seem to be good. Voltages are where they should be. I'm beginning to think that this carbon-comp bashing is hyped up more than it should be, and that issues attributed to carbon comp usage may very well be a result of poor build quality first, then a possible noisy resistor later...so I'm not going to go buy a bunch of carbon or metal film resistors and totally re-build my amp. I've played pleeeeeenty of amps built with carbon comps, and they've been plenty quiet...even high-gain ones, so I'm inclined to think the issues is unrelated to component type and more related to build quality/layout.

To be honest, I'm not sure if this hiss issue is really even an "issue" anymore...that is, maybe this is just the way this circuit is supposed to run... I have a similar amp to compare with mine, but no two are exactly alike so perhaps it is unfair of me to expect this build to be what another build is.

Maybe it is an issue with wire runs? Maybe it is an issue with layout? Maybe maybe maybe.... Anyway, I appreciate everyone's help, but I'm getting weary of trying to figure out this thing and am going to go do some reading on preamp hiss and see what I learn. Who knows...

The amp does sound very nice, though...

Thanks again for your help...

[dehughes]

Well...one breakthrough....I found that the solder joint on the wiper of the EF86 channel volume pot was questionable....this was proving to randomly add a crackly contingent to the noise. This makes sense, as this channel provided the other side of the PI with a ground (the 12AX7 channel was on the other), so if that joint was bad then the whole thing would suffer. I'll go replace that and move on from there...

[steve-o]

Hey David, I just thought of something in response to one of your previous posts on this issue:
You used to have the ef-86 channel wired with a fader/pot, rather than a 7-way selector with individual wires running to the tube. Did you have the noise problem back then? I don't remember any hiss before when you showed me that particular channel, but we may not have had it up loud enough...

[drz400]

Maybe it is an issue with wire runs? Maybe it is an issue with layout? Maybe maybe maybe.... Anyway, I appreciate everyone's help, but I'm getting weary of trying to figure out this thing and am going to go do some reading on preamp hiss and see what I learn. Who knows...

The amp does sound very nice, though...

Thanks again for your help...

Hiss is an issue with too much highs, too much gain or a bad tube
Crackling and hiss can come from plate resistors.
Maybe it is just the nature of the circuit.
Try some 120pf caps across the plate resistors

[dehughes]

Maybe it is an issue with wire runs? Maybe it is an issue with layout? Maybe maybe maybe.... Anyway, I appreciate everyone's help, but I'm getting weary of trying to figure out this thing and am going to go do some reading on preamp hiss and see what I learn. Who knows...

The amp does sound very nice, though...

Thanks again for your help...

Hiss is an issue with too much highs, too much gain or a bad tube
Crackling and hiss can come from plate resistors.
Maybe it is just the nature of the circuit.
Try some 120pf caps across the plate resistors

Thanks man. I'll give that a try... What leads me to believe that this is something before the preamp tubes is that the hiss is present on both the EF86 side and the 12AX7 side. I'll replace some B+ resistors, too...

[lastwinj]

A good average is Precision Resistive Products 1/2 watt audio CF resistors. They are "noise checked" according the factory rep I spoke to.

these are the red ones? arent they metal film? parts connection lists them as such.

Code: Select all

http://www.partsconnexion.com/catalog/resistors.html

germ

[dehughes]

Hey David, I just thought of something in response to one of your previous posts on this issue:
You used to have the ef-86 channel wired with a fader/pot, rather than a 7-way selector with individual wires running to the tube. Did you have the noise problem back then? I don't remember any hiss before when you showed me that particular channel, but we may not have had it up loud enough...

You know, I don't remember, to be honest, but I do know that I didn't crank the amp much back when I first built it, so for all I know the problem was there even then. It really isn't THAT noisy anyway, so maybe I'm just being too picky, but I think it can be better...

I'll go try some stuff...

[dehughes]

PROBLEM SOLVED!!!

The solution: There was no freaking problem....

Really.

Yesterday a friend of mine brought a Winfield EL84 head to practice. VERY nice amp....great, cool, etc.. Immediately after practice I had him turn up the amp all the way (with the guitar volume down...) in order to check out the amount of noise it produced at idle. Hmm...well, it sounded only slightly quieter than my amp...but really not much at all, especially considering the low level of filtering I'm using, and the old-school trannies that hummmmm. So, that was encouraging...

Then, today I went over to a friend's house and he had the "paradigm" Top Hat King Royale head, and a '65 Amps London head/cabinet setup. The first thing I did was turn up the amps as LOUD AS I COULD, and listen for noise/hiss, etc... What did I find? The SAME THINGS that I found in my amp!!!

My problem this whole time was that I lacked any context for the noise in my amp, and it turns out that my amp is just as quiet as these three. So, I'm closing the case on this amp, calling it finished, and thanking God that I didn't die in the process of building it, and that it turned out so very well. Soli Deo Gloria, no question.

So, thanks to all of you for your help! Even though I didn't really reduce the noise level much throughout all of this troubleshooting/tinkering, I did get the amp tweaked to perfection as a byproduct of my work lately. Much appreciated... I now have my AC30 base covered, and am looking forward to entering the world of the JTM-45!