What is causing buzzy distortion in the preamp?

[Mark]

It’s official, I have replaced every component including the power supply cap and it made no difference. The buzziness manifests itself in the highs, I have used a 22uF across the first cathode resistor to get more bass or rather more frequencies other than the ones that are buzzing.

I’m wondering if the valve has less current going through it that a typical 50W Marshall amp and I should look at the bias of the valves?

Someone suggested blocking distortion, though there is a voltage divider after the volume control. I’m not sure what blocking distortion looks like on a class A stage.

EDIT: I put a 33K grid stopper on the second stage to see if it was blocking distortion. The resistor made no difference at all.

[Gainzilla]

I think I was saying to try it in front of the power tube. Again, I'm only basing this on my own experience working with little amps like this. Specifically the P1 High Octane and the Custom 5. I could be wrong, but at this point you've tried everything else.

[cdemike]

Have you tried rolling different tubes through V2? Not necessarily different types of tubes, but different examples of 12AX7/ECC83. If you don't have extras on hand you could just shuffle the positions of the preamp tubes already in the amp. I tend to like JJ's in cathode follower positions, but your taste may differ.

Can you dial out the sound you're not liking with the EQ controls or the guitar's tone knob? My experience aligns with Roe's that too much bass makes for fizzy/buzzy sounding overdrive, so I'd guess tone controls both on the amp and guitar won't make a difference. If that's the case, you might want to reduce the 22nF coupling cap coming off the first gain stage per Roe's suggestion, and I'd also reduce the cathode bypass cap on the second gain stage. I like 680nF there, but you might prefer something slightly larger like 1.5uF; I generally have a pretty strong preference for lead-spec Marshalls, but since your preamp (sparing the tone stack) is bass-spec, you might prefer a larger value than 680nF. In any event, I don't think fully bypassing the second gain stage's cathode is doing you any favors. Later metal-panel era JMPs tend not to have any bypass capacitor on the second gain stage, so that's also an option. The NFB coming from the un-bypassed cathode may help better control the overdrive as well as emphasize overdrive coming from the cathode follower rather than the second gain stage, i.e., emphasize the smoother source of distortion.

Running on Merlin's general emphasis on even-order harmonics, you could also try to bias the second gain stage cooler to ensure consistently asymmetric distortion coming out of V2. I don't know that I'd go all the way to a cold clipper on first try, but 2k2 or 2k7 might be a good starting place.

[LOUDthud]

Looking at the scope shots on page 1, the 1KHz shots look very symetrical. A symetrical wave tends to block out the even harmonics and sound somewhat hollow. YMMD.

One thing to add to what cdemike just said, lowering the resistance of the 100K cathode resistor to 68K or 47K also tends to add even harmonics.

[WhopperPlate]

You say you have replaced every component… with what? Let’s see what the amp looks like now

I can tell you from the original gutshot I would expect nothing less than buzzy harsh tone from that soup of components.

Sure , metal oxides in the signal path aren’t going to help , but the power supply is just as much of an issue for buzzy stiff tone . try a few koa carbon films instead . Same with screen resistors , wire wounds will sound nicer here ime

Power caps all have their own characteristic sonic signature. Different brands are going to shift the response potentially quite drastically , especially with overdriven amps like a Marshall . I like BC Vishays , for modern caps that are readily available they work pretty good.

What signal caps are in the amp now ? 100% polyester is the call . In a Marshall circuit , if any of the current caps are anything else you will likely be troubled. Any mustard clone is going to help.

Cathode bypass caps are huge when it comes to shaping tonal response. Different brands and composition will make or break the sound . In a Marshall , electrolytic caps alone can be too much. Marshall usually used mustard caps for cathode bypass . Big difference. Cathode resistor composition matter just as much .

When all else fails refer to how the original amps were produced. It can be a wild goose chase trying to make most modern components behave. Changing circuit values isn’t really as necessary as you might think; find the correct composition components for each position that adds up to what you want and you can fine tune the stock values if need be.

Fwiw My main amplifiers have always been Marshall circuits , and I have rebuilt them over and over and over trying everything under the sun , and it’s easy to miss the mark. People might argue …doesn’t matter ….everything adds up.

[R.G.]

Yeah, what he said. Worrying about bias points, power supply voltages, grid blocking and all that nonsense about whether the circuit is set up and running properly is nonsense, a time waster. It's the brand of the components, no matter what their actual value, that makes or breaks a circuit's operation. Different brands can make or break the sound.

I - um - read that somewhere on the internet, I think, so it must be true.

[LOUDthud]

Don't you need one of those $500 power cords ?

I tried to play the wav file from post #2 and only heard about 2 seconds of audio. Anyone else have that problem ?

[WhopperPlate]

Yeah, what he said. Worrying about bias points, power supply voltages, grid blocking and all that nonsense about whether the circuit is set up and running properly is nonsense, a time waster. It's the brand of the components, no matter what their actual value, that makes or breaks a circuit's operation. Different brands can make or break the sound.

Nice spin doctoring, but I didn’t say that. I said start with a stock circuit , nail down the appropriate component types according to the original amps, and then proceed from there with deviation in values.

Stock Marshall circuits aren’t inherently buzzy . If he’s changed every component , nothing is actually faulty , and it’s still sounds bad, then it’s down component gastronomy.

We get it RG, you are a die hard , by the book, engineer. Ohms are ohms, capacitance is capacitance, and there’s nothing else to consider but the math and measurements .

All I know is if I lived by that mentality and never questioned that perspective I would be still chasing my tail . Thank God I have ears to hear and the patience to endure the doubt, the patience to actually try things for myself without blindly believing anyone, and thank God for all those throughout the amplifier world that did the same whose shoulders we stand upon

I look forward to hearing and plugging into one of your amplifiers some day

[Mark]

There had been the odd breakthrough moment. I remember trying 1K screen grid resistors on a switch with 100 ohm resistors when building my Rocket amp. I found the 1K screen resistors shaved off the bass response. I decreased the screen grid resistor to 100 ohm and that helped the overall sound of the amp. I’m finding the output stage isn’t delivering the same output as the waveform on the grid. I replaced the output transformer with a different one I have and it didn’t make a lot of difference. I also experimented with a conjunctive filter across the primary of the output transformer, it worked okay but there are limits to its effectiveness.

There is still buzziness in the preamp, but I thought to go over the whole amp.

Thanks for your input everyone.

[R.G.]

Nice spin doctoring, but I didn’t say that. I said start with a stock circuit , nail down the appropriate component types according to the original amps, and then proceed from there with deviation in values.

Perhaps you should read my post again. I didn't say that you said that, did I? Read carefully. Try sounding out the words phonetically. We'll wait. Let's see, where is that eye-rolling icon?
Oh, yeah. I did find some posts where you may not have said the words, but you did over some number of posts say that the brand of resistors directly affected tone directly, in any amp, regardless of the brand of amp, and regardless of the resistor maker possibly changing OEM suppliers that really make the parts, changing machines and process over time.
To cap it off, who exactly is spin doctoring by accusing someone of spin doctoring?

Stock Marshall circuits aren’t inherently buzzy . If he’s changed every component , nothing is actually faulty , and it’s still sounds bad, then it’s down component gastronomy.

Let's play reason for a few seconds. The assumption that "If he’s changed every component , nothing is actually faulty" is factually incorrect on its face.
- First, replacing every component can easily mean replicating the same schematic or wiring mistake faithfully again.
- Second, have you ever been in the situation where you have gone over and over the same problem again and again, and then found that you literally missed something simple because you were so very, very focused on going over and over it again? You may have heard of this condition - it's referred to as not being able to see the forest for the trees. It's common in humans, and shows up a lot. Changing every component will not fix the real issue if you don't know the real issue.
- Third, "If he's changed every component" means that he thinks he's changed every component; maybe he has, maybe he accidentally missed one or two. I have heard variations of that for decades from people building equipment. The only time that every component has actually been changed is on a pluggable breadboard.
I strongly doubt that he's changed literally everything. Probably not the chassis or tag strips/turrets; probably not the grounding scheme; probably not the wires or sockets; likely not any controls or jacks, nor wiring to them. Ever hear what an intermittent socket contact or a cold solder joint does to sound? Of course you have...
The issues with a builder/repairer not being able to fix something after repeated tries of what they think is EVERYTHING! led me to advise builders to instead measure the pin voltages of every component. This does two things. First, if done faithfully, it finds bias and incorrect component placement or connections; second, it forces the person with the soldering iron to stop soldering and take a mental side step that might get them to one tree inside the forest.

So, no, it does not necessarily mean that all that's left to correct is component gastronomy. Interesting idea, though. I'll have some Iskar resistors with a side of pecan-strawberry rutababa sauce. The amp won't sound any better, but it will -smell- GREAT!

In my experience - and maybe in yours, too - the difference between buzzy and not-buzzy distortion shows up on a scope trace.

We get it

You do like spinning yourself, don't you? Nice try with using "we" to imply that you and everyone else is the group, and I'm the outsider. Not true or logically defensible, but a cute application of political spinning.
Then there's your starting statement, using the accusation of spin doctoring to spin. I'm still trying to figure out whether you have a native talent for spin accusations or if it's acquired by practice. If you're not in the politics biz, you may have missed your true calling.
{quote]
We get it RG, you are a die hard , by the book, engineer. Ohms are ohms, capacitance is capacitance, and there’s nothing else to consider but the math and measurements .
[/quote]
First - you say that like it's a bad thing.
Second - you're spinning again, and factually incorrect. Maybe you even self-spin and believe yourself. You're glossing over my previous comments on the human mind doing things that are not measurable. I was championing that, remember? You really, really didn't like the idea things which are NOT measurable may possibly be mental only. Nor the idea that only measurable things exist.

So which is it? Is belief in measurements a good thing? Is belief in mental effects a good thing? Should we do both? Should we -test- what we think we hear? Or should we just throw shade on other people?

On the topic of are real things measurable, and is measuring what we can measure a good thing: is placebo a real, measurable effect? (hint - yes, it is; people have measured the placebo effect)
Do people adopt mental preferences that are sometimes difficult, sometimes impossible to replicate as a real-world effect? Yep. This is a -measurable- effect. Ask a research psychologist.

So let's do some taxonomy. One possible parsing of things is that there are things that are measurable, things that are not -yet- measurable, things that are not ever measurable, and things that exist only in the human mind(s). These last two are not the same, by the way.
- Things that are reliably measurable must truly exist.
- Things that are not -yet- measurable are places to study and learn. Whether they are objectively true in the real universe is still up to question. Maybe they're real, maybe they're imaginary. We (humans) owe it to ourselves to go look and see what's real and what's not. By the way, subjectivists >hate< this idea.
- Things that are never measurable are likely to be imaginary. Things that only one person perceives are likely (by Occam's Razor) to be hallucinatory. Not certainly, but most likely. Maybe if the person makes a good case, some study and learning may prove them correct, eventually. But never, ever measurable means they're only true inside that one person's head, or the heads of people who believe whatever that person says. Look up Jim Jones.
- Things that exist only in the human mind(s) are not in general reproduceable. If you truly, truly believe that one brand or gastronomic combination of parts sounds better, then it will sound great. To you. Probably to a group of people who accept your authority. But to everyone? Humans don't universally like the same things, full stop. The psychology of human preferences is a deep well. Try moderating a debate between jazz guitarists and metal head-bangers sometime. Or people who like vs can’t stand soft boiled eggs.

All I know is

I sympathize. I'm trying to help with that.
Oh wait! In Texas we’d call this the aw-shucks argument. It’s a variant of the appeal from ignorance fallacy.

if I lived by that mentality and never questioned that perspective I would be still chasing my tail .

Again, I sympathize. Maybe some technical continuing-education courses in instrumentation and measurements and or the psychology of music would help.

Thank God I have ears to hear and the patience to endure the doubt, the patience to actually try things for myself without blindly believing anyone, and thank God for all those throughout the amplifier world that did the same whose shoulders we stand upon

Back at work we used to call statements like that wrapping yourself in the flag. No content, no reason, only appeals to motherhood, apple pie and patriotism.

I look forward to hearing and plugging into one of your amplifiers some day

Find a Workhorse amp. Limited runs, hard to find, but very well received by the people who bought them. Like all designs for manufacturing as opposed to gastronomy, there were compromises, and I'm not completely satisfied with all the compromises that manufacturing requires as opposed to crafting another "love potion number nine" amp in the shop. But then manufacturing is a bigger problem than cooking up a new bowl of pecan-strawberry rutababa sauce. I can only console myself with some people preferring Fender to Marshal to Vox to Rutababa.

[WhopperPlate]

Let me quote you again

Yeah, what he said. Worrying about bias points, power supply voltages, grid blocking and all that nonsense about whether the circuit is set up and running properly is nonsense, a time waster. It's the brand of the components, no matter what their actual value, that makes or breaks a circuit's operation. Different brands can make or break the sound.

I didn’t say that, I have clarified what I have said ,and I will continue to stand behind my statements in spite of persistent doubt and arguments. I won’t even begin to address the misattribution and assumption delivered in that last message. Have a nice day RG

[R.G.]

I didn’t say that, I have clarified what I have said ,and I will continue to stand behind my statements in spite of persistent doubt and arguments. I won’t even begin to address the misattribution and assumption delivered in that last message. Have a nice day RG

You're perfectly well entitled to hold your opinions. I strongly support your right to do that.
I personally prefer facts to opinion whenever I can get facts, but that's just my opinion. Well, actually it's a learned response to having to support my opinions with facts so often during my professional career, and having my ... um, anatomy... kicked when I could not support the opinion.
Y'all have a nice day, too.

[WhopperPlate]

I personally prefer facts to opinion whenever I can get facts, but that's just my opinion.

And again for emphasize, and all due respect , the fact remains that I didn’t say what you claim I said . That’s not an opinion .

Thank you for the well wishes, again I wish you a good a day

[Mark]

I didn’t say that, I have clarified what I have said ,and I will continue to stand behind my statements in spite of persistent doubt and arguments. I won’t even begin to address the misattribution and assumption delivered in that last message. Have a nice day RG

You're perfectly well entitled to hold your opinions. I strongly support your right to do that.
I personally prefer facts to opinion whenever I can get facts, but that's just my opinion. Well, actually it's a learned response to having to support my opinions with facts so often during my professional career, and having my ... um, anatomy... kicked when I could not support the opinion.
Y'all have a nice day, too.

I have taken voltages of every point on the amp. Is there any information that you would like me to provide?

The amp uses an EL-84 amp in cathode bias, the rail voltage is 250VDC, it initially used the 6K tapping but I changed that to the 4K tapping as the output was low. I had thought of asking about the correct primary impedance but thought I would try and work it out myself.

[R.G.]

I have taken voltages of every point on the amp. Is there any information that you would like me to provide?

The amp uses an EL-84 amp in cathode bias, the rail voltage is 250VDC, it initially used the 6K tapping but I changed that to the 4K tapping as the output was low. I had thought of asking about the correct primary impedance but thought I would try and work it out myself.

Are your scope pictures taken with AC coupling or DC coupling?