Lend me your ear; am I over analyzing my build?

[_ej_]

Finally got my Mojotone NC3015 build cleaned up and working well. I am hearing a distortion in the low end that isn't pleasant. Is this a typical EL84 tone or is there something I can do about it?

It's one of those things that would probably disappear in a mix but I can hear it and it bothers me. I'm sure you guys can understand

https://on.soundcloud.com/kbH2zXdw7wd8mpav8

[Colossal]

Been following your build. I think the clip sounds good, but you might be hearing something that is bothering you that I am not focusing on. Let me preface this by saying I am not a huge fan of Mojotone's layouts or choices. The voltages they show on the NC3015 schematic seem very high to me. I build 18W 2xEL84 British style amps to about 340-345VDC on the plates and 11-12VDC on the cathodes. Too low a voltage and the low end gets fatter but a bit mushy and less focused, but too high a plate voltage and it can get splatty and too pingy. I love articulation and clarity but want the distortion to be smooth.

There are a couple of things you might do to further fine tune the amp to get what you want from it.

Before suggesting or making further changes, however, please post a current photo of the amp and a table of both the preamp and power amp tube voltages. You said you have cleaned up a bit more. I think lead dress is very important to getting an amp to sound open and clear without crosstalk and a blurry or confused distortion. So let's start there.

[_ej_]

Been following your build. I think the clip sounds good, but you might be hearing something that is bothering you that I am not focusing on. Let me preface this by saying I am not a huge fan of Mojotone's layouts or choices. The voltages they show on the NC3015 schematic seem very high to me. I build 18W 2xEL84 British style amps to about 340-345VDC on the plates and 11-12VDC on the cathodes. Too low a voltage and the low end gets fatter but a bit mushy and less focused, but too high a plate voltage and it can get splatty and too pingy. I love articulation and clarity but want the distortion to be smooth.

There are a couple of things you might do to further fine tune the amp to get what you want from it.

Before suggesting or making further changes, however, please post a current photo of the amp and a table of both the preamp and power amp tube voltages. You said you have cleaned up a bit more. I think lead dress is very important to getting an amp to sound open and clear without crosstalk and a blurry or confused distortion. So let's start there.

Hey man, thanks for the info. I'll post some stuff tonight!

[R.G.]

Before suggesting or making further changes, however, please post a current photo of the amp and a table of both the preamp and power amp tube voltages. You said you have cleaned up a bit more. I think lead dress is very important to getting an amp to sound open and clear without crosstalk and a blurry or confused distortion. So let's start there.

Absolutely on target!

@ _ej_ :
There are a lot of things that can cause an odd sound in an amp without it being just obvious. Colossal is right - check the lead dress as well as the obvious stuff like output tube bias and balance, etc.

Here's a link to some stuff about wiring, what can go wrong and how to do it right. I don't know your tech level, but maybe it'll help.
I actually thought I'd write up a page or so of wiring tips and it grew to over fifty pages. this is a link to the much shorter version with just the pictures and a little explanation.
https://www.dropbox.com/scl/fi/soyg9csg ... zc44p&dl=0

Yell if you want the link to the full article.

[Guy77]

Finally got my Mojotone NC3015 build cleaned up and working well. I am hearing a distortion in the low end that isn't pleasant. Is this a typical EL84 tone or is there something I can do about it?

It's one of those things that would probably disappear in a mix but I can hear it and it bothers me. I'm sure you guys can understand

https://on.soundcloud.com/kbH2zXdw7wd8mpav8

Hi I thought your build sounded great too!!!
Great advise here from Colossal and R.G.

Another thing I look at a lot these days for unwanted noise remedy is shielded cable where its needed.
Its amazing what it can fix in terms of unwanted noises!

Also adjusting voltages as Colossal mentioned can often help get you the tone you are after.
Going with larger or lower values with the cathode resistors in the preamp along with trying different size cathode caps on those resistors can help you dial in the sweet spot too!

Cheers
Guy

[markusw]

Yell if you want the link to the full article.

Would be great to see the full article

[Stevem]

If it’s taking place when two different low notes are played then it could be high levels of IM distortion in the mix with the other things also.

[R.G.]

Would be great to see the full article

Sure!

https://www.dropbox.com/scl/fi/vpijj3al ... x3oay&dl=0

[Colossal]

Would be great to see the full article

Sure!

https://www.dropbox.com/scl/fi/vpijj3al ... x3oay&dl=0

Excellent paper. Thanks for writing and posting it.

[markusw]

Sure!

https://www.dropbox.com/scl/fi/vpijj3al ... x3oay&dl=0

Thank you very much!

[nuke]

I can hear the "raggedness" you're talking about. It's minor, but I hear it.

However, where it comes from.... could be anything. Could be the speaker, or the cabinet hardware, something in the amp construction, a tube that is microphonic, or some form of exotic distortion or oscillation.

Heck, the fireplace doors in my living room had me fooled into thinking I had an amp problem. Just the right note and I swear, it sounded like it was something electronic in the amp.

Time to divide and conquer. Plug another amp into the speaker/cabinet in question. Tap around with a chopstick to see if anything is slightly microphonic. Dummy load, signal generator and an oscilloscope to see if it is electronic in origin.

[_ej_]

I can hear the "raggedness" you're talking about. It's minor, but I hear it.

However, where it comes from.... could be anything. Could be the speaker, or the cabinet hardware, something in the amp construction, a tube that is microphonic, or some form of exotic distortion or oscillation.

Heck, the fireplace doors in my living room had me fooled into thinking I had an amp problem. Just the right note and I swear, it sounded like it was something electronic in the amp.

Time to divide and conquer. Plug another amp into the speaker/cabinet in question. Tap around with a chopstick to see if anything is slightly microphonic. Dummy load, signal generator and an oscilloscope to see if it is electronic in origin.

I have access to some very high-end scopes at work but I wouldn't even know where to start looking for this type of distortion. How would it appear on the scope? What's the best way to generate it in the first place?

[nuke]

I have access to some very high-end scopes at work but I wouldn't even know where to start looking for this type of distortion. How would it appear on the scope? What's the best way to generate it in the first place?

What I usually do is setup a dummy load resistor in place of the speaker and use a signal generator on the input of the amp. Usually about 200mv sine-wave signal at 1khz and put the scope on the dummy load.

The dummy load should be a non-inductive resistor of the same resistance as the speaker impedance (usually 4, 8 or 16 ohms). 8-ohm resistors of this type are fairly easy to find. Just set up the number of resistors in parallel/series to get the value and wattage you need.

Once you're setup, bring up the signal and observe on the scope. You should see a nice clean sine, and as you crank the amp up, you'll see the waveform start to clip. If the amp is a master-volume type with built in distortion, defeat the distortion function, or start the master at a moderately low value and adjust the preamp gain to get a good clean sine on the scope.

Crank up the master volume slowly and watch the power amp go up in power. You should see a nice clean since, which transitions to one with the peaks clipped a little bit as it hits maximum power, and continues deeper into clipping as the volume goes up. Also you'll probably see the waveform in the zero-crossing area show a bit of notch distortion.

What you're looking for are signs of oscillation, which frequently are the cause of "raggedness". A scope will reveal oscillations of this type.

The ubiquitous Fender Blues Juniors do this pretty commonly, the output waveform will have a high frequency wiggle on the edge of one or both sides of the clipping portion of the waveform. The fix is usually nothing more than moving the ribbon cables to the output tubes away from the output transformers.

[GAStan]

To add to @nuke's excellent write-up: You mentioned it's only at the lower frequencies you hear it so you might want to adjust the signal generator's frequency down from the suggested 1kHz accordingly. In fact if it's frequency dependent try at several frequencies. If you see something you're unsure of post pictures. We like pictures