Problem with this fizzy sound

[J M Fahey]

Let's not forget that *all* MV amps sound fizzy with said MV is set at low volume.
As an example, the JCM800, the first "famous" MV amp, was suggested to be set "on 6 and above" by Marshall themselves to avoid fizz.
And on, say, 1 or 2, itīs unbearably fizzy.
We are talking a World Class amplifier here !!

[Chuck H]

JPB and JM, Did you go to page one and listen to the sound file? It's not really a "fizzy amp" thing. It's more of a lingering crackle. I've heard it before. Long ago. I found it unacceptibly annoying then too. Now that I'm a little better at my avocation and have more bench tools I'd love to get one of these NightTrains on my bench and smash the problem once and for all. You really should listen to the sound clip.

[Silvio55]

I'd love to get one of these NightTrains on my bench and smash the problem once and for all.

That would be awesome Chuck! If that happens let us know please!

[Jazz P Bass]

Any chance of obtaining a good scope?
I really would like to "see" this fizzle.
If it only happens when the gain is past 10:00, it has to be in that circuit, no?
Or the increased gain is exasperating something before it.

[J M Fahey]

You beat me on this suggestion.
Agree that there's something strange in the sample, but my ears can not dissect it.
Looking at the waveform can be very useful, to imagine (and later find) where itīs coming from and why it appears.

[Pedro Vecino]

What I would do to try and see what happens (before any other aproximation) is to disconnect R38 (10K) and connect R27 (150K) directly to ground.
Regards

[Chuck H]

That's a good suggestion. I had considered it myself thinking there might be phase shifts within this loop causing minor oscillation problems. I didn't suggest it because I had a hard time seeing this circuits significance to the voicing switch and didn't want to disable it. Worth a try though.

[Steve Conner]

I'd love to get one of these NightTrains on my bench and smash the problem once and for all. You really should listen to the sound clip.

Well, who's going to step up and donate/lend one to Chuck?

[Silvio55]

What I would do to try and see what happens (before any other aproximation) is to disconnect R38 (10K) and connect R27 (150K) directly to ground.
Regards

Iīm definitely trying that! But yesterday one of my preamp tubes went bad (I hope not because of the mods,), so Iīm wating the 2 JJīs 12ax7 I ordered to arrive.

[Chuck H]

How did it go bad?

[Silvio55]

I just turned the amp on and it didnīt worked, no sound at all so I realised that V4 filament didnīt light up, I swaped the tubes to check if it was the tube or the amp, but it was that tube. It was one of the sovteks that came stock with the amp

[Chuck H]

That's a bad tube then. Nothing to do with mods though. The filaments don't care much what the rest of the tube is doing (within reason).

[Silvio55]

What I would do to try and see what happens (before any other aproximation) is to disconnect R38 (10K) and connect R27 (150K) directly to ground.
Regards

Is necesary to conect R27 directly to ground? because I see itīs still conected to R28 then ground (by the way I donīt get why R29 is not conected to ground directly and thereīs this R28, thatīs not a common thing I see in other schematics, I know very little about amps as you may see)

[Silvio55]

Well, the tubes arrived today so I did try disconnecting R38 and connecting R27 to ground. Guess what? nothing again!!!! The fizz is still alive and well.

[wokkel]

Hi all,

while no clear technical solution has been found I was wondering about the effect of speakertype.

I have a lil NT which I use with an EVM12L. Definately not a dark speaker. I have also used it with the Eminence speaker that resides in my Fender Blues deluxe.
This last speaker pronounces the fizz even more.
Is any of you using the NT with the dedicated Vox speaker cabinet with Greenback? Is this better at fighting the fizz?

Harald