Problem with this fizzy sound

[Chuck H]

There is also the unfortunate possibility that it's a parasitic oscillation due to layout. This would be virtually unfixable without seriously altering the tone of the amp.

To change the bias on the PI just change R18 (1.5k) up or down a little. Maybe try 1k and 2.2k there and see if there is any change.

[sickman82]

Ok, that sounds good Chuck. With regard to altering the bias of the PI, is there anything I should be aware of that may damage the circuitry? I mean, will it send the rest of the amp off balance? In terms of longevity of the amp, can I do any damage by changing the bias is basically what I am asking.

I have a funny feeling that it might take some serious modifications to get rid of this problem. Altering the tone of the amp isn't something I am interested in doing.. I'd rather live with the issue to be honest. In a band situation I never notice it anyway. It's just annoying when you are at home, trying to play, and you constantly hear this fizzy decay. There will be a point at which I will say enough is enough though.

[pdf64]

I was thinking that the design is asking for oscillation issues, as the signal goes back to a shorting contact on the input jack socket after 2 inverting stages and potentially masses of gain, ie prime conditions for oscillation. It's an overdesigned input mute function, intended (I guess) to cut hiss at high gain settings when the input lead is removed; normally, the input jack switching just shorts out the input in that condition.
Some 80s Fender amps had a similar arrangement, but the signal was tapped back to the shorting contact after just the 1st stage.
Regarding the added grid stopper and necessity to cut the PCB track to the grid, yes, that would normally have to be done, otherwise the track will just short out the resistor. Should be easy to check with a meter; just measure the resistance of the added grid stopper, it will either be a short or the resistor value.
Pete.

[Chuck H]

Since the FB mute circuit serves no purpose when an instrument is plugged in (and I assume you won't be using the amp without an instrument plugged in) and the only consequence if that circuit weren't there is a little more hiss when no instrument is plugged in, you could open the trace for just that circuit as close to the tube pin as possible. That would eliminate the possibility of oscillation due to that circuit. This is hard to undo should you find there is no consequence to having the circuit intact though.

[Silvio55]

I was thinking that the design is asking for oscillation issues, as the signal goes back to a shorting contact on the input jack socket after 2 inverting stages and potentially masses of gain, ie prime conditions for oscillation

So Pete, do you think that fizzyness can be caused by oscillation? I donīt care for the extra hiss when the guitar is unplugged, and if that doesnīt solve the problem, with a drop of soldering I can restore the FB mute circuit if I cut the trace.

[Chuck H]

I might go to work removing everything un-guitar amp like about the circuit. This has worked for me in the past with Messy Booger amps. WRT guitar amps it seems (at least to me) that it never pays to over complicate the circuits.

[Silvio55]

Ah, ok Chuck, Iīm a newbby with the amp modding thing, but I also prefer to keep things simple, and not only with guitar amps. Other observation, in this amp ALL the leads (coming from the PT, switches, OT, etc.) that are plugged to the PCB are twisted together, can it cause oscillation too?

[Chuck H]

in this amp ALL the leads (coming from the PT, switches, OT, etc.) that are plugged to the PCB are twisted together, can it cause oscillation too?

Probably not. The really sensitive stuff is the preamp circuitry. If there are long leads (or traces) that get near or pertain to the preamp, this is where any real concearn would be. Unless I heard excessive hum I probably wouldn't worry about wire bundles with OT and PT leads twisted together. BUT... If this bundle runs in any proximity to the preamp OR there are any associated traces that run near the preamp OR there are any preamp traces that run close to any of the bundled circuitry, (whew!) that may be cause to suspect oscillation.

[Silvio55]

Probably not. The really sensitive stuff is the preamp circuitry. If there are long leads (or traces) that get near or pertain to the preamp, this is where any real concearn would be. Unless I heard excessive hum I probably wouldn't worry about wire bundles with OT and PT leads twisted together. BUT... If this bundle runs in any proximity to the preamp OR there are any associated traces that run near the preamp OR there are any preamp traces that run close to any of the bundled circuitry, (whew!) that may be cause to suspect oscillation.

OK! well, the first thing Iīm trying will be cutting the feedback trace of the "over complicated" mute circuit, I have located the trace in the PCB already, so I think tomorrow I will post the results if I have time to put the amp together again.

[sickman82]

I'm looking forward to hearing the results Silvio. You're more brave than I! Good luck. I'll get to work on lifting those film caps in the power supply tomorrow, I'll let you know how that goes. I'll also check out those grid resistors if I get chance today.. But I can hear them working, blocking distortion is reduced and it has definitely rolled off a good amount of high end, but maybe I am imagining it.

[pdf64]

'do you think that fizzyness can be caused by oscillation?'
It didn't sound like that to me, but go for it just in case.

[Silvio55]

Well I cut the PCB track of the Mute FB but it didīt made any diference with the fizz, everything was the same, so I put a drop of soldering to repair the track just in case and now weīre back where we started. Iīm ordering the resistors and caps this week to make the other mods.
Sickman, I think itīs all up to you for this days Iīll be wating your posts, good luck man!!!!

[Silvio55]

Trying to find some mods for the night train that take the fizz away I found a thread in another forum (I donīt know if I can link to that page in this forum), but did find something interesting. The "Jule Mod" (Jule is an amp manufacturer and modder) seems to take care for the fizzyness in this amp, he adds a choke and make some changes in the circuit, couldnīt find what this mod really is or what he changes especifically, but this may be interesting:

"The focus is to open up the amp. There are a bunch of second order harmonics waiting in the amp to be heard. Unleashing them is not terribly sophisticated electronically, but does require some changes to the preamp, power supply and the output section. I add some capacitance to the Plate supply and I add inductance as well.

Truth is, the mod is more about the playing and the hearing than it is about components. It becomes a greatly more "touch" sensitive amp. It becomes a more open sounding amp."

Saddly I donīt have that technical knowledge to fully understand what that means, may some of you guys could help?
It will be awesome if we could know the changes he makes in the circuit!

[sickman82]

Ok, so this morning I got to work on the amp. First thing I tried was to measure the grid resistors.. no joy, they were shorted. Pete, Silvio, you were right, you need to cut the trace there

Now, without mentioning any names, as I appreciate that he will more than likely want to remain anonymous.. I have been in touch with somebody who has a great deal of experience with the Night Train. We specifically talked about the problems we are having with the fizzy decay, and the bad interaction with certain pedals.

Firstly, he doesn't think that the fizzy decay is down to clipping. He suggests that it is oscillation, but as of yet he hasn't really given me any clues as to how to fix it. He only suggested I test the amp with an oscilloscope to determine for sure that it is oscillation. He has implied something more by saying that there is too much tone shaping going on in the amp, and for our purposes, less tone shaping is better. He doesn't seem to want to give away too much here, I'm not so sure why, but hey.. I'm not about to force information out of him.

Secondly, he suggested that for better interaction with pedals, we should alter the input stage of the amp.. which I have done this morning. Basically, rather than going through a capacitor and the three resistors (if you include the 1M to ground), just have the input going through a 68k straight to the first stage. So, in other words, remove R20, R22, C19, replace R21 with a 68k resistor and bridge the gap where R22 was.

If you take a look at any Vox AC30 schematic, any AC15 schematic, and indeed I presume most other amps (I checked out a couple of Marshall schematics too), this seems to be common practice. Most amps seem to have input, through a 68k resistor, straight to the grid of the first stage. I wonder why they placed what seems to be another over-complicated circuit at the input? Is this what my 'mystery guy' means by too much tone shaping?

Thirdly, he mentioned the copper strip that I have on the PCB of my amp. It runs about 1/3 of the length of the PCB, and is located in the pre-amp section. This is apparently a late addition by Vox, and allegedly it was included to tame the exact problem we are having, the fizzy decay. The first Night Trains to be sold didn't have this copper covered strip of stuff. Silvio, if you don't have this in your amp, let me know. I'll give you as much information as I can so you are able to fashion this out of something for yourself. Here is a quote from the guy himself..

There is an immediate determination you can make in the area of fizz, and that is to see if your Night Train has been upgraded by VOX with a strip of shielding over some of the circuitry in the Pre-Amp area of the pcb. Early Night Trains did not have this shielding, but VOX found it was needed for current models.

It is some kind of shielding, obviously. But what is it shielding against? This may be a big clue.. but I simply don't have the knowledge to fully understand the implications of this kind of shielding. Either way, it doesn't get rid of the problem, I suspect it just keeps it under control. Having never played one of the early Night Trains, I have no way of comparing the two.

Next up.. lifting those film caps. I'll report back soon once I have tested it out. Cheers guys.

[Steve Conner]

I've heard it claimed both ways: removing the DC block capacitor from the first stage will make interaction with pedals better. Or other people say it makes it worse. One thing is for sure, if you remove it, then some preamp tubes will cause the volume pot on your guitar to make scratchy sounds when turned. It doesn't perform "tone shaping".

The copper strip sounds like a fix for oscillation, by adding capacitance to ground, or screening that area of the PCB against some internal wiring that might be lying around in the neighbourhood.

Some amps sound fizzy by design. It was the designer's idea of a good tone. Also, it is hard to get a lot of gain in a few tube stages without introducing fizz. New strings can help, as the harmonics from them mask the fizz.

The guy sounds like a typical cork sniffer.