Problem with this fizzy sound

[Silvio55]

Hi Wokkel! Yes, Sickman and I are using the NT matching cab loaded with the Greenback, I donŽt have other speaker to compare but I donŽt think the speaker would do much difference.

[Jazz P Bass]

180 posts & this amp still has not seen a scope.

[Chuck H]

184 posts

[Jazz P Bass]

An amp or a scope?

[sickman82]

Well, this thread died a death didn't it!

I've been really busy with work recently. However, I did manage to get the NT on the bench and under the guidance of my local amp tech, we modded the hell out of it. We weren't trying to deal with the fizz issue directly, it was more of a upgrade/improvement task. I replaced most of the components in the signal path for better quality parts, lowered the voltages on the EL84's, made some tweaks to the circuit to get it to a place I had imagined it could be. All in all, I am over the moon with it. It sounds first class, has far more punch and is pronounced and authoritative - all without changing the overall character of the amp. With the tweaking, I got a bit more bottom end out of it.. we went too far initially, but with a bit of experimentation got it to the right spot. It also takes pedals so much better now. Just as an illustration (I would never have the amp set like this), I cranked the gain on the amp, plugged in my guitar to a Big Muff with its volume and sustain cranked up full as well - guess what? The Night Train took it, and apart from the obvious NOISE, it sounded great.. this is the amp that only four or less weeks ago couldn't take a Big Muff with the amps gain set past 11 o'clock and conservative settings on the BM. Your guess is as good as mine here, I haven't added any grid stoppers yet (although I do intend to). I also bought a new cab, a closed back 2x12 with Vintage 30's.. I removed one V30 and put the stock Greenback in. Sounds great and gives the amp a lot more girth.

The only thing I need to add is the choke, it was supposed to arrive this week but didn't (that's Royal Mail for you)- cheers for the tip on Bluebell Audio Pete, they are great people up there. So, you're all wondering if the fizzy decay is still there aren't you?! Well, yes it is.. perhaps not quite as pronounced as before but that may well be what I want to hear rather than what I am actually hearing. I have resigned myself to the fact that it is a design flaw with the amp. My amp tech (and I believe Pete, you picked up on this in the beginning) thinks it is down to a poor master volume. I don't know, he couldn't get his head round it and I am sick of trying to work it out. The fact is, when the amp is cranked, I can't hear that annoying fizzy decay. I bought this amp as a gigging workhorse and it does that job well.. even better now. After spending about Ł70 on parts for the amp, a year and four months after buying the damn thing.. I am finally happy with it!

Cheers for your all help guys, you have all been stellar forumites and have been an integral part if this journey. If anybody wants help with modding their NT, give me a shout at - dannyboyee at hotmail dot com - and I will gladly do what I can. For now, I'm off to play guitar! Cheers folks!

[Silvio55]

Yes, I think this issue will remain unsolved for this time . IŽm quite happy how my amp sounds now, itŽs hard to describe but after all the mods it sounds with more "balls", but still with the same character, I really like it!
IŽm sorry if itŽs an off topic now but I wandered whatŽs the porpouse of R35 that goes between the tone stack and the gain pot, cause I donŽt see this resistor in others classic amps schems, diagrammatiks said "R35: parallel with 220pf remove or increase for more upper harmonic distortion", I donŽt get the part of "remove or increase" maybe itŽs remove the resistor or increase the cap value?

[Chuck H]

If the amp is too hashy and bright remove the 220pf cap. If you want more mids increase the cap value (390pf to 1n). If you want the amp to be a little brighter with less low end gain increase the resistor value. Do not remove the resistor.

[Silvio55]

If the amp is too hashy and bright remove the 220pf cap. If you want more mids increase the cap value (390pf to 1n). If you want the amp to be a little brighter with less low end gain increase the resistor value. Do not remove the resistor.

Thanks Chuck! ok I got it now, but why many amps doesnŽt have this resistor before the gain pot?

[Chuck H]

Many do. It's just one of many design possibilities.

[Ross Grey]

I have tried a bunch of re-voicing mods to address the frizzy decay problem and found that the best I could do is mask the problem for some tone and volume settings. It never really got fixed.

Then I noticed that the phase inverter is getting an input from the cathode of the first preamp stage and started looking into the possibility that the frizz was introduced by the PI and two inputs that were going to be different since they come from different preamp stages.

There has been some discussion about this already, but in my limited understanding, it looks like the PI could operate as a floating paraphase, so I removed R38 to see what would happen when the first preamp cathode input was eliminated. Things still seem to work correctly and frizzy decay is reduced a bit, so I decided to take this further.

Next I tried increasing the values for R28 and R31 thinking that this would reduce the bias voltages (warmer biasing) and reduce the current in the PI while still retaining some balance in the PI output. I roughly doubled these values which I have at 4.7k and 100k respectively.

This may have really distorted the PI balance, but now I have distinct ranges on my gain control where the amp is clean, gets warm distortion and then gets more fuzzy distortion as gain is increased and tubes go into cut-off. This seems to have addressed most of the frizzy decay problem in my amp. See the current configuration below.

I wish I had access to an oscilloscope, but for the time being I'm going to work with this setup until I learn enough to decide were to take this project.

Configuration disclosure:

TYPICAL UPGRADES

Sozo caps in the signal path
10 henry choke for R1
JJ EL84s
Mullard for the preamp tube
R7 and R8 have been replaced with equalivalent value metal oxide film resistors

MODIFICATIONS

C27 removed to take out 'ice pick' highs
C21 changed from 1uf to 33uf to emphasize low frequencies
C7 changed from 2.2uf to 2.2uf to emphasise low frequencies
R9 and R26 changed from 220k to 100k carbon composition
C15 and C30 have been removed and jumpered

MODIFICATIONS now being evaluated for removing 'frizzy decay'

R38 removed to eliminate first preamp input to the phase inverter
R28 changed from 2.7k to 4.7k
R31 changed from 56k to 100k

[Ross Grey]

Clips after PI modifications in my previous post

I don't have an oscilloscope, but I have a cheap mic that is good enough to detect frizzy decay.

Here are some clips made after my most recent attempt to address frizzy decay.

Ross

[Ross Grey]

Ron reminds me that this is not yet Metal Territory. He's right of course.

Ok. This is simply a higher level of gain where some amp stage apparently goes into cut-off and the OD starts to get more brassy/fuzzy.

The point is, that with this current mod, I don't have the funky frizzy decay anymore. I started the clips with some harmonics because it has always been easy to hear frizzy decay on this amp when playing multiple harmonics.

These clips were made with no effects, a single coil pickup at the neck, bright mode and 15w on the amp, guitar controls up full and amp tone controls all at 11-o'clock.

[Silvio55]

Hi Ross! I've been dealing with the fizzy decay for some time until I gave up, probably youŽve read the hole thread and know what I'm talking about! It's fantastic to hear that you've nailed the problem, you did a lot of of tweaking, but I wanted to know: the mods done to fix the "fizzy decay" problem are just these?
MODIFICATIONS now being evaluated for removing 'frizzy decay'

R38 removed to eliminate first preamp input to the phase inverter
R28 changed from 2.7k to 4.7k
R31 changed from 56k to 100k

What volume setting did you use to make the sound clips? Because I have the fizzy sound only with low volume setting (less than 12 o'clock).
Wich value did you use for C7? 'cause you said you changed a 2.2 uf for a 2.2 uf cap!
Thanks!

[Ross Grey]

Silvio,

Yes, I have made a lot of mods, especially to the preamp voicing when I thought that this would fix the frizzy decay -- I probably returned to a 2.2uf cap for C7 after trying a bunch of stuff. Sorry for the confusion.

In spite of the large number of mods I have made, the three you identify are the ones that I believe address the frizzy decay. Here is my thought process.

Removing R38 eliminates a signal path from the cathode of the first preamp stage to the phase inverter. This was probably done because it added interesting harmonics for clean drive, but it actually causes the signal from the first preamp stage to be mixed with the (primary) PI input from the second preamp stage. This looked like a likely place to create frizzy decay because once either preamp stage starts to distort, the mixed signal could distort in undesirable ways -- perhaps frizzy.

Removing R38 took care of most of the problem for me, but when i increased the values of R28 and R31 to reduce the gain of the PI stage, I was very pleased with the result. I now have very nice clean, crunch and OD sounds at any level.

The settings for these recordings were with an unmodified strat, neck pickup, amp tone controls straight up. The volume was at 9-oclock for clean, about 10-oclock for 'crunch', 12-oclock for 'crunchier' and 2-oclock for the file labeled 'metal territory', which was just crunchier yet.

For the complete tally on my mods, I need to get the amp back from a friend to confirm and document, but I strongly suspect that frizzy decay is due primary to the mixing of the first and second preamp signals in the input of the PI and not really fixed by my other mods.

In case you are concerned that dropping the gain of the PI would affect the volume or over drive sounds, I would say that these changes do not noticeably cut the output of the amp and say that the OD sounds with the 'thick' setting are truly outstanding rock sounds, even at low output levels.

As a practice amp, the Vox is now my favorite out of the four that I own. The only mod I would consider now is a control to drop the pickup gain into the first preamp stage to better match the amp with effects boxes, and higher output pickups.

[Silvio55]

I remember that I once removed R38, and still got the fizzy thing, so I resolder it, but what the hell, IŽll try it again and hear more carefully. When you say you changed the setting of the volume knob for the clean, crunch, crunchier and metal, are you talking about the master volume or the gain?