18 watt fizz: please help me diagnose this?

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TomOlsen1
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18 watt fizz: please help me diagnose this?

Post by TomOlsen1 »

Hi Guys!

I recently completed a liteIIb kit with (V)VVR.

First off, the build was a lot of fun, and getting it working gave me a great sense of satisfaction. Steve was, and has continued to be, super helpful and very quick to respond regardless of how small the question was.

However, I suspect I am getting the dreaded 18 watt fizz. I have made some clips, and have also mailed these to Steve. He's sent me a few suggestions to firm up the sound, including the huge cap trick among others.

Is this the sound of the Paul Ruby fizz?

All clips were recorded with a SM57 into a behringer desk straight into my soundcard using Reaper. Mic is off axis.

Cab is a 2061cx, guitar is a PRS custom 22, bridge HB.

Rock on,
Tom

For some reason, I get this message when I try attach a file of only 622KB:

"Sorry, but the maximum filesize for all Attachments is reached. Please contact the Board Administrator if you have questions.2147482127+635971+2147483647-"

Admins - pls help?
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Post by Colin_D »

If you have the clips handy you could post them to stashbox or dropbox very quickly. Then just post the links here so the forum can hear them.
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Post by zaphod_phil »

FYI the admins have cleared some storage space. So hopefully, we should shortly be able to resume uploading of files - I hope.....
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Clips

Post by TomOlsen1 »

Thanks guys for the quick fix! Right... here we go:

Apologies for the use of the Seymour Duncan demo riffs, needed something that would cover all the bases in less than 30 seconds.

The sound comes in over the open E chord at the end of the clip.
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Post by TomOlsen1 »

Also, Steve has recommended I try taking the VVR out and connecting directly from the EZ81 to B+. I'll try this tomorrow and report back if there is any change.
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fizz

Post by Gary »

I'm not qualified to give a technical answer, but when I had a ceriatone 18 watt tremolo clone with London Power PS installed with PPIMV, I found I had to carefully reduce the PPIMV to get rid of exactly the same kind of fizz on the decaying note.

Will await the techie answer with interest, not least because I hope to add Dana's vvr to my Valvepower kit shortly. Perhaps the PI & PA scheme would be an improvement?
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Re: fizz

Post by zaphod_phil »

Gary wrote: Perhaps the PI & PA scheme would be an improvement?
Which raises the question, has the VVR voltage scaling been applied to the whole amp in TomOlsen's case? PI + PA only generally works better.
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Post by katopan »

Just to clarify, VVR on just the power stage can get fizzy at high settings on the volume control if you don't have a PPIMV to wind it back a bit. VVR on the whole amp actually has an effect of reducing the amount of drive to the power valves available as you wind down the VVR. PI/PA VVR seems to get the right balance. Anyway, my point is that if the whole amp is VVR'd then it won't add to the fizz unless there's instability problems with the VVR wiring/layout (that has turned up occassionally).

Sorry to clarify the obvious, but the clips sounded like the problem was there at both settings?
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Post by TomOlsen1 »

Hi guys

@ZP: The whole amp is scaled. I tried connecting directly to b+ and still had the same fizz, so I don't think the issue is the VVR. How would the PI+PA (I assume this means phase inverter and power amp?) be an improvement over scaling the whole amp? Also, I take it that PPIMV is pre-phase inverter master volume?

@Katopan: Correct, the problem is present in both clips. I also recorded clips with the gain at 5, and the fizz is not present in those.

@Gary: you've got me thinking, will elaborate shortly.

Over the weekend I took my amp to Bill Lange, a builder and tech with a passion for cathode bias zero negative feedback designs, focused more on the Fender side of tone. Bill has a freqeuncy generator and scope, so is equipped with the right diagnostic tools.

We haven't investigated for the Paul Ruby crossover distortion problem, but Bill did find a certain amount of phase invertor clipping. Is this normal in the 18 watt?

Anyway, Bill ran the amp through the frquency spectrum, and was quite impressed with the amps response, although he did mention that the high end cutoff was at a far higher frequency than would be found in a fender or modern marshall, and maybe we could tweak that if desired.

Then we plugged the amp into a 2x12" with V30s and used his tele to drive the amp. It sounded fantastic, which was surprising considering that the V30 isn't considered the best match for the 18 watt. Bill was also extremely impressed with the VVR, and has been bitten by the bug. (He also mentioned that this may be the first Marshall amp he's liked)

But, no fizz. Although, we did only plug in via input 1, so no parallel operation. Needless to say, I was stumped. We boxed the amp up, and I went home. Once there, I plugged my PRS and my 2061cx cab and..... the fizz was back.

The two differences were either the cab, or the guitar. I connected my laney VC-15 to the cab, and no fizz of any sort. So it had to be the guitar. I believe that the hotter pickups in the PRS are pushing something in the amp outside of ideal conditions, and when I roll my guitar's volume pot back a notch the fizz disappears.

Our next step is going to be to measure the signal strength put out by my PRS, and pump a low E from the frequency generator at the same signal level, and try measure where the undesired clipping is taking place. Gary, this is why your comment on the PPIMV is interesting, something is being pushed too hard?

I also have clips of the lower gain levels and the Laney. I can post them if they'd help?
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Post by zaphod_phil »

TomOlsen1 wrote:@ZP: The whole amp is scaled. I tried connecting directly to b+ and still had the same fizz, so I don't think the issue is the VVR. How would the PI+PA (I assume this means phase inverter and power amp?) be an improvement over scaling the whole amp? Also, I take it that PPIMV is pre-phase inverter master volume?
Many amps get "brown" as you reduce the preamp voltage. I believe katopan actually meant a post-PI MV. Personally I don't like the ambiguity of the "PPIMV" acronym.
TomOlsen1 wrote: I also recorded clips with the gain at 5, and the fizz is not present in those.
That sounds even more like it's probably the Paul Ruby buzz.
TomOlsen1 wrote:...focused more on the Fender side of tone.
Well no one's perfect. :lol:
TomOlsen1 wrote:We haven't investigated for the Paul Ruby crossover distortion problem, but Bill did find a certain amount of phase invertor clipping. Is this normal in the 18 watt?
Yes, there will be some phase invertor clipping, but not nearly as much as in a Vox/Matchless type amp. Also keep in mind that crossover distortion is an essential part of the 18W's rich overdrive tone. The buzz problem is more about a bias voltage shift and blocking distortion.
TomOlsen1 wrote: Bill ran the amp through the frquency spectrum, and was quite impressed with the amps response, although he did mention that the high end cutoff was at a far higher frequency than would be found in a fender or modern marshall, and maybe we could tweak that if desired.
That shouldn't need to be tweaked. It's what gives the classic 18W crisp definition and bite. Explain to Bill that the Marshall 18W design uses a budget hi-fi OT. And it ain't no stinkin' Fender. :twisted:
TomOlsen1 wrote: (He also mentioned that this may be the first Marshall amp he's liked)
And one of the only Marshalls not to be based on the Fender Bassman architecture. :wink:
TomOlsen1 wrote:The two differences were either the cab, or the guitar. I connected my laney VC-15 to the cab, and no fizz of any sort. So it had to be the guitar. I believe that the hotter pickups in the PRS are pushing something in the amp outside of ideal conditions, and when I roll my guitar's volume pot back a notch the fizz disappears.
You should have also tried your 18W through the Laney's speaker for a true comparison. However, what you're saying suggests that this is indeed a Paul Ruby buzz issue, which is caused by the bias shift and blocking distortion when the EL84 grids are being driven too hard. However, the zener mod may not work when you scale the voltage down using the VVR. A couple of alternative fixes are (a) to use a huge cathode cap (1000uF to 2200uF) to limit transient bias voltage fluctuations and/or (b) lower grid reference resistor values on the EL84s, like 330k or 220k. However, the resistor change will also make the amp breakup later, while the cap change will make the amp feel a bit stiffer. So take your pick. :)
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TomOlsen1
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Post by TomOlsen1 »

First off:
:rol:

Thanks ZP, I'll try each of those out see how I like them. If I use the lower grid reference resistors, will the fizz come back if I hit the amp with a booster?

Also, with regard to the Paul Ruby fix, I am toying with idea of identifying the zener values required when the VVR is at 25%, 50%, 75% and 100% and wiring these up to a rotary switch on the back. What do you think?
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Post by JdJ »

TomOlsen1 wrote:The two differences were either the cab, or the guitar. I connected my laney VC-15 to the cab, and no fizz of any sort. So it had to be the guitar. I believe that the hotter pickups in the PRS are pushing something in the amp outside of ideal conditions, and when I roll my guitar's volume pot back a notch the fizz disappears.
This sounds exactly like what I have been struggling with as well - check out the end of my thread here although I may end up posting relevant info here if no one minds.

I have yet to try a different value for the PR mod zeners, right now I have 15v and my cathode voltage is 12.9 which seems right in the sweet spot, but it is not solving the problem. I was going to try a much lower value just to see if it makes a difference but haven't had the time to yet. I also am using 1/2 watt zeners (which I think PR used in the original paper) but I see other people using 1 watt. Hopefully this isn't the reason I don't hear a difference. It does make sense to me that the increased output of my humbucker equipped custom (not unlike like a PRS or Fender/Baker Robben Ford) would cause the bias shift and resulting distortion - so it does make sense to me that what we're dealing with is the same as in PR's paper. But why then are my 15v zeners/4007's not taking care of the issue?

For the record I also have tried as much as a 1500uF bypass cathode cap with no audible change to the blocking distortion. I am now running the EL84s at right above 80% dissipation (using a 200ohm cathode resistor). I do not have any MV or power scaling, but the unpleasant distortion does clean up when I roll back the volume on the guitar or on the amp. I'm glad there are more than one of us with this problem as I am anxious to figure this out!

Cheers,

Josh
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Post by ontariomaximus »

G'Day all,

re: the Paul Ruby buzz.

I am planning a few 18W builds in the near future but with fixed bias. Every other amp I have built with 6V6's, EL34's, or 6L6GC's have always sounded better with fixed bias. The sound comes from a blacker, velvety background is one way to describe it.

Would fixed bias have any technical advantage in the Ruby buzz area?
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Post by JdJ »

ontariomaximus wrote:
Would fixed bias have any technical advantage in the Ruby buzz area?
My understanding (as slight as it is...) is maybe... It depends on whether your grid voltage exceeds the cathode voltage. Under these conditions (hot enough signal from pi for instance) is when the coupling caps charge from the grid current flow causing the unwanted effect of colder bias and unwanted cutoff. These conditions can be met in either fixed bias or cathode biased amplifiers.

Did I get it?! 8O

-J
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Post by zaphod_phil »

JdJ wrote:I have yet to try a different value for the PR mod zeners, right now I have 15v and my cathode voltage is 12.9 which seems right in the sweet spot, but it is not solving the problem. I was going to try a much lower value just to see if it makes a difference but haven't had the time to yet. I also am using 1/2 watt zeners (which I think PR used in the original paper) but I see other people using 1 watt. Hopefully this isn't the reason I don't hear a difference. It does make sense to me that the increased output of my humbucker equipped custom (not unlike like a PRS or Fender/Baker Robben Ford) would cause the bias shift and resulting distortion - so it does make sense to me that what we're dealing with is the same as in PR's paper. But why then are my 15v zeners/4007's not taking care of the issue?
If the zener mod doesn't fix the problem, then it's almost certain what you have isn't the same issue that Paul Ruby is talking about. There are other things which could be causing this type of problem, such as ultrasonic parasitic oscillations in the amp causing blocking, or tubes that aren't too good. Also have you tried reducing the value of the 470k grid reference resistors or increasing the size of the 8.2k grid blocking resistors?
ontariomaximus wrote:I am planning a few 18W builds in the near future but with fixed bias. Every other amp I have built with 6V6's, EL34's, or 6L6GC's have always sounded better with fixed bias. The sound comes from a blacker, velvety background is one way to describe it.

Would fixed bias have any technical advantage in the Ruby buzz area?
The huge cathode cap trick also makes the amp behave a bit more like a fixed biased amp, since bias voltage fluctuations become more gradual. Fixed bias should certainly kill the classic 18W buzz problem, since there's no shift in bias voltage with increased cathode current, and also because you have to use lower value grid reference ("grid leak") resistors. However, the latter also means that the amp will generally run cleaner, and won't have quite as much of the classic 18W grind.
JdJ wrote:Under these conditions (hot enough signal from pi for instance) is when the coupling caps charge from the grid current flow causing the unwanted effect of colder bias and unwanted cutoff. These conditions can be met in either fixed bias or cathode biased amplifiers. Did I get it?! 8O
Nearly, but not quite. The bias gets colder primarily due to increased current flow through the cathode resistor - which you don't have in a grid-biased amplifier. :)
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Post by ontariomaximus »

zaphod_phil wrote:
JdJ wrote:I have yet to try a different value for the PR mod zeners, right now I have 15v and my cathode voltage is 12.9 which seems right in the sweet spot, but it is not solving the problem. I was going to try a much lower value just to see if it makes a difference but haven't had the time to yet. I also am using 1/2 watt zeners (which I think PR used in the original paper) but I see other people using 1 watt. Hopefully this isn't the reason I don't hear a difference. It does make sense to me that the increased output of my humbucker equipped custom (not unlike like a PRS or Fender/Baker Robben Ford) would cause the bias shift and resulting distortion - so it does make sense to me that what we're dealing with is the same as in PR's paper. But why then are my 15v zeners/4007's not taking care of the issue?
If the zener mod doesn't fix the problem, then it's almost certain what you have isn't the same issue that Paul Ruby is talking about. There are other things which could be causing this type of problem, such as ultrasonic parasitic oscillations in the amp causing blocking, or tubes that aren't too good. Also have you tried reducing the value of the 470k grid reference resistors or increasing the size of the 8.2k grid blocking resistors?
ontariomaximus wrote:I am planning a few 18W builds in the near future but with fixed bias. Every other amp I have built with 6V6's, EL34's, or 6L6GC's have always sounded better with fixed bias. The sound comes from a blacker, velvety background is one way to describe it.

Would fixed bias have any technical advantage in the Ruby buzz area?
The huge cathode cap trick also makes the amp behave a bit more like a fixed biased amp, since bias voltage fluctuations become more gradual. Fixed bias should certainly kill the classic 18W buzz problem, since there's no shift in bias voltage with increased cathode current, and also because you have to use lower value grid reference ("grid leak") resistors. However, the latter also means that the amp will generally run cleaner, and won't have quite as much of the classic 18W grind.
JdJ wrote:Under these conditions (hot enough signal from pi for instance) is when the coupling caps charge from the grid current flow causing the unwanted effect of colder bias and unwanted cutoff. These conditions can be met in either fixed bias or cathode biased amplifiers. Did I get it?! 8O
Nearly, but not quite. The bias gets colder primarily due to increased current flow through the cathode resistor - which you don't have in a grid-biased amplifier. :)
If fixed bias runs a little cleaner before breakup that's fine, and may even be a benefit. Once the volume controls hits 11 o'clock though there will be the wonderful 18W clipping. I am sure of that.
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Post by JdJ »

zaphod_phil wrote:If the zener mod doesn't fix the problem, then it's almost certain what you have isn't the same issue that Paul Ruby is talking about.
I was afraid you were going to say that... I'm starting to wish I had a scope!
zaphod_phil wrote:There are other things which could be causing this type of problem, such as ultrasonic parasitic oscillations in the amp causing blocking, or tubes that aren't too good. Also have you tried reducing the value of the 470k grid reference resistors or increasing the size of the 8.2k grid blocking resistors?
I haven't tried either yet- I see the max series resistance on g1 is 300k so I should be able to raise the blocking resistors without being worried, right?

I am picking up some RF on the tremolo input - should I try more blocking resistors on v3 right at the pin, or shielded cable? I have 22k at g1 of v1 with shielded cable and there is no RF - although there is a whoosh which I assume is from the resistors.

I'll try the blocking or grid ref resistors and post back.
zaphod_phil wrote:Nearly, but not quite. The bias gets colder primarily due to increased current flow through the cathode resistor - which you don't have in a grid-biased amplifier. icon_smile.gif
I'll take a nearly. 2 weeks ago it would have been a "whaaa?"

Thanks for the suggestions!

-Josh
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Post by zaphod_phil »

JdJ wrote:
zaphod_phil wrote:If the zener mod doesn't fix the problem, then it's almost certain what you have isn't the same issue that Paul Ruby is talking about.
I was afraid you were going to say that... I'm starting to wish I had a scope!
When I first heard the kind of buzz Paul Ruby identified, I thought something like a tube retainer inside the amp was rattling, it sounded so mechanical. That's the kind of sound you should be looking out for. Otherwise, it's probably something else.
JdJ wrote:I see the max series resistance on g1 is 300k
That's for fixed-bias operation. In cathode-bias you can go a lot higher.
JdJ wrote:I am picking up some RF on the tremolo input - should I try more blocking resistors on v3 right at the pin, or shielded cable?
Definitely shielded cable.
JdJ wrote: although there is a whoosh which I assume is from the resistors.
Why - are you using carbon comps? If not it may be a high frequency oscillation problem.
Last edited by zaphod_phil on Mon 01/18/10 4:55 pm, edited 1 time in total.
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Post by katopan »

zaphod_phil wrote:I believe katopan actually meant a post-PI MV. Personally I don't like the ambiguity of the "PPIMV" acronym.
Yes, that's what I meant. I'd never thought before about how it could also mean pre-PI MV. :roll: Will have to make that clear in the future. :)
JdJ wrote:Under these conditions (hot enough signal from pi for instance) is when the coupling caps charge from the grid current flow causing the unwanted effect of colder bias and unwanted cutoff.
That IS the mechanism that the Paul Ruby fix is addressing. BUT what Phil described is what would be improved with fixed bias (or a bigger cathode cap). Two different mechanisms that cause the same temporary cooling off of bias.

I assume that with VVR up full, and using the guitar in question, can you have the guitar volume up full and still get a nice clean sound at reasonable volume just by using the amp volume?

I know that EL84s start to sound fizzy when they are overdriven way too much (normally more than what they are in an 18W). But I'm starting to wonder if there's some valves out there that can't handle the 18W amount of overdrive. If the zener fix and large cathode cap don't fix the fizzy problem, and assuming the issue is in the power stage then there's not a lot left..... Unless it's in the PI.
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Post by JdJ »

zaphod_phil wrote: Why - are you using carbon comps? If not it may be a high frequency oscillation problem.
They are 22k carbon comps.
zaphod_phil wrote:Definitely shielded cable.
A la Richie's B I presume...
katopan wrote: I assume that with VVR up full, and using the guitar in question, can you have the guitar volume up full and still get a nice clean sound at reasonable volume just by using the amp volume?
That is the case for me, but I don't have a VVR, that's Tom the OP.

I have tried both Mullard RI (one is microphonic) and Some NOS Russian EL84's I got from Terry - now that I think about it, one of those is microphonic too. Both sets follow the tube as I've tried them in both sockets.

-Josh
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