Help improve pp-18 tone

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Bronsboi
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Post by Bronsboi »

I have built a lite 2b with VVR on PA and PI. Did anyone get round to building and testing the "variable ruby mod" mentioned earlier in the thread?
Just curious as I also get the fizz with the VVR turned down but not when its turned up full.
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Post by katopan »

I don't think anyone got to try it.
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Post by katopan »

Someone on The Amp Garage was having this problem where the Ruby mod fixed the fizz problem at high VVR settings but became ineffective as the VVR was turned down. After trying out a number of different suggestions I was going to post the variable zener circuit which I'd put up in this thread, saying it was very much unproven. Before getting that far he tried zeners with much smaller threshold voltages and seems to have found a very simple answer.

He used 3V 5W zeners in the standard Ruby mod and says that it now doesn't fizz across the whole VVR range. Using zeners of too low a value risks clipping the negative side of the power grid signal before the power valve is cut off. This would introduce unwanted distortion. Paul Ruby's document suggests picking zeners a couple of volts higher than the cathode bias voltage. He aimed to make the clipping around symmetrical with the grid clipping on the positive side. But in theory there's no reason the zeners can't be lower than that as long as they don't clip before the power valve reaches cutoff.

I've never tested the cutoff voltage of EL84s in 18W amp conditions. From the data sheet curves it might be something like -20V. Bias is typically 10.5 - 12.5V, but that cools off under signal even before output clipping. So the 3V isn't that unreasonable. Anyway, something for people to try if the Ruby mod is the only fix to their fizz problems and they want VVR as well.

Here's the thread for those with a login at TAG:
http://ampgarage.com/forum/viewtopic.php?t=18157
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Post by Bronsboi »

Thanks Katoplan for taking the time writing such a thorough reply. I have read the thread and I'm going to trial the lower rated zeners. I will post my findings in a week or two.
thanks again :D
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Post by katopan »

Cool. Definitely let us know how it goes.
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Post by zaphod_phil »

He used 3V 5W zeners in the standard Ruby mod and says that it now doesn't fizz across the whole VVR range. Using zeners of too low a value risks clipping the negative side of the power grid signal before the power valve is cut off. This would introduce unwanted distortion. Paul Ruby's document suggests picking zeners a couple of volts higher than the cathode bias voltage. He aimed to make the clipping around symmetrical with the grid clipping on the positive side. But in theory there's no reason the zeners can't be lower than that as long as they don't clip before the power valve reaches cutoff.

I've never tested the cutoff voltage of EL84s in 18W amp conditions. From the data sheet curves it might be something like -20V. Bias is typically 10.5 - 12.5V, but that cools off under signal even before output clipping. So the 3V isn't that unreasonable. Anyway, something for people to try if the Ruby mod is the only fix to their fizz problems and they want VVR as well.

Here's the thread for those with a login at TAG:
http://ampgarage.com/forum/viewtopic.php?t=18157
Now that's really cool information!
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Post by TomOlsen1 »

I actually tried this out using my rotary zmulti value zener fix. The lower zener value does work over the whole usable range of the VVR pot. (From about 10 o clock)

However, when the VVR is maxed, you do lose a little of the swirl at the end of the note decay. But, not anything you would notice in the mix with a band behind you. I'll try dig up the lowest zener value I used, but from my experience so far, it works.

(Of course, since I've already drilled a hole for the rotary, it stays!)

Thomas
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Post by katopan »

Good to have more confirmation that this works. Thank for letting us know Thomas. :D
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Post by Bronsboi »

Yeah, Thanks Thomas.
I've been meaning to test this for ages. I'll definitely try this out this week and post my findings.
Dave
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Post by Gary »

Thomas

are you saying you prefer a bit of fizz on the decay and that it is eradicated at minimum voltage?

How are you defining usable range of the vvr pot? Mine is usable down to tv-like volume. What does your 10 o'clock equate to?
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Post by TomOlsen1 »

Hi Gary

The swiil is present at max VVR with the usual +/-10v zener, and reducing VVR corresponding with my rotary zener selector. However, some of the swirl disappears at max VVR when keeping the rotary at lowest value (around 4v if memory serves me right)

My lowest usable VVR volume is a bit higher than TV volume, but not more than a loud stereo. Certainly not going to annoy the neighbours too much.

Regards,
Thomas
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Post by Bronsboi »

My lowest usable VVR volume is a bit higher than TV volume, but not more than a loud stereo. Certainly not going to annoy the neighbours too much.
I definitely agree with the above.

I tested this in normal and cascade mod (ZP intergalactic mod) You still get a bit of fizz with VVR at full tilt but nothing compared to when the ruby fix is switched off. With the VVR at about 8-9 oclock it sound great and not much louder than a stereo.

I have installed the 3v zeners and the 13V zeners on a switch...They both are an improvement on the "no ruby fix" tone.
The 3v zeners improve the punch and fullness of sound across the whole range of the VVR... Yes the fizzy swirl is gone, but to my taste, that's no loss!!

I think the 13V zeners are redundant now, cos the 3V zeners improve the sound even when not using the VVR (don't know how!!). I am gonna hard wire the 3v zeners in and get rid of the switch.

I have dialed up the treble a bit to compensate, but that is the only drawback, to my ears at least.

Just one question I could only get 0.5 watt zeners. Is this enough?

Really pleased with how this has worked out. Thanks to Katoplan, Tom Olsen and ZP for all the help and advice. I'm really grateful
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Post by katopan »

0.5W should be fine.
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Isaac Stuckey
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Re: Help improve pp-18 tone

Post by Isaac Stuckey »

Since this thread is SO long I haven't read it all, but fizz can be helped in easier ways I'd think. One that instantly comes to mind, is the 820 ohm resistor in the PI vs the vox value 1k2, (that was mentioned already) and I don't know about this one, but one way I like to tame fizz is just to put a 47pf cap across the plates of the PI tube, it has worked for me many times on several amps. I hope if you hadn't fixed it already, that this will help.


Rock city, regarding your amp being loud, try installing a PPIMV (post phase inverter master volume). Search that on this 18watt forum and you should get lots of older threads regarding it. Hope that helps you, it has helped others, also you could look into getting a good attenuator.
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Re: Help improve pp-18 tone

Post by rock city »

Thanks, I am just scanning posts Read
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Re: Help improve pp-18 tone

Post by 312_Jura »

Isaac Stuckey wrote:one way I like to tame fizz is just to put a 47pf cap across the plates of the PI tube, it has worked for me many times on several amps.
Cheers!
This way?
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Re: Help improve pp-18 tone

Post by Isaac Stuckey »

Yea I've done it on several different amps. Actually never on an 18w based... but I never needed it there. The 1974x's always work for me, they are special. :)
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Re: Help improve pp-18 tone

Post by 312_Jura »

Isaac Stuckey wrote:Yea I've done it on several different amps. Actually never on an 18w based... but I never needed it there. The 1974x's always work for me, they are special. :)
Cheers, I'll give it a try. What kind of cap should it be please? (Aye, I'm green as a leaf.. just assembled the PP18 few days ago)
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Re: Help improve pp-18 tone

Post by Isaac Stuckey »

A regular 47PF will work fine. I have used a 100PF too before I think, but I'm not sure. Either way, assuming I've only ever used 47PF, it has always gotten rid of some fizz/buzz and made it a little smoother. 100Pf will be more extreme, but not bad. Don't go too high otherwise you will get rid of all the highs, anyway, just experiment with it, and try it with more than one guitar, since you could use a tele and it has less treble and it sounds awesome, but it might not be as good with a les paul. Just experiment.
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Re: Help improve pp-18 tone

Post by echuta13 »

Just make sure the cap is of the appropriate voltage for the section. :wink:
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