having trouble visualizing a cathodyne grid stopper

[phsyconoodler]

I'm looking at the valve wizards site and his explanation of the cathodyne PI grid stopper.The only grid is pin 7 in a Princeton reverb,right?
  So you just hang a larger resistor in series there and it gets all civilized when driven hard,right?
 It doesn't seem to do anything for me.

[PRR]

Voltages. Resistances.

Since it's coming off an already high impedance and going to a VERY high impedance, you may need astonishingly high value stopper.

I'm not sure "all" schemes will be affected (depending on bias and relative voltages and impedances). In casual bench racing I could not force a VT40's cathodyne into anything grossly ugly.

And anything will get uncivilized if driven hard enough.

[tubeswell]

I put a 470k grid stopper in Pin 7 in my PR cloney thing and like the effect of it. I have heard of other guys using 1M

[worth]

Grid stoppers don't affect tone.. just get on with your build.

[phsyconoodler]

I'm modding this one for higher gain in the preamp so I may not even have to worry about driving the output that hard anyway,it just didn't do much if anything at all to help the 'blatty' distortion of this PR build.It may be something else altogether.We'll see.

[punkykatt]

"Grid stoppers don't affect tone.."

Do they affect the feel and picking dynamics of the amp? There must be some sort of sacrifice?
Punky"

[phsyconoodler]

Grid stoppers do indeed affect tone.They just are subtle until they get really big,then the tone changes more.Having said that,a 1meg grid stopper in this amp did nothing at all.I'm talking about dimed.

[DummyLoad]

it's going to take a LOT of overdrive to get princeton cathodyne wonky; input threshold is about 45Vp-p & input Z is around 4.8M.

with a 1M grid stopper the F-3dB roll-off working into a 220K load is around 48KHz. up it to a 2.2M and you might start to hear some HF roll off. up it to 3.3M and you'll definitely hear a difference - with a 3.3M grid stopper, -3dB roll-off will be around 14KHz.

--ISO  

[phsyconoodler]

I have implemented the master volume Sluckey was kind enough to share and I'm thinking that it is making this amp sound crappy.I'm going to punt it and see how it sounds.It gets inserted right at the grid of the PI and adding the grid stopper there may not work with this master.
  It's just that I've built LOTS of PR's and none have had any issues with distortion.In fact I crank them and use my giutar volume to get the sweetest overdrive ever.This one gets nasty sounding if you crank the volume and feed it in with the master.Same if you turn up the master and crank up the volume.So I tried adding a 1 med stopper and it made a very,very slight difference but not much at all.
  Maybe this amp needs a 3meg one?It just seems odd that this amp is so different from the many others I've done.None of those needed a PI grid stopper at all.In fact they sound so sweet that adding the grid stopper likely would have made them sound bad.
  I'll get a look at the pattern on the scope at work and see what's going on.

[punkykatt]

OK, so you dont lose highs until the grid stopper is way high.  My question is. Will those grid stoppers affect the feel and picking dynamics of the amp? There must be some sort of sacrifice. 
Punky

[Merlin]

OK, so you dont lose highs until the grid stopper is way high.  My question is. Will those grid stoppers affect the feel and picking dynamics of the amp?

No. Before the point of overdrive there is no difference in the transient voltages reaching the grid; what you play is what you hear. Beyond the point of overdrive the stopper does its job of reducing blocking/frequency doubling.

There must be some sort of sacrifice. 

Why must there? Why this universal assumption that every part added must automatically "sacrifice" some sort of magic quality from an amp, but never improve its performance??

[Geezer]

OK, so you dont lose highs until the grid stopper is way high.  My question is. Will those grid stoppers affect the feel and picking dynamics of the amp?

No. Before the point of overdrive there is no difference in the transient voltages reaching the grid; what you play is what you hear. Beyond the point of overdrive the stopper does its job of reducing blocking/frequency doubling.

There must be some sort of sacrifice.  

Why must there? Why this universal assumption that every part added must automatically "sacrifice" some sort of magic quality from an amp, but never improve its performance??

I'm confused, or I'm missing something......

So if I put a 1meg, or a 44meg, or a 500meg, or a 100giga-ohm grid stopper in there it makes no difference either way, they all will give the same (good?) results & not affect the tone of the amp (in a bad way) or the way it operates....I will see only improvements with no downside?

That's counter-intuitive to me........help me out here.

G

[phsyconoodler]

I think he means within reason.It's like adding a grid stopper on a 5F6A bassman.Some need them and some don't,but as you near 220k the tone changes.It filters out the highs as the value gets larger and the 'chime' if you will,is squashed.
   Frequency on a guitar amp is in a narrow range so the range of grid stopper should be pretty broad before it makes a big enough difference to hear.That's what I hear Merlin saying.
   On his site it looks pretty impressive to say the least,but it doesn't really show the correct values for a common cathodyne.The one example he shows has odd values that a PR and tweed Deluxe don't use.Guess it's best to buy his book.
   It's an odd phase inverter and some more investigation is definitely worth it to get the most out of it.

[punkykatt]

Thank you Merlin for that explaination.  The sacrifice I was thinking about would be the large resister attenuating the signal to the grid resulting in having to turn up the guitars volume  to compensate for the loss of signal. Am I incorrect in thinking that way?  Punky

[PRR]

> I've built LOTS of PR's and none have had any issues with distortion. ....This one gets nasty sounding

So WHAT's different?

I suspect something is not wired right, or some component is not the value you think it is.

[Merlin]

The sacrifice I was thinking about would be the large resister attenuating the signal to the grid resulting in having to turn up the guitars volume  to compensate for the loss of signal. Am I incorrect in thinking that way? 

Assuming you've chosen a value that doesn't interfere audibly with the treble, then there is no attenuation of the grid voltage at all since the grid has essentially infinite input resistance. The grid stopper only does it's job above the point of clipping.

[punkykatt]

Thank you very much for clearing that up for me.  I love this forum. Punky

[phsyconoodler]

Yeah,I'm going through it soon and find out what's amiss.I keep thinking it will fix itself or someone will chime in with "it's this".
  It's kind of an avoidance issue.

[jjasilli]

Maybe post the schematic with voltages.  Some mods are idiosynchratic.  E.g., re my SF Princeton:  The Stokes Mod -- boosting PI voltage by tapping off the 3rd (unused) rather than the 4th filter stage made its overdrive sound really bad.  This is reputed to vary from amp to amp.

[phsyconoodler]

That's why I'm thinking the master volume I used is what's causing it.All the voltages are very close to the schematic.
  I'll lose the master and try again.Maybe I'll add VVR instead of a master.