Killing the hash PO

[Structo]

In regards to my D'Lite and the fizz or hash I am experiencing.

I was reading over at Metro Amps forum and this guy was tweaking his 100 watt super lead.

He was advised to put a 220K resistor on pin 2 of the PI. (grid)

He had what they described as "Cymbal Crash" parasitic oscillation.

He didn't have a 220K so he put a 210K there.
It was pretty impressive the change that he experienced.
He had before and after clips to demonstrate the change.

So it is essentially a grid stopper on the PI and the resistor is inhibiting the high frequency stuff from entering the PI.

I know that there is a difference between the Marshall and my D'Lite but it's worth a try.

Anybody here ever heard of doing this.
I'm thinking about putting a 250K pot there to see if I can dial in a value that helps get rid of the hash I am hearing in my amp.

I tried the treble bleed circuit and it didn't do anything for me.

[Structo]

40 views and nobody can tell me the cons or pros of putting a grid stopper on the PI?

Well I guess it is simple enough to try, I guess I was looking for some kind of confirmation.

[Mark]

Tom

I don't have a clue on this one either, perhaps a recording might be more revealing?

[Structo]

The only form of recording I currently have is with my Canon Power Shot camera.
I've tried to record it but it really doesn't come across very well using the camera.
I can kind of hear it but I know what I am listening for.

Oh well, I will continue to muddle through.

Thanks

[bluesfendermanblues]

Maybe, You should try to track down the source of ocillation, which is occurring prior to the powertubes in the signal chaing.

Marshalls are notorious for having occilation, when you turn up the treble and presence knobs at higher volumes.....

These problems can usually be cured by
- hawing the proper grounding points (like dumble did it)
- also the grid input resistors (on pin5) of the power tubes, should as short as possible

You might also consider soldering the tone capaicitors and resistors, directly on the tone pots (bas, mid, treb), which cuts down on the wiring back and forth. (The latter proporsition could be 'wrong' to some dumble fans, because dumble didnt do it way, but its all a matter of getting good tones ).

[bcook]

I would measure the tube voltages as a quick check for a wiring/resistor error, then run a cable from the preamp out to another amp to see if it's in the preamp or the power amp.

[butwhatif]

I mentioned quite awhile ago about putting a 100k resis there. I always have a series r there, it allows the grid to be more neutral from ground or whatever the source is always, as the grid's hi impedance anyway. Generally the sound's nasty, or fizzly without one.

[Structo]

I did put a 100K resistor on the other side of the .02uf.
Between the Master and that cap per your instructions and I liked the effect.
I soldered it directly to the .02 cap. Was that the wrong place to put it?

Should it be on the other side of the cap, on the tube side?

I saved what you told me in some notes dated 1/03:

Try putting a 100k resistor in series before the .02 PI input cap.
The 100k resistor will make a fair diff in the overall tone, more like D, give it a try, it brings the D tone back. U can use as much as 220k, but 100k will prob work best.


The Metro grid stopper is a bit different but I don't really understand how adding a resistor on the tube pin 2 which already has the 1M resistor on the LTP would make a difference.

Of course the Marshall amps most often only have 470K grids so maybe that helps their amps.

[butwhatif]

what is the scheme you refer to?

[Structo]

Uh, not sure what you mean by "scheme"?

If you mean schematic I am going by the Tweaked Layout that Moss came up with that incorporates most of the suggestions from this forum.

[butwhatif]

Sorry, i couldn't find any posts by 'moss'. R u using a scope? What does the waveform of the od look like w/ a 1kz sine input? ?

[talbany]

Tom
Most of the time those 220k resistors were put there mostly in Fenders.. when 2 signals (Dry/Reverb) were being mixed into the entrance to the pi..Prevents bleed back..Also if on your amp you did a series loop w/ a 220K resistor between the send and return you already have the 220k in there before the PI entrance.. Your better off trying to find the source of the oscillation



Tonyy VVT

[Structo]

I think Moss Hudson (owner of Brown Note) uses brownnote for his username here.

I have the Ironsounds FX loop in this amp.
It has a true bypass button that takes it completely out of the amp.

No scope.
I was actually looking at them on ebay but not sure what specs to get.

I'm sure a 100Mhz would be more than enough.
Actually a 20Mhz one would be more than enough but you don't see those except in antiques.

But like on Tektronix that used to be made here, there is a gazillion model numbers so I'm not sure of the pros and cons of each.

Plus most sellers don't guarantee them so if it is damaged in shipping you are out of luck.

I have also explored the PC interface ones but they are a bunch of money as well.

Yes I need to be more persistent in tracking down the source.
I suppose I am always looking for a magic bullet that would cure it.

Not sure what could be causing it as far as layout goes.
I think the preamp area is ok.
My only questionable area is under the filter cap board where the OT primaries and grid wires are.
It's rather cramped there so I can't route the wires as neatly as I would like.

[butwhatif]

I've had one basically like this for over 25 years, and have fixed over 1k amps with it--
http://mcmelectronics.com/product/TENMA ... 0-/72-6800

[Structo]

Man, not a bad price for a new one.

That would probably work fine for me.

Regrettably I am still not working so it may have to wait unless I can sell something.