Tweed 4 input grid stopper question

[Willabe]

This question is for the 5E3 I'm rebuilding. I'm ready to start wiring it up, have the board stuffed and chassis modified, just one more thing I need to figure out.

If I mount the 68K GS's on pins 2/7 of the tube socket and run shielded cable back to the input jacks, I'd have to run 4 pieces from input to tube socket. If I mount the 68K GS's on the input jacks, I can tie the leads going to the socket together in a Y and run 2 pieces of shielded cable from input to tube socket, but now I don't have the GS's mounted on the socket pins where they do the most good.  

I've also read that any time you use shielded cable to a grid pin you should always use at least a small GS (as good build practice), because of possible problems with capacitance from the cable's shield with/at the tubes grid.

In Merlins preamp book he says as small as 100R can be used effectively. KOC's book's seem to use a 10K as a low end value GS for input set up's, in the schemos (a lot of text to go back through and read to find a reason).    

I want to go with the 2 shielded cable instead of 4 option, but don't know what value GS to use. Large enough to do it's job, but doesn't mess with the input impedance/loading of the pickups.


           Thanks,    Brad      



  
    

[HotBluePlates]

Or...

You can get some 2-conductor shielded cable. Usually, the inner conductors are color-coded red/black. Use the single shield for ground for both jacks.

I have some 2-conductor shielded cable that was pretty cheap, and uses a foil shield instead of a braided shield. The good thing about this for your case is that that foil shield has a "drain wire" which has continuity with the shield, and can be used to make the solder connection to the ground lugs. If you're clever, and cut the drain wire long, you can feed it through one ground lug and then span over to the other ground lug before soldering both.

 

[Willabe]

Thanks HBP, but I went to the trouble of going with a wired star/buss with the ground wire from each (isolated) jack pair going to their own cathode R/BPC ground star point. I split the 2 K's to (if needed) be able to mess with the BP cap's seperatly on each triode.

I do have a roll of 2 wire shielded, 20g, R/B, foil drain wire, but would rather not undue what I've done. I know it's not neede with a little amp like this.    

So what value GS resistor would you recomend.

Here's the layout schemo below.      


           Brad        

  

[HotBluePlates]

Your diagram shows a common ground for each Hi/Lo jack pair. It is that common ground only that I was suggesting to use the drain wire for. You'd still use two cables, but in this case the grid stoppers can stay at the socket.

[edited to make the text and my scatter-brain coincide.]

[Willabe]

     Oooohh.    I get it now.      

I have got to read this stuff slower.

It's perfect, Thanks HBP!

Pretty sad I couldn't figure this out myself. I'm gettin more and more worried about myself as I get older, sadest part is I'm only 52.    


          Thanks again,   Brad    

 
  

[sluckey]

It's a 5E3. You're becoming obsessive about it. Leo did it best. Everything else is just a wannabee.   

[Willabe]

Yeah, I know.               Thanks sluckey.

Just getting some practice in for bigger builds some day. I've always been a worrier and have always over built as a carpenter.    

I'd rather mess this up than something where I've got more $$ in it, including a blank chassis that I've drilled out.


            Brad      

    

[sluckey]

I know. Just pokin' you.   

[Colas LeGrippa]

always thought the 68k WAS the grid stopper, and the 1M the grid leak..why add another one ?

[firemedic]

He just told ya he's overbuilding it!

Just kidding, I do the same thing willabe. Having read the same book, I have started putting extra 4.7k GS's on all my preamp stages.

[HotBluePlates]

Did Merlin/KOC explain the logic behind the low value of the grid stoppers? I haven't gotten Merlin's books yet, but I have/had most of KOC's books.

68k interacting with wiring capacitance and tube input+Miller capacitance is still way above the audio range, much less the guitar range. It seems like much below 10-50k wouldn't have much sure-effect.

You could rightly argue that these are HF rolloffs, and that all of the rolloffs of each stage add up and could be significant. I'm just figuring definite RF rejection would be a desireable thing.

[Willabe]

always thought the 68k WAS the grid stopper, and the 1M the grid leak..why add another one ?

Yes, it is the grid stopper, --- but --- as a grid stopper it should be mounted on the tube socket grid pin, not on the circuit board or input jacks to be most effective. Hence the term/name grid stop?


                  Brad      

[PRR]

The 68K allow two guitars to be plugged into the same amp channel, OR attenuate a too-hot guitar.

100 ohms does not allow this feature.

[Willabe]

I know. Just poking' you.    

I saw your  
Hey, ain't Mardi Grass coming on soon?  


      Thanks,    Brad    

[Willabe]

It's a 5E3. You're becoming obsessive about it. Leo did it best. Everything else is just a wannabee.   

Hummm, wannabee,               Hey I just got it.


       

                                             But I wannabee says Willabe!     


                               
                  Brad