input wiring-conditioning

[shooter]

I’ve been reviewing schematics til my brain hurts, 1 seems to be the typical.  Is 2 *better* by attaching 1/2R directly to each grid?

Been playing with Hf, Lf filter calculators, when I use,  33k grid R, and a .0002, I get 24KHz for Fc at -3db.  Would doing something like that help prevent HF parasitic, and or unwanted stray RF? 
Thanks

[kagliostro]

Standard is 68K or 33K

To attach the resistors directly to the pin of the input tube is better because you leave less wire to act as an antenna

Mesa Boogie also used a Ferrite Bead in place of the resistor (this preserves all your guitar signal level intact and didn't introduce "resistor noise"

My friend tried the "Merlin's" input (not that he invented it, but he explains it in his web page) and is satisfied of the result

it uses a "low value" resistor (10K) + a capacitor connected to ground like in your example




here some explanations about

http://www.valvewizard.co.uk/gridstopper.html

Ciao

Franco

[shooter]

Standard is 68K or 33K

I do the same, sans cap, except I want to split input in 2, so 68k ~33k at each, but the 5 or 6 schems I looked at just used 1M 68K, then split.  cost, simpler, lazy?, just wanna make sure I didn't miss something obvious.  Thanks K

[HotBluePlates]

I’ve been reviewing schematics til my brain hurts, 1 seems to be the typical.  Is 2 *better* by attaching 1/2R directly to each grid? ...

The grid stopper resistor doesn't have a 0.5:1 relationship with the grid reference resistor; it's probably better to say the value of each is determined separately.

... Been playing with Hf, Lf filter calculators, when I use,  33k grid R, and a .0002, I get 24KHz for Fc at -3db.  Would doing something like that help prevent HF parasitic, and or unwanted stray RF?  ...

Reducing RF is one of 3 purposes for the 68kΩ grid resistors in Fender amps.

But if you have 2000pF (0.0002µF) of Miller/wiring capacitance at your input stage, you got problems. It's normally not that large.

[PRR]

> if you have 2000pF (0.0002µF) .... capacitance at your input stage

That's like a 66 foot guitar cord.

You would expect major treble-suck with such a long cord (added capacitance).

The ~~100pFd of a conventional 12AX7 stage is like 3 feet more cord, an acceptable value. 34K+100pFd is 49KHz. "Zero" loss in the audio band, 11:1 loss at the bottom of the AM band. AM 540KHz getting into a 12AX7 does not go far unless the grid overloads and detects the modulation. It would be unlikely to have even 1V of AM on a shielded cord. Cut by 11, 0.1V will not overload 12AX7 grid. Generally this Time-Tested network works unless you play UNDER a large transmit tower, though sometimes a truckstop tavern with illegal-power CB rigs parked around the walls will break-in.

[sluckey]

But if you have 2000pF (0.0002µF)

Too many zeros. 2000pF is 0.002µF

[HotBluePlates]

But if you have 2000pF (0.0002µF)

Too many zeros. 2000pF is 0.002µF

Ack! Ya got me! 200pF it is...

> if you have 2000pF (0.0002µF) .... capacitance at your input stage

That's like a 66 foot guitar cord.

You would expect major treble-suck with such a long cord (added capacitance). ...

But then there were guys like Albert Collins who played with a 100ft guitar cable. If you heard him with his Quad Reverb amps with volume & treble on 10, and playing from the bridge pickup of a Tele, it might be obvious why 100ft of cable was probably a good thing.

(Yes, he did also use it to be able to walk out among the crowd while playing.)

[PRR]

> (Yes, he did also use it to be able to walk out among the crowd while playing.)

"Collins was playing a lengthy solo one night at Antone's and left the building whilst still playing. He returned to the stage still playing the solo and resumed entertaining the audience in person. Shortly afterwards a man arrived at the club and gave Collins the pizza which he had just ordered."

[sluckey]

Look at this...

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

[jojokeo]

Not a fan of Shooter's incomplete diagrams, leaves out important detail and the Devil's always in the details. The divider is a 68k dropping resistor and 68k load resistor removing the 1M in the Lo input condition (for those that may be new or unaware of this). This is not the same as a single input amp using a single 68K:1M combination.

I believe a 33k is about the perfect amount of resistance for the input of a typical single input amp 12a_7 stage, any lower seems to overload & distort the stage too easily especially with humbuckers or pedals feeding into it. However I will go lower with a 22K or 15K on a pentode's grid like an EF86 or 5879. (just my preferences I suppose?)

It is advantageous to use a small amount of capacitance on a tube's input - but on most occasions it is V1b or V2a/V2b and not on the very first tube input/V1a - best to preserve everything initially. On pedals/solid state this is a different issue entirely.

Regarding protection of RF riding into the amp via various means a simple solution is to use a small cap (.01uF/50V) on the input's isolated jack (Cliff type) ground pin to ground using short leads - you cannot use this technique with a Switchcraft type of jack unless using an isolating washer.

sluckey - I always love looking at and reading your schematics and the elegant nuances contained within such as the F voltage, the 6AN8 work, the tranny-trem (which bathroom can it use?), etc... 

[sluckey]

sluckey - I always love looking at and reading your schematics and the elegant nuances contained within such as the F voltage, the 6AN8 work, the tranny-trem (which bathroom can it use?), etc... 

I missed the joke about the bathroom? Can you explain to this oldtimer?

My whole point of posting that link was for shooter to see some more input wiring conditioning. That entire schematic including ss trem belongs to Sunn. Did you notice the triple gang speed pot? Top notch stuff. Very wide speed range and smooth sounding trem (until the optocoupler dies). 

[shooter]

That's like a 66 foot guitar cord.

Thanks guys, I'm in the pencil stage of layout, making sure I leave room if needed, i'll use Sluckeys || if it's noisy, which my last 3 have been dead quiet  with 1M 33k

Reducing RF is one of 3 purposes for the 68kΩ grid resistors

So option 1 and move on?

[jojokeo]

I missed the joke about the bathroom? Can you explain to this oldtimer?

As if the country didn't have enough going on debate rages over transgender rights & bathroom access. Particularly at this time when states are to adhere to the Fed's (Obama's) attempt at blackmail or risk losing $Billions$ of Federal money for failure to comply.

[PRR]

> tranny-trem (which bathroom

"tranny" has many meanings. Transformer. Transistor. Has been a slang word for Transvestite, but that is not what the recent court case was about. I guess the joke is on "Transgender".

[shooter]

the beer, bait, n gas place I work has no *public* bathroom, the govt say's you can't decide for yourself if you wanna use it!  So the folks that get *ugly* with me because we have no bathroom, I simply smile, and say, "we're waiting for the election to see if we need 2 OR 3 doors"

[jojokeo]

I've been in the habit for decades of abbreviating any word beginning with "trans" by using an X as in x-frmr for example. X is also known as the unknown factor or x-factor and in the cases as PRR gives a few - I think is spot on appropriate if used for x-gender and x-vestite, etc. And in the current sociological case of the many (mostly crazy) gender definitions currently trying to be identified, included, & accepted - describing anyone that doesn't seem to fit (or rather psychologically mostly) ether of the old standard M or F should be labeled merely as "X"? However I'm sure that something this simple in concept would be sure to offend somebody in this hypersensitive world that it's becoming? Personally I find it all rather incredulous and laughable but so are many things of this day and age.

Shooter, funny. However the easiest & most simple remedy would just be 1 door...with a lock! What's so hard about that? It's just that no public official would get credit for it that could be used for re-election purposes?
Sorry for the unintentional thread hijack all.

[shooter]

Sorry for the unintentional thread hijack all.

that's ok, thread is gone anyway, the owners solution is 1 door with a spinning arrow

[HotBluePlates]

Reducing RF is one of 3 purposes for the 68kΩ grid resistors

So option 1 and move on?

I'd do (and have done recently) option 2. I just would determine the value of the grid stoppers independently of the resistance to ground.

In my case, I used 1MΩ to ground. I also had two different input tubes from the single jack which were likely to have all the top end anyone could ever want (and more), as well as pretty low Miller capacitance. So I used 100kΩ grid stoppers for each tube.

[shooter]

option 2

Thanks HBP, ch1 will be AU/AX, Ch2 AX/AX.  everything's *maybe* so far.  I'm keeping the 68k-1M ratio for now, wired option2.