input resistor...need to dump it.

[daz]

Ok, first of all, anyone who disagrees with the effect on tone that the input R has need not dispute it here. I already know what it does and don't wish to argue it. I say this because i discussed it elsewhere and got into a big to do about it and never got the question answered. Point it, i have tried removing it and putting it back, and finally bypassing it with a clip cable so i could A/B it quickly and theres no doubt what so ever that in my amp having a resistor there adds brightness. Yes, i do realize it loses brightness at other grids in the amp, but not at the input. Why, i don't know. I just want to ask a related question w/o getting into a debate about whether it does or not...on mine it does.

So if you got this far and aren't angry, answer me this....i have heard of putting a cap from input to ground but that usually dumps brightness to ground. Is there a value that will curb oscillations or radio signals etc that won't change the tone over having no input resistor? I tried a 10k but even that adds too much brightness. It's perfect with no R there. So is there a cap value or another method i can use so the tone doesn't change? (tried a 10k AND a cap to cancel out the R's added highs but couldn't find a good set of values)

[raiken]

It's perfect with no R there. So is there a cap value or another method i can use so the tone doesn't change?

If you don't want a resistor, use a ferrite bead instead, located right at the grid of the first preamp tube. You can buy them either loose, where you slip them over a small piece of buss wire, or you can buy the "bead on a lead" type. You want a small one, about the size of a resistor body, not the great big clamp-on type. Solder it inline with your grid wire and cover the assembly with heat shrink tubing.

Randall Aiken

[daz]

Thanks Randall. I'll go with that, sounds perfect. (and thanks for your pages !!! great stuff)

[Rob Mercure]

Daz,

Since I didn't participate in your "debate" I don't "have a dog in this fight." So, traditionally the input isn't where you're address oscillation problems as they really can't be occuring there unless you're coupling the output from at least two stages down back to the input somehow (some exceptions but not within common guitar amp topology).

Secondly the other response concerning ferrite beads might work for the RF problem - I've never had to use 'em myself - but when I've experienced RF pickup from outside radio sources it's usually been cuz a dirty input jack has formed a diode or there's problems with a grid leak input stage or there's simply a station that's too close, or too "dirty" to get rid of (as a teenaged hobbiest there was a stong commercial AM radio station just up the hill from me - about 1/8 mile line-of-sight - and there were some amps and tuners that I just couldn't work on. So I partially sheilded a closet with aluminum flashing - not a complete "Faraday cage" just to be able to do repairs/experiments. The ironic thing is I would up working for the station about 6 years later!). There are copies of the Radio Designers Handbook volume 4 (RDH4) and other design manuals available now online for download (had to buy mine used for $45 about 15 years ago) that will provide lotsa information on sheilding and preventing RF pickup. Bruce can help on this one as he's a ham (amateur radio - no experience with him on stage <grin>).

Rob

[daz]

Solder it inline with your grid wire and cover the assembly with heat shrink tubing

Randall, or anyone else who knows........i just re-read your post and i don't understand what you said above. "solder it"? The bead just slips over the wire or the wire wraps around a ferrite donut several times from what i'm familiar with. What was meant by soldering it?

[Don Moose]

The ferrite Randall mentioned is quite small. If you get the 'bead-on-a-lead' type, there's already a wire through it, so you'd solder it in place.

If you get a bare bead, unsolder the grid end of the wire, slip it on and resolder.

Example parts from Mouser

Hope this helps!

[jrfrond]

If you are talking about the ubiquitous 68K input resistor, it is really NOT necessary, and only there as a grid stopper to help alleviate spurious oscillation and input noise.

A small capacitor, between 47-100pF across the input and ground will accomplish what you are trying to do. Some people are purists about shunt caps like this and say "no". I say, whatever works for you is right.

[Enzo]

And one reason there are resistors on the two-jack amps is so they can form a voltage divider. The high and low sensitivity jacks like on so many amps. If yuo have one jack, then ther will be none of that.

[Steve Conner]

I had a homebrew amp that used to pick up AM radio on its dirty channel. Radio Moscow, IIRC. It was quiet, but it was definitely there.

It didn't have the input grid stopper, so I tried adding one, and it cured the problem. I used 10k instead of 68k just to see what would happen.

[Paul P]

Could someone please explain how a grid stopper resistor works to cut
down radio interference ? Where does the interference go ?

And how does it compare to a ferrite bead ?

Paul P

[Steve Conner]

The grid stopper resistor makes a low-pass RC filter along with the Miller capacitance of the tube.

http://www.freewebs.com/valvewizard/gridstopper.html

I believe that it's orders of magnitude better at getting rid of RFI than those weenie ferrite beads. I think they only have an impedance of 100 or 1000 ohms, and don't achieve that until maybe 100MHz. AM radio is in the hundreds of kHz up to a few MHz. So I'd say forget the ferite beads unless you're picking up your local airport control tower too.

[Paul P]

Great link. Thanks Steve.

Paul P