Any reason not to use smaller grid stopper at input?

[raiken]

I didn't quite get that. Ar you using the bead after the resistor or how?

Yes, right on the pin of the tube. You can get them either with a hole through them that you can slip over the resistor lead, or in leaded form as an axial-leaded component. We used to used them all the time in the old days of discrete transistor video amplifier stages on the pins of transistors to squelch high-frequency oscillations. You can get them in all sorts of different impedance/frequency relationships. They are great if you don't want to use any grid resistor, but still want some RF rejection. They essentially look like a wire at low frequencies, and a few hundred ohms at RF frequencies.

Bourns and Laird make them - do a search at Digikey for ferrite cores and you'll find the slip on kind in all sizes to fit most any lead or wire. If you search on "ferrite beads" you'll get the axial-leaded kind.

RA

[GainFreak]

I know what you're talking about. I use them as well only wasn't sure if you use them after the resistor or instead of the resistor.
I've seen Mesa schematics where they use them instead of the grid stopper and also before the 1M resistor. I guess the first is better.
Some of those beads have an RF impedance up to 2k2.

[raiken]

I know what you're talking about. I use them as well only wasn't sure if you use them after the resistor or instead of the resistor.
I've seen Mesa schematics where they use them instead of the grid stopper and also before the 1M resistor. I guess the first is better.
Some of those beads have an RF impedance up to 2k2.

There are two applications for ferrite beads - one if you are trying to stop parasitic oscillations or RF at the tube, in which case you want the ferrite bead to be placed right on the grid lead as close to the tube as possible, and the second is if you are trying to meet radiated or conducted emissions standards for FCC, CE, or other regulations, in which case you want the bead placed right at the point where the wire exits the chassis. It won't hurt to put them both places.

RA

[imaradiostar]

Hi Randall- just a quick question. I use various HP analyzers at work (3562A is my favorite). What software are you using to replace your HP 3580? I've seen many spectrum packages and often use Spectraplus but I'm looking for a more complete swept sine measurement package. What are you using?

Jamie

[Wombaticus]

For a bit of completely unscientific fun (I don't have a matching clip with a different input setup for comparison), here is a clip of an amp (please excuse the many clams) that I recently completed that actually uses a 10K grid stop / 220pF cap at the filter. When tweaking I also tried a 500pF cap, the sonic difference was very minor indeed.

[trobbins]

I think your pickup model will become very misleading above the first SRF, and hence extrapolating results in to the 100's kHz is prone to gross misrepresentation.

[walkman]

So what are the best type of guitar cables to use , with out having to pay for mojo? and if you are using pedals that roll off lows and highs does it really matter that much anyway?

[trobbins]

Just to add that the mid-f dip is related to the speaker diameter, where the cone changes from being effectively a rigid piston to having breakup modes. Hi-fi speakers at 12" get rolled off before or near that dip, and the breakup modes are dampened heavily so that those higher-frequency artifacts aren't so noticeable even when driven by a crossover attenuated signal level. How a guitar speaker controls its breakup modes provides probably the major differentiator in sound character, and its inherent ability to cut-through a wall of band sound :-)

As indicated, you will only experience those high-level graphical dB peaks when your ears are on-axis, as the response level will be very beamy at such high frequencies for a 12" speaker.

[J M Fahey]

Exactly, that was my point.
You will find that huge, unstoppable around 1500Hz dip on *all* 12" guitar speakers.
It comes from the cancellation caused by the mechanical wave travelling from voice coil to edge and back ... 180º out of phase.
You will not find it (or much attenuated) in Hi Fi or PA speakers (such as EV or JBL) because they use much thicker and curvilinear/expopnential cones. Fine.
But the classical guitar speaker uses a straight cone (not curved), split in sections with circular coupling rings (usually called ribs, which they are not).
And yes, a huge 8 to 10 dB presence peak between 2000 and 3000 Hz is part of the signature guitar sound.