Re: Piazza Mod (for GBs)



The JT-30 (and other crystal/ceramics) are a special case, ultra high
impedance.  They're not even spec'd as "ohms", but as "the equivalent to a
390 pF capacitor".  Combining this with any finite resistance results in a
high pass filter.  Even though the spec sheet says otherwise, Astatics
engineers recommend one meg or higher.  One meg results in a -3dB point of
400 Hz, rather nasally sounding.  The Rod Mod, if run DIRECTLY into the grid
(and no bias resistor, so the pot is used for grid leak), results in a more
acceptable cutoff of 80 Hz, still not exactly hi fi but better.  This would
usually be a modified amp, as it would requre a 5 meg mic.  You wouldn't be
able to use another (lower impedance) mic with this without a special grid
leak biasing circuit.

Running the Rod Mod into a typical tube amp input (with cathode or gridleak
bias built in) will result in a higher -3 dB point.

Running a JT30 into one meg results in a -3 dB rolloff @ a very midrangey
400 Hz.  They're actually at their best running into an infinitely high
impedance,  Anything less is a compromise.  Even Rods 5 meg pot loads the
element, reduces low frequency response from the ideal, and requires a
little compensation.

Rods mod is more or less based on circuitry popular in Little Walters day.
It's better than most modern gear for this purpose, but it's still not
optimal.  Even though the "purists" will shoot me for saying this, the best
device to run a JT-30 into is a solid state op amp configured as a voltage
follower.  The output is shorted to the inverting input, and replaces all
power losses, resulting in a very real infinite impedance.  Running through
this simple but extremely effective circuit, the JT-30 actually sounds full
enough to sing through, but it still has that nice "natural" distortion
breakup when tightly cupped.

Green Bullets (controlled magnetic, controlled reluctance), while considered
"high impedance", are MUCH lower impedance than a crystal/ceramic.  A GB is
quite happy running into an impedance as low as 20k, although it's better
through a somewhat higher impedance like 50-200k.

So basically, no you wouldn't want to use the "Rod mod" on a "medium"
impedance mic like a Green Bullet.

- ----- Original Message -----
From: "Jp Pagan" <coquiblues@xxxxxxxxxxx>
To: <harp-l@xxxxxxxxxx>
Sent: Friday, March 21, 2003 11:21 AM
Subject: Piazza Mod (for GBs)


>
> hey all,
>
>    the talk about the Piazza Mod (the VC cap) always refers to crystal
> JT-30s (or at least, it has recently.) my question is, does the same mod
> work for Green Bullets? does it work differently for a Controlled Magnetic
> element or a Controlled Reluctance? would the cap value need to be
different
> depending on the GB element type, and if so, what values work best?
>
>    thanks,
>          -- Jp
>
>
>
>
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