From reaike–(at)–nnova.net Tue Jul 21 10:01:51 CDT 1998
Article: 116933 of alt.guitar.amps
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From: Randall Aiken
Newsgroups: alt.guitar.amps
Subject: Re: 68K input resistors, what purpose?
Date: Mon, 20 Jul 1998 21:55:23 -0400
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plexibreat–(at)–y-dejanews.com wrote:
>
> Here’s a question for all you amp gurus, The resistor from the input jack to
> the first grid of a tube amp, it’s usualy 68K. I usualy see these in amps
> with more than one input and have assumed that it’s to give a lower level
> input alternative. On Mesa boogies with one input there is no resistor here,
> I’ve assumed this is because with only one input there’s no need to provide a
> lower level alternative. What I’d like to know is, are there other reasons
> for that resistor? What would the reprocussions be to removing this resistor
> and going straight from the input jack to the grid, (of course keeping the
> 1Meg grid resistor), other than the one I’ve mentioned above?
>
> JKB
>
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Those resistors are not just put on the control grid for signal level
attenuation purposes; rather, they also act as a very high frequency
low-pass filter in conjunction with the input capacitance of the triode
(which is a sum of the grid-to-cathode capacitance and the Miller
capacitance).
This low-pass filter does a couple of things: (1) it helps prevent high
frequency parasitic oscillation in the tube itself; (2) it helps prevent
radio frequencies from getting into the input stage, where they can be
rectified and lowpass filtered (AM detection) and become audible at the
amplifier output; and (3) it can limit grid current when the tube is
driven into the positive grid region.
You will notice on two-input amps, such as Fenders, when you plug into
the low-level input, the 1MEG grid resistor is shorted out and the two
68K resistors act as an attenuator to cut the input signal in half. The
downside of this is that the input impedance drops from approximately
1MEG down to approximately 136K, which is a rather heavy load for a high
impedance guitar pickup.
If the resistor is connected in series with the input jack and before
the 1MEG grid resistor, there is a small attenuation of the input signal
(0.94 times). If the low-level input attenuation is not necessary, the
resistor is best placed after the 1MEG grid resistor and should be
soldered directly to the grid pin instead of back at the input jack.
This will be the best position for RF and parasitic oscillation
attenuation.
Bottom line: leave it on there.
Hope this helps,
Randall Aiken
reaike–(at)–nnova.net
