From kee–(at)– Thu May 2 11:08:59 CDT 1996
Article: 14183 of alt.guitar.amps
From: kee–(at)– (R.G. Keen)
Newsgroups: alt.guitar.amps
Subject: Re: Bypass Cap values, cathode followers
Date: 2 May 1996 12:33:24 GMT
Organization: Adhesive Media, Inc.
Lines: 34
Message-ID: <4maa2k$pk--(at)>
References: <4m8smd$pu--(at)>
X-Newsreader: TIN [version 1.2 PL2]

LarrySB (larrys–(at)– wrote:
: In studying the differences between amps, I began wondering about the
: values used to bypass cathode resistors in the preamp stages.

: Some amps, like Fenders, typically use 25uf to bypass the cathode
: resistors. Others use much larger values, on the order 250uf. I’m just
: wondering about the effects on overall tone from such different caps.
The capacitor bypass on the cathode causes the gain to be higher at
frequencies where the capacitor impedance is less than the resistor. The
turnover point is the spot where the cap impedance equals the resistor,
or F=1/(2*pi*R*C); for 1500 and 25uF, this is 4.2Hz if I got the decimal
point right. The lowest note on a concert pitch guitar is 82 hz, so even
a 2.5uF cap should not cause much of an effect in lows. 250uF would
rollover at .42 Hz.

: On many classic tweed era Fenders, a cathode follower is used to drive the
: tone stack. Marshall persisted with the cathode follower, while Fender
: dropped it in the BF and beyond era. Any thoughts on this topic as well?
: Tonal differences? Pet theories?
The follower definitely contributes to overdriving the next stage and
crisping up the frequency response. A neat trick for mod’ing amps without
followers is to stick in an IRF820 power FET as a source follower to
drive the tone/vol stack. The FET is so fast that it follows the driving
tube almost perfectly, and provides strong low impedance drive to the t/v
stack. The saturation/cutoff limits of the 12AX7 driving section are
still well within the limits of the FET linear region, so you can’t get
semiconductor clipping from the FET at any drive level. Best of all, you
only need the FET and a 10K-100K resistor to do the mod. The IRF820 is a
600V device, so it works by connecting drain to B+, gate to the 12AX7
Plate, and source to the (added) source resistor to ground. The old plate
capacitor connects to the source. No other changes are needed.



Buy the Book!

I cleaned up my tab for Sonny Boy's Help Me and made it into a short book. There's a Kindle version for 99 cents, and if you buy the paperback you get the Kindle free.

Playing "Help-Me" In the Style of Sonny Boy Williamson II: A step by step, note for note analysis of some of Sonny Boy's Signature Riffs

I also write Science Fiction, so you can sample some of my best stories. Also available in Kindle format.

Error Message Eyes: A Programmer's Guide to the Digital Soul

Supro 1624T Dual-Tone 24W 1x12" Tube Guitar Combo Amp

End Date: Sunday May-26-2019 16:58:05 PDT
Buy It Now for only: $1,199.99
Buy It Now |
Vintage 1960's Valco Supro Super Guitar Tube Amplifier

End Date: Sunday May-26-2019 19:48:37 PDT
Buy It Now for only: $679.15
Buy It Now |
Vintage 1966 Supro 1688T Guitar Amplifier

End Date: Sunday May-26-2019 21:51:43 PDT
Buy It Now for only: $1,999.00
Buy It Now |
Supro 1600 Supreme [DISPLAY MODEL] All Tube Amplifier 2-Channel 25W 1x10

End Date: Monday May-27-2019 7:28:12 PDT
Buy It Now for only: $849.00
Buy It Now |
Supro 1685RT Neptune Reverb 25-watt 2x12" Tube Combo Amplifier New in Box

End Date: Monday May-27-2019 8:27:25 PDT
Buy It Now for only: $1,149.00
Buy It Now |


The JT30 Page Popular links

I began collecting data about the microphones used by blue harp players before there was an internet. I began organizing it into in the late 1990s. I accumulated more stuff than I remember. This is some of it.

Street Theory

A Harp Player’s Guide to Music Theory

Learning Harp

Picking Up Blues Harp

A guide to learning to play Blues Harp

Microphone Information

Usenet Articles

Harp Amps

I've been collecting Harp Amps for a while. This is the old website. There is lots of information here. Here a coupld of links.

Harp Tab

A collection of songs and riffs that I’ve worked out over the years, plus some libraries of stuff I’ve converted to tablature. I’ve included most of the notes and instructions that helped me when I was learning to play blues harmonica.

Basic Riffs Simple harp tabs for songs Blues riffs and phrases.

Harp-L Archives 1992 to 2002


Harp Frequently Asked Questions