Fender 6G15 Reverb Unit Dry Path

[W5FH]

Hello-
      In studying the Fender 6G15 reverb unit the dry signal path is of interest to me. An AC coupled, voltage divider biased type cathode follower is used in the dry signal path. I wanted to ask players, and anyone with technical knowledge of this circuit their opinions. The reason I ask is that I am considering using a cathode follower circuit like this as the first stage in an amp build. The purpose of it would be to provide a very high impedance, and a very low capacitance input to the guitar pickups which should accomplish two things:
    1. Reduce or even eliminate the loss of high frequencies when guitar volume control is at low settings, due to the very low capacitance  of cathode follower input. The common cathode gain stage is about 100 pF input capacitance (Miller capacitance) as compared to about 2 pF for cathode follower (neglecting stray capacitances).
    2. Could allow an amp to do a much better job with piezo pickups due to the higher impedance (much less loss of bass).
Am I missing something here or could there be a benefit in designing this cathode follower, or similar circuit, into an amp to serve as input stage?
Thanks!

[PRR]

Guitar cable is 30pFd per foot or more. Unless your cord is 3 feet, cord capacitance swamps tube capacitance.

Also you "want" the 2-pole low-pass filter to take the string inharmonics off. A truly flat signal path sounds nasty.

[pdf64]

Adding a CF at an amp’s input  may tend to increase noise.

[W5FH]

Thanks guys for this enlightenment. Learning is what it is all about. I was not thinking about anything preceeding the amplifier input. I recall common coaxial cables in RF work were around 27 pF per foot. Had not occurred to me in regards to guitar cable. OK on the noise concerns. Will file all of this away!
Thanks.

[tubeswell]

The function of the CF bypass in the 6G15 isn't really about being an input stage for the dry signal, so much as being a current driver/impedance buffer for the dry signal between the 6G15 output and the actual guitar amp - its buffering a guitar pickup level signal which isn't amplified prior to that.


Also, the fixed bias CF in the 6G15 has a 1M1 input impedance to be sure ( 2 x 2M2 in parallel as seen by the signal at the grid), but that isn't as high as a cathode-bias CF could be. (A cathode-bias 12AX7 CF with a 1M grid leak, bootstrapped to a 100k cathode load resistor, has a much higher input impedance)

[Williamblake]

My guess and trying on this was to use a gain stage for wet and dry and mix web and dry on the input of a cathode follower. I am very happy with the result as i have a wide range of sounds and do not have to dial knobs to get a good enough sound. Don't want to badmouth the 6G15, it has surely made a dent in history.