Comments on: The Cathode Follower Mod

By: John

John — Wed, 11 Aug 2010 15:34:14 +0000

Actually it turned out I had a bad solder joint in the tone stack. Getting essentially no bass or mids! After fixing that, it sounds just great.

PS – Bill is correct in his advice not to use a cheap soldering iron…

By: Golney

Golney — Tue, 10 Aug 2010 11:23:03 +0000

Hi bill

I think I have the necessary resistor for this mod, its definitely 100k but is there anyway of telling if it is a 1/2 watt resistor? I received it in a byoc kit for an overdrive, it is blue and about 6mm in length. this may be a stupid question!
I’ve just ordered the to-20 OT and an input jack, along with coupling caps. any chance you could include one of these 1/2 watt resistors if this isn’t correct? ill happily pay for it, would save me having to pay extra postage!

By: bill

bill — Mon, 09 Aug 2010 13:49:04 +0000

Your tone stack sounds thin with the .1µF bass cap and .015 mids cap? Have you changed the coupling caps to .02µF?

By: Shot

Shot — Fri, 06 Aug 2010 07:24:27 +0000

i think changing the tone stack’s slope resistor to 68k might make it a bit better. i’ve noticed that after a cathode follower, standard fender tonestack tend to sound thin, and if you look at a 5F6 tone stack, the slope resistor is 56k. we can experiment with that too.

By: John

John — Sun, 18 Jul 2010 21:30:26 +0000

Additional observation – I usually run the amp with no fat. This makes the output impedance of the second preamp stage 68k. The output impedance of the 12AU7 half of a 12DW7 in the cathode follower is 425R. According to Duncan’s tone stack calculator, under my typical paramters (TMB all on 12) going from 68k to 425R gives a 6db boost in treble, 4db in mids and 1db in bass. Your results may differ. Turning treble down to about 9 fixes it.

By: John

John — Fri, 16 Jul 2010 15:08:14 +0000

Just did this mod. I like it. To my ears, there’s a definite difference. The overall tone balance is definitely brighter, and the bass is much cleaner.

A 12DW7 in v2 leaves the overall sound a little cleaner. A 12AT7 in v3 makes the amp a little quieter. A 12AY7 in v1 makes it quieter still, and also pushes the breakup to a higher volume setting.

For jazz and clean fingerstyle strat, the alternate tubes are great. The clean boost mod compensates nicely for the drop in volume. Crunchy strat needs a 12AX7 in v1. The breakup is much more uniform across the frequencies with this mod. Before the bass would tend to breakup before the treble.

By: bill

bill — Wed, 14 Jul 2010 23:56:51 +0000

I can send what you need if you’re buying other kits.

By: bill

bill — Tue, 13 Jul 2010 02:06:39 +0000

On the stock amp, the unused cathode, plate, and grid elements are grounded together. There’s no voltage present. So the cathode may be hot, and may even poison itself, but if that half of the tube is unused, I guess it doesn’t matter.

By: Cliff

Cliff — Tue, 13 Jul 2010 00:14:29 +0000

Leaving a tube on with no plate voltage/cathode current whatsoever will wear down the tube alot faster than in normal use. (See: Cathode Poisoning)

You’d think that they would start making 6C4′s again if they needed one triode (1/2 of a 12AU7). (or, could just use a 6/12AV6 and ground the diode plates)

Excellent mod! I myself hate having unused tube sections. (Even in AM tube radios where one diode plate in a 12AV6 is left unused)

By: Craig

Craig — Sat, 10 Jul 2010 19:43:52 +0000

Where do i purchase the capacitors and wire for this mod?
Thanks