changing the cathode follower into a gain stage

cb · Post by cb » Wed Apr 13, 2005 4:24 pm

I remember a while back looking at the schemo for the 1959slp RI and wondering if you could change the cathode follower into a gain stage. Then I got TUT and I saw just that very thing in a couple of diagrams. So I'm thinking about trying it but I have a couple of questions:
1. Is there any negative affect on the amp's tone?
2. If there is no downside to doing this mod, why does Marshall do it the current way? Is it just "good textbook practice" or something?

rjgtr · Post by **rjgtr** » Wed Apr 13, 2005 5:48 pm

cb,

If you do it right the amp can sound just fine, but maybe not the same. The tone controls might work a little different due to the different impedence; a lot of people change out the 500p treble cap for a 250p.

And Marshall does do this in their DSL and TSL lines. Other manufacturers also do this (Rivera is one off the top of my head).

I have also used this in some amps I've designed and I actually like it a lot.

If you've got a PTP board in your amp, you can always try it and see if you like it.

cb · Post by cb » Wed Apr 13, 2005 10:41 pm

First rj, props to jax - I was born there! Thanks for the info. After reading your comments and based on the TUT stuff, it's not a question of 'should I', but now it's simply 'when'?

My SL RI board is not PTP - will that be an obstacle?

cb · Post by cb » Wed Apr 13, 2005 10:44 pm

First rj, props to jax - I was born there! Thanks for the info. After reading your comments and based on the TUT stuff, it's not a question of 'should I', but now it's simply 'when'?

My SL RI board is not PTP - will that be an obstacle?

tonejones · Post by **tonejones** » Wed Apr 13, 2005 10:47 pm

Hey rjgtr,
Love the signature bro!!!!

videocat23 · Post by **videocat23** » Thu Apr 14, 2005 12:27 am

would it change the tone if you added a solid state buffer before the tone stack? in theory it shouldn't, as it would be the same thing as the cathode follower. But you wouldn't need a tube for that. A high end op-amp should do the trick without affecting the freq response.

I haven't tried it, but in theory, that should work and your amp would just have an extra gain stage.

Billy Batz · Post by **Billy Batz** » Thu Apr 14, 2005 9:43 am

A lot of things to do with linear and transparent gain and buffering happen in theory but it never truely works out that way.

If you change the CF driving the TS in a Marshall then your just changing to another type of stage driving a TS. It will sound very different like these guys are sayin. TSL/DSLs and Riveras have more gain stages anyway. But if your talkin about changing V2b from the CF into a regular k biased gain stage, and you do want to add a buffer after that to drive the tone stack then all your really talking about is adding another gain stage to a 1959 which is basically a 2203/4. With the stack still on the k I wouldnt know how that would sound.

The TUT series is full of many mods to 1959s and 2203/4s that were big in the 80s that dont sound so great. Im guessing they were huge then because the sound of more and more gian was unique then. Now everyone and their illigitimate sons have a high gain amp. Theres plenty of designs using 4 and more gian stages that sound great. The challenge these days with gain seems to be squeezing it out of an amp with as few tubes as possible and retaining the complexity, harmonix and dynamics.

Brentsp · Post by **Brentsp** » Thu Apr 21, 2005 11:43 am

This is exactly what I'm trying to figure out to.....turning the CF into a gain stage but so far no one knows how or can explain or provide a diagram on how to do it.

videocat23 · Post by **videocat23** » Thu Apr 21, 2005 2:24 pm

just convert the cathode follower look like V1a or V1b or V2a.

now after that, it's up to your ears I guess. The CF is just a buffer so the tone stack doesn't affect the V2 gain stage. So if you didn't put a buffer there, your gain on the new stage (now V2B) would be dependent on the resistance of the tone stack, Rk, and Ck that you pick for that stage (820ohms, 0.68uF for example).

Now if you wanted a tube buffer, you would need to have a new tube put in and just set up the first half of that tube the same as the CF, and don't use the second half of the tube(i'm assuming you would use a 12AX7 for this since they are dual triode, but if you found a single triode it would work too).
So really, the schematic for it is already there in the origianl amp schematic, you're just duplicating the second gain circuit.

Billy Batz · Post by **Billy Batz** » Thu Apr 21, 2005 5:05 pm

Brentsp wrote:This is exactly what I'm trying to figure out to.....turning the CF into a gain stage but so far no one knows how or can explain or provide a diagram on how to do it.

I can explain how just fine. The problem is, as I said in your thread on the topic, you would then have no tone stack- no tone controls unless you wanted to have it on the new gain stage off the plate like Fenders. Sounds like a project.

Brentsp · Post by **Brentsp** » Thu Apr 21, 2005 8:51 pm

Ah I didn't see that you responded. I sent you an email Dan. I know you know how but I have to see it laid out as I'm not familar with the circuit by someone just telling me to wire the plate to the B+ voltage with
a 100k. Use a 10k cathode resistor and wire the grid
to the previous plate with a .022 cap and a pair of
470k's in a voltage divider.........as George has told me.

Heres a diagram if anyone wants to draw out what you guys are talking about
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Billy Batz · Post by **Billy Batz** » Thu Apr 21, 2005 9:46 pm

I sent you a layout of how to do it. The only problem would be fitting it on a standard board. If you are going to try it let us know how it sounds. Id be curcious to know as well.

myker · Post by **myker** » Fri Apr 22, 2005 3:57 pm

yeah, im interested too. take some pics along the way...please
mike

Guest · Post by **Guest** » Fri Apr 22, 2005 7:13 pm

By the way. Its likely youll need to change the cap values and slope resistor.

Dan

Brentsp · Post by **Brentsp** » Fri Apr 22, 2005 10:29 pm

ugh......I may have to just start off with the stock 69 specs first and replace/ add whatever needs to be done.