I read in Megantz' book, one is needed before a Cathodyne PI, and he mentions the Fender Deluxe, but the 5E3 has two regular cathode biased gain stages leading up to it. Then I pick up an old phono amp (Webcor MT-1755), with 6v6 P-P and a Paraphase PI, and IT has a cathode follower stage before it (after a standard cathode bias input stage). What gives? I want voltage gain, not current. Show me a guitar amp with a cathode follower - I need to see when and why it's used. Thanks!
It's usually used as a buffer to drive heavy loads like tone stacks. It does this because it has such a low output impedance compared to a normal gain stage.
When directly coupled from a previous gain stage the interaction of the two is more akin to the Marshall sound (or other high gain amps). Basically the positive side of the waveform gets compressed more than the negative which causes lots of second order harmonic distortion.
Merlin has some great info about cathode followers on his site I would recommend reading it.
When directly coupled from a previous gain stage the interaction of the two is more akin to the Marshall sound (or other high gain amps). Basically the positive side of the waveform gets compressed more than the negative which causes lots of second order harmonic distortion.
Merlin has some great info about cathode followers on his site I would recommend reading it.
All well and good, but not seeing it used that much in guitar amps, and my wanting to do just that with this little phonograph amp I am playing with I am gonna turn it into a 2nd gain stage and see if I can swap headroom for a little distortion. Would like to make it switchable but will see how many poles on a switch I would need.....
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Thanks! I figured someone would find a schematic with a follower stage....but they have the luxury of 4 stages of AX7 to play with..... BTW, the AX84 P1 amp has only two gain stages (one tube), and uses both for gain, also before the tone stack. I only have one tube for gain on my amp, and a SAMS Photofact shows it having two gain stages, whereas what I actually have (see hand drawn pic) is one gain stage and then the follower. And notice the tone stack is after the phase inverter! Webcor must have changed it's design in mid-production... There is some tone stuff in the preamp, to give an old ceramic phono cartridge some semblance of RIAA correction.... So, I just gotta try it! Thanks again!
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I'm now a Cathode Follower "Follower"!
That brings up the question: do you want distortion in the preamp or power tube(s)? I just sub'd out my 5Y3 for two diodes (B+ on this amp is lacking, was only 244 at the rectifier). Now it's 295v! and plate current went from 25 to 38 mA, this amp rocks my 12" cabinet! I'm liking this cathode follower circuit. Have tons of headroom in the preamp, the PI isn't getting pushed hard at all, and the scope shot is at full vol (at spkr). The amp is only 7-1/2 x 3-1/2 x 2" high (less PT). But what a mini-brute! 10 watts diss/tube. And with a pedal......
That brings up the question: do you want distortion in the preamp or power tube(s)? I just sub'd out my 5Y3 for two diodes (B+ on this amp is lacking, was only 244 at the rectifier). Now it's 295v! and plate current went from 25 to 38 mA, this amp rocks my 12" cabinet! I'm liking this cathode follower circuit. Have tons of headroom in the preamp, the PI isn't getting pushed hard at all, and the scope shot is at full vol (at spkr). The amp is only 7-1/2 x 3-1/2 x 2" high (less PT). But what a mini-brute! 10 watts diss/tube. And with a pedal......
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I like the sound of the power amp distortion much better than just the high gain preamp sound. A good rock amp for me has distortion in all stages. I actually dislike the sound of high gain cascading preamp stages so much that I use a fuzz pedal with big clean amps.
Having multiple gain stages can be a daunting task to get to sound good IMHO. You can center bias all stages but multiple gain stages will really sound much better when you alternate bias of each stage like:
Hot --> Cold --> Hot --> Cold
To make good use of the cathode follower I would direct couple it from the previous gain stage and then drive the tone stack.
Having multiple gain stages can be a daunting task to get to sound good IMHO. You can center bias all stages but multiple gain stages will really sound much better when you alternate bias of each stage like:
Hot --> Cold --> Hot --> Cold
To make good use of the cathode follower I would direct couple it from the previous gain stage and then drive the tone stack.
The cathode follower has different sound depending upon if it is DC coupled to the preceding gain stage or not.
Refer the "Wiz"
The Valve Wizard
Cheers,
Ian
Refer the "Wiz"
The Valve Wizard
Cheers,
Ian
"The cathode follower has different sound depending upon if it is DC coupled to the preceding gain stage or not." That's what I wanted to hear! It sounds different.... and direct coupling.....is OK, even meaning your putting plate voltage into the grid of the follower? Will check out the "Wiz". Thanks
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