cathode follower

[cbass]

I am not familiar with any amp that uses a cathode follower after the PI. Not with a long tailed pair PI, anyway. Perhaps you are thinking about the split load PI (aka cathodyne) used in the Fender 5E3 and other similar amps which takes opposite phase outputs from both the plate and cathode to drive the push-pull pair.

http://ampwares.com/schematics/deluxe_5e3.pdf

I think several amps use CF's after the PI Super twin,SVT and some other high powered amps

[Tillydog]

Cathode followers are used to drive high impedance loads, like a tone stack. Why do you want to use one to drive the PI? I really don't think it is necessary.

**LOW** impedance loads...

I don't. I was looking at an amp that has it in the design. Wondering what it meant

Is it DC coupled to the previous stage?

If so, it might be included for the compression/distortion that this circuit can give, rather than the low impedance output / drive capability.

See Merlin's site for a good explanation

http://www.freewebs.com/valvewizard1/dccf.htm

[cbass]

Yes .DC Coupled more compression is the only reason I can think of .Or just a use for that triode

[bluesky636]

Delete

[bluesky636]

Cathode followers are used to drive high impedance loads, like a tone stack. Why do you want to use one to drive the PI? I really don't think it is necessary.

**LOW** impedance loads...

I've already acknowledged that error.

[Firestorm]

This potentially interesting thread got a little derailed.

Fender used the CF as an impedance buffer and (luckily) got more than they bargained for. We also see it used as a current amplifier to drive lots of grids, as in the SVT. But what might be the effect of using one to drive a PI? Maybe bandwidth. What else?

[dave g]

There is really no reason to drive a LTP with a CF because the input impedance of the LTP is already extremely high. The Concorde does it, but it's DC coupled to the 3rd stage and is presumably there to change the clipping characteristics of that stage as was previously mentioned.

[romberg]

Yep. From what I've read on the "net" the amp in question uses a DC coupled follower after the third gain stage. Merlin talks about this general arrangement in his preamp book and here:

http://www.freewebs.com/valvewizard1/dccf.htm

In his book there is a nice example of the gain stage being bias so that the "bottom" of the wave is clipped by grid current limiting of the gain stage and the top is clipped by the cathode follower. The grid stopper controls the amount of "top" clipping. The clipping is very smooth/gradual.

I'm gonna put 1 of these stages into my next build.

Mike

[talbany]

if you were modding an amp and someone told you to add a cathode follower between the last stage and the resistor that goes to the PI input, how would that be drawn on a schematic?

A Cathode follower before the PI would be a good place to generate some 2nd order (fatten up the tone) Harmonics, and some compression since there is plenty of signal to drive it..This would be the only reason I would consider using it..(before the PI)

Hope this answers your question..

Tony

[roberto]

I think several amps use CF's after the PI Super twin,SVT and some other high powered amps

Yes, VHTs (now called Fryette) use this configuration too.

[Ken Moon]

When I was playing around with Marshall-style preamps, I built one with an LND150 source follower after each of the 3 gain stages, and a cathodyne PI. This had a super smooth high gain marshally sound, using only 2 preamp tubes. Harmonics sounded fuller and the overdrive was less ice-picky with the added followers (sorry if that sounds like TGP talk )

The Marshall JCM-1C 1-watt jcm800 amp has an LND150 source follower driving the tone stack, and a cathodyne inverter and P-P 12AU7 power amp, for a total of 3 tubes. I'm about to build a similar amp, but I'll probably add the LND150 source followers after the first two stages just for fun.

Source/cathode followers smooth out the overdriven sound nicely, but as Merlin mentions in his book, a CF doesn't have this effect if the B+ is too low (under 300V in my experiments - the JTM45 CF B+ is 310V or so, '59 Bassman was about 325V). This last observation is not universally shared, so try it for yourself and trust your ears

[bluesky636]

'59 Bassman was about 325V).

59 Bassman (5F6A) B+ was spec'd at around 435 VDC.

[img:800:465]http://farm9.staticflickr.com/8349/8224 ... e585_c.jpg[/img]

[JazzGuitarGimp]

'59 Bassman was about 325V).

59 Bassman (5F6A) B+ was spec'd at around 435 VDC.

[img:800:465]http://farm9.staticflickr.com/8349/8224 ... e585_c.jpg[/img]

Hey Bluesky, Ken is talking about 325V on the CF plate, not the 5881 plates. If you look at the CF in the schematic you posted, you'll see its plate is connected to the +325V node in the power supply.[/code]

[bluesky636]

'59 Bassman was about 325V).

59 Bassman (5F6A) B+ was spec'd at around 435 VDC.

[img:800:465]http://farm9.staticflickr.com/8349/8224 ... e585_c.jpg[/img]

Hey Bluesky, Ken is talking about 325V on the CF plate, not the 5881 plates. If you look at the CF in the schematic you posted, you'll see its plate is connected to the +325V node in the power supply.[/code]

My apologies. I fixed on the "B+" which I've only used to reference the full power supply source before the choke or first dropping resistor.

[M Fowler]

I build a great sounding amp so smooth but can get up and rock. My new version is built into a Mojotone Bassman chassis with new mojotone faceplate.

https://tubeamparchive.com/viewtopic.ph ... ht=swamper

https://tubeamparchive.com/files/swamper_final_207.pdf Schematic