How many stages in front of the cathode follower?

[rexindigo]

I`m preparing to build sort of a bassman clone and was wondering why there are allways at least two triode stages in front of the CF in all the classic designs?
is it possible to realize a cathode follower circuit with just one 12ax7 (one stage in front of the CF) ?
any suggestions?
thanks for You help, this forum helps me a lot!

[loudthud]

A Bassman 6G6A used a cathode follower on the first stage. I used one in a bass amp I built because I used a 470K plate load and was driving a treble and bass boost network around the volume pot. The low driving impedance changes how the typical tone stack sounds depending mostly on the slope resistor.

[rexindigo]

Thank You Loudthud!

That sounds great. I've studied the 6G6A schematic and this is very interesting
But I'm still not sure if I can just realize a working Amp with just one 12ax7 in front of the PI, is that possible for Example:
V1:
- first Triode: classic Fender input stage
- Second Triode: cathode follower "followed" by the tone stack
V2:
- Phase Inverter
V3+4
- Power Tubes

Just because less is more and i want to keep parts minimized

[loudthud]

I like the minimalist approach but I think you are going to be a little short on gain. You can get more gain out of the first stage by increasing the plate load to 220K or 470K and the cathode follower will preserve it. For the PI, if you use the common cathode stage driving a concertina (aka split load), again, you can increase gain with a higher plate resistor and not suffer because the concertina has a very high input impedance. Choice of power tube then comes into play. EL84's have the highest gain, followed by EL34's, the 6L6 family and then 6550's. So unless you want to spend a month doing computer modeling, just build it and see if you like it.

By itself, the 5F6A Bassman tone stack sounds kind of dark. The cathode follower helps some but you also need additional brightening at the volume control or other place in the circuit. Most designs I see that are restricted to two preamp tubes use two common cathode stages in front of the PI and drive the tone stack from the plate. A little tweeking here and there will get it to sound good. One option discussed on this board is the MosFet source follower. (Do a search) I've never tried it but it's tempting. The blackface tonestack is not as dark as the 5F6 so that in another option.

[rexindigo]

Thank You Again, Loudthud,
thats my approach to the thing, "just build it" instead of to much theory and planning. (thats what I've done since getting addicted to Tube amp building)
Tubes seem to be very tolerant concerning "experiments" and who cares if maybe the circuit is not according to the book, as long as it sounds good....
So thats what i will do: i just try it!
i can still try out different versions of the tone stack , maybe even something very simple like in the princeton.
I'll keep You informed, but it may take some time

[CarlZ]

I'm onboard with loudthud. If I understand properly what you're trying to build, you're going to be really short on gain by a ton and here's why...

Let's say you're using a typical first gain stage, 100k plate load and bypassed cathode resistor feeding a pot. This is going to give you a gain of roughly 56. That translates to a gain of 35dB at the maximum.

Now, that signal is sent to the CF which will cost you a few db but not too much so lets round it down to 30 dB of gain at the input to the tone stack...and here's where the problems really start! The FMV stack is passive and you'll have anywhere from 15 to a max of around 30 dB of insertion loss at mid-band. What you've got left feeding the power amp is at best 15dB and at worst 0dB of signal. In other words...not much more than what your guitar pickup is putting out. This is never going to drive the power tubes to anywhere near full output.

Now, if you want to only use a single preamp tube, you could drive the tone stack off the plate with typical Fender values. Then use the second triode as a gain recovery stage and noodle the gain there till the output is high enough to properly drive the power amp.

I don't always explain things as clearly as I'd like to so I hope you understood what I was trying to say and I'm sure somebody else will come along and clarify things a bit if you're still a little confused.

-Carl

P.S. Yea, the numbers are off by a hair but not by much, and is intended to illustrate the idea more than anything else.

[MWJB]

Ditto what Carl & Loudthud say. If you play guitar a single 12AX7 configured as a CF will not give you enough gain. I have used this configuration on a couple of harp amps and the volume control ends up pretty well dimed...compared to on a regular tolex style 12AX configuration that would be on 2.5 to 3 with a 1Meg volume control.

As Loudthud says you could try a BF style preamp, or you could have the first triode feeding the volume control and the second triode feeding the tonestack from the plate, no recovery triode before the PI. This second route may take some experimenting to get appropriate gain?

[rexindigo]

Thank You Guys, this is very helpful!
After building some Tube amps, I'll start to learn the theory.
I understand that the Cathode follower circuit does not at all amplifiy the signal, and therefore a single 12ax7 configured as a CF will not at all drive
the powerstage to acceptable volume.
I just tried it with my "sandbox" amp, and whats very surprising it sound very much like a "Marshall" Type of amp (of course no overdrive and at very low volume, but with a complete different sound than the usual Plate driven preamp)
Think i will build a classic design then, Bassmann or JTM45
Thank You again!