CF as First Stage

[Steve A.]

The rhythm channel of the Mesa Boogie Maverick has an interesting Bright/Fat switch shown in the attached snippet. In the Bright mode the signal going to the tone stack is fed by the anode of the first stage; in the Fat mode the signal is fed from the cathode of the second stage which is wired as the cathode follower half of the DC-coupled pair (one name for the two stages in the classic Bassman/Marshall cathode follower arrangement.)



After "jonesing" for a Maverick since the late 90's I finally got one last year when a used one reached a price point I could not refuse. Once I got it I was not as impressed as I was when I had first tried them out. So I ended up rewiring the Rhythm channel as a Lonestar (which for all intents and purpose is a tweaked Fender BF design.) After doing that and being very pleased I decided that the Lead channel which had a sound to die for was really just a one trick pony (basically the same as the Lead channel on the DC-3.) Looking at architecture of the Lead it is just begging to be rewired as an SLO. I did some preliminary drawings for that but moved on to other projects (sure sounds like OCD to me! ) The hardest thing about all of that was translating the Maverick schematic to the printed circuit board and making up layout drawings which will eventually be transferred to detailed pix of the board.

Steve Ahola

[loudthud]

Has anyone run the 6G6A or 6G6B through the tone stack calculator? Huge loss in the bass frequencies before the volume control. Not much difference if you change the source impedance from 1K to 38K. The 1 Meg slope resistor sets the bass turnover frequency pretty low with the .25uF caps. There is some adjustment with the Bass pot but it falls off the bottom of the graph. The Treble control kind of has the opposite effect. It goes from almost flat to a huge treble cut that acts as a bass boost.

[Amp Kat]

A follower as a first stage may be good or bad for other reasons, but it's decidedly not the best way to go for noise. The general rule for low noise is to focus on the first stage as a low noise design and getting the signal amplified up enough to get out of the noise floor. All of the noise of the first stage is amplified by all the following stages. When you're assigning the total needed gain to stages, put as much as possible in the first stage so that the following stages amplify its noise as little as possible.

There are other ways to get a low output impedance.

The input impedance of a normal triode stage is generally good enough for a magnetic guitar pickup, although there are ways to boost it too.

While I agree with that RG what do you think about this scenario of shunting the grid circuit noise of the next stage to ground in a 1st gain stage cathode follower because : The cathode load resistor is in parallel with the next stages grid leak resistor . The Cathode output impedance is equal to the reciprocal of the gm so placing either of those internal values in parallel with the Rk and Rg will shunt external noise to ground mainly because the cathode impedance is so low.

[R.G.]

what do you think about this scenario of shunting the grid circuit noise of the next stage to ground in a 1st gain stage cathode follower because : The cathode load resistor is in parallel with the next stages grid leak resistor . The Cathode output impedance is equal to the reciprocal of the gm so placing either of those internal values in parallel with the Rk and Rg will shunt external noise to ground mainly because the cathode impedance is so low.

That's reasonable, if you consider only the noise added by the grid circuit of the amplifying stage.

The problem then becomes that the noise of the cathode follower's grid circuit is buffered and lovingly transferred at low impedance to the grid of the amplifier stage. If the grid circuit noise of the CF is about the same as the grid circuit noise of the second and amplifying stage, you've only added the somewhat-shunted noise of the CF driving the second stage grid to the same noise you've have gotten if you hadn't used a CF.

This concept puts you into the game of picking amplifying devices to match the source impedance driving the input. For a given source impedance, you need a certain input impedance of the amplifying device to get the best signal to noise on the input. If you're stuck with a specific kind of amplifying device that doesn't work all that well with the impedance of the signal source, you can sometimes win by using a buffer in front if you can use buffers that are better suited to the signal source.

The game becomes looking for the lowest RMS sum of input source impedance noise, input voltage noise, and input current noise with the necessary biasing networks. There's generally a chapter or two on the topic of picking amplifying devices to match the source impedance for lowest-noise compromises in every book that addresses amplifier noise.

There are unusual situations and exceptions, but in general, low noise amplification requires you to get as much gain as possible in your first stage.

[Amp Kat]

That's reasonable, if you consider only the noise added by the grid circuit of the amplifying stage.

The problem then becomes that the noise of the cathode follower's grid circuit is buffered and lovingly transferred at low impedance to the grid of the amplifier stage. If the grid circuit noise of the CF is about the same as the grid circuit noise of the second and amplifying stage, you've only added the somewhat-shunted noise of the CF driving the second stage grid to the same noise you've have gotten if you hadn't used a CF.

This concept puts you into the game of picking amplifying devices to match the source impedance driving the input. For a given source impedance, you need a certain input impedance of the amplifying device to get the best signal to noise on the input. If you're stuck with a specific kind of amplifying device that doesn't work all that well with the impedance of the signal source, you can sometimes win by using a buffer in front if you can use buffers that are better suited to the signal source.

The game becomes looking for the lowest RMS sum of input source impedance noise, input voltage noise, and input current noise with the necessary biasing networks. There's generally a chapter or two on the topic of picking amplifying devices to match the source impedance for lowest-noise compromises in every book that addresses amplifier noise.

There are unusual situations and exceptions, but in general, low noise amplification requires you to get as much gain as possible in your first stage.

Yes, good answer and I mean obviously this is the way everyone does it and has done it for years so there is no real advantage as I was trying to answer the question of topic but if I had to use one I would bootstrap the input resistor and drop the value to maintain virtually the same load as the pick-up. The main problem then would be output swing and headroom limitations which is one of the reasons they canned the 6G6 in the 1st place.

[J M Fahey]

Well, some fully SS amps add a useless CF stage at the input simply to claim "Tube Preamp".
One famous such case is a Bass Amplifier maker whose brand I won't mention but which may be spelled with the letters E ... D ... E ... N
Grid is ground referenced, cathode resistor goes to the -70V rail, and plate to the +70V one.
I bet the beancounters vetoed a properly fed and biased *real* tube gain stage.