Cathode follower "gain" question

[mlp-mx6]

Hi folks,

I've been trying to find equations around the type of cathode follower circuit seen in a Bassman/Marshall - regular triode tied directly to the follower. I'm using this as the first gain stage in my 3-channel ODS-style amp. I'd like to get a little more output from this tube - and was wondering if I could change the 100K to 220K - on just the first triode's plate, or on the follower's cathode, or both. Anyone experimented with this kind of thing before? (I know that I *can* change it - I was just hoping that someone else could speak authoritatively before I whip out the soldering iron. )

Thanks!
Michael

[oldhousescott]

The gain of the cathode follower section is slightly less than one so it's not providing any increase in voltage amplification. It does, however, provide a lower output impedance to the tone stack so there should be slightly less signal loss there. You can raise the plate resistor and/or reduce the cathode resistor on the first stage to increase the gain. Assuming the cathode is fully bypassed (22uF or greater) you may not notice much difference reducing the cathode resistor.

Another way to increase gain would be to just parallel the triode sections on that first tube. I believe Matchless did this on some of their amps (actually paralleled triodes throughout the amp).

Oh yeah, check out Aiken amps technical notes if you haven't already for detailed analyses of various circuits.

Hope this helps...

[mlp-mx6]

Scott,

Thanks. I am aware that the cathode follower itself has a gain of less than unity. (that's why "gain" is in quotes in the topic! )I am really referring to my first stage as the "regular common-cathode triode PLUS the cathode follower" as a unit. I chose to use the cathode follower because I am driving two tone stacks from this one stage.

Fender did something similar to what you describe in the Rivera-era Twin II and Concert. In fact, they drove 2 tone stacks off that paralleled 12AX7. So that approach remains an option - but I like the lower output impedance driving the tone stack, so I'd like to keep the first stage architecture as I have it.

Bottom line - I'd like a little more output (voltage swing) from my first stage. Is the easiest thing to just increase the plate resistor? Or the follower's cathode resistor? Or both? Or something else? Have you ever seen a guitar amp with a cathode follower cathode resistor larger than 100K? (just curious)

BTW, I have the first triode's cathode bypassed with a 4.7uF cap. The follower's 100K cathode resistor is not bypassed.

Didn't see anything on Aiken's site on followers - I checked before coming here. Perhaps I missed it?

Michael

[Darkbluemurder]

FWIW, 220k plate and 3k3 cathode is a good combination. Compared to the 100k plate/1k5 cathode it seems to have a bit more gain and also a bit more headroom but it's not much.

[oldhousescott]

Changing from 100k to 220k plate resistors calculates out to about a 1.25 times increase in gain, or about 2dB. I don't know if that would be enough for your application. I've only ever seen 100k or 56k as the CF cathode resistor in conjunction with 12AX7's. You could use a 12DW7 in V1, using the 12AU7 section for the CF. Setting the cathode resistor down to 10k, this would reduce the output impedance slightly (down from about 800 to about 300 ohms) and provide more current drive for the tone stacks.

Or, you could always just cascade one triode into the next with a voltage divider. The losses due to the tone stacks should be overwhelmed by the voltage gain of the cascaded stages. Not the direction you wanted to go, I know, but maybe the easiest to get the amount of drive you want.

Once again, HTH...

[mlp-mx6]

Thanks guys. As time permits, I will probably change the 100K on the normal triode to 220K. We'll see if that does the trick.

My car's alternator belt broke on my way home, so I won't be doing much amping until she's back on the road.

[UR12]

I just thought of another option and that is to adjust the plate voltage to a higher value by adjusting one of the resistors values in the power supply voltage divider, or just connecting the plate to a higher voltage source from the power supply. Like I said ....just a thought

[jaysg]

...cathode follower circuit seen in a Bassman/Marshall - regular triode tied directly to the follower. I'm using this as the first gain stage in my 3-channel ODS-style amp.

hmm...be prepared to be do a lot of testing with different first stages. I tried parallel triodes and a cathode follower in my 2 channel Odious. I found that each had a severe effect on tone down the line. I ended up with two triodes with independent plate and cathode treatment, with the grids tied together. The parallel triode made everything thicker in the ODS channel. ymmv

[macula56]

hey OldHouseScott, where are you in SC? i'm in charleston.

[mlp-mx6]

Update on this - I tried other values for that resistor, but they all screwed up the tone for me. I ended up going back to the 100K resistor over the regular common-cathode gain stage.

BTW - macula56 - I was raised in Charleston (on James Island). We'll be down at Folly Beach for a week's vacation in a couple weeks. Where are you?

I was also there for the CRBR on the new bridge on 4/1. (Cooper River Bridge Run) That's a LONG uphill!

Regards,
Michael

[MarkB]

If you increase the cathode follower cathode resistor, you have to consider the heater-cathode voltage limit. You'd need to check the data sheet and check the voltage drop across any new value resistor.

[oldhousescott]

Macula56, I'm in the great metropolis of Clover, about a half-hour south-west of Charlotte, NC. Haven't been to Charleston in many years, may have to visit soon, now that the kids are old enough to appreciate it.