CF as First Stage

[clyde1]

Fender did this on the 6G series of Bassmans and never again as far as I know. The gain stage had a 220k plate resistor, 2.7k bypassed cathode and 100k cathode resistor on the follower. Any ideas as to why it was discontinued-the infamous grid current problem perhaps? Maybe just economics.

[Chuck H]

There are a lot of ways to make an amp. This circuit wasn't better or worse. They probably changed it for economy. It's a tube stage that's not amplifying anything. The ideology for doing it in the first place was probably to reduce the impedance for feeding the tone stack. In the early stages this seems to make more sense for bass than guitar since there would be less likelyhood of heavy bass notes swinging so wide as to clip the following stage. There wouldn't likely be any clipping at that point with a guitar, so none of the cathode follower mojo we associate with the later Bassman and Marshall amps. If you've heard one and liked it you could build one.

[Tage]

I have a bass preamp that was essentialy the same a one half of a bassmans inputs, so i had an extra triode, and i just hooked it up as a cathode follower, it doesn't realy add or detract from the sound, it is a pretty clean bass preamp, i feel that it takes pedals quite well, fuzzes and such, although i don't think that has anything to do with the cathode follower dv coupled to the first stage.

In hind sight my anp is set up virtualy the same as the 6G bassmans, although i got the idea from the Traynor YBA300

[Amp Kat]

Fender did this on the 6G series of Bassmans and never again as far as I know. The gain stage had a 220k plate resistor, 2.7k bypassed cathode and 100k cathode resistor on the follower. Any ideas as to why it was discontinued-the infamous grid current problem perhaps? Maybe just economics.

They used it in the Bassman 5F6 & 5F6A and in the 80's and 90's with the reissue 59 series. Not sure if there is a difference in a 6G or if you are talking about the 6G6 & A. which was a direct coupled Cathode follower driving the tone stack but it had a 56k plate resistor that sent 250v to the plate. The 5F6 was quite different but is did use a Cathode follower to drive the tone stack. The main reason as Chuck pretty much said was headroom however the Bass players really didn't go for it nor like it so Leo scrapped it but it was to late for Marshall as they had incorporated it into all of their amps and the rest is history.

Would like to see a schematic of the 6G if you have it cause I couldn't find it. Just the 6G6 which actually uses the Cathode follower on the second stage. I still think the follower on the very 1st stage does little except shunts some noise to ground which in a Bass amp is almost negligible but could benefit a high gain amp.

[clyde1]

All 3 of the the 6G series Bassmans had a cathode follower as the first stage as described in the original post. Since there's none of that mojo, as Chuck calls it, to be had, I guess you do benefit from low impedance to the tone stack and tons of headroom. I just bread-boarded it and it sounds fine, nothing special and I'm not sure, like Leo, that it's worth wasting a triode over. A parallel stage would be more to my liking.

[Bruce / Mission Amps]

All 3 of the the 6G series Bassmans had a cathode follower as the first stage as described in the original post. Since there's none of that mojo, as Chuck calls it, to be had, I guess you do benefit from low impedance to the tone stack and tons of headroom. I just bread-boarded it and it sounds fine, nothing special and I'm not sure, like Leo, that it's worth wasting a triode over. A parallel stage would be more to my liking.

I can't find any Fender amp using a cathode follower for the first stage.... what are you guys looking at??

[exclamationmark]

Uhh... I can't actually find a schematic, but I'm fairly sure the first stage is just a gob standard gain stage directly coupled to a cathode follower. To legitimately have cathode follower as the first stage it would require some sort of blocking cap at the input side. But that's just me being pedantic Since it's a direct coupled cathode follower it really deserves special attention and the two stages can be rolled into one, as the behaviour of the circuit depends on both triodes. I suspect you wouldn't get the much sought after compression effect that the direct coupled cathode followers do really well at only the first stage. If there was a tone stack directly after, that would sort of make sense though. Also, reliability may have been an issue if the heaters weren't elevated, as it most likely violated the cathode-heater voltage limit.

[clyde1]

The bass channel on all the above-mentioned. What do you see Bruce?

http://www.thevintagesound.com/ffg/s...g6-b_schem.gif

[Wombaticus]

The bass channel on all the above-mentioned. What do you see Bruce?

http://www.thevintagesound.com/ffg/s...g6-b_schem.gif

There is no Fender amp with a cathode follower as the first stage. You are looking at a conventional cathode biased gain stage with a fully bypassed 2.7K bias resistor, and a 220K anode load feeding into a direct coupled cathode follower at the second stage. The CF is *not* the first stage except insofar as the operating point for the gain stage will influence the behaviour, and potentially the amount of grid current developed by the second stage CF.

[clyde1]

Sorry, my bad. I was referring to the complete triode, as I usually do, as a cathode follower. I realize that it has 2 stages. Thanks for that. Would it even be possible to have a follower as a first stage?

[Steve Conner]

I did this many years ago.

Still happy with it

[Amp Kat]

Sorry, my bad. I was referring to the complete triode, as I usually do, as a cathode follower. I realize that it has 2 stages. Thanks for that. Would it even be possible to have a follower as a first stage?

Yes it would but you would get very little out of it as far as gain or an advantage anywhere and at best a tad less than unity even if you bootstrapped it like the example that Steve threw up which I like btw Steve and quite interesting and I bet it's a pretty quiet overdrive with the follower in the 1st position.

[Wombaticus]

Would it even be possible to have a follower as a first stage?

There is no reason in principle why not. On the other hand, you should also ask yourself "why bother". There's maybe one decent answer to that question. Most of the time we consider including a cathode follower in a circuit when we wish to achieve a low output impedance, but that is not the only potentially desirable feature of such a stage. Another potential feature of the cathode biased cathode follower is the very high input impedance that can be achieved. This might be used to great advantage in cases where the amplifier is intended for use with piezoelectric pickups, which must work into very light loads / high impedances. However, another very desirable feature of an an amp input is to lift the signal as far as possible above the noise floor, as early as possible. So in practice it probably makes more sense to design a stage which offers both a high input impedance as well as some gain. That looks to be what Steve's input stage does. Note that R16 in his schemo is powerfully bootstrapped by the signal at the junction of R15 and R12, so the input impedance of the input is much higher than the 1Meg suggested by the value of R16. However, by virtue of retaining an anode load at R18, the stage still offers modest gain. The best of both possible worlds, really.

EDIT: Actually, on brief reflection, in Steve's particular setup, it will actually attenuate the signal. However, you can achieve what I was describing by implementing kind of the inverse of his stage -- with an anode resistor that is larger than the unbypassed cathode resistor

[Steve Conner]

My design originally started out with the first stage as a cathode follower. It gave a voltage gain of 1 and just lowered the impedance to drive the Baxandall tone stack.

That worked great with active basses, but I soon found that I wanted some more gain. So I changed it from a cathode follower, to a classical gain stage with a very big cathode resistor. The gain is a little more than 1 when the "gain boost" pot is in its lowest setting, and increases from there. (At the lowest gain boost setting, Rp is 22k and Rk is 48.5||22 = about 16k)

I never thought about the input impedance at the time, however I can see now there is some bootstrapping action going on. I made the anode load resistor 22k as a compromise between achieving more gain, and a low output impedance.

The gain boost pot also introduces a high-pass filter effect with C7 as it is turned up.

[R.G.]

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.