Title says it all, but in my ignorance of amplifier design - and the fact that only one of my three guitar amplifiers is cathode biased, I'm curious.
What does a cathode follower do?
- Thread starter Ed Storer
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Wyatt
Tele-Afflicted
- Joined
- Nov 3, 2004
- Posts
- 1,547
Cathode Follower is irrelative to Cathode Biasing.
In guitar amps a cathode follower is most often used in place of a gain stage, it gets its name because the signal is outputed through the cathode instead of the traditional plate output. CF's are buffers, pure and simply buffers, with high impedance input and low impedance output; their super low output impedance allows them to maintain the signal integrity while driving the next stage.
Most traditional Marshall circuits (and Tweed Bassman) use a CF instead of a gain stage for driving the tone stack while producing a rather unique overdrive (hot-rodded Marshalls usually swap this to a gain stage instead for more drive); Dumble used CF's after the PI in some of his Steel String Singers to give the power amp more clean drive; and almost all tube-buffer FX loops use a CF to match the outgoing impedance of the SEND circuit (coupled with a Gain Stage for boosting the RETURN signal).
In guitar amps a cathode follower is most often used in place of a gain stage, it gets its name because the signal is outputed through the cathode instead of the traditional plate output. CF's are buffers, pure and simply buffers, with high impedance input and low impedance output; their super low output impedance allows them to maintain the signal integrity while driving the next stage.
Most traditional Marshall circuits (and Tweed Bassman) use a CF instead of a gain stage for driving the tone stack while producing a rather unique overdrive (hot-rodded Marshalls usually swap this to a gain stage instead for more drive); Dumble used CF's after the PI in some of his Steel String Singers to give the power amp more clean drive; and almost all tube-buffer FX loops use a CF to match the outgoing impedance of the SEND circuit (coupled with a Gain Stage for boosting the RETURN signal).
Old Tele man
Friend of Leo's
Lots of things:
1) provides impedance isolation (hi-Z input to low-Z output)
2) provides power gain (but not voltage gain)
3) provides current gain (but not voltage gain)
4) provides one-HALF of Cathodyne PI output (the in-phase half)
1) provides impedance isolation (hi-Z input to low-Z output)
2) provides power gain (but not voltage gain)
3) provides current gain (but not voltage gain)
4) provides one-HALF of Cathodyne PI output (the in-phase half)
Hey old tele man, I hope all is well!
Are you sure about #2 and 3 above?
I thought current gain and the resulting power gain is only in the realm of the power tubes.
pre amp tubes can either gain voltage, not gain voltage, or even lose voltage but it's not their place to be growin the power.
I could be wrong but I thought that was the basis of it all.
Are you sure about #2 and 3 above?
I thought current gain and the resulting power gain is only in the realm of the power tubes.
pre amp tubes can either gain voltage, not gain voltage, or even lose voltage but it's not their place to be growin the power.
I could be wrong but I thought that was the basis of it all.
Old Tele man speak truth!
Your thought wrong, you basis wrong. You missunderstand power.
The 12AU7 makes a pretty good power tube for low output amps. It's all in the supporting circuitry.
The cathode follower in its DC coupled form also can add some interesting overdrive flavor when pushed. It provides progressively greater compression and even order harmonics as the signal level rises. I hear its effects strongly in one of my amps; as gain starts to get towards max the overdrive gets actually less harsh/aggressive and more fat and thick.
For more reading:
http://www.valvewizard.co.uk/dccf.html
There's a couple of pics in the link that show the asymmetrical compression effect.
For more reading:
http://www.valvewizard.co.uk/dccf.html
There's a couple of pics in the link that show the asymmetrical compression effect.
3-Chord-Genius
Poster Extraordinaire
I don't know but whenever I see a word with "cath" in it I think of a catheter in my weenis, and I absolutely don't like that. I'm cringing.
Ok, a preamp type tube could be wired like a power tube, and then it would be a power tube.
But, I'm talking about stock fender tube amps. The reason they call the power tubes the power tubes is because they are in the power section of the amp and that's where the power is generated.
Everything I've ever read about tube amps states that the voltage gain takes place in the preamp including the PI, and the gain in current and power takes place in the power tubes.
But, I'm talking about stock fender tube amps. The reason they call the power tubes the power tubes is because they are in the power section of the amp and that's where the power is generated.
Everything I've ever read about tube amps states that the voltage gain takes place in the preamp including the PI, and the gain in current and power takes place in the power tubes.
Thanks for the responses. BTW my one cathode biased amp is the only one with a cathode follower.
Ed Storer, which amp is that that has a cathode follower and cathode biasing of the power tubes? ...curious I am....
Old Tele man
Friend of Leo's
Power is voltage x amps: P = V × A.
Cathode follower provides current gain, but not voltage gain; in fact, it has a slight voltage loss (Vi/Vo < 1.00), no better than µ/(µ+1).
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I'd bet it's something like an AC30 or Matchless Lightning. The Brilliant channel of the AC30TB has a DCCF driven tone stack.
Gain with a following load is usually quite a bit down on the "slightly less than 1" gain that is usually accredited to the CF.
For a good basic understanding, you can't get a better description than from Norman Crowhurst.
Try his 1954 article in Dec "Radio Electronics" p.50-51
Radio-Electronics/50s/1954/Radio-Electronics-1954-12.pdf
And the follow-up in May 1958 p.50-53
Radio-Electronics/50s/1958/Radio-Electronics-1958-05.pdf
Other good reading from his English equivalent - Cathode Ray (the pseudonym of M G Scroggie)
Wireless World nov. 1945
Wireless-World-1945/The%20Cathode%20Follower.pdf
and his follow-up on CF biasing and loadlines in june 1955 p.292-296 (p.42-46)
Wireless-World/50s/Wireless-World-1955-06.pdf
and later about CF distortion in may 1961 p.261-265 ( p.25-29)
Wireless-World/60s/Wireless-World-1961-05.pdf
That should do for now, .....but as Old Tele man pointed out, a CF is half of a Split-load PI, hence its cathodyne by-name, and a lot of understanding about the one can be usefull in understanding the other.
For a good basic understanding, you can't get a better description than from Norman Crowhurst.
Try his 1954 article in Dec "Radio Electronics" p.50-51
Radio-Electronics/50s/1954/Radio-Electronics-1954-12.pdf
And the follow-up in May 1958 p.50-53
Radio-Electronics/50s/1958/Radio-Electronics-1958-05.pdf
Other good reading from his English equivalent - Cathode Ray (the pseudonym of M G Scroggie)
Wireless World nov. 1945
Wireless-World-1945/The%20Cathode%20Follower.pdf
and his follow-up on CF biasing and loadlines in june 1955 p.292-296 (p.42-46)
Wireless-World/50s/Wireless-World-1955-06.pdf
and later about CF distortion in may 1961 p.261-265 ( p.25-29)
Wireless-World/60s/Wireless-World-1961-05.pdf
That should do for now, .....but as Old Tele man pointed out, a CF is half of a Split-load PI, hence its cathodyne by-name, and a lot of understanding about the one can be usefull in understanding the other.
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Yup. I use a 12AU7 in one of my Fender Gretas as the output tube. Stock was a 12AT7. Any similar tube will work as an output tube - tone can vary, but they definitely can be "power" tubes.
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Sidebar - The never-ending debate isn't new! From a Zenith ad in the 1958 Radio Electronics:
"3 reasons why there are NO printed circuits in a Zenith TV chassis"
I swear - it's there!
The purpose of the preamp tubes? Raise voltage.
The purpose of the power tubes? The Signal voltage from the preamp is imprinted on the high current flow thru the power tube, greatly increasing the current and power.
How is it known for sure that the power tubes supply the power and not the preamp tubes?
Think about it.
What does the power tube need in the way of signal to operate? VOLTAGE ONLY
The purpose of the power tubes? The Signal voltage from the preamp is imprinted on the high current flow thru the power tube, greatly increasing the current and power.
How is it known for sure that the power tubes supply the power and not the preamp tubes?
Think about it.
What does the power tube need in the way of signal to operate? VOLTAGE ONLY
Endless Mike
Friend of Leo's
So he speaks truth to power?
Endless Mike
Friend of Leo's
It's like the paparazzi, it follows cathodes around and takes their pictures.
Ten Over
Friend of Leo's
No need to stress-out about it, you are right about the function of the preamp and the power amp. The fact that a cathode follower can provide current gain and, therefore, power gain is irrelevant. In fact, the current gain is a severe annoyance when designing buffers and there is the need to balance current consumption with the desired performance. The current gain from a cathode follower buffer in the preamp is in no way used for the power output of the amplifier.
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