Gain stage triode questions

[Randall]

Looking at the Bass channel in this modded Bassman schematic, I wonder why there is no cap between the plate of the second triode and the grid of the third? Wouldn't this put plate voltage directly on the grid? And if a cap is omitted, what would be a proper value?

Secondly, why is there no cathode bypass cap on the third triode, or maybe why do some preamp triodes have bypass caps and some don't?

http://hasserl.com/Amp_Mod_Docs/Fend..._mod_schem.pdf

[Gnobuddy]

Wouldn't this put plate voltage directly on the grid?
<snip>
Secondly, why is there no cathode bypass cap on the third triode?

The third triode is operating as a "cathode follower". The incoming signal goes to the grid, as usual, but the output is taken from the cathode, and not the anode. Notice that there is no anode resistor?

Because the output signal is actually taken from the cathode, there is no cathode bypass cap - you do not want the cathode signal shorted to ground via a bypass cap in this type of circuit.

Since the output is taken from the cathode, we would actually like the cathode to be at somewhere around 1/3 to 1/2 of supply voltage (that might be 100V - 150V), rather than the typical 1.5V you find in a 12AX7 common-cathode gain stage. Connecting G1 directly to the previous stage's anode is a cheap way to get roughly the right cathode voltage, as it eliminates a few resistors and capacitors.

Cathode followers are usually used as impedance buffers. They have a voltage gain of about unity - the signal coming out is very slightly smaller than the signal going in, instead of fifty or sixty times bigger. Leonidas' plan was to use the cathode follower to drive the tone stack, while buffering the preceding gain stage so it doesn't get loaded down by the low (and frequency dependent) impedance of the tone control circuitry.

Leo got the details badly wrong, by the way - this particular cathode follower is biased far too "hot", and distorts like crazy as the signal level goes up. This was definitely not what Leonidas wanted - he was always focused on clean amps - but this increased distortion is one of the reasons why the Bassman, a failed bass guitar amp, turned into a successful blues guitar amp. Blues guitarists liked having the distortion that Leonidas was trying so hard to eliminate from his amps!

Why do some preamp triodes have bypass caps and some don't?

Sometimes there is a bit too much gain in a guitar preamp, and a cathode bypass cap can be omitted to drop the gain down a little bit.

Another reason to omit a cathode bypass cap is if the intention is to apply negative feedback to the cathode.

-Gnobuddy

[Randall]

Darn, I totally missed that it is a cathode follower. My bad. Thanks for the great answers!

[Leo_Gnardo]

this increased distortion is one of the reasons why the Bassman, a failed bass guitar amp, turned into a successful blues guitar amp.

By some coincidence was also the circuit copied into early Marshall amps, thereby launching another branch of the amp building industry.

[Randall]

Also by coincidence that I was asking in another thread about the tone stack being fore or aft the third triode in this circuit. I still am not really grasping that.

[eschertron]

Also by coincidence that I was asking in another thread about the tone stack being fore or aft the third triode in this circuit. I still am not really grasping that.

Just my take on this, but assuming a fair degree of distortion/clipping in the preamp, a tone stack early in the signal chain affects the character of the distortion, while a tone stack late in the signal chain acts more like a proper EQ. Some amps have tone controls in both locations.

[soundmasterg]

Another thing to consider and something you may have noticed is that amps with the tone controls early in the signal chain like a blackface Fender tend to allow more noticeable tonal changes while one later in the signal chain such as a Marshall doesn't seem to do much when you move the tone controls.

Greg

[Chuck H]

Another thing to consider and something you may have noticed is that amps with the tone controls early in the signal chain like a blackface Fender tend to allow more noticeable tonal changes while one later in the signal chain such as a Marshall doesn't seem to do much when you move the tone controls.

Greg

I actually notice the opposite when an amp is clipping because early alterations are more thoroughly swamped by subsequent clipping stages. As far as clean tones I agree with your observations, but I think the reason is in the actual tone circuits rather than having to do with circuit topography. The Marshall tone stack, as run from a cathode follower, having a smaller slope resistor, larger treble cap value and smaller mid and bass cap values and being wired at the mid pot such that there is minimum floating resistance for the circuit has about half the range of the Fender BF circuit.