Shared and split cathode in Super Bass/Super Lead

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VanMalmsteen

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Good evening, Rig-Talk. I am curious; I was reading a thread elsewhere and came across a thread about a Marshall amp. A comment was made about how it was unusual for a superbass to have shared cathode like a Superlead (I may have that backwards). Can anyone tell me what is typical for those, and in layman’s terms, what’s the impact of shared vs split as far as the feel/audible changes.
Thanks everyone.
 
Dale, the shared cathode was a direct copy in 1967 (Marshall JTM) from a Fender design.

Jim Marshall must of realized that splitting the cathode for "guitarists" would allow more "Gain & Tone" options from the dual Triode 12AX7 tube as the lower resistance of the cathode resistor more gain can be finessed (Biased) out of that triode gain stage while using bypass caps to remove low bass signals as treble is often perceived as gain.

Soon followed the NFB circuit which really got guitarist going from 27k to 47k to 100k and selecting different impedance taps 4/8/16 made what were then considerable changes to feel and gain as well as attack and response. This varied due to who populated the boards, supply of components and who was soldering in the ohm tap selector that day.

Transformers played and even greater part as evolution occurred to suit players needs.

In summary: It is easier for guitarist to clip the amps preamp. Bass players wanted the clearest signals over all.

This was the beginning of catering to guitarists by Marshall. Glad Jim decided on 4 X 12 Cabs as well.

I am sure someone with vastly more knowledge than I will drop in and perhaps shed a different light as to your question.

Just my $.02

David...♫
 
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David covered it pretty good. The shared cathode resistor is half the value of what an equivalent single cathode resistor would be since it has double the current flowing thru it and thus double the voltage across it. Some guys love the shared, I think Michael Audette did and maybe still does and he got ripping tones with it.

Here's a calculator that shows you a graph of the output based on the cathode setup.

I just finished building a Superlead which allowed me to make my Superbass more of a true Superbass instead of pulling double duty as a SuperB-Lead. But what the fuck is a 'true Superbass' anyway, Marshall always seemed to be transitioning during the golden years (late 60s).

And what about the cathode cap on V2? If SuperLeads had it and Superbasses didn't that would throw another monkey wrench in the feel/tone differences. That was one of the few changes I made to mine, I really like no cap there for tone...but I like the gain it brings so it's good I get both again.
 
D
NewReligion said:
Dale, the shared cathode was a direct copy in 1967 (Marshall JTM) from a Fender design.

Jim Marshall must of realized that splitting the cathode for "guitarists" would allow more "Gain & Tone" options from the dual Triode 12AX7 tube as the lower resistance of the cathode resistor more gain can be finessed (Biased) out of that triode gain stage while using bypass caps to remove low bass signals as treble is often perceived as gain.

Soon followed the NFB circuit which really got guitarist going from 27k to 47k to 100k and selecting different impedance taps 4/8/16 made what were then considerable changes to feel and gain as well as attack and response. This varied due to who populated the boards, supply of components and who was soldering in the ohm tap selector that day.

Transformers played and even greater part as evolution occurred to suit players needs.

In summary: It is easier for guitarist to clip the amps preamp. Bass players wanted the clearest signals over all.

This was the beginning of catering to guitarists by Marshall. Glad Jim decided on 4 X 12 Cabs as well.

I am sure someone with vastly more knowledge than I will drop in and perhaps shed a different light as to your question.

Just my $.02

David...♫
Click to expand...
Ah, I see. Have you seen JMP Bass models with split?

So that negative feedback circuit climbing essentially gave it a brighter or more ‘gainy’ tone, where it would have been darker or rounder with the NFB 27k?
 
SpiderWars said:
David covered it pretty good. The shared cathode resistor is half the value of what an equivalent single cathode resistor would be since it has double the current flowing thru it and thus double the voltage across it. Some guys love the shared, I think Michael Audette did and maybe still does and he got ripping tones with it.

Here's a calculator that shows you a graph of the output based on the cathode setup.

I just finished building a Superlead which allowed me to make my Superbass more of a true Superbass instead of pulling double duty as a SuperB-Lead. But what the fuck is a 'true Superbass' anyway, Marshall always seemed to be transitioning during the golden years (late 60s).

And what about the cathode cap on V2? If SuperLeads had it and Superbasses didn't that would throw another monkey wrench in the feel/tone differences. That was one of the few changes I made to mine, I really like no cap there for tone...but I like the gain it brings so it's good I get both again.
Click to expand...
I’m no tech, but I enjoy the inside of amps. I feel like half the time Insee a Marshall from mid-late 60s to 70 or so they always have variances. So what is a Superbass? Good question, I think you and Dave nailed it; depends on the person and the day of building!
 
VanMalmsteen said:
So that negative feedback circuit climbing essentially gave it a brighter or more ‘gainy’ tone, where it would have been darker or rounder with the NFB 27k?
Click to expand...
More NFB = less gainy/less raw/tighter bottom, and transition from clean to dirty less smoothly
Smaller NFB resistor = more NFB
Higher OT tap = more NFB

EDIT: and many Marshalls have a 0.68uF Presence cap. That essentially turns the Presence into an NFB control. If you've ever had an old Marshall that just got meaner and more raw as you turned up the Presence instead of just getting brighter...that was probably one of those.
 
psychodave said:
I played Chris Merren’s stock 1967 Super Lead that had a shared cathode and it had a shit ton of gain. We cranked it up through an original pinstripe 4x12 with 20 watt Celestion. It was glorious.
Click to expand...

That had to be an experience. So damned loud but too great to turn down. Lucky you.

Do you recall what material you rolled through on it?

David...♫
 
SpiderWars said:
More NFB = less gainy/less raw/tighter bottom, and transition from clean to dirty less smoothly
Smaller NFB resistor = more NFB
Higher OT tap = more NFB

EDIT: and many Marshalls have a 0.68uF Presence cap. That essentially turns the Presence into an NFB control. If you've ever had an old Marshall that just got meaner and more raw as you turned up the Presence instead of just getting brighter...that was probably one of those.
Click to expand...

0.68uF presence cap was essential to getting a more full sound and adding aggressiveness to my amp. The cap construction matters as much as the value in that spot - very important not to overlook the impact of the presence cap for tuning how an amp sounds, each one is different.
 
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