cathode follower vs. plate driven tone stack

[fusionbear]

I have heard and read a bit of this discussion with regards to the advantages in tone in a cathode follower tone stack vs. a plate driven tone stack in Marshall type high gain circuits. I have built both and maybe my ears are going, but I like the rawness of the plate driven circuit more. Is there any thing "wrong" circuit wise with the plate driven tone stack? I have heard that the cathode follower is hard on tubes. Is this true? Maybe one of you more seasoned fellows could give me some references to read up on this and or comment?

Thanks in advance...

[diagrammatiks]

you're more likely hearing the difference between series overdriven regular stages versus the overdriven cathode follower more so then the effect of tone stack loading.

The difference to the tone stack in a marshall circuit is almost negligible but not having that cathode follower is going to have a big difference on the sound.

[teemuk]

I have heard that the cathode follower is hard on tubes. Is this true?

Maybe, maybe not. I think that in the preamp stages the currents those stages will be passing through are so tiny that there really won't be so significant power losses that could result into untimely tube death.

But generic cathode followers do introduce a high cathode-to-heater voltage, which some tubes out there simply do not tolerate.

[Reeltarded]

Nothing wrong with it for us, because we are going to tweak from a baseline. The plate driven is flatter if you plot the curve, and of course the tone knobs do different stuff at some higher R. I like both.

[tubeswell]

I have heard that the cathode follower is hard on tubes. Is this true?

Depends on the tube. Modern production tubes tend to be highly variable in quality. Whereas in the golden age of manufacture, they had a lot more quality control in the assembly process and rejected bad stuff outright. The main determinant of whether a tube will work well as a CF is the h-k insulation. Many NOS 12AX7s will run comfortably with the cathode idling at 200, whereas you may have to use heater elevation on modern tubes (and even then that might not work, depending on the tube)

[Colossal]

The difference to the tone stack in a marshall circuit is almost negligible but not having that cathode follower is going to have a big difference on the sound.

Absolutely. That cathode follower is part of the magic and is a source of added compression. I'm a squish junky and like the sound. I elevate cathode followers to about 50-80VDC.

[renshen1957]

I have heard and read a bit of this discussion with regards to the advantages in tone in a cathode follower tone stack vs. a plate driven tone stack in Marshall type high gain circuits. I have built both and maybe my ears are going, but I like the rawness of the plate driven circuit more. Is there any thing "wrong" circuit wise with the plate driven tone stack? I have heard that the cathode follower is hard on tubes. Is this true? Maybe one of you more seasoned fellows could give me some references to read up on this and or comment?

Thanks in advance...

Hi,

The plate driven EQ allows the controls to be more effective; variable loading on the gain stage produces a larger range of variation as the controls are manipulated. Whether someone likes this or a CF better is a matter of preference.

The Cathode follower was part of a feedback controlled stage to drive the EQ in the 5E6-A Bassman. The loop provided a virtual earth mixing function, and the output impedance of the stage would be reduced and drive the EQ more easily. A CF follower was the easiest way to drive the loop and consequently more accurate when driven by a low impedance.

This gain stage configuration changed when the 5F6 eliminated the loop and had the PA feedback bootstrapped to the EQ which lessen the load on the CF. The 5F6-A eliminated the PA feedback connection.

Marshall and Traynor copied the 5F6-A Bassman and perpetuated the CF in their designs. Some view the CF as an essential part of the Marshall, or Bassman tonal signature. Others regard the CF as a vestigial tube stage of an earlier circuit.

Go with your ears and what you tonal signature you like.

[gingertube]

renshen1957 has it right - the change with frequency of the tone stack impedance changes the anode load and so the anode driven tone stack is more "reactive". You want to watch the minimum impedance of the tone stack when driving from the anode which means keep that slope resistor at the higher end of values like the typical Fender value of 100K.

If wanting that mid gutted sound you get with a 33K slope resistor then you are pretty much forced to drive it with a cathode follower.

The cathode follower imparts some additional compression (as explained in Merlin Blencowe's book) but I worry a bit about it running in its grid current region with very high tube current (from a tube longevity/reliability point of view) so I generally don't use it.

Cheers,
Ian

[Structo]

Good stuff guys!