Two tone stacks parallel after cathode follower

[d95err]

If Marshall can make it work with 3 tone stacks plus the clean channel volume control in parallel from a regular 12AX7 cathode follower, it obviously works with two tone stacks.

The key is the series resistor (39k) between the treble viper and volume to avoid interaction with the channel mute switching.

[bea]

I was simply skeptical that the impedance of a 12AX7 as a cathode follower would be really low enough. Therefore my suggestion to spice before building.

Another possibility to consider might be to check wether an ECC832 could be used instead (i.e. if the pinout matches so that the 12AU7 system of the tube would be in the position of the cathode follower.)
Or if an ECC81 which combines high µ and low output impedance would work better.
But always: spice before taking the iron.

[pdf64]

I think that the waveform at the output of an overdriven dc cathode follower will be affected by adding more tone stacks to load it, in the same way that it is significantly affected by adding the first tone stack (compared to its unloaded output); the cathode follower's ability to sink/source current with large signals being the issue, rather than its small signal output impedance.
But it will very likely sound fine and not be a problem, as per the Marshall 410.
Whereas a different tube type for the cathode follower may be more likely to result in a difference that the user may notice.

[bea]

But the Marshall 410 was just an example. The question was generic.

[cbarrow7625]

We would never want to do anything in a guitar amp that makes it perform in a non-linear fashion; that never works out well. ;p


To paraphrase my main man Jim Croce, "If it sounds good do it. If it sounds really good, do it twice."

(None of this is aimed at you PDF64, just seemed like a good place as any to interject).

[Merlinb]

Although a 12AX7 CF would probably drive two Marshall tone stack simultaneously without problems, I would use Fender tone stacks (but tweaked for a similar frequency response), i.e. higher impedance.

[Dave H]

Why not use two CFs, one for each tonestack?

[bea]

What would You do with the remaining triode?

[nickb]

Seems we are trying to discuss in a data vacuum so I've got some info to add fuel to the fire

I've done three things and I'm making the assumption that only one half of 12AX7 is available.

1. Determine the input impedance of a Marshall tone stack. I'm calling this nominally 50K ohms - see the top chart below.
2. Charted the output impedance of a 12AX7 with plate to cathode voltage of 150V vs plate current. On the second chart below the output impedance is 420 ohms with a plate current of 1.5mA and 350 ohms with 2.5mA. Gm goes up with plate current.
3. Set up four sample cathode followers. All are fed with 140Vpp from with a source impedance of 50K. All are loaded with two tone stacks represented as 25K in series with 1uf ( for DC blocking). The supply is 300V and the source offset is 150V. See the bottom chart.
   1. Cathode follower with 100K cathode resistor (1.5mA plate current)
   2. Cathode follower with a 1.5mA constant current sink (CCS)
   3. Cathode follower with 60k i.e. 2.5mA plate current to try to lower output impedance and increase drive current
   4. Cathode follower with a 2.5mA constant current sink

Conclusions:

1. The 100k case only manages approx 20 Vpp out. This might be enough, it depends on the levels in your particular design. If your signal is small enough this is the way to go. Some compression.
2. The 1.5mA CCS does a little better at 28 Vpp (at the expense of complexity). This happens because the high AC impedance of the CCS means all the signal current flows through the load. Least compression.
3. The 60K case gets you to 72 Vpp but there is significant compression due to grid current.
4. The 2.5mA CCS has 112 Vpp out, with moderate compression.
5. The CF output impedance is so low that interaction between tone stacks will be minimal. I can't see that being an issue in practice.

'course this is just a simulation and is therefore subject to the setup, interpretation, models used, my mistakes and other random effects. Just my opinion on compression. Some might argue more is better

[Dave H]

What would You do with the remaining triode?

Sorry, I don't understand the question. It was just an idea. Are you thinking I'd have a spare triode because I'd have to add a 12Ax7 for the extra CF? In that case I'd use the existing 12AX7 for a parallel gain stage and the added 12AX7 for the two CFs.

[Mike Sulzer]

1. The CF output impedance is so low that interaction between tone stacks will be minimal. I can't see that being an issue in practice.

If the CF runs out of current for any reason then it no longer has low impedance, and there is interaction. Since a tone stack stores energy, the interaction can take the form of mutually altered frequency responses.