One cathode follower, 2 tone stacks

[Ray Barbee]

Running both stacks in a channel switching config. attached to one cathode follower, outputs switched after stack and master. Anyone done this? My questions would be:

1) resistors to isolate each stack?

2) better to do this, or put a 1uf dc blocking cap after the cathode and just switch stacks with a relay?

3) effect of stack 2 on stack 1 tonally?

I've worked on Diesels that did this, but never disconnected one stack to see how much it affected the other.

[pdf64]

Adding a 2nd tonestack to the DCCF has an obvious, significant effect on its large signal waveform.
The difference may not be particularly audible though, it’s apparent but certainly it doesn’t change the tonal character.
I decided not to follow that option though.

[rangdipkin]

could you use a DPDT relay for an extra contact to lift the ground off the mid pot on the stack not in use?

[cdemike]

could you use a DPDT relay for an extra contact to lift the ground off the mid pot on the stack not in use?

You definitely could, but I think you may still get some wave form changes if the tone stack outputs are still connected since the CF still "sees" the load presented by the treble circuit. Probably less audible than parallel circuits, and I'm not really even sure it'd be audibly different than a single tonestack. If you're using relays, you could just use 2x DPDT relays to basically make a 4PDT switch with the "send" and "return" for each of the 3 pots going to the 2 different tone stacks and to feeding the input of the PI. Otherwise, I know parts express has 4PDT toggles for sale.

[dorrisant]

could you use a DPDT relay for an extra contact to lift the ground off the mid pot on the stack not in use?

You definitely could, but I think you may still get some wave form changes if the tone stack outputs are still connected since the CF still "sees" the load presented by the treble circuit. Probably less audible than parallel circuits, and I'm not really even sure it'd be audibly different than a single tonestack. If you're using relays, you could just use 2x DPDT relays to basically make a 4PDT switch with the "send" and "return" for each of the 3 pots going to the 2 different tone stacks and to feeding the input of the PI. Otherwise, I know parts express has 4PDT toggles for sale.

This is how I've done it... With anti-pop resistors between NO and NC on each relay. Works great.

Hi Ray... Been a long time since the Amp Expo. Good to see you are still kicking!

[Ray Barbee]

What about a 1uf cap to block the dc from the cathode follower and a relay to switch the output between the stacks? Any foreseeable issues there?

The 1uf should be more than large enough to be transparent. I see what you're saying about using 2 dpdt to switch the output of each cap, but that assumes two tone stacks using the same cap values and slope resistors.

[dorrisant]

I'm not trying to tell you what to do - I may be misunderstanding, but, I'd use a 0.1µF cap to feed the A side COM of a DPDT relay... A side NO and NC would go to the two individual TSs. Jump NO of the A pole of the relay to the NC of the B pole, then jump from NC to NO. Connect the B side COM to GND. If you are using two Masters, run the wipers to the NO and NC of a second relay. Connect the COM of one stack to the PI entrance cap, and the other to ground. Do the jumpers... connect a 1M resistor across the two wipers to eliminate the pop... or not.

The schem below is a perfect working example. The 0.1µF is plenty large enough to pass all that you need, or at least it has worked a bunch of times for me with no ill effects.

With this method, you isolate and ground the unused TS. So it is not affecting the circuit when using the active channel.

Am I missing something?

[cdemike]

What about a 1uf cap to block the dc from the cathode follower and a relay to switch the output between the stacks? Any foreseeable issues there?

The 1uf should be more than large enough to be transparent. I see what you're saying about using 2 dpdt to switch the output of each cap, but that assumes two tone stacks using the same cap values and slope resistors.

You're right, I absolutely did make that assumption -- good point. I think dorrisant's schematic looks good if you want to have two dissimilar tone stacks. Out of curiosity, what two tone stacks were you planning on? My current build should be able to switch between the 250pf/56K and 500pf/33k tone stacks, but I do love an out-of-the-box tone network.

[Ray Barbee]

I'm not trying to tell you what to do - I may be misunderstanding, but, I'd use a 0.1µF cap to feed the A side COM of a DPDT relay... A side NO and NC would go to the two individual TSs. Jump NO of the A pole of the relay to the NC of the B pole, then jump from NC to NO. Connect the B side COM to GND. If you are using two Masters, run the wipers to the NO and NC of a second relay. Connect the COM of one stack to the PI entrance cap, and the other to ground. Do the jumpers... connect a 1M resistor across the two wipers to eliminate the pop... or not.

The schem below is a perfect working example. The 0.1µF is plenty large enough to pass all that you need, or at least it has worked a bunch of times for me with no ill effects.

With this method, you isolate and ground the unused TS. So it is not affecting the circuit when using the active channel.

Am I missing something?

Yeah that's what I was talking about. I haven't had popping issues switching wipers. The resistor there would change the taper of the pot(s). Also, the dc blocking cap is essentially in series with your tone caps, so I'd be worried .1uf might make an audible change .1uf in series with .022 for example is .018 ish. The slope resistor would affect that but I'd want to clip things in and out to verify if I could hear it. 1uf is so big vis a vis the tone caps that it shouldn't cause an audible effect.

[Ray Barbee]

What about a 1uf cap to block the dc from the cathode follower and a relay to switch the output between the stacks? Any foreseeable issues there?

The 1uf should be more than large enough to be transparent. I see what you're saying about using 2 dpdt to switch the output of each cap, but that assumes two tone stacks using the same cap values and slope resistors.

You're right, I absolutely did make that assumption -- good point. I think dorrisant's schematic looks good if you want to have two dissimilar tone stacks. Out of curiosity, what two tone stacks were you planning on? My current build should be able to switch between the 250pf/56K and 500pf/33k tone stacks, but I do love an out-of-the-box tone network.

Really could do any two tone stacks, or no 2nd tone stack if the stack for that channel was before the CF. In a 3 stage to CF standard 2204 topology, if you switch in a fender type stack after stage 1 and go right to stage 3 and CF after the stack, you get a quite nice Fendery clean channel. Then on that channel, the CF output can go right to the PI.

Just comes down to the best way to do it. I wanted to bounce those ideas and see what people's experiences were, as I wasn't convinced the Deisel method was very transparent in terms if tone preservation.

[Rogan]

The Marshall JVM 410 and 210 amps run two tone stacks off of one cathode follower with no isolation. There is a 39k resistor between each treble wiper and volume pot, but I think that's to isolate the channel mute relay from the other channels, in case both volume and treble is at max.

I've only played a JVM410 once, so I don't know if there is any interference between the tone stacks. The solution is good enough for Marshall though...

https://www.drtube.com/marshall-jvm/