Split channel,off of first plate?

[sonofmickel]

Is it ok to split off of the first gain stage plate to two separate circuits?

[roberto]

Not the best idea, as they will interact each other, plus the load will be low.

[sonofmickel]

Would coupling caps off the plate on each circuit isolate them?

I actually have an amp that is like this that I am blueprinting now. It has two channels off the plate that each go their own separate ways, then they each go to a separate effects send, to a passive channel switch, then return to a a 12ax7 recover stage, then to the PI. I don't hear much if any contamination from either channel.

[tubeswell]

Would coupling caps off the plate on each circuit isolate them?

No. AC load goes 'through' coupling caps.

[pdf64]

As long as R5 and R7 are large, eg >=100k then the loading should be at a level to allow reasonable gain, and any grid clipping at subsequent stages shouldn't bleed through (eg by affecting signal swing at the 1st plate).

[stiltamp]

Is it ok to split off of the first gain stage plate to two separate circuits?

Hook in a Cathode follower at the Anode of V1 to lower the output impedance, both channels are extensive decoupled then.

[sonofmickel]

R5 would be Fender value 100k R6 would be a Marshall treble peak 470k/470p. I am trying to make a Blackface/Naylor SD60 two channel amp using four preamp tubes(3 if we leave out the PI). I'll finish drawing it up(it's actually my early Phaez amp) and see what you guys think. Feel free to poke fun, show disgust whatever.

Stiltamp: are you saying to split the channels off of the cathode like a marshall cathode follower tonestack? I will investigate my PT and see if it can take another tube. If it can, I can do a fully tube buffered effects loop. Or...should I use the fx loop return tube as a Ctahode follower and put in a completely passive effects loop?

[pdf64]

Hook in a Cathode follower at the Anode of V1 to lower the output impedance

I don't see that it would be worth it.
As I see it, the main source of interaction would be the signal level and control settings being such that one of the channels enters grid clipping at V2 or V3, most likely V3.
If R7 was 0 and R8 on max, then the output of a cathode follower would get distorted.
Granted it would be less so than for a common cathode.
Plus the lower output impedance of a cathode follower wouldn't provide the standard BF tone control range.
If there's a spare triode, surely it would be better to just have an individual 1st stage for each channel?

[xtian]

What about taking one split from the anode of V1 and the other from the cathode of V1? See attached.

I'm not suggesting this will work, just asking a question.

[roberto]

No, it doesn't work. you have no gain on the cathode, plus the cap to ground.
Also the CF after the first gain stage idea is not good, you'll loose all the fender sound.

You can use a split plate load on the first stage if you want, anyway the relay switching would be my choice.

[stiltamp]

What about taking one split from the anode of V1 and the other from the cathode of V1? See attached.

I'm not suggesting this will work, just asking a question.

It seems you misunderstood me I meant to adding a CF as shown on attached schematic.
The output impedance of the CF is about 600 Ohms,
add the red drawed 39 kOhms resistor to maintain the output impedance for the Fender style circuit.

[roberto]

The problem is that the CF will change the sound even if the output impedance is similar.

[V2]

your original plan looks good to me. R7 is in series with R8, so there won't be a load problem on that arm.

Kevin O'Connor does something similar with his Standard preamp design.

[teemuk]

Both methods will work.

Both methods will also suffer from the inherent drawbacks of the circuit: Common cathode amps should preferably be coupled to a load that is high enough to make it rather "invisible" in comparison to load presented by the plate resistor. Complex and low-impedance loads (like tonestacks) will cause the gain stage to "interact" with them. Purely resistive high impedance loads will cause practically no interaction at all. Oh, resistors to "isolate" both outputs from the plate are advised to minimise interaction. Anyway, I had the impression that the common cathode amp with two outputs was a rather common circuit for input stage in modern dual-channel designs...?
Cathode follower amps have less load interaction due to their lower output impedance and current driving capability. They tolerate lower impedance loads but proper "impedance bridging" is still advised. However, their gain is less than zero. Additionally headroom requirements of the cathode follower will limit the linear gain you can extract from the plate output. Some tubes may not tolerate the elevated cathode-to-heater-voltage difference.

Both both circuits will in practice work. Somehow. Depends a lot on implementation whether the performance is hideously interactive with channel controls or highly un-interactive.

[pdf64]

The output impedance of the CF is about 600 Ohms

I get about two or three times that?
eg set unloaded CF output to 1V (to avoid excessive distortion) and loaded it until it drops to 0.5V.
TSC has it as 1k3.

Cathode follower amps .. their gain is less than zero

Ha, sorry, the pedant in me had to pick up on that.
I'm sure everyone knows what you mean but I think that should read 'CF gain is less than 1', using a normal definition of gain (V out / V in).