hi all - can someone with greater experience than me please cast an eye over the following schematic to see if it'll successfully switch between two separate tone stacks on the single cathode follower - any pops/issues ?
and are the 100k mix resistors enough on the outputs of the 2 master volumes ?
I can get around this problem by adding another 12ax7 but dont have the room in the amp
two channels/one cathode follower
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phillyhudson
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two channels/one cathode follower
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Re: two channels/one cathode follower
Your cathode follower can't work because the grid has no bias voltage. See https://www.valvewizard.co.uk/accf.html.
Be aware that regardless of how you bias the CF (either by direct coupling, fixed bias, or bootstrapping) you will end up with a pretty big dc voltage on your relay.
Be aware that regardless of how you bias the CF (either by direct coupling, fixed bias, or bootstrapping) you will end up with a pretty big dc voltage on your relay.
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phillyhudson
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Re: two channels/one cathode follower
ah right - thanks for pointing that out - back to the drawing board
Re: two channels/one cathode follower
AC coupling the CF would be a lot easier to figure out in terms of switching without issues versus switching inputs on a DC coupled CF. Although there will still be significant DC on your relay depending on how you bias the CF, at least with AC coupling there would be not direct relationship between the switch and the bias condition for the CF, which would be especially important in light of many 12AX7 cathode followers being set up to draw significant grid current.
Is the inclusion of the CF aimed at imparting compression or just to drive the tone stack from a low output impedance stage? If it's the latter, you could always use a source follower from a MOSFET for one channel and use the triode for the OD channel in order to introduce compression. From there, you could keep it fairly similar to how it is as it stands currently with the pre-master volume mixing resistors, although I'd suggest placing the mixing resistors after the volume rather than before it. Raising the values may also be a good idea to reduce interaction between the parallel tone stacks -- my knee jerk would be to reach for, 220, 470k, or something in between.
Is the inclusion of the CF aimed at imparting compression or just to drive the tone stack from a low output impedance stage? If it's the latter, you could always use a source follower from a MOSFET for one channel and use the triode for the OD channel in order to introduce compression. From there, you could keep it fairly similar to how it is as it stands currently with the pre-master volume mixing resistors, although I'd suggest placing the mixing resistors after the volume rather than before it. Raising the values may also be a good idea to reduce interaction between the parallel tone stacks -- my knee jerk would be to reach for, 220, 470k, or something in between.
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phillyhudson
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- Joined: Fri Dec 21, 2007 11:35 am
- Location: UK
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Re: two channels/one cathode follower
thanks for reply - I,ve tried a mosfet CF but it doesnt sound the same to my ears , and yes it's the compression/sound of the CF that I like
the idea was to have a clean sound separate from a marshally - OD sound in the same amp but I cant get the clean sound how I want - the answer would be to just add a 5th valve but that's a whole new project
I'm experimenting with a CF after the first triode and its better than NOT having one but not as good as a CF in front of two triodes
the idea was to have a clean sound separate from a marshally - OD sound in the same amp but I cant get the clean sound how I want - the answer would be to just add a 5th valve but that's a whole new project
I'm experimenting with a CF after the first triode and its better than NOT having one but not as good as a CF in front of two triodes
Re: two channels/one cathode follower
If I'm understanding you right, it sounds like the CF is primarily there for the OD channel and that you may not be committed to the clean channel having a CF. If that is the case it may make more sense to just experiment with asymmetric mixing resistors (high values on the lower impedance, CF-driven side).
However, if that's a misunderstanding, though, there are a few ways to get an extra triode into the circuit somewhere. The two that I think would make the most sense would be:
- Replace a gain stage with something like a LND150 MOSFET (input gain stage being most common since it won't be overdriven in most conditions); or,
- Use a triple triode compactron like 6AC10 (or 6C10 if you're feeling spendy...)
Either should provide the extra triode you need to get you the CF. 6C10s are very similar to 3x 12AX7 triodes in one tube, but they are extremely difficult to find and extraordinarily expensive. 6AC10s, on the other hand, are much more accessible ($8.70 right now from CE Dist) and are very similar to having 3x 12AT7 triodes in one bottle. Gain stages using 6AC10 would yield approx 7/10 as much gain as a 12AX7 stage, but if you find that's an issue when the amp gets built, you can get more gain from those stages with some revisions elsewhere in the signal path. The first things that stand out would be moving the 22k resistor feeding the grid on the triode labeled "drive" channel and increasing that same stage's grid leak resistor to 1M. I'd probably accompany those changes with an effort at shifting that same triode away from center-biased and more like a cold clipper to compensate for the smaller signal.
The compactron route wouldn't necessarily add any extra cost since many are still cheap enough to make up for the increase in price for a 12-pin socket vs a 9-pin socket. Lead dress, on the other hand, may be difficult to get right given how, uh, compact the layout would be using a compactron.
However, if that's a misunderstanding, though, there are a few ways to get an extra triode into the circuit somewhere. The two that I think would make the most sense would be:
- Replace a gain stage with something like a LND150 MOSFET (input gain stage being most common since it won't be overdriven in most conditions); or,
- Use a triple triode compactron like 6AC10 (or 6C10 if you're feeling spendy...)
Either should provide the extra triode you need to get you the CF. 6C10s are very similar to 3x 12AX7 triodes in one tube, but they are extremely difficult to find and extraordinarily expensive. 6AC10s, on the other hand, are much more accessible ($8.70 right now from CE Dist) and are very similar to having 3x 12AT7 triodes in one bottle. Gain stages using 6AC10 would yield approx 7/10 as much gain as a 12AX7 stage, but if you find that's an issue when the amp gets built, you can get more gain from those stages with some revisions elsewhere in the signal path. The first things that stand out would be moving the 22k resistor feeding the grid on the triode labeled "drive" channel and increasing that same stage's grid leak resistor to 1M. I'd probably accompany those changes with an effort at shifting that same triode away from center-biased and more like a cold clipper to compensate for the smaller signal.
The compactron route wouldn't necessarily add any extra cost since many are still cheap enough to make up for the increase in price for a 12-pin socket vs a 9-pin socket. Lead dress, on the other hand, may be difficult to get right given how, uh, compact the layout would be using a compactron.