How easy is it to switch in additional gain stages?

[Pott]

Basically I was thinking of the fact that my ideal amp is one of the 6V6s Fender down to the clean channel, but with footswitchable preamp gain...I was thinking of two independent stages (one additional 12AX7), each voiced slightly differently and with their own gain pot. One would be a little bit thicker and bassier, the other thinner and spankier. Both could be turned on individually or together, or none for a clean sound.

How easy/not easy would that be to do?

[SatelliteAmps]

It could be done. Depends on your definition of "easy." The gain stages aren't hard to work out, nor are the relays needed for the foot switch, if you have experience doing this kind of thing. If you are asking about a new build from the ground up, it would be considerly more costly than a single channel, non switching amp. Modifying an existing amp might be more of a pain due to increased needs of the PT and the layout issues.

[PRNDL]

The easier method would be to switch in a gain stage (instead of switching between).
Some boutique builders boast switching between a Fender clean and Marshall lead channel.

The hard part is designing the amp so that both sound great, plus adding a second set of tone and volume pots, plus a master volume.

I believe the the "Overtone" is available as a channel switching kit.

Does anyone know of a fantastic channel switching kit?

[Tone_Terrific]

The easy way is to get an OD pedal.
Somebody else has done all the r&d and you get to buy your preferred tone, no hassle.

[Pott]

Actually I do concur on the OD part, it's what I do now, but I find amp distortion when well designed has a more pleasant clean sound when I back off the volume pot, and well it'd lighten up the pedalboard hehe...

[epluribus]

It could be done. Depends on your definition of "easy." The gain stages aren't hard to work out, nor are the relays needed for the foot switch, if you have experience doing this kind of thing. If you are asking about a new build from the ground up, it would be considerly more costly than a single channel, non switching amp. Modifying an existing amp might be more of a pain due to increased needs of the PT and the layout issues.

Hey Adam.

I've been fiddling with this idea a ton lately, way too much fun. I have an old Kalamazoo that has an unused triode in it, and I diverted the power from the vibrato triode to my new gain stage, which makes power supply issues easier. Sorta fiddly, but it's a bench rat for doing experiments so no problem.

But for a serious build for stage use...what kind of relays would you recommend and how would you hook them up? This is an area as yet unexplored in my budding electronics career. :)

--Ray

While we're on the subject of additional gain stages, I'm under the impression that a 12AX7 triode doesn't tax the power supply all that much, something like 1/2 watt IIRC. This is another issue I've not explored, owing to the already-set-up power supply in my Kzoo. Generally, how do you approach the power considerations when adding gain stages?

[FortinAmps]

+1 for the od pedal. You can even look into those dual OD pedals to get your different voicings. That's the quickest and easiest implementation.

Cheers,
Mike

[donnyjaguar]

What Toneterrific and PRNDL say. What you are basically proposing is adding a second channel. This will need its own set of tone controls to be useful. Of this I'm certain.

[SatelliteAmps]

You don't need to add a separate set of tone controls or volumes. Doesn't sound like that is what you want anyways. It all depends on where you put the new stages. I would run them as first stages before any other volume or tone controls so that the later controls would still affect the signal. If you run them later in the signal, you would have to preset everything or have controls to affect them (which isn't that much more difficult, but it is more work).

I've done this for a number of touring musicians, where I build the front end of one of my amps into a foot pedal. The idea is that if someone was doing a fly-in gig and didn't know what kind of an amp they were getting, they could at least have a decent front end that could be plugged into any amp that could be made clean, and get a decent tone out of it. I have built them with just a volume, a volume and tone, and a volume and gain controls. Same idea really.

Since you have it on the bench, I would probably try and wire all this up without worrying about the footswitching yet. Tackle the gain stages first and see how you like them. Since you already have the amp on the bench, I would mount a little piece of metal to mount a few switches on (so you don't have to drill the front of the amp.), and I would wire the gain stage(s) to that. You could run just one half of the 12AX7 as stage one, and have it kick in the other half as stage two. You can wire it up as any standard gain stage.

If you decide you like how it sounds, then the issue of footswitching comes into play. The main issue is that you don't want to have 30 feet of signal drapped across the floor while you use a foot pedal to switch it off and on. So you need to set up a relay. What this does is the footswitch will trigger just the relay in the amp (no signal at all through the footswitch), which will then trigger an internal switch to add in either or both gain stages. The same system is used in some channel switching amps. If I was doing this for myself, I'd look at someone else's schematic and borrow that part. It would be easier than working it out yourself. Let me know if you need help finding one.

And the power requirements for a 12AX7 are lower than some similar tubes, but it really matters what we are comparing it to.

[epluribus]

You don't need to add a separate set of tone controls or volumes. Doesn't sound like that is what you want anyways. It all depends on where you put the new stages. I would run them as first stages before any other volume or tone controls so that the later controls would still affect the signal. If you run them later in the signal, you would have to preset everything or have controls to affect them (which isn't that much more difficult, but it is more work).

I also concluded that, when stacking up gain stages, I like my tone stack later in the chain. So I too simply tack additional (tuned) stages on the front end. If I need EQ earlier on, I much prefer an EQ pedal to the complexities an early tone stack introduces. (Also makes for a good clean boost, depending on the pedal. Another gain stage!)

However, I also learned that it was far easier to make the tone stack moveable if multiple locations are desirable. Just gotta pay attention to impedances and where, if any, the recovery stage is. Though the technique works well, it's easier to leave the tone stack near the end of the preamp where it sounds best anyway and use the EQ pedal if needed earlier.

On the Fender/Marshall thing, I found another handy goodie...wiring two slope resistors in paralell in the tone stack. You put one on a switch (the other hard-wired) and mark the positions Fender-ish and Marshall-ish. You can also do this with the various cap values, but the switches start piling up...

I've done this for a number of touring musicians, where I build the front end of one of my amps into a foot pedal. The idea is that if someone was doing a fly-in gig and didn't know what kind of an amp they were getting, they could at least have a decent front end that could be plugged into any amp that could be made clean, and get a decent tone out of it. I have built them with just a volume, a volume and tone, and a volume and gain controls. Same idea really.

I stick a Weber Mass in my speaker line, zero the output, and use the Mass's line-out to turn the FrankenAmp into a pedal. Makes a wonderfully responsive pedal that way, great tube-y tone.

Since you have it on the bench, I would probably try and wire all this up without worrying about the footswitching yet. Tackle the gain stages first and see how you like them.

Kinda how I'm proceeding. If I ever decide I like something enough to commit to a finished amp, this will definitely be handy stuff to research. However, the short version you outlined tells me enough to know how to design with such future additions in mind.

Since you already have the amp on the bench, I would mount a little piece of metal to mount a few switches on (so you don't have to drill the front of the amp.), and I would wire the gain stage(s) to that. You could run just one half of the 12AX7 as stage one, and have it kick in the other half as stage two. You can wire it up as any standard gain stage.

Glad you said that. I've used a piece of light angle iron drilled and clamped firmly to the chassis to hold my switches and pots and all that. Fortunately the tube sockets are already in place, so that part is easy. But I feel better knowing my angle-iron approach isn't totally preposterous.

If you decide you like how it sounds, then the issue of footswitching comes into play. The main issue is that you don't want to have 30 feet of signal drapped across the floor while you use a foot pedal to switch it off and on.

Wondered about this, 'preciate the tip. Sounds like an RF magnet to me otherwise.

So you need to set up a relay. What this does is the footswitch will trigger just the relay in the amp (no signal at all through the footswitch), which will then trigger an internal switch to add in either or both gain stages. The same system is used in some channel switching amps. If I was doing this for myself, I'd look at someone else's schematic and borrow that part. It would be easier than working it out yourself. Let me know if you need help finding one.

Thanks Adam, really nice of you to offer. I may take you up on that at some point.

And the power requirements for a 12AX7 are lower than some similar tubes, but it really matters what we are comparing it to.

Power supply design is a part of amp design I've not really researched yet, beyond calculating power demands vs. available power--learning to build cascading gain stages and couple them properly has been really challenging so far. Kinda cool that there's so much room for sculpting tone and response just doing that alone.

A bit more recently I started spending more time on the tone stack and all the different ways to build them as well. (I like the old Vox AC30 two-knob stack. Strikes me you don't need a recovery stage or a cathode follower like you do with a three-knob stack--but I have a lot more number-crunching to do on that.)

If I ever hard-wire this beast I'll definitely have to QC the available power in this amp and the way I'm regulating and smoothing it--trying to work within the confines of what's in there as a learning tool and all.

Pott, I hope all of this is useful to you as well. Sounds like we're both abominating the same things, more or less. :)

--Ray

[RedMan]

It sounds like a great idea that you have there. I also very much prefer all tube drive over solid state for many reasons including what you have stated. Most any amps supply can handle one additional 12AX7 so I wouldn't worry about that too much. It's pretty straightforward to use switching relays as already stated but there are a few things you must watch out for to eliminate switching noise. One thing that would be cool is either a some sort of way to blend the 2 gain stages. You could either use 2 separate volumes, a balance and volume or or 2 balance controls to balance between the clean and mean and either extra gain stage. Sounds like a fun project.

[epluribus]

It sounds like a great idea that you have there. I also very much prefer all tube drive over solid state for many reasons including what you have stated. Most any amps supply can handle one additional 12AX7 so I wouldn't worry about that too much. It's pretty straightforward to use switching relays as already stated but there are a few things you must watch out for to eliminate switching noise. One thing that would be cool is either a some sort of way to blend the 2 gain stages. You could either use 2 separate volumes, a balance and volume or or 2 balance controls to balance between the clean and mean and either extra gain stage. Sounds like a fun project.

Level controls on each gain stage are a thing of beauty. When I had all four hooked up with a pot for each stage this thing looked like a porcupine. But it's so cool to be able to shape saturation so exactly...the variety of balances you can achieve are downright amazing.

I'm really getting a kick out of this project, btw, thanks...and it's been going on for, oh, goin' on two years maybe? Every time I read about a cool circuit, I run out in the garage and rewire this poor thing. Already had to replace one tube socket. :dunno

The neighbors are totally mystified as to the notion of when it might be "done." Their dog likes it when it oscillates too. :)