re-configurable amp - Princeton, Top Boost, JCM, yada

[pvsage]

First, here's my (limited!) experience building tube amps:

My first amp was a Valve Junior copy with switches to change component values. The switches change cathode bias resistors and bypass capacitors, the first stage coupling capacitor, and a treble bleed cap on the voltage divider before the volume control. This worked fairly well, but the effect on the tone was more subtle than I expected. (The component values were chosen based on the "Voxy", "Fenderish", and "Marshallized" mods listed on the turretboards.com Valve Junior page.) This is still one of my favorite amps.

My second and most recent amp (told you my experience was limited!) has two(!) additional gain stages, a TMB, and a master volume, using the 2.2uF 93Hz shelving cathode bypass caps suggested in the Marshallized mod on the aforementioned page. I quickly learned that four gain stages is *way* too much, so I removed the last gain stage from the circuit. This is my current practice amp (when I practice with an amp).

Then I perused some of the schematics at schematicheaven.com, and was duly humbled. Hence, my current project, which is still on paper:

I'm currently in the process of laying out an amp which, by way of relays on the circuit board controlled by a fairly complex footswitch system, will hopefully be able to bypass gain stages and voltage dividers, switch bright caps in/out, and yes, change cathode bias resistors, cathode bypass capacitors, and coupling capacitors, turning the preamp into a copy of a JCM 1959, a slightly modified JCM 2204, a Vox TB, a Fender Princeton, or any of literally thousands of (hopefully) usable amp sounds that incorporate elements of each of these four well-known amps.

My question is: Has anything like this been done before? Does it even sound feasible?

I'm posting a copy of the schematic, which will hopefully clarify what I'm trying to do on the amp side of things - the "one-of-N" latching circuit for the pedal board still needs some kinks ironed out. The relays are drawn as simple switches; three of them are DPDT bypasses, but that should be fairly obvious from the wiring.

[Bruce / Mission Amps]

First, here's my (limited!) experience building tube amps:

My first amp was a Valve Junior copy with switches to change component values. The switches change cathode bias resistors and bypass capacitors, the first stage coupling capacitor, and a treble bleed cap on the voltage divider before the volume control. This worked fairly well, but the effect on the tone was more subtle than I expected. (The component values were chosen based on the "Voxy", "Fenderish", and "Marshallized" mods listed on the turretboards.com Valve Junior page.) This is still one of my favorite amps.

My second and most recent amp (told you my experience was limited!) has two(!) additional gain stages, a TMB, and a master volume, using the 2.2uF 93Hz shelving cathode bypass caps suggested in the Marshallized mod on the aforementioned page. I quickly learned that four gain stages is *way* too much, so I removed the last gain stage from the circuit. This is my current practice amp (when I practice with an amp).

Then I perused some of the schematics at schematicheaven.com, and was duly humbled. Hence, my current project, which is still on paper:

I'm currently in the process of laying out an amp which, by way of relays on the circuit board controlled by a fairly complex footswitch system, will hopefully be able to bypass gain stages and voltage dividers, switch bright caps in/out, and yes, change cathode bias resistors, cathode bypass capacitors, and coupling capacitors, turning the preamp into a copy of a JCM 1959, a slightly modified JCM 2204, a Vox TB, a Fender Princeton, or any of literally thousands of (hopefully) usable amp sounds that incorporate elements of each of these four well-known amps.

My question is: Has anything like this been done before? Does it even sound feasible?

I'm posting a copy of the schematic, which will hopefully clarify what I'm trying to do on the amp side of things - the "one-of-N" latching circuit for the pedal board still needs some kinks ironed out. The relays are drawn as simple switches; three of them are DPDT bypasses, but that should be fairly obvious from the wiring.

Hundreds and hundreds of builders have passed that way before. Sorry old man, although fun, ....you have broken no new ground.
But, your second and third triodes will be broken when you switch around and bypass. There will be no grid loads on them and could turn a pretty color before they eventually fizzle out for good.
Like many before you, you'll find that there are only a few settings that sound good and are usable so, you'll probably redo it later with less switching and options... most of that just ends up being a bunch unneeded stuff getting in the way of good tone.

[hasserl]

Remember the KISS principle

[pvsage]

Hundreds and hundreds of builders have passed that way before. Sorry old man, although fun, ....you have broken no new ground.
But, your second and third triodes will be broken when you switch around and bypass. There will be no grid loads on them and could turn a pretty color before they eventually fizzle out for good.
Like many before you, you'll find that there are only a few settings that sound good and are usable so, you'll probably redo it later with less switching and options... most of that just ends up being a bunch unneeded stuff getting in the way of good tone.

I figured there had to be a reason why you never see this...oh well. At least now I'll save some time and money by *not* building this!

Thanks for the quick feedback.

[Bruce / Mission Amps]

I figured there had to be a reason why you never see this...oh well. At least now I'll save some time and money by *not* building this!

Thanks for the quick feedback.

Well you have a decent start just eliminate some of the switching around stuff... choose either the classic tweed Bassman/Marshall/VOX layout (with or with out the extra gains stage) or the brown face/black face Fender layout and go for it.

[chipaudette]

Whoa whoa whoa...you should definitely build something. Just because people have tried it before, and just because most of your sounds won't be very cool, you will find this kind of amp an amazing learning experienece about how different elements affect your tone. That kind of experience is totally killer.

It's clear that you've got the amp-building bug, so it's likely that this amp will not be your final amp. When you get the bug to build your 4th amp, the knowledge you gain from this amp will let you make very informed decisions about what you want your 4th amp to look like.

So, add some 1M resistors from the grid of each tube to ground (maybe you can even get away with 3-5M to affect the preceeding tone shaping less...others around here might now how high its safe to go). The goal is to make sure that the grid of over tube ALWAYS has a path to ground no matter what the switch status.

Other than that, you'll probably be good. It'll be fun!

Chip

[chipaudette]

Two issues that I see (besides the no grid resistors mentioned by Bruce) are:

(1) Relays (as opposed to switches) often introduce noise into your sound. There have been several threads on this recently. I'm also still struggling with it in my amp. You might consider prototyping with switches and the swap them out with relays once you know that your amp itself is hum free.

(2) Kicking a cap into and out of your circuit with switches (or relays) can sometimes cause a pop or thump in your sound as it initially charges or discharges. It matters most with caps that hold back some DC voltage like your coupling caps. I don't think that it'll hurt your amp...it'll just sound annoying. I'm not sure the best way around this...I think that it involves NOT switches the cap completely out of the circuit but instead swapping in a giant 10M resistor so that it is effectively out of the circuit but that DC can still leak to it and keep it charged. I'm not sure on this. If you have a problem, others around here can help.


Chip

[chipaudette]

Man, three posts in a row on the same thread. Sorry, I just keep getting new thoughts on your layout. My new thought is that, as freakin' switch-tastic as your amp is, you're missing out on a whole class of amp sounds...Fender.

In your schematic, I see that your tone stack is at the end and its driven by a cathode follower. This is surely a classic Marshall / Fender Bassman configuration. Cool. But, it won't sound anything like any of the classic Fender brown/blackface amps. Fender amps all (nearly) use tone stacks after the first gain stage and do not use a cathode follower to drive them. To make up for the loss in the tone stack, they then follow the tone stack with another basic gain stage. The component values in the tone stack are different from your tone stack as well.

So, if you feel like adding more switches, you can either add switches to move the location of your current tone stack (though you'd still have the wrong component values) or you could use switches to remove your current tone stack (and the cathode follower) and add a new tone stack earlier in your chain. For this last option, you'd also need another set of TMB pots.

If you do move the tone stack, and if you add a 4th (and maybe 5th) gain stage (but no cathode follower), you'd also capture the configuration of the lead channel of the Mesa Boogie Mark series (II, III, and IV). Sweet sweet overkill.

Switch-tastic!

Chip

[trevorus]

Off Topic: I have a question. What do you use to draw your schematic?

[pvsage]

Whoa whoa whoa...you should definitely build something. Just because people have tried it before, and just because most of your sounds won't be very cool, you will find this kind of amp an amazing learning experienece about how different elements affect your tone. That kind of experience is totally killer.

It's clear that you've got the amp-building bug, so it's likely that this amp will not be your final amp. When you get the bug to build your 4th amp, the knowledge you gain from this amp will let you make very informed decisions about what you want your 4th amp to look like.

So, add some 1M resistors from the grid of each tube to ground (maybe you can even get away with 3-5M to affect the preceeding tone shaping less...others around here might now how high its safe to go). The goal is to make sure that the grid of over tube ALWAYS has a path to ground no matter what the switch status.

Other than that, you'll probably be good. It'll be fun!

Chip

I know that many people who build solid state distortion pedals like to shunt their inputs directly to ground when bypassed. Would this be a safe practice with tube grids?

In my first amp, I actually used the switching to add resistors and capacitors in parallel. For example, the fixed bypass caps were 2.2uF, and I had 22uF caps that could be added in parallel by switch. There was only a little bit of a pop when switching; this would probably be the safer way to go for this circuit as well, and it wouldn't take up any more space on the circuit board.

5 gain stages?! My second amp started out with four, and I was getting strong preamp distortion with the volume control at about *one*! I basically used four Valve Junior stages back-to-back-to-back. I'll have to look up the Mesa/Boogie Mark series...I assume at least a couple stages actually have their gain reigned in a bit, like the first two stages of a JCM-800 model 2204. The weird part is that, even with the master volume turned way down, I was getting feedback through the strings.

As for the tone stack...I considered making it movable, but the logistics were a bit complex, and I could practically *see* some fairly high parasitics no mater how I laid it out. The obvious alternative is to have separate tone stacks, and consider the one after the CF as part of that "effect circuit" as far as switching is concerned, with the Fender stack switched in when the Marshall/Vox circuit is switched out.

I had settled for a "nearly-Fender" TMB, with the treble cap and slope resistor tweaked just a bit to give a response curve a little closer to a Vox when the midrange is turned to 10K. I considered making the tone caps and slope resistor switchable as well, but there's the matter of the different pots in Fender, Marshall, and Vox tone stacks.

The comment about relay noise is a good point! The relay coils will be energized using a well-filtered DC power source, and I'm sure I can track down ways to suppress any remaining ripple noise.

"Answers are easy! The hard part is asking the right questions."

[Steve Conner]

Sounds like a cool project, but building it would be a *lot* of work! You might want to consider making something like the Seymour Duncan Convertible instead:

http://www.diyguitarist.com/GuitarAmps/Convert.htm

It's like Lego with tubes. If you don't like that concept, you could make a "mainframe" containing the transformers, power tubes and PI, with a drawer where an entire preamp plugs in. Like those old Tektronix scopes with the plugins. Have two identical drawers so you can load any two preamps you like, and switch between them with a footswitch.

Finally, can you think of a way to change what might be the most important and characterful element of all: the speaker and cabinet?

[pvsage]

Off Topic: I have a question. What do you use to draw your schematic?

Actually, I use the crappy little Paint utility that comes with Windows. It was simply a matter of drawing a few basic component shapes, then laying them over a grid. The grid just made it a lot easier to make sure that Point A connects to Point B - after finishing the circuit, I erased the grid. Not the easiest way to do it I'm sure, but considering I already had Windows, the price was right.

[chipaudette]

On the Mesa Boogie gain structure...yes, 5 gain stages can be stupid and ridiculous amount of gain. But unlike your valve JR, the Boogie doesn't just pile gain on gain. Instead, there's a ton of signal shaping between gain stages. This tone shaping "throws away" lots of the gain.

For example, the Boogie Mark IIc uses a standard Fender blackface setup at the beginning: guitar -> gain stage 1 -> Tone Stack -> Vol Pot 1 -> stage 2.

When you then kick in the overdrive circuit, the output of stage 2 get routed over to another volume pot (plus a big step-down resistor) to control the gain and then it gets passed to stage 3. Stage 3 boosts the signal...but it has a puny cathode bypass cap and so doesn't have as much gain as it could. Between stage 3 and stage 4 are some high cut and bass cut elements which further cut the gain.

It then goes into stage 4 which has no cathode bypass cap at all, really cutting that stage's gain. After stage 4 are more treble-cut and bass-cut elements which cut some more gain.

Finally, the overdrive signal then dumps back into the main signal pathway (ie, the pathway shared with the clean channel). Right there is gain stage 5. Gain stage 5 is also in the signal path for the clean channel pathway, but for the clean channel, there is a 3M resistor decimating the signal. The overdrive channel, though, jumps around this 3M resistor is bypassed so the full overdrive signal is slamming stage 5. After stage 5 are the master volume controls and all that. Basically stage 5 is the end.

So, yes, the Mark IIc has 5 gain stages, but tons of the available gain is either not used (no cathode bypass caps) or is "thrown away" (tone shaping elements).

Chip

[pvsage]

Sounds like a cool project, but building it would be a *lot* of work! You might want to consider making something like the Seymour Duncan Convertible instead:

http://www.diyguitarist.com/GuitarAmps/Convert.htm

It's like Lego with tubes. If you don't like that concept, you could make a "mainframe" containing the transformers, power tubes and PI, with a drawer where an entire preamp plugs in. Like those old Tektronix scopes with the plugins. Have two identical drawers so you can load any two preamps you like, and switch between them with a footswitch.

Finally, can you think of a way to change what might be the most important and characterful element of all: the speaker and cabinet?

If you're talking about simply re-routing the input signal from the jack to the discrete preamp "blades", I had actually started sketching out something similar last night. It may be easier to make multiple pre-configurable preamps and switch the signal from one to the other as though I were switching between effect pedals. (That was my initial idea with the gain stage and CF, by the way - bypass them just like external stomp boxes.)

Most of the hard work was the circuit board layout. Well, that and figuring out the latching switch circuit that would let the user change from one of many configurations to another by tapping a single foot switch...but it turns out that this, like almost everything else, had already been figured out by someone else. Actually building the thing should be no harder than building any other amp, although it will be a bit more tedious, especially the wires for the relay coils on the circuit board and the switch arrays in the pedal board.

Changing the speaker and cabinet? Only thing I can think of would be either a rotary switch similar to a speaker impedance switch, or a lower-current rotary switch re-directing the current through power relays. Actually wouldn't be too difficult to add something like this to the pedal switch array. The micro-seconds during which the power transformer has an open secondary may be a bit hard on the coils...

[pvsage]

I just found this schematic under "similar threads":
http://www.kilback.net/homebrewtweak...hem-od3_r3.gif

This doesn't seem too dissimilar from my bypass switching - I just need to make sure there is a path to ground for the grid of each stage when bypassed...right?