Mesa Bottle Rocket - Circuit ?s

[BrassElephant]

My friend has this pedal and it sounds killer for alt-rock/grunge heavy tones. I want to build one. It will be a long time before I can find the time (new baby) but I wanted to get my head around a couple things.

I have a question about the busy resistor/capacitor network between the treble wiper and the output. I'm assuming this is a tone shaping network, is that correct? Does it shave off a lot of high frequency content? The sound is on the darker side.

Separate inquiry; if I wanted to match this preamp to a power amp, I know there's 2 ways about it. I could calculate the voltage gain of the circuit, then pick output tubes to match. Or I could choose output tubes and desired output wattage, then increase/decrease voltage gain thru the circuit to better match that compliment. The problem is I don't know how to calculate voltage gain. I'd love to build this into a head with an output wattage of around 20W (6V6s?). So as the circuit is documented, is it going to need adjustment to play well with a 20W output? Or just as is?

Or better yet, can someone learn me how to calculate voltage gain with this circuit as an example?

Thanks for reading, thankful for the knowledge here in these forums.

Cheers

[tubenit]

I would suggest taking a look at Mark Huss's 6V6 Plexi and studying the power amp of that design. 


And also at Hoffman's  6V6 Plexi.   Hoffman Amplifiers Stout (el34world.com)


With respect, Tubenit

[BrassElephant]

Thanks tubenit, I appreciate you posting those references here.

I'm not having any trouble coming up with a PA design. I'll definitely go with something tried-n-true. The issue is that I want to know how to tell if a particular pre circuit is gonna give enough voltage gain to adequately drive a particular PA circuit. I don't know if this particular circuit will barely excite a 6V6, or immediately clip a KT88. I know I could build-it-and-see, but I would like the knowledge of how to tell from a schematic.

My gut tells me the output was cut way down since this was a pedal design. But maybe not. I'd like to know how to tell, and that will better inform me of where to make adjustments so it will play nicer with a given PA circuit.

Thanks for your response!

[PRR]

> match this preamp to a power amp

We usually treat the power tubes AND their driver as a "unit". Most power tubes are scaled so their grid drive is about as much as dedicated voltage-amp can deliver un-hindered. You are unlikely to drive any real power tube through that long lossy tone network.

A "preamp" with four triodes and four tone networks is a real pain to compute, even approximately. And gain is clearly different at 200Hz, 1kHz, 5kHz. Except in this case I'd be pretty sure this is intended as a "guitar cord effect", that the output is a really loud guitar level. IAC, such questions are often easier to answer by experiment than by brain pain.

[BrassElephant]

> match this preamp to a power amp

We usually treat the power tubes AND their driver as a "unit". Most power tubes are scaled so their grid drive is about as much as dedicated voltage-amp can deliver un-hindered. You are unlikely to drive any real power tube through that long lossy tone network.

A "preamp" with four triodes and four tone networks is a real pain to compute, even approximately. And gain is clearly different at 200Hz, 1kHz, 5kHz. Except in this case I'd be pretty sure this is intended as a "guitar cord effect", that the output is a really loud guitar level. IAC, such questions are often easier to answer by experiment than by brain pain.

Ok gotcha, I had a hunch it might be as such. So the answer in this particular case would be to mock up the circuit and just experiment until joy. Understood.

In the meantime I'll do some research on calculating voltage gain and see if I can't learn me something.

I may just build this as a pedal like intended. If I built it into a head, I would have to add another dual-triode for a clean channel. This circuit sounds awesome but does not clean up well. One trick pony territory.

[d95err]

The circuit after the tone stack is a series if different low-pass filters, as well as reducing the overall volume.

If you want it brighter, you could experiment there. For example, by reducing the cap values or removing some of the caps.

[BrassElephant]

The circuit after the tone stack is a series if different low-pass filters, as well as reducing the overall volume.

If you want it brighter, you could experiment there. For example, by reducing the cap values or removing some of the caps.

Awesome, thanks. Maybe changing values there and also addressing some of the values in the previous stage's voltage dividers will increase the volume enough to drive a PA circuit. I'm afraid changing some of the voltage dividers may change the behavior of some the clipping in the circuit, but it'll be fun to experiment and find out.

EDIT: If I change the 330k grid leak before V2A to a more typical 1M, I feel like that would increase volume quite a bit. And that final stage is typical center bias, so I don't feel like it would change the character of the circuit too much.