Bass to Guitar Amp Conversion???

[Jacob]

Hi All,

I’m considering building the Mojotone 50 watt Bass Amp to use for guitar (for a high headroom pedal platform type thing)

I emailed Mojo asking if there was anything they would recommend I change for use with guitar.
They said that many people do this, and the only thing that would need changed is the Grid Stoppers on the input (which I had assumed)

Are there any other tweaks/changes anyone would recommend?

I’m thinking this may be a good platform to experiment with since it is relatively simple; maybe I’ll end up with a couple different versions.

If I’m understanding correctly this is something like an AB763 preamp with a JTM 45 power section as it sits?

Any ideas/advice would be greatly appreciated

Thanks!
Jake

[thetragichero]

possibly tweak tone stack, maybe lower the 100nf coupling caps off the phase inverter to 22nf or 47nf? I'd probably use a 4k7 or 10k on the power tube grids just because i have a ton of em

[Jacob]

possibly tweak tone stack, maybe lower the 100nf coupling caps off the phase inverter to 22nf or 47nf? I'd probably use a 4k7 or 10k on the power tube grids just because i have a ton of em

Thanks! The tone stack is an area I was wondering about.

I was comparing it to a Deluxe Reverb and noting the differences.

Is there another Amp to borrow from, or specific values you would recommend there?

[thetragichero]

I'll often compare values between existing designs and tweak around in this: http://www.duncanamps.com/tsc/

[Willabe]

I'd add a 4th B+ filter node.

Yes, there's only 2- 12 _ _ 7 tubes on node C. But I'd add a 20uF or even a 10uF cap after node C with at least a 10K R in between node C cap and node D cap.

So power tube OT CT B+ = node A, power tube screens = node B, PI = node C and preamp tube = node D.

Use a radial e-cap for node D and put it right on the board close to the preamp tube circuitry.

Maybe move the 220pF cap up on to the pots and play around with moving the coupling cap. You do have a few extra eyelets/turrets already open there.     

[Jacob]

I'd add a 4th B+ filter node.

Yes, there's only 2- 12 _ _ 7 tubes on node C. But I'd add a 20uF or even a 10uF cap after node C with at least a 10K R in between node C cap and node D cap.

So power tube OT CT B+ = node A, power tube screens = node B, PI = node C and preamp tube = node D.

Use a radial e-cap for node D and put it right on the board close to the preamp tube circuitry.   

Thanks Willabe! That 4th filter cap would make it more like a standard Fender setup right?

[Willabe]

Yeah, kind of, but not going for that.

Just a little better filtering and isolating the tubes from each other. I wouldn't use anything bigger than 20uF there, just my preference.

Radial caps are small in size, very low in price. And that node will have to handle very little current. You have the room.

They don't list the DCV's on the schematic/layout. You may need a 500v e-cap there. But you might be able to use a 450v e-cap. If you need 500v you can get a couple 250v or better yet 300v/350v caps and put them in series with a couple balancing R's across the 2 caps. The lower the voltage the cap, the smaller their size. So 2 will still fit.

See if you can find out from Mojo where the B+ sits at?

Not an absolute must, but if it were mine, I would.

[Jacob]

Yeah, kind of, but not going for that.

Just a little better filtering and isolating the tubes from each other. I wouldn't use anything bigger than 20uF there, just my preference.

Radial caps are small in size, very low in price. And that node will have to handle very little current. You have the room.

Gotcha! The filter caps were just a difference I noticed when comparing it to a Deluxe Reverb, that makes sense, thanks!

[Willabe]

They do have a lot of filtering, and they are using a choke, that's good. 

I just don't like the PI on the same B+ filter node with the preamp tube. Others here would probably be fine with it?

[Willabe]

Sorry, but you should go back and re-read reply's 4 and 6, edit/additions.

[Jacob]

I just don't like the PI on the same B+ filter node with the preamp tube.

That makes sense! Sort of a similar concept as having separate grounding points for the preamp and power amp?

[Jacob]

Sorry, but you should go back and re-read reply's 4 and 6, edit/additions.

Thank you so much! Really great info

[Willabe]

I just don't like the PI on the same B+ filter node with the preamp tube.

That makes sense! Sort of a similar concept as having separate grounding points for the preamp and power amp?

Yes and that amp has a negative feed back (-FB) loop on the output stage, OT secondary to PI. So the PI should be grounded with the power tubes, not with the preamp.

http://www.valvewizard.co.uk/Grounding.html   

[Willabe]

And the -grid bias supply should not be grounded with the preamp tube as they have it drawn, it should be grounded with the power tubes ground.

Read the grounding link I posted just above from Merlin.

[Jacob]

Yes and that amp has a negative feed back (-FB) loop on the output stage, OT secondary to PI. So the PI should be grounded with the power tubes, not with the preamp.

Ah! That is a really good detail I might have missed since they don’t really show the grounding points on their layout, thank you!

[Jacob]

And the -grid bias supply should not be grounded with the preamp tube as they have it drawn, it should be grounded with the power tubes ground.

Read the grounding link I posted just above from Merlin.

That’s great! Thank you 

[Willabe]

Yes, they show the grounding points on the layout.

Their using a random grounding to the chassis.

Read the link from Merlin I posted on grounding, much better. Our host Doug, his grounding works fine, power amp ground, preamp ground, but you can do more.

The less points on the chassis you ground to the better, and tie the grounds from each tubes circuit to the B+ filter cap nodes ground that feeds that tube.   

[Jacob]

Yes, they show the grounding points on the layout.

Their using a random grounding to the chassis.

Read the link from Merlin I posted on grounding, much better. Our host Doug, his grounding works fine, power amp ground, preamp ground, but you can do more.

The less points on the chassis you ground to the better, and tie the grounds from each tubes circuit to the B+ filter cap nodes ground that feeds that tube.   

That’s a great tip, I really appreciate the advice!

[jordan86]

I’d be curious to hear this with guitar if you have a way of making a recording when you are done. Sound like a great spin on a Bassman/JTM45. Good pedal platform.

[Jacob]

I’d be curious to hear this with guitar if you have a way of making a recording when you are done. Sound like a great spin on a Bassman/JTM45. Good pedal platform.

I will definitely do that! And that is what I'm hoping.

Seems like a Bassman/JTM45 power section with more of a Blackface Fender front end.

The only thing Mojo said to change was the grid stoppers, but as suggested above, I would think the Tone Stack could optimized more for guitar compared to how they have it set up for Bass.

[jordan86]

I’d probably build the tonestack as spec’d in the schematic just to see if you like it. Plus it decreases the number of variables in the build phase. Less things to screw up. If you dig it, leave it. If you want to experiment later, it wouldn’t be too hard to change it stock AB763 values after the fact.

[Jacob]

I’d probably build the tonestack as spec’d in the schematic just to see if you like it. Plus it decreases the number of variables in the build phase. Less things to screw up. If you dig it, leave it. If you want to experiment later, it wouldn’t be too hard to change it stock AB763 values after the fact.

Exactly what I was thinking!

Figured the tone stack would be easy to change out after the fact if it didn’t suit