Pedal Mule 30 - Prototype build

[stephenl]

Yes, the cap multiplier is in Merlin's power supply book, which is out of print. I'll post a pic of a cap multiplier when I get a chance.

[stephenl]

Here is a cap multiplier example, I drew it up quickly...so check my work

R2 is not necessary but it will give you better filtering...and higher dissipation through the mosfet. R1 and R2 form a voltage divider for referencing the mosfet and can be set to lower the voltage out - be mindful of dissipation. R3 is a gate stopper and can vary depending on the mosfet you pick. The 12v zener may not be needed if it is included in the mosfet package. The transistor and R5 provide active current limiting and a nice ramp up for B+. The current limit is set by Vbe/Rs.

Bolt the mosfet to the chassis w/ insulator or heatsink.

[stephenl]

For some reason I can't get the pic to attach to the post

[stephenl]

Cap Multiplier Pic

[stephenl]

Merlin's book example used and IRF820 mosfet with a 100K gate stopper and a 2N3904 transisitor without a stopper.

[John_P_WI]

Hmmm, at first glance is there a ground connection missing on the image above? I have not played with that exact arrangement and may be wrong.

I like IRFPE50PBF or IRF840 mosfets, the IRFPE50PBF has a Vds of 800 volts and nearly the same working current parameters as the IRF840, cheaper too.

[stephenl]

Yes, the 22uf cap should be connected to ground.

[stephenl]

If people try this and like it, it would be kinda cool to design a standard pcb power supply board around it w/modern snap-in elytics. Group design/buy?

[ToneMerc]

If people try this and like it, it would be kinda cool to design a standard pcb power supply board around it w/modern snap-in elytics. Group design/buy?

How about moving this " cap multiplier " discussion to your own thread in the technical section instead drifting this TW amp thread build any further?

TM

[stephenl]

Apologies, RJ, it wasn't my intent to derail your thread.

[RJ Guitars]

No problemo... I have been on the road for a week and ended up sick so I was watching with some curiosity but dead to the world for the most part. I'm still mostly dead but maybe if I can find "Miracle Max" he can do his magic so I can at least drive home?

Right before I left I made a little more progress on the amp... a bit premature on this amp but maybe it is the inspiration I need. Once I feel up to it I'm going to make sure i have all my component values right, maybe look for a bad coupling cap or some fundamental design flaw... I figure there has to be something that I am missing?

Thanks for Merlin info - I'll follow this along if it takes on a life of it's own.

[RJ Guitars]

I stayed sick for 10 days and then finally gave up and went to the doctor - she says I have pneumonia but it's likely I'll survive... with drugs and a couple weeks of amp therapy I figure I'll be back to my old self soon.

So after a bit of a miserable road trip I finally got home and after extensive thinking about that stupid hum in the Mule amp, reading just a bit, and then taking another look at it it seems like the right thing to do was move the center tap grounds on the B+ and Heater wiring away from each other. Viola, that did it!

So maybe Gerald Weber's note about "a choke is a choke" is valid? I'll have to test that again but I'm not putting the Fender choke back in this build to verify that.

The amp has a very respectable clean tone and is surprisingly loud... I am actually shocked by the amount of gain it has.

I put my favorite low budget Russian 6SL7 tube back in it and it really sounds good.

I also went with a vintage 12AX7 for the preamp tube. It just had a better and fuller sound than the 12BZ7...

I liked the JJ El84's better than the Saratov equivalents... seems like they worked well with the very clean sound of this amp.

The amp has a really great sounding clean tone with a huge amount of bass available.

It does actually break up when you you press it hard.

Next I'll test my pedal boards and after that an acoustic guitar through it.

[ToneMerc]

Looks great RJ

TM

[stephenl]

Great looking build, hope you're feeling better.

[RJ Guitars]

Thanks guys - With the help of modern medicine I am starting to feel normal... this must be really good medicine since I was never normal to start with??

Anyway, I've called this "done" a half dozen times so now I am just tweaking. I read up on the capacitance multiplier circuit that Stephen posted and also grabbed a few more things from google about taming hum.

I went with the extreme in conservative grounding with each stage going back to it's filter cap and it's either better or no harm done.

I changed the V1 cathode bypass cap to a 47uF based on some additional schematics that were floating around... can't say it made any difference but again at the least - no harm done.

I also added an antenna to the heater lead on the 6SL7 and put the antenna up next to the input lead to form a humbucker.... it was very crude but as soon as I get the antenna near the PI inputs you can hear the hum decrease... I'm sure someone tell me why but it goes against my intuition for sure! Zoom in on the 6SL7 and you can see how I did this.

This has the noise floor getting down to almost nothing but once I began hunting down all sources of hum I wanted to keep going just to see how low I can get. DC heaters is still an option but certainly not needed now.

A few observations and your comments requested:

1) The negative feedback does almost nothing even going down to a 56K feed resistor, The little effect it has kinda dulls the tone. Why?

2) The bright cap doesn't not have a huge effect, even though the value of it is huge - why?

3) Why did such a simple amp require so much tweaking to eradicate hum - Far beyond what I've done on any of the other Wreck circuits?

4) This amp has a very clear and pure tone but maybe just a hint of compression until you get it loud enough to hear just a hint of distortion. Then the notes ring a bit longer. Up to that point they seem to sorta die off easily.

5) I tried the amp with pedals and it works nicely. I think it gives you the sound of the pedal very clearly and doesn't add that much flavor of it's own. I would almost suggest a boost pedal early in the chain even if you don't want to press into a hard distortion. I'm not sure an effects loop is needed either.

6) T tried the amp as an acoustic guitar amp and it works amazingly well. If you've ever played an acoustic through a good sounding electric guitar amp you know it can be brittle and harsh. This amp has a lot of tone control available and you can dial it in nicely as an acoustic amp.

7) I played a bass through this amp and it's almost like it was made for it... very versatile amp overall.

My voltages were just slightly higher (~1%) than those believed to be taken from an authentic SW-30 build. My wall voltage was right on 120V today.