Pedal Mule 30 - Prototype build

[RJ Guitars]

Thanks guys. I really wanted to develop a standard combo cab that worked with the Wreck sized chassis. Ultimately I was pleased with the look and size of it... it sorta reminds me of the early days of Mesa Boogie. I don't mind sharing the drawings for those that want to diy one for themselves... send me an email.

This amp does really well with the alnico speaker and the open back format since it has lots of natural mids and lows.

[lumox0013]

Thanxx for the great work RJ !! i love seeing more of kens work being explored and I will be starting and octal version myself did you make your final adjustments on the schematic ,I cant wait to try this amp I'm all about pedals so this one might be great for me.

[dorrisant]

I gotta build one of these... Maybe some 6L6s or EL34s... All octal though.

I have the ugliest head cab to use... it will be beautiful.

Thanks for the build thread rj. My madness will continue in this direction.

[RJ Guitars]

Hey Tony, No problem. I was actually surprised how good this amp sounds. I've been playing it regularly at church the past few Sundays. It does excellent with Pedals and has a low noise floor.

Check out Gary's thread on the octal version http://ampgarage.com/forum/viewtopic.php?t=27813

His reports on his octal version confirm that the circuit really works. With the minimal parts count for this amp in an Octal version it is moving to the top of build list as well.

[sunnydaze]

RJ,

Sorry to resurrect this great thread. Thinking about building an amp based on this circuit. It looks there is a resistor across the bass pot in the build pics. Didn't see it on the schematics - am I missing something?

Would appreciate any clarification you or anyone else can give.

Thanks
Mike

[RJ Guitars]

No apologies needed... these old archives are meant to be used later by whoever needs them but we can add to them as we learn a bit more.

IIRC - the schematic calls for a 3 Meg pot and I didn't have one so I found a 5 Meg pot and jumpered a resistor across it to drop it down to 3 Megs. I'm not sure that pot worked like it was supposed to because the volume knda took off early and I had to have a gentle touch to get it set where I liked it. I think there are some 2.5 Meg pots available that could be worth a try?

I gotta say that I love the tone of that 6SL7 octal preamp tube but it was a challenge to find one that didn't hum. If I did the amp again I might change that to a 12AX7 if I was to make the amp a little easier to build for next time.

I think one of the coolest things I did on this is mentioned on page 2 of the thread. I had to add a really large value (.022uf) bright cap to get enough added brightness to notice it. Then I also added some negative feedback and I used some combination of things to get a Deep setting. I'm still trying to remember how all this worked so what you see in the final photos is more authentic than what I recall in my mind. What I do recall is that it was a very clever way I covered up the fact I biffed the faceplate layout and simultaneously made a very cool addition to the tone circuit.

I'd be curious how your amp turns out. I made about a half dozen amps last year and almost all of them were really good sounding units. However, I think this was probably the very best of the bunch. I am wondering how many of these are out there and what folks think of them?

[sunnydaze]

Thanks RJ,

That's helpful. I'm thinking about a variant of this circuit. I have an express type chassis with three preamp tube sockets. Might use the second as a built in fx loop. Just need to find the time. Pretty sure I have a 3 meg dual potentiometer from an solid state stereo from the 70's..

Hopefully, more to come.

Thanks
Mike

[stevenocon]

Thank you all for this great forum! I just joined and have been exploring the archives which led me to this thread. Great build RJ! Though it seems odd to make my first post on such an old thread I feel like you've left off with a hum issue that was never fully resolved. I do believe you've hinted at the right answer but never pulled the trigger on executing it.

What you describe reminds me of a very similar issue I ran into in a rewire of an Ampeg Super Jet. Because the original preamp tubes passed through the chassis to mount to a pc board the chassis knockouts were too large to mount 9 pin sockets. We decided to expand the holes and use 6SL7s / 6SN7s. Just like you I swapped through dozens of tubes trying to chase down the cause of a low annoying hum. I finally decided to try a dc filament supply. Problem solved!

I haven't explored an answer as to why but 6SL7s are notorious for coupling filament hum to the audio path. I'm not sure if you ever tried elevating your heaters but that might be a good place to start. It was too long ago to recall if I ever tried this in the Ampeg rewire but elevated heaters are pretty much standard practice in all my builds now. If that doesn't work see if you might have a 6V dc wall wart you can quickly rig up to test heating your 6SL7 with dc - I bet that will silence that hum that's been bugging you.

You're better off than I was in this case since you've only employed the 6SL7 at the PI stage. Add a 6SL7 at V1 and it would be a different story - too significant to brush under the carpet. I think it's this quark of the 6SL7 / 6SN7 family of tubes that accounts for the relative lack of interest in them. There's a ton of NOS ones out there and they sound great if you can overcome the hum.

[RJ Guitars]

Thanks... And welcome to the forum. Ultimately I swapped out 6SL7 tubes until I found one that sounded great and didn't hum... They are a pain but sound so good. Not sure I would repeat this in the next build but was cool to do it once and succeed.

[freakshow__]

resurrecting this old thread!

So I have built one of these and I am having a few issues....

1 is it hums quite loudly, It also has a pretty loud "POP" when I turn it off. UPDATE: i tried the amp in a different location and it was noticeably quieter….like dead silent quieter! must be my poor power situation at my place. It does still make a substantial ‘POP’ at power off though….

2 the deep switch doesn't seem to do anything? the bass and treble controls themselves dont seem to do much until you max them out. Bass will actually increase the amps volume at max, and treble will give you a pretty dull tone at max. (it IS a cut correct?) Anything in between though and these controls don't seem to do much....pretty mild.
UPDATE: i ended up ixnaying the deep switch for a switchable NFB circuit. seems to be a little more effective to my ears.

I built it according to RJ's schematic at the beginning of the thread. There are a few differences between that and the final picture he posted of the amp though....

Here's a few pics if anyone would like to chime in? any help would be greatly appreciated!

[freakshow__]

amp2.jpg

amp1.jpg

amp3.jpg

[RJ Guitars]

I'll fire mine up and see if I have those same things happen... plus look at my notes and see if there are any changes that are not documented. Stay tuned

[Colossal]

Freakshow, that's a nice looking build. With a cursory look at the photos, the only thing that bothers me is the 50R cathode bias resistor and cap grounding (through) that steel tube socket cover. There may be some unwanted contact resistance there between the cover and the chassis. Maybe not though. But perhaps try grounding directly to the chassis.

[dorrisant]

What were your differences from RJ's original?

[Littlewyan]

- Your Treble Control pot taper is logarithmic, which for that control works best if you swap the leads around so it increases treble as you turn it up. Take the lead from the outside lug and put it onto the lug on the otherside. This should increase treble as you turn it up and give you a much smoother control.
- Where does your circuit ground to the chassis? Looks like it's meant to be on the lug near the input. If the circuit isn't grounded to the chassis then this could explain the hum and the pop and is also quite dangerous, please check it.
- Why have you got two wires coming off of the negative lug on your IEC Socket? Can't work out what the second wire is for.
- I would personally reroute the wires to the bass control to be away from the input socket, but leave them for now as you might get away with having them there.
- I would also move the volume pot to be next to the bass control, it'll make your lead length so much shorter and less susceptible to interference. But again, leave it for now as you may get away with it.
- Is that glue holding the 50R Cathode Resistor down?
- The deep switch going by the schematic won't do much if I'm honest. You'll need to lower that capacitor value to a max of .001uF for it to make any noticeable difference. I'd go straight to 500pf, experiment though and see what you like. Even mess with the resistor value, try taking that down to 100K or even short it out so it makes more of a difference. Might be worth leaving some resistance there though in case it makes a popping noise when you use it.

Overall though, really nice looking build. Great work.

Edit: Just noticed you said you got rid of the deep switch. Has your amp got NFB? I can't see where it's connected in your photos.