The Mule Driver - aka Fischer Meets Dumble

[RJ Guitars]

No noise issues in my 5879's. All mine are marked RCA, but i heard regardless of brand they were all made by RCA. What date codes or lots are you having issues with?

I finally found that the random noise was actually coming from the AC power and it is not the 5879 tubes... I just had the misfortune of using two amps with 5879 tubes in them on two consecutive weekends and picked up the same noise. Turns out here are several pieces of equipment on the stage sharing the same circuit and the noise was not in the tubes but in one the other pieces of equipment and somehow bleeding into my amp.

[RJ Guitars]

I was able to do a little more on this build and put together a circuit board. I did have to tweak it a bit from the layout drawings. I'm not sure what was going on with my design on the dropping string of the power supply but I implented a change there. I think I had terminals 4 and 5 reversed and I've gone with a more convention idea there... look for an updated set of drawings to be posted before I call this done.

[max_lwedge]

I really like the idea, the minimal loss but useful bass and treble are nice. I have been scheming on something similar thing based on a Supro, with its paraphase, 6973s and quirks. Looking at the schem, I am wondering if the volume pot in the circuit feeding the OD will be too interactive. A change in overall volume could make both the OD gain and master need adjustment. At low volumes, you might not have enough juice for the OD.

[RJ Guitars]

I really like the idea, the minimal loss but useful bass and treble are nice. I have been scheming on something similar thing based on a Supro, with its paraphase, 6973s and quirks. Looking at the schem, I am wondering if the volume pot in the circuit feeding the OD will be too interactive. A change in overall volume could make both the OD gain and master need adjustment. At low volumes, you might not have enough juice for the OD.

I'm not sure yet what to expect but hopefully I'll be able to tell you how well it works before too long. There is an empty hole to add a tube if it comes to that - I'm hoping it doesn't need it.

[jelle]

Looks like a fun project!

[dorrisant]

rj, I did a similar project a while back., mating the Pedal Mule preamp and #124 OD (nearly identical to your schem) into a single 6V6. I don't mean to walk on your thread... but I did have some issues with the gain of the overdrive being too much, to the point of squealing feedback with the OD pots at levels around noon. I need to scope my build to find out where the feedback is beginning. I did add some resistors at the entrance to the OD triodes and got very usable results. I would like to see if there is a different way to do it. Ready to experiment... and thinking maybe two minds looking directly at and tweaking concurrently will speed things a bit. My build has a paralleled 12AX7 for a gain stage before the 6V6 input grid. This is just to replace the gain of the PI you have in your schem. I may have to get the signal levels to match more of what you are seeing and compare apples to apples as much as possible.

Sorry my build is p-t-p... kinda hard to make out the connections.

[jelle]

Dorrisant, add the 270pF snubbers on the OD tube and report back....it may help.

Jelle

[Colossal]

Very nice build Tony. Love the point-to-point layout.

[RJ Guitars]

Tony, I like the PTP build. Don't feel bad about walking on a thread, I am stoked that you have some useful info that may save me some steps in tweaking this thing. I like it that similar ideas are being developed simultaneously... although I'm not sure if it means we are both very clever or equally clueless??

It's nice to have some other folks weighing in on this as well.

[didit]

Sorry my build is p-t-p... kinda hard to make out the connections.

Apology not excepted. That's a fine work of art.

Best .. Ian

[dorrisant]

Thanks guys... Much appreciated.

Jelle, I did put in the 270pf snubbers a month or so ago. That alone didn't do much to stop the squealing. I used with a decade box to bleed some of the signal at the OD Drive pot to get the squeal down. 68k was just low enough that the OD pots could both be turned up to about 9 before there is any hint of feedback. I feel that this is just a band-aid though. I saw that rj was talking about this build and thought that I should wait until now to present this... By the time of these pics I had also installed a switch for alternate cathode bypass caps for V1... stock value of 1µF in the center position with 25µF and 100µF added by the switch. Here are a couple more pics at different angles...

rj... I'd like to think we are mostly clever with momentary lapses of cluelessness... is that even a word?

[xtian]

Hey, Tony, set your amp just at the onset of feedback and chopstick all the components. Coupling/tone caps can sometimes be microphonic.

[RJ Guitars]

I hit this thing pretty hard this weekend, thinking I was nearly done... I wasn't. However, I kept at it and finally I have made it to a major milestone. I have the Normal mode wired up and functional. I found a couple things but generally it was pretty close just following my schematic and layout. I will adjust those to the "As-Built" once I get a little more certain I've got things working well.

I determined that I didn't have the bright/boost switch I needed so I don't have those features operable yet. I used a set of Russian 6N3C tubes in the power tube holes and some old 12AX7's in the preamp holes just in case I had a wiring issue. I didn't want to toast my new KT66's in the startup efforts.

Once I verified that I had DC voltage everywhere I was expecting it and things looked safe I powered it up and it definitely sounds decent with the starter tubes. I replaced the old preamp tubes with some new Tungsol 12AX7's and it seems all is well on the normal channel... well sorta. I had the bass pot wired backwards and I don't like how it works. It's sorta ineffective until you get to 90% then it's doing it's thing O.K. Gotta look at that.

I'm happy for now and will continue to finish it up and try the OD channel as time allows, then start tweaking. Stay tuned.

[didit]

Taking shape nicely RJ. Please keep at it.

Anyone recommending sources for a 3M pot as spec'ed for the bass control? There is an unsuitable but commonly available CTS reverse audio that seems is for tremolo speed control.

Best .. Ian

[dorrisant]

I have found very little sonic difference between 3M and 2M. Try for yourself. I also like the metal shaft better than the plastic. I wouldn't say there is no difference... Just not a deal-breaker. Linear taper functions better too, IMHO.

https://www.cedist.com/products/potenti ... 38-bushing