The Mule Driver - aka Fischer Meets Dumble

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RJ Guitars
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The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

Hello All,

I wasn't sure where to put this post since I am probably going to equally offend the Dumble and Trainwreck faithful so I went for the crowd that tends to be more experimental and chose to put it here in the Technical Discussion section.

I had a customer ask me about a simple but great sounding clean amp and then add an overdrive option. Hands down the best clean amp I ever built has to be the Pedal Mule http://ampgarage.com/forum/viewtopic.php?f=1&t=26872 which is based on Ken Fischers interpretation of the VOX AC-30 "normal channel which he called the Songwriter 30. Komet in recent years has created a production version of the Songwriter that was cloned from a Fischer original.

This amp package indeed adds a foot-switchable Dumble styled Overdrive into a Trainwreck Songwriter based clean channel. However, we've also decided to go with a pair of KT-66 output tubes in a cathode biased configuration. This is something I really grew to like in the Rockster amps and wanted to try it with a Pedal Mule preamp.

Among the the interesting things that Ken Fischer did with his Songwriter was employ clever ways to control the tone without loading down and thus losing any signal from the preamp stages. I've kept most of those innovations in this amp as well.

I built a pair of barebones amp packages since I want to build one for myself and one for the guys that conceived the idea initially. I just shipped out the package so at this point I am as far along on mine as the other guys... but I have another project on the bench for this week and I'm already sure I'll finish last in this event. Should be fun... Lemme know if you see anything that I ought to reconsider.

Thanks,
rj

Edit October 20, 2017 - Updated Schematic and Layout replacing the previous versions
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Last edited by RJ Guitars on Fri Oct 20, 2017 10:57 pm, edited 1 time in total.
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M Fowler
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by M Fowler »

That looks great Bob. So 200R is enough to bias those KT66's?
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

M Fowler wrote: Mon Aug 21, 2017 12:34 am That looks great Bob. So 200R is enough to bias those KT66's?
That's what I used on the Rockster amps and it worked well for me... I'll do the math and see is I have it in a good spot or just lucky it didn't melt down on me. Cathode biasing is very forgiving you know. I like the KT66's because they sound a bit like an EL84 but with more of everything.
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TUBEDUDE
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by TUBEDUDE »

Looks interesting. I haven't built anything with KT-66s. I'd be tempted to kill the feedback and use a 5879 for the hot channel though. Looking forward to this and the comments from the builders. And of course the eventual soundclips. 😁
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by M Fowler »

Also, I would think I would like this circuit with one of your higher powered PT's Bob 325v-0-325v or 350v-0-350v to get that plate voltage up for more clean stage volume.

Does this 300-0-300 PT get the job done? I built a Reverb Rocket (Normster's) with 300-0-300 PT and 5k2 OT it sounds fantastic with some great bloom but not enough headroom.

Your amp looks like a fun amp. I would also like to try your octal version.

Mark
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

I really like 5879 tubes. I think they sound absolutely fantastic. However, the first batch that I've worked with all seem to have a little random noise in them. Nothing really bad but in a quiet moment I hear a little noise and an occasional pop... it's happened in three different amps so far and all the tubes are from the same lot. I'm not sure of this is typical of the tube or maybe (hopefully) I just got into a batch of them that are not quite right. I'd be curious if anyone else has had this issue?

The thing about Edcor transformers is that they always seem to run a little high in voltage so I expect to see somewhere between 320 and 330 volts on each side of this particular power transformer. So maybe I'm going to be very near that voltage area that Mark mentioned. I also have an expectation of a little more fullness and chime with the lower voltage B+ (some of the rationale behind the Trainwreck strategy) so I don't want to go too far in creating headroom if it will cost me some of the sweetness of the KT66's. I also tend to build from the prejudice perspective that I personally never get to play at any venue where I can push an amp into natural distortion so I always have plenty of headroom.
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TUBEDUDE
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by TUBEDUDE »

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?
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gingertube
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by gingertube »

Just for info:
I've just completed a AWV PA Amp Conversion. Being an Aussie Amp I wanted to keep an Oz flavour.
Counter intuitively Oz flavour pentode front ends means English (Pommy) 6BR7 . Looking critically at the data sheets tells me that 6BR7 is closer to the 5879 then EF86 or other options.

A friend had tried a vintage Gibson GA40 Les Paul Tremolo Amp with 5879 Pentode inpu tubes, and loved it but bulked at the >Oz$3000 asking price which was way beyond his means and asked if I could build one of those.

In was embarking on a vintage AWV/Philips PA Amp conversion at the time so I built the GA40 Les Paul Circuit exactly but substituting 6BR7 for the 5879. Preamps and "dodgy" phase splitter exactly as per the schematic, all circuit values kept the same.


I also substituted 6DQ6 for the 6V6 output rubes which meant 30-40W out vs 15W Out for the 6V6. I ran these fixed bias which required an extra R and a C in the inverter triode of the phase inverter. Else it is a GA40 with 6DQ6 Outputs for 30-40W in lieu of the 15W from a pair of 6V6. It is a great amp.

The last substitution was 1/2 a 12AX7 for the 6SQ7 tremolo oscillator, again no other circuit value changes.

Aside:
The Tremolo scheme of varying the anode voltage ogf the input pentode (with fixed low screen voltage) sounds great.
I'm not sure why - BUT when looking at the waveforms on the oscilloscope I noted one interesting thing.
When tremolo was switched off the I had a particular signal voltage level, when tremolo was switched ON I had a signal level which varied (according to the trem oscillator frequency) above and below that Trem OFF signal level. That is it was "expanding" on the positive Trem frequency excursions and compressing on the negative Trem frequency excursions. That is different to most other Trem schemes and MAY account for its different (superior in my opinion) sound.

So don't forget the Pommy 6BR7 - great tube and not too hard to find.

The GA40 Les Paul Schematic:
https://elektrotanya.com/PREVIEWS/63463 ... .pdf_1.png

Cheers,
Ian
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RJ Guitars
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

So here it is about a month later and I'm getting started on the wiring for this project. I learned previously that it's very useful to wire up all the AC connections and turn the power on as soon as it's visually verified to be in order. If it powers up without any fuses blowing or creating smoke, then I follow that up with the preliminary voltage measurements. Indeed, it powered up and the light came on.

124 VAC - Power In
651 VAC - B+ Supply
7.4 VAC - Heater Supply
5.5 VAC - 5V PS supply

5V DC - Output of 5V supply

Stay Tuned...

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Re: The Mule Driver - aka Fischer Meets Dumble

Post by M Fowler »

Bob,

Great to see all the builds your doing bud.

Love the Dave (colossal) relay boards and your green or seafoam board is cool.

Enjoy.

Mark
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

After I got caught up a bit on other things I went back to this and wired up the output transformer. This particular OT has dual primaries of 6.6K and 5.2K and I've decided to go with the 5.2K on this amp. I did leave a little tag end of the 6.6K pair if I change my mind sometime in the future. Nothin special going on here, just trying to keep the wires orderly. I'll wire up the B+ supply and run the heater wires next.

I do have a question about the 5V supply I'm using for the relay switches. Has anyone wired up those switches while also using the 5V to run a Belton relay module? I wasn't sure if I'd get noise that could get creep into the reverb unit. I still haven't decided whether to add that option or not but I did want to explore it a bit more.
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roberto
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by roberto »

Do you mean Belton Reverb module?
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

Yes, I think some folks refer to them as a "Belton Brick". I would be curious if I could slip one in here with minimal trouble? I currently use a reverb pedal but this amp has me curious if I could get the Overdrive and reverb on board and travel a little lighter?

I did some more work on the amp over the last couple days. I've got some deliveries to get out so I'll have to set it aside for a while until I get things caught up a bit...

Meanwhile the Power Supply is wired up, the heater wiring is done (yes, I used some funky colored leftover wire), and then tonight I went ahead and wired up the power tubes. Since this is a reverse image of typical Trainwreck chassis, there were some differences from what I call normal. I was happy with it overall but now that I've done it once I might be able to tidy up and simplify things for the next iteration.

Unloaded I have about 445 volts DC of B+

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roberto
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by roberto »

You could dedicate the 7805 to the belton brick and use this circuit for the relay:
http://www.edn.com/design/components-an ... gn%20Ideas
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by pompeiisneaks »

There are also multiple variations of a one tube reverb that work quite well. You may want to look into that, seems that chassis has room for one more tube imo :)

~Phil
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