imo1 wrote: ↑Sun Jul 06, 2025 2:51 pm
-I wasn't envisioning needing a clean Master. I was imagining being able to set the clean amp for a base sound that worked for me and then using the internal trim to set the input accordingly to the OD section. It seems that this would accomplish a similar thing since the circuits are in series. Is there another reason for having a master before the trim?
The so-called second generation Dumbles work that way, with a fixed OD input attenuation set internally, and one master volume for clean and OD at the PI input. The clean output either goes straight into the OD entrance or direct to the MV. Some people have added a clean master, and found that to be useful. That's following the evolution of the ODS by adding the ability to use the clean volume to set the level going into the OD, and have a separate control for the clean volume in clean mode. The first iteration added an OD volume (at the OD output) that allowed the relative Clean/OD levels to be controlled, and the overall level still set by the MV. Finally, separate masters were used, one at the output of the clean and one at the output of the OD, with each going direct to the PI input as determined by the OD relay.
imo1 wrote: ↑Sun Jul 06, 2025 2:51 pm
-The higher voltage is a bit of a question mark for me. I don't know why Mojotone is selling a 350V PT when line voltage has gotten higher over the years. I like the idea of eliminating the rectifier tube, and I like the idea of the larger filtering. I don't want to push the power tubes too hard. I have an early silver face Deluxe Reverb that sounds good around 440-450 with a GZ34. I wouldn't be unhappy with that voltage and sound, but 470 is pushing it. I am looking to make a road-worthy amp, so if the MOSFET adds a degree of unreliability, I don't want to chance it. I was trying to come up with a solid way to get in the low 400's. I can buy another PT, and maybe that is the solution. I was hoping to get there mostly with the components in the kit.
For the DR I built, I used a Hammond 290BX PT (330V Spec), and a 5U4GB to drop the B+ ~50V to ~400V. Unloaded, it produced something like 490 VDC. Ian's MM proposal is even better at 315. Mojo used to sell kits without transformers, IDK if they still do that.
Thanks for the explanation. I'm going to build it without an additional master for now. I could always use the back "ext speaker" slot for a master if I need it. I'm curious to build the OD circuit and see what it can sound like as a possible option. I like the idea of theoretically traveling without a pedalboard! It may not be something that works for me. I know that the OD circuits that I built didn't give me the sound I was looking for. I'm a big fan of a nice Fender right at break up with a pedal to push it just a bit. I've fallen in love with the Kingtone duellist for just this, and it may be hard to top that sound.
Unfortunately Mojotone does not do this anymore. That was the first thing I asked them when I was looking at the kits. I get a discount, so it will work out as a wash with the OT and PT. I like having all of the hardware taken care of as that part of it takes up a lot of brainspace I would prefer to be putting elsewhere.
So if you delete the clean MV, and give up the reverb dwell control you have enough holes in the faceplate. I don't think full-size pots can be placed side-by-side in the vibe channel input jack holes, so that might be an issue. I'd be wanting to make a new faceplate with correct text, so I'd ignore the existing labels and arrange the controls in a logical order:
Input Jack - Cl Vol - T - M - B - OD Drive - T - M - B - MV - Rev Level - Trem Speed - Trem Intensity
Mojo gets their transformers from Heyboer, so a DR set would probably be pretty easy to unload or use somewhere down the line.
imo1 wrote: ↑Sun Jul 06, 2025 4:34 pm
Unfortunately Mojotone does not do this anymore. That was the first thing I asked them when I was looking at the kits. I get a discount, so it will work out as a wash with the OT and PT. I like having all of the hardware taken care of as that part of it takes up a lot of brainspace I would prefer to be putting elsewhere.
Unless a brief scan through their website this morning missed a detail they still sell each of the parts separately---chassis, faceplates, and boards vs the entire kit. That's what made me think you'd be easily able to opt for other iron. You'd then might resort to Amplified Parts or other for the misc. passives etc. but that's not a serious obstacle.
imo1 wrote: ↑Sun Jul 06, 2025 4:34 pm
Unfortunately Mojotone does not do this anymore. That was the first thing I asked them when I was looking at the kits. I get a discount, so it will work out as a wash with the OT and PT. I like having all of the hardware taken care of as that part of it takes up a lot of brainspace I would prefer to be putting elsewhere.
Unless a brief scan through their website this morning missed a detail they still sell each of the parts separately---chassis, faceplates, and boards vs the entire kit. That's what made me think you'd be easily able to opt for other iron. You'd then need resort to Amplified Parts or other for the misc. passives etc. but that's not a serious obstacle.
..
I misunderstood what you were saying. They used to offer the kits without transformers. They do not do that anymore.
I just got home from a tour 4 days ago and leave again on Wednesday. I am forced to move quickly. I do my best to do due diligence in planning, but my life is organizationally like a military maneuver. I have to plan ahead for the 2 weeks in August that I will have to not only build the amp, but have it completely tweaked and road ready for a 2-3 month tour. This necessitates making decisions like buying the kit. Those hours saved are hours I can use planning, and when building, doing my best to be methodical in making a road worthy amp. I consider the time spent here talking to you guys, as well as researching the endless rabbit holes that come up in the process, as the best use of my time, so make a conscious decision to do that.
And, I am very grateful for the knowledge and information! As I've said before, as a builder I am an amateur. I am firstmost a player, and I can contribute to the forum through my experience using gear in endless settings that often reveal strengths and weaknesses of design . I hope that my contributions are helpful in that context
martin manning wrote: ↑Sun Jul 06, 2025 6:16 pm
So if you delete the clean MV, and give up the reverb dwell control you have enough holes in the faceplate. I don't think full-size pots can be placed side-by-side in the vibe channel input jack holes, so that might be an issue. I'd be wanting to make a new faceplate with correct text, so I'd ignore the existing labels and arrange the controls in a logical order:
Input Jack - Cl Vol - T - M - B - OD Drive - T - M - B - MV - Rev Level - Trem Speed - Trem Intensity
Mojo gets their transformers from Heyboer, so a DR set would probably be pretty easy to unload or use somewhere down the line.
You are probably right. As I told the other gentleman, I am always in pretty tight time constraints, so need to be realistic to if I could pull that off now.
I could see it working using the holes provided and relabelling. It would follow your order. I need to find an affordable way to get another faceplate made
imo1 wrote: ↑Sun Jul 06, 2025 7:36 pm
I need to find an affordable way to get another faceplate made
I have had good luck with this company: https://indcom.graphics/ I sent them a .pdf and they made a very nice aluminum faceplate, maybe 0.030" thick. Mine was about 5E3 Deluxe size, but they can probably do DR size. I drew mine up using PowerPoint, and then saved it as a .pdf. No vector graphic software needed. TAG moderator Aaron Lyon put me on to them.
Thanks for the recc. I have an email to Mojotone to get the specs for the chassis.
It will look a little uneven, but I'm fine with that. As long as it sounds good!
imo1 wrote: ↑Sun Jul 06, 2025 7:23 pm
I have to plan ahead for the 2 weeks in August that I will have to not only build the amp, but have it completely tweaked and road ready for a 2-3 month tour.
Tight schedule, and you can hardly afford to find yourself missing a few parts or wanting to change the design on the fly.
Here's how I would do it. I've kept the shared cathode resistor and cap on the reverb recover tube, but separated the ones on the two channels. You should be able to squeeze another resistor and electrolytic cap in between the ones connected to pins 3 on V1 and V2. I would run shielded cable to the grids, and put terminal strips on the V1 and V2 tube sockets to mount the grid resistors at the pins. I set supply voltage at 400 and let the rest fall out. There are separate filters for the clean and OD stages, so you'll have to find room for one more. The clean channel is Fender, OD is D Bluesmaster, and the reverb is Fender. I took my best shot at setting signal levels and tweaking the clean and OD frequency responses so they looks about right.
PS That bias circuit is the same as I put in my recent DR build.
Note: See revised schematic later in this thread.
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Last edited by martin manning on Sun Sep 14, 2025 1:34 pm, edited 1 time in total.
Here is the circuit with the updated bias circuit. I also got the Weber 'wz34' as well as a plug in with no voltage dropping. That makes it easier to play with a couple of different B+ voltages, and also, should I decide I don't like the solid state route, can go to a GZ34 and just change a couple of caps in the power supply.
Trying to think ahead to changes I can make relatively quickly in making this amp work for me.
Hmm. Just saw your post as I was about to post this. I'm going to put it up for posterity and reference. Curious as to why you used this bias circuit and not the other one with the wiper to high?
Oh, and man..Thanks for the schematic work. I'm gonna need to dig into this later today.
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imo1 wrote: ↑Sun Jul 06, 2025 7:23 pm
I have to plan ahead for the 2 weeks in August that I will have to not only build the amp, but have it completely tweaked and road ready for a 2-3 month tour.
Tight schedule, and you can hardly afford to find yourself missing a few parts or wanting to change the design on the fly.
Here's how I would do it. I've kept the shared cathode resistor and cap on the reverb recover tube, but separated the ones on the two channels. You should be able to squeeze another resistor and electrolytic cap in between the ones connected to pins 3 on V1 and V2. I would run shielded cable to the grids, and put terminal strips on the V1 and V2 tube sockets to mount the grid resistors at the pins. I set supply voltage at 400 and let the rest fall out. There are separate filters for the clean and OD stages, so you'll have to find room for one more. The clean channel is Fender, OD is D Bluesmaster, and the reverb is Fender. I took my best shot at setting signal levels and tweaking the clean and OD frequency responses so they looks about right.
PS That bias circuit is the same as I put in my recent DR build.
Two quick questions, if I may-
-Why the bigger pot on the bass in clean?
-Any downside to mounting a reverb 1M dwell pot on back in place of the resistor?
The bass pot is a leftover from the Dumble circuit I started from. Either one would work, with the 500k delivering a bit more bass. Sure, you could put the dwell pot on the back.
Ok! Martin!!
A couple of questions on this if you don't mind...
-The 47k in "clean" tonestack. Is this to mitigate the PAB jump? Isn't it just similar to having a big mid pot in there? 72k to ground? So it would be a mid boost of sorts, as opposed to a full bypass of tone controls?
-They seem to both work, but is there a reason you changed the wipers on the clean/OD switch? My wipers were "in" and "out" . It seems that they are all doing the same thing, but perhaps there is a functional reason you routed them this way?
-I'm a bit confused by how you are doing the "OD master". Is ground coming from wiper of 100k trim, high coming from wiper of "drive" and the wiper going to the connection of C12 and the high side of the treble pot?
-the 10u is similar to what I've done in other amps, though I split it to a couple of small(4.7uf) early and then standard 22uF later in circuit.
