Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Harvester on May 09, 2025, 01:51:52 pm
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Hi all,
First, I would like to thank Doug Hoffman for creating this space (and selling me many of the parts for this build) and SLUCKEY for the layout. I built a MOJOTONE 5e3 kit last fall and settled on this little amp as a second but wanted to scratch build.
I was able to make this little circuit in a box. Will post a bunch of pics. Worked first try just like my 5e3 (I do have a doorstop pedal project that has been set aside tho...), and I have had great fun running it through different guitars and speakers.
The verdict: compared to my 1973 Bronco amp, this thing is a dog. But cute, like a puppy.
Including the schematic which shows that the preamp cathode goes directly to ground and a few other unorthodox bits. I am guessing that if I add a resistor and bpass cap I will get more gain, but will need to change a few other values like that 6.8m resistor and the cap next to it?
Any advice on improving, modding this little circuit welcome.
Thanks much.
Darrin
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pics
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+ pics
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What would your goal(s) be in modding the amp?
'Thicker tone' can be got by running the tubes hotter - especially the output tube, which running in SE cathode bias, can be safely pushed to Pmax.
Running a low-efficency, low power speaker and help this too, esp an Alnico 10-15W.
The input stage is grid-leak bias, and changing to cathode bias with fully bypassed cathode will increase gain, as will increasing plate resistors to 150k or 220k or 330k. But losing the grid-leak bias will make V1 run colder.
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Thanks tubeswell for taking the time. I thought I was clear on my points of dissatisfaction in my head, but I just played it for an hour and I am fairly pleased. I had been using hotter pickups and just tried some less heat humbuckers and still think a little more gain would be great. I will leave the input stage alone and experiment with the bias to add heat.
I have a Heppner 15 watt alnico that is good as you suggest. I also found that the outtake speakers from modern "Supro" amps are really fun as they are voiced like a field coil speaker. I have never used a field coil speaker but this 10" Supro dk10 is a nice blurry match as well.
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Have you tried swapping in different tubes? Worth a shot before firing up the soldering iron. I'd try bypassing the 6V6 cathode resister with a 20 to 50 uf cap. Exact value is not critical. Although it seems counter intuitive, I'd try a 5751 or 12ay7 for the preamp.
And if you do decide to change V1 to cathode bias, I think the values used in the Gibson GA-5 (the single ended 6V6 variety - Gibson used the same model number on several amps :w2:) are a good place to start.
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have you seen this thread
https://el34world.com/Forum/index.php?topic=27455.0
I discovered that I cannot access the Sluckey amp site anymore. Too bad was a great place.
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Thanks for the guidance. Will order some more tubes for trial.
Also, I saw that thread long ago and should have gone back to it. Fixes the hi/lo inputs.