The Amp Garage

I've been thinking about this approach some more in context of a Hiwatt build I did. I used the "SAP" preamp (https://hiwatt.org/Schematics/DR_Pre2InputSAP.gif) with an added SPST in series with the footswitch in case the footswitch breaks, I forget it packing for a gig, etc. The switch was installe...

Turns out my hasty decision to remove the preamp and negative feedback at once hid an important clue. The oscillation is much, much better without NFB. I was able to redo the power supply to decouple the OT from the preamp in a way similar to maxkracht's drawing with some value changes. The stages a...

When it finally came to completely disconnecting the preamp section (including the NFB lead), there was no more problem. Not sure if I am following. Have you tried just disconnecting the nfb? If no, you should try reversing the primary or secondary of the OT. Weirdly, the output section got biased ...

Y'all are the best -- thanks so much for helping me figure this out. So I tried gradually reducing the power supply as suggested and found the following didn't resolve the oscillation: -Disconnecting the Vox side completely -Disconnecting the parallel 30uF capacitor at the reservoir (didn't fix the ...

Thanks everyone for the help so far! I'm currently reading through the Champ article -- thanks for passing it on, R.G.! [Edit: just finished the article: I think the lack of decoupling may be the issue here -- would decoupling the output transformers be a good starting point? Since the issue appears...

Just finished wiring up a "space saver amp" for my wife and I to do shows together. The idea of the amp was to have one amp that has 2 truly separated channels so we could play small coffee shop gigs together and haul less. Channel 1 is basically a brown panel Princeton (6G2) and channel 2 is a Vox ...

Ahh I see. I didn't realize you were plugging the footswitch into the "low" input (should have read your first post more closely -- my mistake!). Very cool idea to use the stereo jack. I wonder if you could add a switch onto your guitar to switch the cathode bypass from there if you also installed a...

I really like the idea of bigger circuit changes being affected by switches at the jacks. I'm planning a plexi build with a similar scheme to jumper channels and to route an integral JFET preamp. I'd be curious how different the sound is with the cathode bypass capacitor. If I understood Merlin's bo...

a 1202-132 here seems overkill, expensive and heavy. the -84 sounds different from the -132 due to different Z. That's a good point -- certainly at odds with going with a toroidal PT for the weight savings... Do you have a suggestion for an alternate solution to get the amp to sounds more like a 10...

Very relieved to hear how painless working through mounting and current surges was for you. Really cool artwork on the top of the chassis, by the way! The next hurdle I'm facing is "options paralysis" in terms of the output transformer specs. Question area 1: I'm seeing differences between manufactu...

These are great! Thanks so much. The calculator seems to indicate that 325V may be better for an on-spec B+ of around 440V. I'm somewhat hesitant to bump the B+ too far, but that's primarily a function of uncertainty over what would happen if the OT primary impedance is too far below the "rule of th...

I'm taking on a challenge of trying to downsize the amount of equipment I have and have a "capsule collection" of versatile amps (floor space issue in a small-ish apartment). I love my Marshall SV20, and I'm selling a Fargen Mighty Plex that I really enjoy (especially since it's cathode biased, so t...

Good article on the SE design based around an EL34 here: http://www.valvewizard.co.uk/se.html cdemike, have you seen this? https://el34world.com/Forum/index.php?topic=21867.0 I haven't seen that -- thanks for passing it on! I'm consistently surprised by how much gain is possible from triodes. I'm s...

Congrats on the gift! I'm currently designing a similar amp for my wife, who currently plays through a Fender Champ 600 reissue. I generally play a Les Paul, so between the fixed black panel tone stack having too much bass and a general move to down-size our gear, we're going to replace it and a Mar...

Thanks again everyone for helping me find the problem. Dorrisant, I think you're likely right that the OT would still be usable. During the disassembly and teardown process, I ended up cutting the plastic/paper insulating layers over the primary taps, so it'd involve a more extensive repair than tap...