Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bbmade on April 26, 2022, 12:49:02 pm
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Built a Mojotone 5E3 kit years ago (Rev 1 schematic). Instructions for that kit were nonexistent and made my share of mistakes. It was noisy and eventually I worked through the grounding scheme with the help of Hoffman info and Rob Robinette info. Amp is quieter now.
I had a 12AX7 in V1 and V2 and the amp would sort of collapse into itself as you turned it up and/or played hard. Sort of cutting out or voltage sagging in a bad way. Swapped the V1 to the 12AY7 it should have, and the collapsing/cutting out when played hard went away.
Just curious if this scenario sounds like anything common you all could point me in the direction of. Not expecting a “solution” or magic pill. Have not tested the 5Y3 but it’s a new JJ. Tried different 6V6’s and the problem was always there with the 12AX7 in V1.
Thanks for any insights!
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I would verify that the kit you built is wired to the instructions they have now or a schematic of the original fender. Then I'd check voltages against known good ones for the vintage amp and see whats happening there. Most people who have experimented with a 12ax7 for V1 have said it completely redlines the preamp.
A popular modification, one thats even pushed by mojotone themselves, is to swap the 0.1uf coupling caps for 0.022uf caps. This helps tame some of the low end that can send the amp off the rails as well. On a non-kit build of that circuit I had to do the cap swap and get a 15 inch speaker capable of handling the bass to play at any volume and it not fart out.
Go with the 5y3 rectifier. Make sure you are biased correctly. Have fun. It's a great amp.
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Thank you very much. I was surprised how much better it got with the 12AY7 and I shouldn’t be now that I tried it. I need to compare the Rev1 I built to their current Rev4.
Good idea on the coupling caps too. My buddy was convinced he wanted to buy it on Friday after we swapped that tube and I’ve been telling him for months I’m not convinced he really wants a 5E3. Instead of selling it Friday I told him to take it to band practice and let me know. Today he says it’s not want he wants just like I’ve been saying!
Glad he finally knows it’s not the circuit he’s after. Now I get a chance to work on it more.
Thanks again and anyone else feel free to chime in too.
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If the 5E3 still has a little more then wanted heavy bass or muddy tone after the .1 to .022s cap change, try changing the 25uf cathode cap on the first stage 12AY7 to say 4.7uf, if better but still a little bassy, change the other side to say, 4.7uf or 10uf. These changes are called de-farting the 5E3.