Mojotone 5E3 With No Gain

[Chodrick1]

Im not looking for a high gain amplifier. I just want a little crunch, and when I need more then I can plug in my Tube Screamer. I really dig the tone of the new '57 reissue Deluxe, and the circuit is almost identical to a vintage 5E3. The only differences are the 12AX7 preamp tube and then they put grid stopping resistors on the power tubes for some gain, but I am assuming that the grid stopping resistors would be a minor tweak. I havent tried it yet, but I am wanting to get my amp stock before I decide to alter it any. My only issue is that I dont believe that it sounds like it should.

[Justin Thomas]

Voltages being a bit high doesn't alarm.l me. But I'd be more concerned with V1 being low plate/high cathode. Pulling a bot too much current. I'd suspect a wiring error, a partial short, or some kind of oscillation. Oscillations can manifest in tons of dofferemt ways, low output and clean being one. It all just depends on what is oscillating next to what... And yes,an open cathode.bypass cap could do that, too. A lot of times, when working on an old cheapie amp or a PA or HiFi, all you have to do is add a ctahode bypass cap or two to get instant megagrind. Or maybe a resistor is mis-marked.

Justin

[Chodrick1]

Thanks Justin. The way I put the cathode cap and resistor into the eyelet board will make it hard to perform a capacitance test, but Ill have to pop them out one by one and check them. To me it sounds like the bypass cap is derectly responsible for the gain loss, but I dont have an oscilliscope at the moment to check. But based on the decrease in voltage, I think its ratted itself out. I have measured continuity to ground and then to pins 3 and 8. Everything checks out fine here. I don't believe it is my tube, because when I swapped it with the 12AX7, it still had no gain. At this point I am just crossing my fingers that the bypass cap has crapped out. At least it would be the end of my suffering haha.

Im dedicated to fixing this Deluxe this week. I will have more free time in the mornings to fiddle with it. Thanks in advance.

[Justin Thomas]

No need to check the individual parts. If your resistor leads are too short to span the eyelets, tack two resistors of half the value each in series into place and listen. Then tack a cap across both resistors and listen again. If that fixes the problem, then make another cap/resistor up. You'll know the difference, if that is indeed the problem. No need for the test parts to look pretty or be precise - close enough is good enough. I do nearly everything myself point-to-point because my fhassis recycling does not allow boards. I understand having to get creative.

And those grid stoppers, if they're on pin 5, are for stopping oscillations, not adding gain. Item a protection measure, not a sound measure.

Justin