I actually swapped the 12ax7's and it doesn't follow the tube. I thought it was also tube, but it seems to be following the socket. The quick heater light up is only happening when the treble potentiometer is turned completely counter-clockwise and the buzzing is only happening when that socket has a 12ax7 inserted. Here are some more pictures of my amp. Sorry the wiring isn't color coded...just the grounds.
I took another look at my layout and the schematic and it looks ok. A couple things I notice...the treble pot seems to reduce the top when turned clockwise, while the bass pot increases the low end when turned clockwise. Almost like the treble pot is reversed in the schematic or I'm reversed. With nothing plugged in and all controls down there is a buzz. When the volume pot is turned up, buzz still stays but reduces and static hiss starts to increase.
The other thing, in regards to the heater light glowing bright at power up, when I power down and restart the amp after a minute or so, the heater doesn't flash bright like it does on power up after the amp is sitting for a while. This doesn't happen on any of my other amps.
The pictures obviously show a mess of wires...maybe my layout needs reworking. The other amp I build is dead quiet, but this one seems to be pretty noisy. Any thoughts on reworking the layout? Grounding? Shielding some cables? Moving preamp tubes further from output tubes?...
So I put the amp to the side that last couple days and just reworked the wiring etc...now I'm getting a hum. Great. This happens when the amp is turning on. The hum increases in the speaker as the amp is warming up, but I shut it down before I could measure any kind of voltages at the power tubes.
Before I powered up, I checked the voltages coming off the P/T secondary and they were fine. Then I checked the negative bias voltage and it was fine. Then I put the rectifier tube in and the plate voltages at the power tubes were 520vdc at pin 3 and 4 and -25 at pin 6.
I've been looking for loose wiring etc..., but can't find any. I pulled all the electrolytic capacitors, thinking one had failed, but all test fine with my capacitance meter. Could one of the capacitors have failed already, but test ok? Not really sure how to troubleshoot this hum. I'm pretty sure it's 60 cycle, but...
So I finally got my SB-12 bass amp up and running. I'll post some pictures tomorrow. Sounds pretty sweet at low volumes. When I do start to turn things up volume wise, before half way, the speaker rattles on the low notes. Took off the back baffle of the cabinet and it sounds like the speaker itself is the culprit. It especially rattles when I pluck the E strings and then hit the A string. It almost sounds like the speaker is folding or can't handle the low end information. Not sure.
What I do know is that this Jensen field coil has probably seen better days. Not sure if I want to go through the hassle of trying to find what is wrong with it. Any other suggestions to find the rattle? And if not, any recommendations on a new replacement speaker? I went to fliptop.net and they are recommending the Eminence Beta-12A-2. There is also the Jensen C12N. I don't have any experience with bass speakers.
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