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What am I missing here? I am working on a home-built ac-4 with vibrato. I used a schematic of the web and made my own eyelet board. My issue is with the vibrato portion of the circuit. On fire-up, everything seemed to work fine. No hum, little hiss at full volume, and a working vibrato. I was planning to use an rca out and use a footswitch to operate the trem, but for start-up I just ran a switch to ground on the back of the chassis. For tubes I used spares from other builds. The 12Ax7 vibrato tube was a sovtek that I pulled out of my 5f1 build. Later, I remembered that I had a chinese 12ax7 in a drawer and I swapped the sovtek for the chinese one and that is when the trouble began. With the china tube in and the virbato on the circuit oscillates, but also has a nasty hum -with the vibrato off, the hum stops and the amp seems okay. I put the sovtek back in and all works fine again. I couldn't beleive that tube brand would make a difference, so I swapped in a JJ from another amp, and that didn't work well either and it also produces a lot of noise and hum - put the sovtek back in and all is fine again. All three tubes behave differently, all three tubes "work" in the 5F1 as preamp tubes, so I don't think they are bad. I didn;t think there was any real difference in tubes - I thought oscillation was a pretty straight-forward job for a tube and shouldn't make any difference which brand I put in if they are all 12ax7 tubes. Obviously, I don't want an amp that only works with a specific brand tube. So, my thought is that my hookup must be off somehow. I didn't ground the center pin on the socket - would that make a difference?
EDIT: Okay, after playing with this some more, it is really two things I hear going on, and it really does vary by tube, but I have some unwanted oscillation, and I am picking up some hum. I can "cure" the oscillation by touching my voltmeter probe to the first 12ax7 plate (pin1), or by putting a cap from that plate to ground. Chopsticking seems to point to the EF86 input (always a favorite place for hum) and the line to the "speed" pot.
EDIT: Okay, after playing with this some more, it is really two things I hear going on, and it really does vary by tube, but I have some unwanted oscillation, and I am picking up some hum. I can "cure" the oscillation by touching my voltmeter probe to the first 12ax7 plate (pin1), or by putting a cap from that plate to ground. Chopsticking seems to point to the EF86 input (always a favorite place for hum) and the line to the "speed" pot.
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