Sorry again about the schematic but yes, the caps in the bias supply are the ONLY electrolytic in the whole thing, and yes checked and double checked, + to ground.the. As soon as they start to oscillate they flash a light blue, looks like it's coming from in between the cathode and the grid, or grid and control grid.
But when these are running without the oscillations there is no glow. What is it that causes the blue flash? I'm familiar with common blue glow, from imperfections in the vacuum of the tube. The only time I have ever had a 2Hz oscillation problem like this was feedback problems, but I don't see where they would be coming from. So, anyhow here's the schematic, revised to show what's actually going on now. I know these 6BG6's are pretty sensitive to changes, especially at these voltages. As long as the PS was well filtered, which it is, they were fine. I have made other amps with light decoupling like this. I really want to try what the poster said about decoupling the stages a bit more. I think the topology is good, I'm just missing something. I just wanted to, and like, splitting phase while the signal is still small. This topology is pretty often seen on 2 stage amps. Seems like I should be able to play with these things without sending the finals into oscillation mode, right?
#BIAS AMP 2 AMP LIST DRIVER#
where do I have the CCS set, what cathode R value do I have in driver position, etc. Like I said, it's very dependent on how the rest of the amp is set up, I.E. In tests when I had the oscillations stopped, she sounded great.but obviously I'm not exactly sure what's causing them, or why they come and go. Leaky Tube? Nope, all test NOS or better on a verified tester Leaky coupling cap? Nope, they're all blocking fine. 75K is much better but the problem still occurs sometimes in testing Lower grid R's on 6BG6.I started out with 100k. Lower B+ (turn down variac so that b+ is only 350).still oscillates, tubes still get blue/white "flashes" I feel like I've checked over most of what I knew could cause it. I'll post the schematic, if someone could help me cross some stuff off the list I'd be thrilled. If I unhook the front end and just leave the 6BG6's hooked up just like this, at 50ma bias, they will sit there stable all day long. Get the CCS set for too much or too little current on the first 6SN7 for example, and it oscillates. It's very sensitive to the rest of the amp. The bias/current draw on the tube oscillates between the quiescent 45ma and about 75ma. You know the type, blue flashing in the tube. I am getting some oscillation problems in the 6BG6's. The 6BG6GA's are negative biased, with 75K grid resistors. Voltage amp/phase splitters are LTP, CCS, 6SN7. I have built from scratch this Push Pull 6BG6GA amp. I'm hoping to get a little advice, and know there are many of you here more knowledgeable than I
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