I agree again
John, on all points. and I had thought about it. Although I'm confused, you say I need grid leak resistors on both grids? There are grid leak resistors on both grids of the drivers. As far as the LTP on the input...well, that doesn't need grid leaks to ground on both so you must mean the drivers. There are many ways I could make the layout more efficient, better sounding. But the way it is now, I would think, it should work fine. Already, having an LTP for phase splitting, having negative bias, having good PS filtering...it is better than so many more mediocre schemes that some manufacturers got away with long ago.
At this point, it's just a curiosity thing. Before I go taking things out and re-wiring/rewriting the circuit, I would like to understand for my own knowledge and satisfaction just why the oscillations are occurring.
As I stated before, I can take the tubes out of circuit on the input and still get it to oscillate. At that point the only thing on the front end with any connectivity to the rest of the amp are the plate resistors to the LTP and the coupling caps.
Thanks again, I am appreciating all these responses very much
John, on all points. and I had thought about it. Although I'm confused, you say I need grid leak resistors on both grids? There are grid leak resistors on both grids of the drivers. As far as the LTP on the input...well, that doesn't need grid leaks to ground on both so you must mean the drivers. There are many ways I could make the layout more efficient, better sounding. But the way it is now, I would think, it should work fine. Already, having an LTP for phase splitting, having negative bias, having good PS filtering...it is better than so many more mediocre schemes that some manufacturers got away with long ago.
At this point, it's just a curiosity thing. Before I go taking things out and re-wiring/rewriting the circuit, I would like to understand for my own knowledge and satisfaction just why the oscillations are occurring.
As I stated before, I can take the tubes out of circuit on the input and still get it to oscillate. At that point the only thing on the front end with any connectivity to the rest of the amp are the plate resistors to the LTP and the coupling caps.
Thanks again, I am appreciating all these responses very much
Put a scope on it and you'll find out what's happening when the flash occurs. Could be arcing over.
I did today
Put the scope on it. Made a few small changes, i.e. larger dropper for the drivers, shortening leads on the grid tie downs on the input, etc. I'm thinking it was a problem, as was stated early on, with grounding. Redid all the grounds and dialed in the CCS on the inputs. Sounds great now, reminds me of the amp the iron came from, the Scott 299-C. These 6BG6's have a bright sound to me, or, maybe it's the iron, but the scott sounded that way as well. It's not unnaturally bright or unbalanced, just...well place fancy audio adjective here:
Thanks everyone for the replies, I really appreciate it. Only thing left to do now is the hum. This amp is set up with a volume pot, and I wired it so that the source always has the 50K load and the grid is to wiper. All the way down (grounded grid) and no, no hum. That's good in my book, means it's getting picked up on the grid of first tube and amplified through system. Even at full volume with 50K on the grid (well, more like 40, the 50K pot and 100K tie down), it's not very loud at all. I don't want to go with a grid stopper, so I'll try moving some things around hopefully can get rid of it. Any last advice for stopping the hum is always appreciated as well. Obviously, one doesn't know the things that one isn't thinking of 🙂
LC
Put the scope on it. Made a few small changes, i.e. larger dropper for the drivers, shortening leads on the grid tie downs on the input, etc. I'm thinking it was a problem, as was stated early on, with grounding. Redid all the grounds and dialed in the CCS on the inputs. Sounds great now, reminds me of the amp the iron came from, the Scott 299-C. These 6BG6's have a bright sound to me, or, maybe it's the iron, but the scott sounded that way as well. It's not unnaturally bright or unbalanced, just...well place fancy audio adjective here:
Thanks everyone for the replies, I really appreciate it. Only thing left to do now is the hum. This amp is set up with a volume pot, and I wired it so that the source always has the 50K load and the grid is to wiper. All the way down (grounded grid) and no, no hum. That's good in my book, means it's getting picked up on the grid of first tube and amplified through system. Even at full volume with 50K on the grid (well, more like 40, the 50K pot and 100K tie down), it's not very loud at all. I don't want to go with a grid stopper, so I'll try moving some things around hopefully can get rid of it. Any last advice for stopping the hum is always appreciated as well. Obviously, one doesn't know the things that one isn't thinking of 🙂
LC
Not sure if it would help your hum, but since the right/undriven grid of your PI is both at ground potential and ac grounded, you could simply tie it to ground directly.
One less high impedance, possible hum pickup point.
It saves you a 1M resistor and a cap as well. 🙂
One less high impedance, possible hum pickup point.
It saves you a 1M resistor and a cap as well. 🙂
I thought that
And it had been suggested by a few other local builders. I have had luck with it this way before, but I think I'll try tomorrow. Why is it that some designs show it this way and some the other, in what case would you need the grid to only be at AC ground potential, and need the 1M resistor?
Thanks much
And it had been suggested by a few other local builders. I have had luck with it this way before, but I think I'll try tomorrow. Why is it that some designs show it this way and some the other, in what case would you need the grid to only be at AC ground potential, and need the 1M resistor?
Thanks much
'Traditionally' a large resistor is used to have a high enough impedance (long tail) for the LTP to work. This means that there is much more dc voltage on the cathode than is needed for proper bias. In other words, the grids need to be at a higher dc level as well to get the correct grid-cathode voltage.
Example with fictional numbers, just to get an idea:
- Cathode through CCS tail @ +5V. Grids at 0V. Grids wrt cathode -5V.
- Cathode through big tail resistor @ +50V, grids need to be @ +45V to get the same -5V bias. Both grids need caps to connect them to the source resp ac ground.
Example with fictional numbers, just to get an idea:
- Cathode through CCS tail @ +5V. Grids at 0V. Grids wrt cathode -5V.
- Cathode through big tail resistor @ +50V, grids need to be @ +45V to get the same -5V bias. Both grids need caps to connect them to the source resp ac ground.
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