Hi everyone,
Happy holidays! I was messing around on the bench last week trying to come up with a good use for some 4:1, 10k/600 OPT's I have laying around. In a standard push-pull configuration these really aren't the right transformers to be using for any off the shelf triodes, but I had the idea of trying an AC coupled cathode follower in push pull and this is what I ended up with. I didn't include the AC caps in the schematic.
According to the limited information I've found on the internet about cathode follower push pulls, I've read that they are incredibly difficult to implement. However, this sounds and performs fantastically, which is why I think something is really wrong, and this is just a big accident and I'm not seeing a glaringly obvious mistake here.
I don't know why it works, but it does. The numbers all make sense voltage wise. I've included the voltage measurements at different points. Measuring current from my ammeter, from CT to 47k tail resistor, I measure 4mA. From tail resistor to ground I measure 12mA. What doesn't exactly make sense to me is why this is working.
I can't seem to wrap my head around creating a load line for this as it's kind of a mix of a reactive and resistive load...
1. I don't have much experience with push-pull stages, but I understand them and how they work, but bench testing them, I simply don't have the experience. One general question about any push-pull valve stage, if I probe one of the OPT primaries, would I actually see the signal cut off on my scope during the transition from class A to class B if it was operating in class AB? On this circuit, I can send 20v peak-peak signals into the grids, scope the cathodes, and never see a cutoff of any sort even with the grid bias at -0.5v. There isn't any visible crossover distortion on the output making me think this might be running in class A push-pull? I have a scope probe that measures current as well, should I be using that instead?
2. Frequency response is flat with -3db points at 9hz and about 50khz no matter the secondary load. Which is again why I think there is something wrong. There is only .005% THD+n unloaded, and don't get above 1% unless I really load the secondary down with something crazy like 130ohms, which ends up giving me -6db, which I'm also guessing loosely assumes the output impedance? (I know in reality it's more complex than this)
3. If I probe any of the grids with my 10M probe, the output signal completely collapses into an undesirable mess. Almost like the signal on one side is grounded out. What's up with that? I've never had that happen on any other valve stage before. Maybe the input impedance is super out of whack? I haven't tried any other value of grid leak resistors, but that will probably be next on the list.
4. As to the fair frequency response, I had thought maybe my 50ohm signal gen was perhaps doing all the heavy lifting here, so I built a few different PI circuits before this to feed it, with more of a high impedance output, but it didn't change the performance at all. This made me think either A, something is really wrong here, or B, I just kind of whipped out something that works great.
4. No doubt the DCR's are creating the negative grid bias, that math works out. But at such a low bias voltage I'd assume I'd see some sort of signal cut-off on the cathodes?
Anyways, I'd be curious to what everyone thinks is going on here. I'm humbly confused, and humbly not ashamed to admit when I've overlooked something really dumb here.
Happy holidays! I was messing around on the bench last week trying to come up with a good use for some 4:1, 10k/600 OPT's I have laying around. In a standard push-pull configuration these really aren't the right transformers to be using for any off the shelf triodes, but I had the idea of trying an AC coupled cathode follower in push pull and this is what I ended up with. I didn't include the AC caps in the schematic.
According to the limited information I've found on the internet about cathode follower push pulls, I've read that they are incredibly difficult to implement. However, this sounds and performs fantastically, which is why I think something is really wrong, and this is just a big accident and I'm not seeing a glaringly obvious mistake here.
I don't know why it works, but it does. The numbers all make sense voltage wise. I've included the voltage measurements at different points. Measuring current from my ammeter, from CT to 47k tail resistor, I measure 4mA. From tail resistor to ground I measure 12mA. What doesn't exactly make sense to me is why this is working.
I can't seem to wrap my head around creating a load line for this as it's kind of a mix of a reactive and resistive load...
1. I don't have much experience with push-pull stages, but I understand them and how they work, but bench testing them, I simply don't have the experience. One general question about any push-pull valve stage, if I probe one of the OPT primaries, would I actually see the signal cut off on my scope during the transition from class A to class B if it was operating in class AB? On this circuit, I can send 20v peak-peak signals into the grids, scope the cathodes, and never see a cutoff of any sort even with the grid bias at -0.5v. There isn't any visible crossover distortion on the output making me think this might be running in class A push-pull? I have a scope probe that measures current as well, should I be using that instead?
2. Frequency response is flat with -3db points at 9hz and about 50khz no matter the secondary load. Which is again why I think there is something wrong. There is only .005% THD+n unloaded, and don't get above 1% unless I really load the secondary down with something crazy like 130ohms, which ends up giving me -6db, which I'm also guessing loosely assumes the output impedance? (I know in reality it's more complex than this)
3. If I probe any of the grids with my 10M probe, the output signal completely collapses into an undesirable mess. Almost like the signal on one side is grounded out. What's up with that? I've never had that happen on any other valve stage before. Maybe the input impedance is super out of whack? I haven't tried any other value of grid leak resistors, but that will probably be next on the list.
4. As to the fair frequency response, I had thought maybe my 50ohm signal gen was perhaps doing all the heavy lifting here, so I built a few different PI circuits before this to feed it, with more of a high impedance output, but it didn't change the performance at all. This made me think either A, something is really wrong here, or B, I just kind of whipped out something that works great.
4. No doubt the DCR's are creating the negative grid bias, that math works out. But at such a low bias voltage I'd assume I'd see some sort of signal cut-off on the cathodes?
Anyways, I'd be curious to what everyone thinks is going on here. I'm humbly confused, and humbly not ashamed to admit when I've overlooked something really dumb here.