Really? Ballpencil pointed out near the beginning of the thread that the datasheet says that 400k would be fine for fixed bias. Seems to not be particularly picky.
But if it really needs a low DCR path, a grid choke is a way to do it. I just was curious why mr2racer felt the need to make the change to the design, and I wanted to introduce the idea that there might not be much (or any) improvement over the 220k resistor.
But hey, sometimes I do something just to try it out and see if I can. Don't let me stop that. I'm just trying to understand the why.
When I first listened to the amp a while back it was distorted in a way I have never heard before. Some frequencies were fine, voice, others, bass and high frequencies were distorted. Grid chokes become more necessary as bis voltage increases. The amp required about -78 volts of grid bias. I built this same amplifier with 2A3, (@-40 volts) and it was great right out of the box. The only difference was the output tube. The voltages and currents were perfect and yet it sounded really strange. I have the grid chokes in correctly now but the bias PSU choke wasn't up to it and while testing it got really hot. I have a new PSU choke on the way.
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This doesn't give me a lot of confidence that you have correctly identified the problem or have chosen the correct solution for it. You think you have a frequency-dependent distortion problem, so you are taking out a component which behaves well throughout the frequency band of interest and are replacing it with one whose performance will vary substantially with frequency. Your own calculations earlier in the thread reveal that you will have worse LF performance with the choke. HF performance might be better but only by a tiny bit since the 220k resistor isn't a big load for the 6SN7s that are already loaded by a 33k resistor in the plate.
Who says that "Grid chokes become more necessary as bias voltage increases?" I've built amps that require similar levels of negative bias without a grid choke. They aren't necessary at all. They are simply one option among many. They have pros and cons. They should be used when the pros outweigh the cons.
You made one major change when you changed tubes, you put a tube in that requires much more driving voltage. Voltages aren't labeled on the Uniamp schematic. Are you sure you aren't just clipping the driver? What is the driver supply voltage?
I would hazard a guess that most people don't know how to employ them correctly..I use lots of chokes and transformers in my designs; iron phobia is so unbecoming..
But surely you agree that if he is clipping the driver stage, a grid choke won't magically fix that.
I'm not afraid of iron either, but I'd prefer to use it where it won't cause more problems than it creates. If I suffered from iron phobia I wouldn't build tube amps.
But surely you agree that if he is clipping the driver stage, a grid choke won't magically fix that.
I'm not afraid of iron either, but I'd prefer to use it where it won't cause more problems than it creates. If I suffered from iron phobia I wouldn't build tube amps.
Absolutely 100% in agreement, and it could well be the problem here. I'm a little overboard in the iron department these days but very pleased with the overall system performance.
Yep, I think it was a 50 that I was thinking of. Once I was perusing the local antique store and found a very old Cunningham CX350 marked $5. I bought it and it tested good when I got it home. That's probably the only time I have ever bought a tube and felt like I stole it.
grid chokes have been proposed for a push pull amp in this 1948 article
....http://dalmura.com.au/projects/Phase Inversion Circuits Radio Craft 1948.pdf
....http://dalmura.com.au/projects/Phase Inversion Circuits Radio Craft 1948.pdf
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