This is my third foray into the world of hi-fi-ish amplifiers, the first two being simple single ended affairs. I've collected a handful of 832A and GU32 tubes and really like the look of them. I plan to run them cathode biased to keep things simple. According to the RCA datasheet, the grid resistors should not exceed 50k per section, but I have seen audio designs as high as 390k. What do you guys think about this?
For the front end, I want to use the 9002 tube with a 6C4 phase splitter. I came up with 2 options.
AC coupled:
and DC coupled:
For the AC coupled design, I think I picked a pretty reasonable operating point for the 9002 tube, but then there's that 0.1uF coupling cap. However, I got that 0.22uF cap coming off of the wiper of the potentiometer. I've seen opinions stating that the 0.22uF cap is unnecessary, and I've also read that it's good to keep DC off of the pot to prolong it's life. What do you guys think?
For the DC coupled design, the operating point of the 9002 tube is kind of wonky, I think. Also, will the protection diode on the 6C4 be necessary as I have the 6CG3 diode in the power supply providing a slow start-up?
I have absolutely no reason for the 6CG3 diode except to drop a few volts and to increase the tube count. I have plenty of filament current available so why not? More tubes = more better, right?😀
Thank you all for your time.
For the front end, I want to use the 9002 tube with a 6C4 phase splitter. I came up with 2 options.
AC coupled:
and DC coupled:
For the AC coupled design, I think I picked a pretty reasonable operating point for the 9002 tube, but then there's that 0.1uF coupling cap. However, I got that 0.22uF cap coming off of the wiper of the potentiometer. I've seen opinions stating that the 0.22uF cap is unnecessary, and I've also read that it's good to keep DC off of the pot to prolong it's life. What do you guys think?
For the DC coupled design, the operating point of the 9002 tube is kind of wonky, I think. Also, will the protection diode on the 6C4 be necessary as I have the 6CG3 diode in the power supply providing a slow start-up?
I have absolutely no reason for the 6CG3 diode except to drop a few volts and to increase the tube count. I have plenty of filament current available so why not? More tubes = more better, right?😀
Thank you all for your time.
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AC coupling shouldn't be necessary unless your source has DC offset for some reason. The protection diode is going to cause problems.
? Merlin says it's "obligatory".
I never seen a schematic with the screen of the output tube bypassed to the cathode, only signal pentodes. I'd always thought the filtering cap in the power supply took care of that. Interesting thing is that according to RCA's datasheet, there is a capacitor between the screen and cathode.
I measured the tubes that I have, and the capacitance is around 30-40 pF. I assume it's related to it's role as a high frequency tube.
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If you are a Newbie why did you pick such an unusual tube lineup? All but the 6C4 were never common & might be difficult to replace if you should destroy them. Is the proposed amp to listen to or to look at?
Better to try something with common tubes if this is your first amp build.🙂
Better to try something with common tubes if this is your first amp build.🙂
Newbie maybe the wrong term. I've been building guitar amps for some time, but that a whole different approach than the hi-fi world. I've done some builds with pretty strange tubes, 717A, 6F5, 6G6G, so weird stuff does not bother me. I know the basics pretty well, but when I read Merlin's books on the more advanced stuff, my eyes glaze over. I hope I'm beyond the stage where I know just enough to be dangerous, but I'm nowhere near having the amount of knowledge and wisdom that I've seen on this forum.
Plus, I've got ears of stone from years of loud guitar. If the amp does not squeal and amplify music, it's a win in my books. 😀
You can send the screen cap to ground if you'd like, but you'll want a cap there either way.
The tube datasheet indicates lots of capacitance there, but not enough to be considered a screen bypass.
The tube datasheet indicates lots of capacitance there, but not enough to be considered a screen bypass.
OK go for it. Early on I did build an audio amp with a PP 815. And another with PP 6146s. Both were driven thru an IT. Wouldn't do that now, don't like HV above the chassis. Never had cats to worry about & children all gone with no injuries.😀
The elements of a good amp are the same whether for instruments or HIFI. Quite a few instrument amps are built as linear as a HIFI amp & the FX is simply something between the instrument & the linear amp. Why not, that works.🙂