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Old 24th January 2007, 07:23 AM   #1
qq is offline qq  United States
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Default add gain before or after phase splitter?

I've built a little stereo amp around a pair of gov't surplus 815 tubes I found at a junk shop. The tubes look cool and are dual beam power tubes designed for push-pull outputs.

My one-tube 12ax7 phase splitter works great, but with a little bit of feedback from the OPT output doesn't have enough gain for a normal 1 volt input to drive the 815 to full power, which i think should take something like 30vrms.

Can someone educate me on the advisability of a) adding a preamp before the phase splitter vs. b) adding drivers after the phase splitter to drive the output tube grids?

I have really no experience in audio design or really even anything analog. I'm just amusing myself with a new-to-me technology. It sounds a lot better than I'd expected, actually!
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Old 24th January 2007, 07:35 AM   #2
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If you add gain before de phase splitter you will spend one double triode for both channels. If you do this after the splitter you will use one double triode for each chanel.

This is the first thing you should think.
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Old 24th January 2007, 08:18 AM   #3
Merlinb is offline Merlinb  United Kingdom
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If you need quite a bit of gain I would say add the gain stage before the PI. Then apply NFB to the cathode of that stage instead. Any distotion taking place within that stage will then be identical at each power valve grid.

If you only need a little bit of extra gain (like a gain factor less than 10) I would say add gain stages between PI and power valves. You can apply local feedback on these stages to increase their input impedance and linearity so that the PI is always constantly loaded, and so that you obtain a very low output impedance to drive the power valves. Then global NFB can be brought back to the PI and you have 'nested' feedback and a nice little amp.

DC coupling would be a nice touch in both cases.
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Old 24th January 2007, 11:40 AM   #4
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If I were you I'd do both. To get the best out of the 815, you can run it in Class AB2 and that needs a direct coupled cathode follower (or source follower) driver to supply the grid current.

This would provide also a high impedance load for the 12AX7, which is a very wimpy tube to be used as a driver for beam tetrodes, even in class AB1, because of the low resistance they need in the grid (usually 100k or less, especially with fixed bias). Stick a good linear voltage gain stage on the front to feed the 12AX7 splitter, direct coupled, and use global NFB to the cathode of the first stage.
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Old 24th January 2007, 05:13 PM   #5
qq is offline qq  United States
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I actually only need a small amount of gain, perhaps a factor of 6 or so. I experimented with a simple triode gain stage in front with lots of local feedback to reduce the gain, however that seems to result in a big decrease in high-frequency response.

Merlinb suggested DC coupling - to get the output and inputs at the same DC voltage level, I take it I add resistance below the cathode of the following stage to bring the whole thing up to the plate voltage of the driving stage?

Is the benefit of DC coupling that it eliminates frequency-dependent phase shift caused by the coupling capacitors? If so - what is the harmful result of that phase shift?
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Old 24th January 2007, 07:23 PM   #6
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Quote:
Originally posted by qq


Is the benefit of DC coupling that it eliminates frequency-dependent phase shift caused by the coupling capacitors? If so - what is the harmful result of that phase shift?
Ringing or even oscillation
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Old 24th January 2007, 07:33 PM   #7
SY is offline SY  United States
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A few more details might help, especially what sort of phase splitter it is. A schematic would be most helpful.
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Old 24th January 2007, 07:34 PM   #8
DougL is offline DougL  United States
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If I were you I'd do both. To get the best out of the 815, you can run it in Class AB2 and that needs a direct coupled cathode follower (or source follower) driver to supply the grid current.
I am in violent agreement
A random thought is that the loading of the output could be affecting gain enough that with followers, additional gain may not be required.
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Old 24th January 2007, 07:55 PM   #9
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Can your splitter provide enough of undistorted signal to drive output tubes?
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Old 24th January 2007, 09:45 PM   #10
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by DougL
I am in violent agreement
Wow, does that mean you'll beat the *!#@ out of anyone who is not?

Sheldon
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