Troubleshooting Push-Pull 2A3 (please help) - diyAudio
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Old 24th August 2009, 01:56 AM   #1
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Default Troubleshooting Push-Pull 2A3 (please help)

I just finished wiring an amp today which was constructed from the schematic shown here: http://www2u.biglobe.ne.jp/~tossie/F2A3PP-E.html

I have not yet plugged the amp into speakers for a couple of reasons; for one, I don't have any spare banana plugs right now, which my speakers require (Vandersteen 1C), and I am also having a problem with one channel of the amplifier.

On one channel, all voltages measure ok. On the other, when the input is connected to the grid of the 76, the 2A3's start producing this awful hissing, buzzing, high frequency sound. Of course I turned off the amp as soon as I heard that. With no input connected, there is no noise from the tubes, but the voltages are also quite off (about 38V on the 76 plate and 295V on the 6A6 plates). With the input connected, the voltages are fine, though I have not left the amplifier on that way for more than a second or two after B+ comes up since I'm not sure what's going on with the 2A3's.

I've switched every tube around. No matter which tube I use in what location, the amplifier's behavior is exactly the same. I have looked and looked, but I cannot see any difference in wiring between the two channels. When the input is connected, AC voltage on the 2A3 plates shoots up and approximately doubles.

That brings me to another point. The AC voltage on the 2A3 plates with no input signal is about 70V on both channels. This goes to about 140V when the input is connected on the bad channel. I'm not sure exactly what this implies; I would have expected very low AC voltage on the plates with no input signal applied, so this seems bad to me.

I should also mention that I previously had the channel that is not giving me problems wired and working before I wired the other channel (was missing some parts). I did not measure the AC voltage on the 2A3 plates at that time. The only difference between the wiring now and the wiring then, is that I connected a 1.5k 25W resistor in parallel with a .10uF capacitor between the 16 ohm tap of the OPT and the junction of the 3.3k and 47R resistors on the cathode of the 76. Is this ok? There is no value specified for the capacitor in the feedback loop.

I'm lost here. Does anyone have prior experience with either of the two problems I mentioned? Thanks, and sorry if my post is a bit long!
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Old 24th August 2009, 06:21 AM   #2
rutcho is offline rutcho  Bulgaria
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It seems the problematic channel is going to self oscillation.
First - do not apply power unless transformer outputs are unloaded! Power tubes can be seriously damaged. Connect a ballast resistors (8 Ohm/20W or more) to both outputs.
Then remove a Negative Feedback from the problematic channel. Using ON/OFF switch as shown on the schematics is not enough. Disconnect the wire from the output transformer side.
Apply the power and check if the problem still exist.
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Old 24th August 2009, 06:27 AM   #3
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.1uf for the feedback 'speed up' cap seems awful high. This cap is usually a few hundred pf,IME.

Also,be sure that you have the phase of the output transformers correct,if it's reversed you no longer have Negative feedback,but instead Positive feedback,essentially turning the amplifier into a large power oscillator(!).

If you disconnect the NFB wire from the output transformer,does the amp behave? If it's OK with no feedback connected,and goes wacky with the feedback connected,then it's out of phase (swap the plate leads around.)
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Old 25th August 2009, 07:52 PM   #4
Mush is offline Mush  United States
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Is the feedback on the problem channel phase reversed from what it should be? That's where I'd look.
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Old 26th August 2009, 02:48 AM   #5
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DigitalJunkie is right. 0.1 ufd is way too big. Start with no capacitor at all. Then when things are working correctly, the feedback trim capacitor is chosen for best square wave rising corner using a 2KHz square wave. You'll need an oscilloscope and a non inductive load resistor for this. The quality of the output transformer determins the capacitor's value, which is typically 50 to a few hundred picofarads.
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