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Old 17th April 2006, 07:49 PM   #1
gary f is offline gary f  Canada
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Default 6v6 PP - No sound

Hello

I'm stucked again. Building Poindexter's 6V6Gt push Pull amp.
Everything is finished, power supply seems to work, but no sound out from the amp.

Here is the schematic. I have 267 volt DC at the plate of 6v6 PP which is lower than the 315 volt specified. But 6v6gt specsheet specify 285 volt in push-pull triode configuration. Am i within specs?

Bias circuit seems OK, voltages are close.

5965 driver circuit is also low in voltage, 255 volts at point B instead of 295 volt specified, but the driver triodes seems to work, I measure signal at the 6V6 grid, approximately 10X higher voltage than the input. But still no sound.

Thanks for any help guys.
F
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Old 17th April 2006, 07:58 PM   #2
anatech is offline anatech  Canada
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Hi Gary,
What's the voltage across the 15R bias test resistor?

Lower plate voltages will only reduce the power a bit, they will not kill the sound at those levels.

-Chris
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Old 17th April 2006, 08:15 PM   #3
gary f is offline gary f  Canada
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The voltage drop across the 15 Ohm resistor is 0,8 volt approx.

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Old 17th April 2006, 08:20 PM   #4
anatech is offline anatech  Canada
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So that's about 26 mA per tube. A bit low. You should get some kind of noise out of it.

Turn it off and run your signal generator straight into teh output transformer and listen to the speaker. You should get something.

Don't forget to discharge the supplies first!

-Chris
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Old 17th April 2006, 08:47 PM   #5
gary f is offline gary f  Canada
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If I put input signal on the input of the amp (0,3 volt approx), I get approximately 10 volt AC on the primary wires of the output transfo. So in theory, I should get some signal out I guess.

Output trans are hammond 1608. Here is the input schematic from manufacturer:

I connected the blue and brown to 6v6 plates, and red is 267 volt DC. The SG taps are not connected to anything right now.

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Old 17th April 2006, 08:49 PM   #6
gary f is offline gary f  Canada
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The output transfo are wired like this on the secondary. I grounded the neg side of each channel also.

Something wrong?
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Old 17th April 2006, 09:05 PM   #7
anatech is offline anatech  Canada
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Hi Gary,
Your hookups are correct according to the Hammond catalog. You could ground, or not. There is no feedback to worry about. You should be getting something.

Do you have an oscilloscope handy? The circuit may be oscillating ultrasonically. I'd still expect some noise though.

Do me a favour and do the above test to make sure the output and speaker connections are okay.

-Chris
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Old 17th April 2006, 09:10 PM   #8
gary f is offline gary f  Canada
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I can have an oscilloscope, but don't have it today. I will make the test later today or in teh next few days. Thanks for the info.

Also, the power supply caps discharge quite quickly when i put the power off. All the voltages are down to zero in maybe 10 seconds when tubes are in.

Is this normal?
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Old 17th April 2006, 09:15 PM   #9
anatech is offline anatech  Canada
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Hi Gary,
I don't have a full schematic so I can't tell you. It seems like a fast discharge to me unless it has aggressive bleeder resistors.

-Chris
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Old 17th April 2006, 09:39 PM   #10
gary f is offline gary f  Canada
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No bleeding resistor... I might have a short circuit, or a discharge path somewhere. I will check circuit again tomorrow.
Thank you very much for your help.
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