90 watt Head is 10 watt issue all channels

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jdgebha
Mark I
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90 watt Head is 10 watt issue all channels

Post by jdgebha » Fri May 03, 2019 4:13 pm

I usually put my channels in 45 or 10 watt mode when practicing at home. Lately when i switch any channel to 10 watt mode the sound fades out to nothing. Not sure if this is a relay issue but is there a chance a faulty tube could be doing this?

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Old BF Shred
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Re: 90 watt Head is 10 watt issue all channels

Post by Old BF Shred » Fri May 03, 2019 6:53 pm

I remember hearing that the Phase Inverter (driver) tube can cause this. I believe it is V7, but the manual will tell you for sure. Hope that helps. :)
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Don
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Re: 90 watt Head is 10 watt issue all channels

Post by Don » Fri May 03, 2019 10:47 pm

I agree with BF- look at the phase inverter tube. I had a problem where my Express 5:50 would cut out if I switched it to 5 watt single-ended mode. If I understand it correctly, being used as a driver in a single-ended amp is more demanding the being used as a phase inverter in a push/pull amp.

If 10 watt mode is single-ended then that could be your problem.

APEMAN
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Re: 90 watt Head is 10 watt issue all channels

Post by APEMAN » Sat May 04, 2019 4:27 pm

After checking V7 I would look for the rectifier tube as well. it is active in all 3 channels running in 10Watt. Good luck!

bandit2013
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Re: 90 watt Head is 10 watt issue all channels

Post by bandit2013 » Sat May 04, 2019 9:02 pm

You may have a weak preamp tube in the phase inverter. Like others have suggested I would start there.
Rectifier tube is active all the time but will be bypassed in 90W mode but will still be conducting current some current since the heater is also the cathode but the contributing current is negligible when it is bypassed by the quad of diodes. 10W power mode does use the tube rectifier to reduce the plate voltage for the class A (cathode biased pair of tubes, uses two 7W 360 ohm resistors). If the Rectifier is not operating properly the voltage will sag as if you turned the amp off but the LED's will be active since they are sourced by a different coil on the power transformer. The voltage bias circuit is also removed from the two tubes but not removed from the unused pair tubes since they are acting as a load ballast. Are you running 6L6 or EL34? If EL34 what type are they if not Mesa? If it was the tube rectifier, you may get the same result using 45W power setting on CH1 or CH2 with tube tracking selected.

jdgebha
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Re: 90 watt Head is 10 watt issue all channels

Post by jdgebha » Sun May 05, 2019 11:49 am

Looks like it was the rectifier tube. Thanks for all the responses.

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