Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rutledj on December 13, 2023, 12:02:56 pm
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The amp sounds fine but I have a problem with it eating power tubes. So far, two different sets have had one tube redplate. Switch the two tubes around and the same one red plates so it is definitely the tube going bad.
When I replace the tubes, it works fine again. Voltages look good.
Plate: 405v
Screen grid: 409
Grid: -37.8
Voltage through 1 ohm resistors on cathode read 23 v.
I only play this at bedroom levels but still use a Bugera attenuator between the amp and speaker to allow me to set the volume on about 4.5. It probably doesn't have 50 hours on the tubes before the go bad.
Is it just coincidence that I've had two tubes go bad?
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Voltage on 1 ohm resistors should not be read in volts, but milivolts.
Is the -37.8 volts on each power tube grid, or just one?
How about a close up picture of the guts on the power tube end of the amp?
Dave
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Maybe just a coincidence. However, even though you're playing a bedroom level, you're running the amp pretty hard. So, using the attenuator is probably a factor too.
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My bad. It is 23mv. And yes, both tubes grid voltage are the same.
Here is a pic:
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When the tubes have redplated, was it in the same socket both times?
What brand of tubes are you using? Are they new?
Dave
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Not positive if it was the same socket. Good idea to check that though. Tubes are EHX. I just put a set of JJs in there.
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If you are getting good negative bias voltage on the grids of the power tubes, and the tubes are good, they should not red plate.
I would check the tube sockets and make sure they are tight. Could be that you have good bias voltage, but it never actually makes it to the tube grid because of a bad connection between the tube and the socket.
Dave
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The amp sounds fine but I have a problem with it eating power tubes. So far, two different sets have had one tube redplate. Switch the two tubes around and the same one red plates so it is definitely the tube going bad.
Once a valve has redplated significantly, it's knackered, probably won't be able to run stably again.
I only play this at bedroom levels but still use a Bugera attenuator between the amp and speaker to allow me to set the volume on about 4.5. It probably doesn't have 50 hours on the tubes before the go bad.
Is the correct OT secondary tap being used for the load presented by the attenuator?
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Is the correct OT secondary tap being used for the load presented by the attenuator?
Yes. 8 ohms. It only has the one pair of output wires.
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Here is a pic:
Lots of questionable work here. Looking at your bias supply the bias voltage and ground wires are coming through the same hole with lots of insulation stripped back. They could be making contact.
Also the plate connection, brown wire, looks like it's not making good contact with the socket.
Finally, the bias pot wiper looks corroded or burnt.
I think you're losing bias and therefore redplating. Either the pot is intermittent or you're shorting bias voltage to ground.
Go over that entire bias circuit with a magnifying glass correcting bad connections and ensuring proper creepage and clearance to prevent losing negative voltage reference.