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Old 2nd April 2008, 12:24 PM   #1
FSHZ:42 is offline FSHZ:42  United States
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Default Need Help For Tube Amp

I had recently upgraded my intergrated tube amp. I replaced the original .47uF 400V caps with Sonicaps .47uF 600v caps. I then bypassed the larger value electrolytic caps with some Jantzen .10uF 400V caps. I then changed out the oem binding posts with some gold plated ones. Next, I installed 4 winged C's,EL-34's, matched quad set from tubesandmore. I then swaped out the oem pre-amp tubes with a set of Russian 6n3p-eb's purchased from boiaudioworks. I then was told to set the bias on the pot's to 400mV. After setting all 4 pots, I checked the voltage from the #5 pin (blue wire) to ground getting approxiamatley
-22volts. All was working well for a few hours of listening, so I turned the amp off. The following day, I turned it back on, and let it warm up a bit. I started to play a disc and noticed that only the right speaker was playing. I then looked at the amp and noticed that the rear left pre-amp tube wasn't working. So, I shut it off.

Could it be that the tube was defective? Is this 400mV setting for the power tubes too high?

Any help would be appreciated!
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Old 2nd April 2008, 01:17 PM   #2
SY is offline SY  United States
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When you say "rear left preamp tube not working," do you mean that it's not lit up?

Without knowing the value of the cathode sensing resistor in the output stage, it is impossible to comment on whether 400mV is a safe value.
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Old 2nd April 2008, 01:41 PM   #3
FSHZ:42 is offline FSHZ:42  United States
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Quote:
Originally posted by SY
When you say "rear left preamp tube not working," do you mean that it's not lit up?

Without knowing the value of the cathode sensing resistor in the output stage, it is impossible to comment on whether 400mV is a safe value.

The pre-amp tube does not light-up.

I believe the cathode sensing resistors are 10ohms
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Old 2nd April 2008, 02:51 PM   #4
SY is offline SY  United States
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OK, you need to check the heater wiring. Make sure that there's continuity to that socket, then check the heater voltage. If that looks OK, swap left and right channel preamp tubes and see if the problem moves with the tubes.

With a 10R cathode resistor, your idle current is 40mA per tube, which doesn't seem at all excessive.
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Old 2nd April 2008, 03:03 PM   #5
FSHZ:42 is offline FSHZ:42  United States
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Quote:
Originally posted by SY
OK, you need to check the heater wiring. Make sure that there's continuity to that socket, then check the heater voltage. If that looks OK, swap left and right channel preamp tubes and see if the problem moves with the tubes.

With a 10R cathode resistor, your idle current is 40mA per tube, which doesn't seem at all excessive.

I skipped the heater wire check and continuity to socket, and just swapped out left and right preamp tubes. The tube that had no glow still has no glow, but the other tube glows. So I would assume that the preamp tube that went out was perhaps faulty?
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Old 2nd April 2008, 03:25 PM   #6
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It's always a good idea to have extra preamp/power tubes laying around in case of situations like this.
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Old 2nd April 2008, 04:25 PM   #7
FSHZ:42 is offline FSHZ:42  United States
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Fortunately, I had ordered a set of 5 from boiaudioworks. Hmmmm, kinda makes you wonder! Anyhow's I still have the original (4) Chinese brand preamp tubes that actually are pretty good. So I guess I will install the extra tube from boiaudioworks and see how that works! Hopefully it was just a faulty tube. I'll also go back into the amp and make sure the bias setting is o.k. Another thing I might try is to mis-match the preamp tubes. I think that maybe I'll put one Chinese tube with one Russian tube on each side!
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