I decide to change a pair of the power tubes in one of the MKIV's tonight, and when I tried to set the bias, I was only getting 6 ma with the pot wide open, on either tube. I checked the other amp and it was fine. It seems that the reading is off by a factor of 10, so I immediately thought resistor. I recently put in a switch to change from UL to triode in each amp. I checked all the wiring and resistors on both amps, and every thing is the same in both amps. I am not sure if this is the new part gumption trap, where I should have checked the tubes before dropping them in, which I am now doing. Any ideas on whats happening here? The DVM I use is auto-ranging so that should not be a problem
Problem with the bias readings in a pair of MKIV's
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Bob Latino- Admin
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Re: Problem with the bias readings in a pair of MKIV's
by Bob Latino Sun Mar 04, 2012 12:06 pm
Michael,
Check the bias again with another meter that is not "auto ranging". Sometimes an auto ranging meter does not auto range properly - OR - See if your meter allows shutting off the auto ranging feature. If it does, then set your meter manually to 0 to 2 volts DC and try it again. If the bias cannot be set properly then possibly one tube of the pair or one of the tube sockets has an issue. The rectifier could have a problem also.
Bob
Check the bias again with another meter that is not "auto ranging". Sometimes an auto ranging meter does not auto range properly - OR - See if your meter allows shutting off the auto ranging feature. If it does, then set your meter manually to 0 to 2 volts DC and try it again. If the bias cannot be set properly then possibly one tube of the pair or one of the tube sockets has an issue. The rectifier could have a problem also.
Bob
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Thing is, the meter reads fine on one amp but not the other. If the rectifier has a problem, would both tubes be effected ? I am getting the reading on both tubes which check fine on the tube tester, but of course they are not any where close to real world voltage. Thanks for the help!
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Bob: Very strange thing has happened, the amp that would bias last night, will not bias now. Amp that would not bias last, now will bias. Even though the old rectifiers tested OK. I put in a different pair, new power tubes in one amp. The amp that does bias, hits 40ma when the trim pots are at about 3/4 of their travel. At that same point on the other amp, the bias reading is about 17ma, and the power transformer is really starting to hum. This is really crazy. The only change recent change other than the triode/UL switch, was to install 3 watt 6.8K metal films on the power supply cap in each amp. I had 1 watt, and Roy suggested going to the 3 watt. This all seems very strange
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