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I'm rebuilding a pair of MK III's. Just now working on the first.
My plan was to replace all tubes AND: -1 Replace selenium stack w/ diode & bias caps/resistors with a kit from Uncle Neddy. Test voltages. -2 Replace quad cap with triple cap + 1 cap with another kit from Ned. Test voltages. -3 Replace driver board with stuffed board from Classic Valves. Test voltages. Enjoy some Dave Brubeck on my rebuild 60 honkin' watt amps. Of course, I never took baseline voltage measurements (for I am RAT Boy... and I knows it all). I replaced the bias circuit (being very careful to get it all wired back correctly). Using original tubes, I powered it up and checked the biaset... 1v, 1.9v 2v, 4v, 26v!!! PULL THE PLUG! I powered it up again and began to check the voltages and compare them to those listed in manual... 5AR4 voltages are OK on pins 4 & 6 but pins 2 & 8 are sky high. PULL THE PLUG!!! Replaced the Quad filter with Ned's (triple + 1) kit... same issue. Is my PT fried? What should resistances at PT leads be? Could the choke be causing this? Ideas? |
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