My money is on the inrush current being the culprit. So about 71 mA draw on the power supply at idle. I got a 515V B+ (spot on) and 7.1V drop across the 100 ohm resistor before the first node. Powered it up on the variac again from 60VAC up to 115VAC. It also looks like it should have a 10H input filter choke to be running the tube into a 40uF filter within this range. That should be closer to 75 ohms from the chart I looked at. The plate to plate resistance of the secondary is about 63 ohms (31.4 ohms to CT). It appears to have a ~960VAC RMS plate to plate secondary. I've been looking at the 5U4GB data sheet and it seems like this amp is just pushing it way too hard. But that's not an acceptable fix as my B+ would be in the ballpark of 570V on line voltage. That might be why it's arcing.Ĭome to think of it, I did get it to work once on a cold start with a JJ 5AR4 and the primary voltage turned down to 110VAC so I didn't exceed my cap ratings. With 60VAC on the primaries I'm seeing almost 480VAC on the HV secondaries while operating with a load. You know, I've never checked the secondary voltages on this amp and they aren't listed on the schematic. (I flipped the switch as soon as the arcing occurred.) It doesn't draw any excess current when ramped up on the variac to 120 nor does it appear to when started cold on 120. It didn't hit me until now that I could have just used the extra 20uF section on the can. I've tried three 525V CE cans and two different 20uF caps on the fourth node. The rectifier arced as it starts conducting. I didn't bother including that detail but should've. I installed a new 525V CE can and a new 350V 20uF radial cap this evening before blowing a new JJ 5U4GB. You know, I thought that may have been an issue too. I don't want to ruin any more good tubes. This thing sounded so beautiful with my ES reed organ and now it's a paperweight money pit. It worked great on the variac when I ramped up the voltage slowly. So I tried employing a 100 ohm 10W resistor before the first node. But if I plug the primaries right up to 120VAC it'll arc through a new 5U4GB.Īt this point I thought that the issue might be that the rectifier was directly coupled to the first filter node. The amp will power up correctly if I ramp it up on the Variac. I put in another known good 5U4GB and POP. The tube tested good but that doesn't always tell the whole story. I didn't mess with the tone stack and only changed one large 1000uF out in the Specratone relay setup as a precaution since that is it's own separate circuit.Īnyways, I powered the amp up expecting it to be better than ever and POP! It arced through the 5U4GB as soon as it started conducting. I also changed out all of the coupling caps (C8, C9, C10, C11) with equivalent values. I replaced C6 with an identical value 350V 20uF. I went ahead and changed the three section 500V 50/40uF + 400V 20uF can (C7C, C7B, C7A on the schematic) for a CE 525V 40/40/20/20uF and employed the 40/40/20 sections respectively. The thing was singing! Regardless of that, it was long overdue for a cap job, especially with a 530V B+ at idle. The 7027A's needed biased so I rebuilt the bias supply and hit the nail on the head with a cool 33mA. There was a little bit of power supply hum present but surprisingly little for its age or its output. After powering it up on the variac it worked like it should despite being all original and dating to the '50's. First I tested all the tubes and they showed at 80% or better. I bought the thing off of CL to mate to my Wurlitzer 4600 organ. I've repaired/hod rodded several dozen amps and even designed and built my own amps but this one simply eludes me. Hi guys, I have spent months fussing with this amp out of a Wurlitzer tone cabinet and figured it's time to call for outside help to see WTF I am missing.
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