B+ Issues in Tweed Twin Build

[Zer09]

I just recently finished a build of a 5e8a tweed twin. This was not a kit but a conversion of an old Conn organ amp. The only problem with it was I had to tap into the b+ to create a bias supply and the PT could only support the current draw of one rectifier tube (no big deal). I found another PT on ebay with both a bias tap and 2 5v, 3A taps for rectifiers. The PT is rated at 335v@220ma, has 13 amps of 6.3v filament capability.

I installed it today. Did my usual power up process...no tubes, then rectifier. I had 340 AC rectified to 460 DC, no load. i tried to check b+ under load to make sure things were not thrown off too much. When I switched standby to on, voltage dropped to about 190 volts and rectifier got extremely hot(NOS RCA5u4gb) same with an NOS Sylvania and a Weber copper cap I had laying around. It does wether or not it is loaded with tubes. Any ideas?

[loudthud]

Measure the ohms to ground at the output of the rectifier(s). (Power unplugged, caps discharged, Standby in Play mode.) You should see a high resistance like 10K or more. Most meters will not give a steady reading but one that slowly increases. If you see a lower resistance like 500 ohms, there is a partial short to ground, at a filter cap or a defective choke. Another possibility is a short inside the transformer. Disconnect each winding and check the resistance to the transformer frame. It should measure infinite.

[Zer09]

Thanks Thud. I'm leaning towards a a short to ground somewhere in the rectifier-to-standby-to-filter caps connections. When standby is switched to play the voltage drops 200+ volts down below 200vdc. At first cap it's actually down to millivolts. I'm thinking something was disturbed when I swapped the PT and something is shorting to ground. If not, I appreciate the suggestions for checking the PT for shorts. Will do that if I cannot find a short which I'm really, really hoping for. I'll be really pissed if I found a PT that perfectly fit my chassis AND has dual rectifier filament taps but it's got shorts. Thanks again.

[Bruce / Mission Amps]

Is this a 340v PT (170v-0-170v)? Or is it actually a 340vct PT (340v-0-340v)?
However, it looks like a simple wiring short if both PTs are doing it...

[Zer09]

Bruce,
It's 340-0-340. Actually rated 335@117 volts ac but I have slightly higher out of walls. It's a vintage organ PT that supported 2 6l6's and a whole bunch of preamp tubes. It is only doing this with the "new" PT. The previous one didn't have a bias tap and it annoyed me having to tap into the b+ to create one. What happened is things are pretty tight around the PT because of the length of my chassis so things were disturbed when replaced it. Pretty certain that the problem is in filter caps because there is something strange going on there. The voltage from the filter caps feeding the power tubes is exactly the same as that feeding the preamp section, despite the 10k dropping resistor. Have hardly touched it since my post but took off half a day today and I'm about to start sorting through it

[Zer09]

Problem fixed. As I said, my board is a tight squeeze lengthwise. So much so that the board actually covers the 2 inner PT mounting bolts which are longer on the new PT and were actually making just enough contact with one of the turrets for a filter cap to short it partially. I removed the turret and installed it to the side a bit so there was no danger of contact and all of a sudden I have 410v on the plates biased at 44 ma. Just need to decide what to build around the old PT now. I love these forums. Thanks guys.