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AL1
04-22-2005, 06:32 PM
I just put in a new Mercury Magnetics Axiom FBFPP power transformer in my blackfaced Princeton Reverb and the vdc from pin 8 of the GZ34 rectifier is only 320. It should be about 420. VAC from wall is 120.5. Anyone have any ideas why the voltage is so low? Thanks.
John Phillips
04-22-2005, 06:53 PM
Do you have the first filter cap connected to the rectifier? Disconnecting the cap drastically lowers the measured voltage because the meter tries to average the voltage in the half-wave ripple. Once the cap is connected, the voltage stabilises at the full peak voltage.

Just a thought! If you haven't disconnected the cap (or fitted a standby switch) that isn't it...
AL1
04-22-2005, 06:57 PM
John, everything is connected. The only thing I changed was the transformer.
AL1
04-22-2005, 07:04 PM
John, I didn't put the2- 100 ohm resistors that are by the lamp. Would this make a difference. A friend of mine has a 1964 Princeton Reverb and it doesn't have these 100 ohm resistors.
John Phillips
04-23-2005, 02:51 AM
No, that shouldn't make a difference. Those are the 'virtual center tap' resistors for the filament winding, which on the original transformer doesn't have a real one. If the new one does, those resistors should be removed. (Even some of the original amps without a center tap don't have them, either one side of the winding is grounded via the lampholder, or it's done elsewhere.)

Check the AC voltage between ground and the plates (pins 4 and 6) of the rectifier tube. It should be around 340VAC on each.

Also check the ground connection to the HT secondary center tap - if you've reconnected it to one of the big solder points on the chassis, it can sometimes be hard to get a good connection there since the chassis is a very good heat sink.

If the center-tap connection is good, and the AC voltage at the rectifier plates is right, the problem is elsewhere (bad rectifier tube maybe?); if the AC voltage is low, it's the transformer.
AL1
04-23-2005, 09:42 AM
John, do I put the black tester lead on say pin 4 and the red one on pin 6? Not sure how to test for the 340 VAC.