Fender Twin Power Transformer - is she dead Jim?

[wizard333]

Amp was putting out nothing and voltages were very odd (250 ish B+ but over -100V bias voltage) so I disconnected the HV and bias taps from the rest of the amp and here's what I get (unloaded, disconnected from rectifier diodes):

Bias Tap ~190VAC, HV Secondaries: one side to ground (center tap Red/yellow is grounded) 480VAC, other side: 150 VAC.

Should be seeing roughly the same on either side of the secondary correct?

The amp is not blowing fuses and the PT is not getting abnormally hot (just warm the the touch) but that sure doesn't look right.

[Lee_ranaldo]

Amp was putting out nothing and voltages were very odd (250 ish B+ but over -100V bias voltage) so I disconnected the HV and bias taps from the rest of the amp and here's what I get (unloaded, disconnected from rectifier diodes):

Bias Tap ~190VAC, HV Secondaries: one side to ground (center tap Red/yellow is grounded) 480VAC, other side: 150 VAC.

Should be seeing roughly the same on either side of the secondary correct?

The amp is not blowing fuses and the PT is not getting abnormally hot (just warm the the touch) but that sure doesn't look right.

i had the same issue months ago with a blown PT from a marshall clone. The voltages were totally random and not even close to the nominal values.

Remove the protective shells on your fender PT transformer and check if the core of it has serious signs of over-heating. I seriously recommend getting it rewound by a professionalk shop
there are a few parameters and including the heat produced by the transformer that should not be ignored. Making a transformer is not just winding some wire around a core.

You might need to check the bias circuit of your amp . In my case the amplifier had old caps and they literally shorted the B + to ground creating an excesive current consumption. The transformer was home made so it was beyond 40°C to the touch.

[wizard333]

Caps are new that isn't the issue. Those things were checked.

Was more a simple question of: I should be getting near identical voltages off each HV secondary, I'm not, there is no load to interfere, so that means dead right?

[Lee_ranaldo]

Caps are new that isn't the issue. Those things were checked.

Was more a simple question of: I should be getting near identical voltages off each HV secondary, I'm not, there is no load to interfere, so that means dead right?

Sry mate i though you were getting 480VAC on one side and 150VAC on the other side of the center tap.


I got a twin amp here.. i can measure the voltages if you need them .

[wizard333]

i though you were getting 480VAC on one side and 150VAC on the other side of the center tap.

That is correct.

[Lee_ranaldo]

That is correct.

btw is this a 94 twin amp? with od channels and presence control?

you could get 250 Volts on the B + if you set the switch for "low power"

if you already disconnected everything including the heaters
measure every voltage across the terminals
not grounding it to the amp's chassis

you should see around (estimated value) 640 VAC center taped (320 0 320) volts AC
around 60 VAC for the bias and 6,4 volts AC center tapped (3,2 0 3,2) for the heaters

[wizard333]

Its a silver faced non ultra linear twin.

Fender schematic shows the transformer at 340 0 340 loaded. Loaded is very different than unloaded. One leg of the secondary, as I mentioned, is 480VAC unloaded, not out of line with what I'd expect, the other is only 150VAC. Leg to leg is 670VAC ish, but again, that is with no load.

Also, if the center tap is chasis grounded, how is measuring from either leg to chasis different than measuring from either leg to center tap?

With *ONLY* rectifier diodes hooked up, NO LOAD, the VDC is about 390VDC. That is very low with no load.

I haven't seen that many PTs fail, and when I have seen it, its the perpetual high-current fuse blow kind of fail. Never have seen one just get weak, but it seems to me to be the case with this one. I'm not a transformer expert, not sure if anyone here is unfortunately.

Another other odd thing is that, with only rectifier diodes hooked up, measuring VDC for B+ and Bias, the bias voltage starts out very low and creeps up to -100VDC or so, the B+ starts over 400 and creeps down to under 400 as the bias comes up. Keep in mind this is with *nothing* but the rectifier diodes for the bias and B+ connected.

[Enzo]

340 AC loaded should not rise 140v unloaded. And no matter what, 350 should not measure out at 150, especially unloaded.


If you connect ONLY the rectifiers to the 350vac, then you get unfiltered pulsing DC. It will measure a lot lower than filtered. YOu need to connect a filter cap of some sort after those rectifiers to get anything meaningful.