4.6V heater voltage on Fender Pro Tube Twin

[harry]

Hello all,

My arch nemesis Fender Pro-Tube Twin (4x6L6, 7x12AX7 1x12AT7) is causing a problem again.

The amp stopped working suddenly during a show and I have since diagnosed the problem to be a heater voltage of 4.6V when all the tubes are installed.

This amp belongs to my bandmate, and I noticed it had a low heater voltage last time I worked on it, about 5.7 volts, but couldn't see a way of rectifying it, and it seemed within tolerance, so left it.

If I disconnect the heater taps and leave the HT connected, the heater winding reads 6.5V.

If I disconnect the HT and leave the heater taps connected, the heater winding reads about 5V.

If I disconnect the HT and leave the heater taps connected for the preamp valves only, the heater winding reads about 5.8V.

Does anybody have any suggestions? Has the PT finally given up? The amp is regularly used on full volume so is often "under stress".

Any help/suggestions would be much appreciated!

Harry

[52 Bill]

Is it just the heater voltage that is low or is it both heater and HT?

The only thing that I can think of is the primary wiring including the inrush limiter and the connections to the dual primary windings.

There are 7 primary taps for this transformer. Have you checked the connections and jumpers for the correct input voltage setting? There are settings for European 230vac and a different setting for UK and Australia 230-240 vac.

[harry]

Hi Bill,

I don't remember the exact figure but HT was definitely 400V+. I have checked the primary connectors; they seem fine. Previously, the heater voltage did increase over time as the amp warmed up. Perhaps the inrush limiter is at fault? I will try bypassing it.

[Enzo]

Try this: wire it all up like normal. But take one of the 6v transformer wires and leave it disconneted. Set your meter up for AC current and clip in in series with that loose wire and its destination. How much current is the circuit drawing? And just for science, disconnect your heater center tap. Might have to just lift the two resistors. You know what current each tube OUGHT to draw. I get 4x0.9A plus 8x0.3A, for a total of 6 amps. Oh plus a bit for the pilot light. SO then we can determine that the tubes do indeed try to draw 6 amps and the transformer can;t handle it, or we find the transformer is dragged down by excessive current.

I'd pull the wires for the pilot light off the circuit board. A shorting light socket could be involved.


ALternatively, if you have a heavy 1 ohm resistor - like 35 watts or more - pull the green wires off the board and connect the resistor between them and see if the voltage behaves the same. If the transformer cannot feed a 6A load all by itself, then it has some problem. Actually, clip your meter to it, and power up for just long enough to read voltage and back off, you can probably get away with a smaller resistor.

And don't overlook the primary side. I'd carefully verify the mains side wiring, the schematic is detailed on this. I'd also clip an AC voltmeter across the thermistor - P200 to P201 - to make sure it isn't dropping much voltage. DO this with tubes insstalled so the tranny is loaded.

[Enzo]

Ah, you guys covered that while I was writing.

[harry]

Thanks Enzo.

The inrush is only dropping 2.5Vac, so can't be having a massive impact there.

I paralleled together 2.7, 4.7 and 4.7 ohm resistors for an approx total of 1.25 ohms - I measured 5.9V across the heater taps with just this connected, which equates to a current of 4.64A.

I unwired the pilot light too, no change.

[harry]

One thing I noticed about this amp that I found strange last time, is that it has a 5A fuse in for the heaters, when on paper it should draw 6A, as Enzo says. I've actually run it with a 4A fuse for some time without realising.

[olddawg]

Do you have a variac? You could up the AC mains and see if the heater voltage goes up. (Make sure you monitor the other supplies. You don't want to pop filter caps). Also you could turn it down and sub the heater taps while reading the current draw. You could also sub your heaters with a DC bench power supply.

[Jazz P Bass]

In the original post it was stated "The amp stopped working suddenly during a show "
Would 4.6 volts on the heaters 'stop' the amp?
It sounds to me like the transformer is bad.
At least the heater winding.

[MarkusBass]

That's right. The OP stated that:

The amp stopped working suddenly during a show and I have since diagnosed the problem to be a heater voltage of 4.6V when all the tubes are installed.

I don't think that 4.6V heater voltage would stop the amp from working. Lately I was checking an amp that had 4.5V heater voltage (power amp) and it worked surprisingly good. And the reason for the low voltage was incorrect transformer used (the voltage was always low). Does the amp work now and what exactly are the symptoms?

Mark

[harry]

In the original post it was stated "The amp stopped working suddenly during a show "
Would 4.6 volts on the heaters 'stop' the amp?

Ah - perhaps there is another problem too. The amp has sounded increasingly dull over the past few months. There is now no output whatsoever.

Can a transformer "wear out"?

Thanks for the suggestions all. When I find a 6V 6A power supply I'll try subbing it in for the heater winding and see what happens.

[Steve Conner]

Try replacing the heater fuse with a fresh one of the right rating. Also make sure the contacts in the heater fuse holder are clean and well tensioned, and the solder joints holding it to the PCB are good.

If you had a 4A fuse in there, it would have been loaded near to breaking point for a long time. It could have got itself and the holder very hot, and developed a bad contact.

[Randall]

I also don't think that a 4.6v heater voltage would crash an amp, but I do think it would make it dull sounding, as previously mentioned.