Fender Twin Pro Tube low heater voltage

[htrd]

Hello.

I have an '07 Twin Pro Tube series (one with bias tremolo) and it looks like I have a problem similar to this thread. All tubes are fresh and under 25-30 hours of work, Sovtek 5881/6L6WGC in PA, Sovtek 12AX7LPS in PI, 12AT7EH in the reverb driver and Tung Sol RI 12AX7 in the rest.
One day my tremolo stopped working, I pulled out the tube (TS), one triode developed a really low emission, I've checked it in another amp. I thought that it was due to the CF position, but yesterday I've reechecked this tube and it was working as new. Later my lead channel (yes I'm really using it) had lost its gain, almost clean sound with full gain. I swapped the tube, everything back to normal. Reverb has lost a bit of its volume - tube swap fixed the problem, but all those "dead" tubes were working fine in my Mesa Rectifier V1, tested twice with a completely fresh one.

So the problem is in the amp itself.
All HT (B+, Z+, Y+, X+) are fine, but heater voltage was down to 4.4-4.6V at the V1 and something about 5.4 at the xformer taps (after the fuse). Few days after I retightened the fuse and checked once again, 6.7V with the unloaded xformer, 6.7 with unloaded and fuse, but lower with full load - 6.0V on the pilot light, 5.9V at the closest tube socket and 5.5 at the far ones (V1 and power tubes) with all values in between.

Looks like the PCB heater wiring was a bad idea. I'm thinking about soldering a separate heater wire, bypassing all PCB traces and going straight to the sockets. What do you think of this? May be I'm missing something?

Thank you.

Twin (Pro Tube) Amp SchE45.pdf

[Chuck H]

Here's a link to the schematic if it helps anyone.

https://schematicheaven.net/fenderamps/pro_tube.pdf

[Jon Snell]

Measure the voltage between P214 & P215 whilst in standby. The most common issue with these connectors is a bad crimp or poor soldering of the spade connector on the pcb.

[htrd]

Measure the voltage between P214 & P215 whilst in standby

6.1V between spades. Connection looks tight and sturdy, nothing loose.

I pulled out 4 preamp tubes (V2, 3, 5, 6) and got 6.2V between P124/215 and 5.8V on V1 and power tubes.
All preamp tubes out - 6.3V on spades and 6V on power tubes.
All tubes out - 6.6V everywhere.

Underspec'd power transformer?

[Helmholtz]

Generally heater voltage being low by 10%, i.e. 5.7V, is no problem and should not degrade amp performance.

What is your actual mains voltage measured at the PT primary?

[htrd]

Maybe I'm overthinking it. I should try another set of power tubes or, if it won't help, just pull out 2 of them. 50W is definitely enough for bedroom playing.

Generally heater voltage being low by 10%, i.e. 5.7V, is no problem and should not degrade amp performance.

What is your actual mains voltage measured at the PT primary?

Problems were with ~4.4V. It was fixed somehow (maybe bad crimp or fuse connection), but right now it is close to the lower limit and I'm a bit worried.

227V at the wall outlet (combo is 230V EU version).

[Helmholtz]

227V at the wall outlet (combo is 230V EU version).

What voltage drop do you measure across the heater fuse (measured at the fuse holder contacts)?

[g1]

What voltage drop do you measure across the heater fuse?

Yes, check with all tubes installed. The heater fuse is not shown on schematic, if it is the black in-line holder shown in the photo, find where the wires go and measure from there.
Also, all tube socket heater pins, and all those green wires marked 'fil' should be re-soldered.

[Helmholtz]

The heater fuse is not shown on schematic,

It is shown with 230V PT wirings.
Probably because of different safety regulation in some export markets.

[catalin gramada]

I had the same issue in one of my Peavey amp. Measuring good at transformer terminal and some voltage drops over the tubes. I found the pcb traces had enough resistance to limit the curent and drop substantial voltage. I did a hard wiring for power tubes and soldered 20 awg wire over original preamp traces and get a consistent supply for my heaters. You may imagine even a very litle resistance in respect with the heater current may determine substantial voltage drop. Just do a solid track doubling the traces with solid wire. You don't need a fuse in heater circuit. If it put in short-somehow- the fuse in primary side will blow first.

[htrd]

What voltage drop do you measure across the heater fuse (measured at the fuse holder contacts)?

~0.195V with everything installed.

Fuse is T10A 250V. I can bypass it, actually.

Also, all tube socket heater pins, and all those green wires marked 'fil' should be re-soldered.

That PCB is monstrous and held by front panel potentiometers. It would be much faster to solder a whole new heater wiring.

I had the same issue in one of my Peavey amp. Measuring good at transformer terminal and some voltage drops over the tubes. I found the pcb traces had enough resistance to limit the curent and drop substantial voltage. I did a hard wiring for power tubes and soldered 20 awg wire over original preamp traces and get a consistent supply for my heaters. You may imagine even a very litle resistance in respect with the heater current may determine substantial voltage drop. Just do a solid track doubling the traces with solid wire. You don't need a fuse in heater circuit. If it put in short-somehow- the fuse in primary side will blow first.

Yes, that was the original idea. But right now I'll try to bypass the fuse and pull out 2 power tubes as an easy way.

[htrd]

Update.
5.8V at power tubes without fuse.
6V without V10 and V12. PT begins to hum, but very quietly.

[g1]

That PCB is monstrous and held by front panel potentiometers. It would be much faster to solder a whole new heater wiring.

I won't argue that for the preamp tubes. But I see the power tubes socket's solder is all accessible, and the joints don't look great. Quick resolder, as well as the green wires going to that board.

[htrd]

Agreed, that's possible.
Green wires from the main PCB to power tubes are the signal ones, from PI. Heaters are twisted black and white wires in the middle.