problem with filament rectified supply

[JC@]

Hello I have a problem here, this is a single ended dual power amp.
It has a rectified power supply for the ECC82 on both channels, both tubes share this filament supply.
The amp stopped making any sound and I traced it to the filament supply, reading only a mere 1 volt DC. Now, while I was taking some measurements, the filaments begun to glow and there was about 5.9 volts on them. The leads read correct continuity but to be really sure I have to unsolder quite a few cables and take out a board because the filament leads are soldered on the side I can't see.

What about the rectifier (one of those little round ones with four legs), or the filter cap?
Could they be the cause of the voltage drop?

Thanks for your help.

[Amp Kat]

I doubt it unless it's loose and not making good solder contact which seems to be your problem. Either a bad solder connection under the board or somewhere around the filament itself. If the filament is using pin 9 as ground it could be any ground that is connected to that source. Try moving some wires and see if you can get it to start and stop.

[JC@]

Could it be the filament winding for the preamp tubes?
With the tubes out of their sockets, the reading is 7.5 vdc or so
With just one tube in, it reads 6.3 vdc
With both tubes, it is 5.8 or so.
It changes quite a bit from tube to tube of the same lot

I've seen it just once with the problematic 1.something vdc with the first set of ECC82, but I can't reproduce it any more.

[Enzo]

If it were the transformer winding, ALL the tubes would be affected, not just a couple. If the DC for the preamp heaters comes and goes, then yes, look for poor solder on the rectifier bridge or cracked traces.

[JC@]

Hi Enzo. The power transformer has two filament windings, one for the power tubes and one for the preamp tubes. I will try resoldering the rectifier bridge and let the amp on for a few hours and watch for any changes. Well I guess I will have to pull that board out too.

If the amp comes back I can always fit a little filament transformer somewhere.

[JC@]

It's stuck in "fault" mode right now.
With the tubes out, 7.7 vdc
As soon as I plug any of the two ECC82 in any of both sockets, the voltage slowly climbs to 1.7 vdc and a little ac component appears, just 140mV

There are no transistors or IC or voltage regulators in the circuit.
On each tube one of the cathodes is tied to ground across a 3K3 resistor, the other across a 3K3 resistor bypassed by a 100uF 25 volt electrolytic cap. I don't see any voltage across them.

[JC@]

it is fixed now. It was a bad rectifier bridge.