Amplification breaks up after some time

[Herr Grau]

Hi guys,
I got an old guitar amp here. Friend asked me to look it over real quick. I can only say that the heaters work and that I can measure B-Voltages about right with my crappy multimeter. I've changed most of the tubes against new ones I had on shelf. I also tried different speakers and made sure there are no visibly loose connections. There are no schematics to be found and I dont have an oscilloscope on hand.
The problem is this: When I switch on the amp, it needs a little to heat up, then it works fine. After a minute or so the sound starts to break away. The amp only amplifies weakly, only very loud input realy goes through.
I wondered if that sounded familiar and whether you could point me in the right direction.
Thanks

 

[doozerdave]

Could be a failing plate resistor in the preamp. Those commonly fail on old amps. Check B+ at the preamp plates on the tube socket and see if it drifts as the amp warms up.

 

[DF96]

Open-circuit grid resistor?

 

[Enzo]

Open grid return is a good shot, my first thought was leaky old coupling caps between stages.

You may not have a schematic, but it would be helpful to us if you mentioned the make and model anyway. You might be surprised at the resources we have.

 

[indianajo]

You can check the voltages on the tube sockets at about 70-80% of the maximum rated on the tube datasheets. Derate more if the maximum B+ voltage is a lower percentage than that. A grid voltage way off would be a sign of a resistor or capacitor problem, possibly. Tube datasheets are on triodeelectronics.com for example. With the power off and B+ zero you can also measure the resistor values against the color code without a schematic diagram.