Problem with my 81 Ampeg V3

[deanamite]

I have an 81 Ampeg V3. it has the problem that if i switch on the hard setting which is the setting that is supposively the jcm 800 clone setting the distortion doesn't ring out for very long. it kinda falls flat after a while and it sounds as if there was something not working right. it does sound great with a distortion pedal but on it's own it cuts out to early. i switched out the 6550 power tubes but that made no difference. could it be one of the other tubes? bias? speaker? i have only very limited knowledge and understanding of the matter but am very willing to learn and try myself.
just want to say that i also own a V7 which is the 100 watt head version of this 50 watt combo and the V7 played thru a marshall cab doesn't have that problem
thx

[cortezthekiller]

The old Ampegs didn't have adjustable bias control, so usually a trimpot is added in to compensate for the higher voltages of today and to have the amp running its best.
It could be a weak preamp tube that isn't amplifying well enough to drive the rest of the amp.
The filter caps could also be due for replacing if it hasn't been done already.

[deanamite]

hey thx cortez. i'll try to replace the pre-amp tubes and see where that goes. if that doesn't work i'll bring it in the shop cause i love the amp and don't want to repair it kaputt.
thx

[PapaMidnite]

I have a vl 503 and the rectifier tube was bad it only affected Ch 3 Not 1 or 2 .I would go with the pre amp tubes

[deanamite]

so i replaced all the pre-amp tubes. that brought the second channel back to life. however, i still have a problem with the distortion on channel 2 as it gets really whacky and cuts out early if i put the channel 2 volume above 4. i have also noticed that channel 1 is not as loud. maybe i need to replace the power tubes with new powertubes? the ones i put in were s swetlanas i bought off ebay so they might be just as shot as the jj's that were in there before.
anybody?