Fender Pro Tube Twin Low Output

[harry]

(getting about 58 watts, 100 watts max on the onset of clipping now... if I have to call it a day on this one, it's loud enough and it's louder than it was, so no great loss...)

[pdf64]

What's the heater voltage with no tubes in?

[harry]

It's 6.6 or 6.5V depending on where you measure it...

I'm going to call it a day - I shifted the PT to the 230V tap, removed the tube for the distortion channel that he doesn't use: getting about 65-70 watts clean, about 110 distorted.. It's probably for the best that it doesn't go above this - he tends to play the amp on 7 or 8 as it is and I'd rather the amp isn't capable of putting out wattages that will stress the tubes!

[MWJB]

PT should use the 240/250 tap.

6L6s can be stressed at 10W output in some amps, it's high plate dissipations and hard use that kills them, 4x6L6 in a twin at appropriate plate currents should comfortably handle a couple of years of regular gigging at 100W RMS output before tone degrades, typically they'll last longer before they actually fail.

Your heaters shouldn't be dropping below 6.3VAC with a full load.

Something would still appear to be amiss with the amp.

[harry]

well.. I've just run out of ideas.

As I say, there was a 4A fuse on the heaters before, and as far as I know, the amp has always been like that... the amp has obviously never been drawing the right amount of current for the tubes.

He's had it since new, and the local tech has looked at it a few times in it's life, but I don't see why he would have changed anything with the result that a 4A fuse doesn't blow.

[pdf64]

It would take a long time at 5A for a 4A fuse to blow - have a look at the characteristic curves sometime.
http://docs-europe.electrocomponents...6b80111b84.pdf
I would still be looking where the heater voltage is getting lost.
Wherever there's a measureable volt drop, tack a wire in parallel, ie from the pilot light to the power tube sockets.
My buddy's PTTA benefitted greatly from warming the bias up a little, to 40mA per tube.
BTW are you sure this amp isn't still under the 3 year Fender waranty?
Peter.

[harry]

haha - oh, you'd laugh if you saw it - it's a mess cosmetically.. we've taken it around europe 5 or 6 times, last tour was 50 shows in 10 weeks.. played on 7 or 8 every night. The tolex got ripped off it in Germany! I think it's from 2002; maybe 2001. He's probably done close to 300 shows with it.

And it's never blown a fuse... that 4A fuse has been in there the whole time.

I'll try tacking a wire in parallel.. that's a good idea.