Fender SF Twin Reverb, funny problem...

bren-hur · Newbie Alert Joined: 20 Oct 2008 Posts: 3

Hi...so I have been rebuilding-revitalizing a friends '73 twin reverb. It was pretty broke and sounding bad....it's on the way back to life, but I hit a bit of a roadblock. I did all the necessary blackface conversions, filter caps are good, and I rebuilt the whole circuit board swapping out the "turd" caps for sozo's and checked all the resistors and made them all match to AB763 blackface spec's. I changed out all the tubes and ran it through my external 2x12. Now...this ALMOST fixed it, reverb and vibrato came to life and it almost sounds RAD....BUT...the volume aint what it should be. My modded 50watt Bassman is considerably louder, through the same cab. Also, there's a funny distortion going on....when i hit the low e string hard, the amp kinda farts out into an ugly distortion, like the sound of a broken speaker. This happens real early on to, like the volume on 3. Real wierd, it's like when it hits a certain low frequency it craps out into bad distortion. I thought bias....but I tried all sorts of bias adjustments and it was the same in all circumstances. The tubes are getting the correct negative voltage so I dont think its something in the bias circuit. It is happening on BOTH channels as well....which leads me to believe its something in the phase inverter/power section. I checked all the resistor values and changed the caps...everything LOOKS ok...but I dunno...I am beginning to suspect the output transformer....anyone hear what a broken output transformer sounds like? is it anything like I described? anyway...any help appreciated!!!

Thanks all!

tw001_tw

bren-hur · Newbie Alert Joined: 20 Oct 2008 Posts: 3

So I went through the amp again on a techs advice and checked all the voltages on all the power tube pins....all voltages match and look ok going into the output transformer....SO...I carefully re-anylzed exactly what the amp was doing....and here it is, slightly different than my preivous post..

On volume 1-2 it sounds ok, a little soft but clean. At 3 instead of good old fender breakup, the amp produces an ugly broken speaker type fizz at the clipping point, a clean undertone is still present beneath the fizz. at 5-6 the amp reaches maximum volume, not really much louder than 3, you can hear where the amp is breaking up, but like before instead of nice solid crunch it clips out with a fizz, not a very loud fizz either. Again there is an almost separated clean undertone underneath, the clean is slightly louder than the fizz. At volume 10 its exactly the same as 6. Even at 10 the amp really isn't that loud, comparable to like 3-4 on my 50W bassman....

Output transformer? I am beginning to think it could be almost nothing else....any thoughts???

thanks!

GrampaTbird · Little Guppy Joined: 27 Mar 2007 Posts: 30

I noticed my amp doing similar behavior, a new set of tubes cured it.
I've had bad luck with tubes, until my buddy drops of a tube tester.
A guy on ebay tried to unload a quad of very weak 6v6's on me.
With the tester I set him straight.

My bottom line is I always suspect tubes first at least I am learning that, but with a tester, life is soo much easier.
Try tubes first then the transformer. Good Luck