..... I pulled V4 it had a little white halo around the top .
That means the tubes vacuum went bad, sprayed silver getter did it's job, absorbed the atmospheres gasses and turned white.
....I don't know why the AB568 has an aura of distain about it; it seems very darned fantastic to me.
CBS ran those amps colder, too cold, than the Fender black face amps.
Here's what CBS changed;
1. CBS put in 2000pF/.002 caps on the 6L6GC grids to kill off any parasitic problems caused by bad lead dress.
(These make a large difference in the amps tone, for the worse. They bleed off a lot of signal/harmonics/sparkle/chime to ground. This is why you can't just bias the power tubes hotter to warm up the amp like a black face. If their in your amp, try snipping the ground end and see if it stays stable. If not stable then you have to play around with the lead dress. Fender/CBS put them in just as a precaution. CBS/Fender was selling sooooo many amps and they didn't want to take the time with trouble shooting them or taking the time to do the lead dress correctly. Not all those amps are unstable without them. Some say only ~50% are unstable without those grid bleeder caps.)
2. They changed/lowered the PI (12AT7) plate R values from 82K/100K down to 47K.
(You can try changing those and listen if you like it better. They will add a little more drive/fuller/fatter tone.)
3. Lowered the 6L6GC grid return R's from 220K down to 100K, in part because of the bias balance.
(You can try changing those to 220K
IF you also change to adjustable bias. Or if you stay with balance bias you could try 180K, maybe even 200K, lower value because of the resistance in the bias balance circuit. They will add a little more drive/fuller/fatter tone.)
4. They used a combination of grid and K bias.
(Snip out the 150R's and cap on the 6L6GC's K and then run a wire from the K's to ground. Much mo' better, put a 1ohm R between the 6L6 K and ground to measure bias current, should be done anyway. Then you have to re-bias. Some say the combination bias effects the amps tone for the worse. I suspect that cap, run from K to K, more than the K R's.
)
5. They used bias balance instead of adjustable bias, some/many prefer adjustable bias.
(This can be changed to adjustable bias pretty easily, if you want/prefer. With matched tubes you don't really need bias balance. With adjustable bias you can set the 6L6's to run hotter or colder. Plus, different tube sets, even the same brand, will all run hotter or colder with the same -dcv (bias voltage) setting.)
You won't know if you like it any better then it sounds now if you don't try these things.
If you do try them, do them 1 at a time and listen for changes. (Except for 2 and 3, I'd do those at the same time.)
The parasitic caps on the power tube grids will most probably make the most noticeable difference, #2&3 next most difference, then #4.
If it were my amp, I'd do all the changes I listed without hesitation.
It fattens up/warms up the amp and is more lively, gives it a little more drive. CBS ran them colder than Fender did and CBS sales plummeted when they made these changes because of it.
Topic: Guess what followed Me home AB568?? (Read 5744 times)