1968 Twin Reverb AC568 To AB763

[jy67]

Hello All!

In the process of converting a 1968 Twin Reverb AC568 to AB768 specs.

Mainly concentrating on the bias and phase inverter circuits.

I've read that Fender also changed the dropping resistors to 2.2k and 10k in order to achieve more clean headroom?

 Is it safe to change these to the AB763 values(1k and 4.7k) as well?

The power transformer is the 022756 (345-0-345-310ma).
Any insight is greatly appreciated!

[Latole]

Show schematics of what you are talking about

[mresistor]

Hi it's AB568  and AB763. My thought is why are you converting this amp?  There isn't much difference in sound comparing them.

[Willabe]

Hi it's AB568  and AB763.

It's a simple courtesy to the others that you are asking for help to post the schematic(s) so they don't have to go look for them. 

My thought is why are you converting this amp?  There isn't much difference in sound comparing them.

There is a difference in tone/sound and feel/touch sensitivity.

PI plate R's, top PI K R, and the dreaded 2000 pF power tube grid cap. I'd set it to BF specs. And that screwy mixed bias. Get rid of the extra R and C added to the power tubes K's.   

[jy67]

I've buttoned most of this up.

After checking some voltages though, I think it's actually an AA270; The AC568 had a similar mixed bias and PI circuits as well, but 460V on the 6L6's, while the AA270 is right around 400V(different transformer?). I've attached both schematics for comparison with the AA270 layout marked with current voltages.

The schematic has the modifications marked up.
 
I started off just wanting to remove the mixed bias circuit and do normal maintenance; cathode bypass and filter caps, I figured why not revert to the bulletproof AB763? I was already in there, and it cleans things up considerably.

The change in the dropping resistors raised some of the voltages on the preamp tubes some(noted on the scan), but the amp sounds divine!


https://photos.app.goo.gl/LPtsg7fsCZb72xWt6

[Willabe]

The schematics were too big to upload:

If you scroll down to the bottom of any page in Doug's forum, you'll see the link to his tube amp schematic library.

Just copy the link to the schematic in there you want and past it here, won't be too big. 

[jy67]

The schematics were too big to upload:

If you scroll down to the bottom of any page in Doug's forum, you'll see the link to his tube amp schematic library.

Just copy the link to the schematic in there you want and past it here, won't be too big.

These are marked up with the changes I made and voltages.

[Willabe]

After checking some voltages though, I think it's actually an AA270; The AC568 had a similar mixed bias and PI circuits as well,

The AA720 has those 330K grid return R's on the PI. 1M will sound better and have better touch/feel.   

[jy67]

After checking some voltages though, I think it's actually an AA270; The AC568 had a similar mixed bias and PI circuits as well,

The AA720 has those 330K grid return R's on the PI. 1M will sound better and have better touch/feel.

Yep. Did that.

My original concern was the dropping resistors; I changed them from 2.2k/10k to 1k/4.7k.

Any issues here?

Thanks-

[Latole]

Hi it's AB568  and AB763.

There is a difference in tone/sound and feel/touch sensitivity.

You may be right, but I know from experience that, for example, several AB768s or AC568s can be very different from one another.

[mresistor]

I agree that the AB763 bias circuit is simpler/better.  I just don't play the big amps at any appreciable volume level so to me there wasn't a whole lotta dif. Many of you'all have probably practiced or played out with one and know alot more about the difference in sounds between them. The one I played had JBLs in it from the factory,

[BrianS]

https://el34world.com/Forum/index.php?topic=30675.msg338064#msg338064

Above is a link to a thread regarding a AC568 Bassman I worked on a bit over a year ago.  I guess it's probably irrelevant since you already did the mods, but my testing found the mixed bias design to be very useful, and the amp sounded great as well.  I'd never recommend converting this design, as it seems like a waste of good engineering. 

[Willabe]

but my testing found the mixed bias design to be very useful, and the amp sounded great as well.  I'd never recommend converting this design, as it seems like a waste of good engineering.

I'd recommend adding a bias set pot to the bias balance, and getting rid of the K bias R and C.   

In your like to your thread on a Bassman, PRR and HBP explained why Fender probably went with the bias balance and the mixed bias.

Your not addressing not being able to set the bias for current draw % at idol so the tubes don't red plate, draw too much current. 

The bias balance can be a good thing, but you still want to be able to set the bias. Fender has it set up so you can only balance the 2 output tubes.

Adding a bias set pot to the bias balance that's already there is the way to go.

I drew these bias/balance circuits from Kevin O'Connor's TUT books.   

[Willabe]

On your marked schematic, it looks like you took out the 6L6 1500 grid stoppers?

[jy67]

On your marked schematic, it looks like you took out the 6L6 1500 grid stoppers?

Typo. They're in place.

[stratomaster]

My original concern was the dropping resistors; I changed them from 2.2k/10k to 1k/4.7k.

Any issues here?

Thanks-

No issues, so long as they are spec'd take the power dissipation. The goal should be to match the schematic voltage +/- the tolerance of the amp spec you're wanting to convert to--so not necessarily copy the same resistor values unless the power transformer specs are identical to the target amp.

[jy67]

Updated/Corrected Schematics of the mods and voltages.

I think the power transformer voltages "should" be higher.

Also the voltages on V5 seem high.

Not sure what else I'm missing.

[stratomaster]

Updated/Corrected Schematics of the mods and voltages.

Also the voltages on V5 seem high.

Are the V5 measurements with the trem bypassed?  See note 4 on the schematic drawing.

[jy67]

Updated/Corrected Schematics of the mods and voltages.

Also the voltages on V5 seem high.

Are the V5 measurements with the trem bypassed?  See note 4 on the schematic drawing.

I'm not sure I understand; The trem circuit was not "on".

Usually I would use a shorted rca jack like this to engage the tremolo.

I tried re-measuring with the jack installed, but had to dime the speed knob to get the voltages to stabilize. I ended up with 346V on pin 6 and 259V on pin 1.



Also, the reverb tank was not plugged in either.

[SEL49]

The fluctuating voltages means the tremolo oscillator is working.