AB763 vs AB568 on spec?

onstage76 · August 25th, 2008, 01:38 PM

Hi guys - one for the techs I guess.

Have the opportunity to buy a MINT 1968 SF bandmaster head which really suits me right now. Tube chart says AB763 but as you know, fender had a very poor practice of 'using up' existing tube charts during transition periods which makes amp tube charts - especially from a year like 1968 - unreliable.

The advertisement has photos of the circuit - is there a way for me to tell easily if the amp is AB763 or AB568?

Please help if you can!

Cheers fellas

Richard

PhatTele · August 25th, 2008, 03:58 PM

The key things to look for are over on the power tube and bias side of the board. BF circuits tend not to use the small solder lug on the back/butt end of the bias pot (and perhaps an extra solder post with a flying resister or two). Also, BF circuits tend not to use extra solder posts and flying resisters between the power tubes and the main board. If you see those things, you can bet that the left hand side of the board is SF era. The rest of the board (the preamp side) will look pretty much like a BF board.

If you see that extra stuff, don't worry. It's easy to get rid of and turn to BF spec. Just factor that into your price negotiations. Others disagree, but I think it's worth the mods to get the bias circuit back to BF specs.

onstage76 · August 25th, 2008, 07:24 PM

Quote:

Originally Posted by PhatTele

If you see that extra stuff, don't worry. It's easy to get rid of and turn to BF spec. Just factor that into your price negotiations. Others disagree, but I think it's worth the mods to get the bias circuit back to BF specs.

Great - so an AB568 is easily converted to 763 specs?

Sounds good - your info is very ueful.

Richard

PhatTele · August 25th, 2008, 08:30 PM

I think it's easy. Plus, those drip edge amps are still really cleanly put together (not like later SF amps). So, it's actually a pleasure to work on them.

pbenn · August 25th, 2008, 10:56 PM

The Gerald Weber books are into this stuff, he did articles for Vintage Guitar magazine. It's probably on the net, too, but I'm too old.

Ectually, there was a Guitar Player magazine article on this in the 1970s or something....

Tremo · August 26th, 2008, 02:10 AM

the 568 uses a combination of fixed and czathode bias on the 6L6s. Look for the cathode resistors. If they are they're, it is a 568. If not, it's a 763.

Yes, the 568 is easily converted to 763.

onstage76 · August 27th, 2008, 01:55 PM

Thanks guys (in particular phatTele) for all your help. Questions asnwered. Great forum!

Scott S · August 27th, 2008, 06:58 PM

As you go through the blackfacing, it might not be a bad idea to leave the bias feed resistors (220K in the BF schematic) at 100K -- it's a little protection against bias runaway on old or uncertain tubes. In addition, it can help with high-volume fartiness.

- Scott

August 25th, 2008, 01:38 PM	#1 (permalink)
onstage76 TDPRI Member Join Date: May 2006 Location: melbourne australia Posts: 34	AB763 vs AB568 on spec? Hi guys - one for the techs I guess. Have the opportunity to buy a MINT 1968 SF bandmaster head which really suits me right now. Tube chart says AB763 but as you know, fender had a very poor practice of 'using up' existing tube charts during transition periods which makes amp tube charts - especially from a year like 1968 - unreliable. The advertisement has photos of the circuit - is there a way for me to tell easily if the amp is AB763 or AB568? Please help if you can! Cheers fellas Richard

August 26th, 2008, 02:10 AM	#6 (permalink)
Tremo Friend of Leo's Join Date: Mar 2003 Location: Silicon Valley, CA, USA Posts: 3,767	the 568 uses a combination of fixed and czathode bias on the 6L6s. Look for the cathode resistors. If they are they're, it is a 568. If not, it's a 763. Yes, the 568 is easily converted to 763. __________________ Exposing the CONs. www.myspace.com/59tremolux

August 25th, 2008, 03:58 PM	#2 (permalink)
PhatTele Tele-Holic Join Date: Jun 2003 Location: Richmond, VA Posts: 744	The key things to look for are over on the power tube and bias side of the board. BF circuits tend not to use the small solder lug on the back/butt end of the bias pot (and perhaps an extra solder post with a flying resister or two). Also, BF circuits tend not to use extra solder posts and flying resisters between the power tubes and the main board. If you see those things, you can bet that the left hand side of the board is SF era. The rest of the board (the preamp side) will look pretty much like a BF board. If you see that extra stuff, don't worry. It's easy to get rid of and turn to BF spec. Just factor that into your price negotiations. Others disagree, but I think it's worth the mods to get the bias circuit back to BF specs.

August 25th, 2008, 08:30 PM	#4 (permalink)
PhatTele Tele-Holic Join Date: Jun 2003 Location: Richmond, VA Posts: 744	I think it's easy. Plus, those drip edge amps are still really cleanly put together (not like later SF amps). So, it's actually a pleasure to work on them.

August 25th, 2008, 10:56 PM	#5 (permalink)
pbenn Tele-Holic Join Date: Apr 2007 Location: Toronto Age: 52 Posts: 593	The Gerald Weber books are into this stuff, he did articles for Vintage Guitar magazine. It's probably on the net, too, but I'm too old. Ectually, there was a Guitar Player magazine article on this in the 1970s or something....

August 27th, 2008, 01:55 PM	#7 (permalink)
onstage76 TDPRI Member Join Date: May 2006 Location: melbourne australia Posts: 34	Thanks guys (in particular phatTele) for all your help. Questions asnwered. Great forum!

August 27th, 2008, 06:58 PM	#8 (permalink)
Scott S Tele-Afflicted Join Date: Dec 2005 Location: SW CR IA US NA PE Age: 28 Posts: 1,732	As you go through the blackfacing, it might not be a bad idea to leave the bias feed resistors (220K in the BF schematic) at 100K -- it's a little protection against bias runaway on old or uncertain tubes. In addition, it can help with high-volume fartiness. - Scott