It is currently Mon Mar 16, 2020 9:05 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 5 posts ] 
Author Message
Post subject: 1968 AC568 Twin Issues
Posted: Mon Jul 10, 2017 4:41 pm
Offline
Hobbyist
Hobbyist

Joined: Mon Jul 10, 2017 4:37 pm
Posts: 2
Hey guys, new to the forums but not to Fender amps. I'm having a strange issue on this AC568 Twin I have. I have this weird gating effect going on when I hit certain notes together. I recapped all the electrolytics with everything the schematic calls for except for the 70uf filter capacitors, I replaced those with 100uf ones. I've also verified all the values of the coupling capacitors in the amp with a multimeter and they test within spec. I have a sound clip here which demonstrates the noise. Thanks for any and all help to clear this up!

https://soundcloud.com/adamlightmusic/6 ... ms/s-aA6jF


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: 1968 AC568 Twin Issues
Posted: Fri Jul 14, 2017 8:00 pm
Offline
Professional Musician
Professional Musician

Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1023
I couldn't discern the problem with the clip, but I have old ears.
While old caps can cause ghost notes, most problems with a particular note or combination of notes can be traced to mechanical issues with speakers or microphonic tubes.
First, ensure the speakers are in phase by placing a 9volt battery across the speaker plug. Both should move in the same direction.
Plugging into a different speaker cab may expose a bad driver, loose dust cover or paper cracks around the rim.
Microphonic preamp tubes can cause strange noises.
Don't forget to check for loose hardware on the chassis and amp and secure the grille frame.
Also, check nearby clocks, pictures and other sources of sympathetic rattles.
Noisy capacitors can measure good. A tester will only show a completely dead one. Tap on all the coupling caps with a plastic probe while operating. Look for noises, especially the disc cap to the phase inverter. Don't shock yerself, it makes me look bad. Good hunting


Top
Profile
Post subject: Re: 1968 AC568 Twin Issues
Posted: Mon Jul 17, 2017 10:39 am
Offline
Hobbyist
Hobbyist

Joined: Mon Jul 10, 2017 4:37 pm
Posts: 2
Thanks for the reply. I can guarantee that it's not my speakers, as I have plugged it in to two different cabs, as well as a loadbox directly in to my recording console. However, it sounds like maybe and impedance mismatch to me. I don't know for a fact that this amp was a Twin or a Dual Showman. The back panel is missing so I don't know what it is spec'd for. Do you know what loads they want to see? I've been using a 4 ohm load.


Top
Profile
Post subject: Re: 1968 AC568 Twin Issues
Posted: Mon Jul 17, 2017 11:01 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
4-ohm total load should be fine. Both Twin and DS run two 8-ohm speakers in parallel = 4-ohms total load.

I can't hear this "gating effect" you mentioned. You aren't running any effects boxes in the signal path?


Top
Profile
Post subject: Re: 1968 AC568 Twin Issues
Posted: Wed Jul 19, 2017 10:42 am
Offline
Professional Musician
Professional Musician

Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
Quote:
I recapped all the electrolytics with everything the schematic calls for except for the 70uf filter capacitors, I replaced those with 100uf ones.
The schematic calls for 35uf(2x70uf in series). You have 50uf w/ 2x100uf in series, increased the capacitance by 42%. Maybe try dropping those 2x 100uf down to
2x 80uf= 40uf in series (14% capacitance increase), would provide some relief from what you are hearing.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 5 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: