It is currently Mon Mar 16, 2020 7:35 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 17 posts ]  Go to page 1, 2  Next
Go to page 1, 2  Next
Author Message
Post subject: 1967 Fender Bassman AB165 blowing fuses
Posted: Thu Mar 10, 2011 5:33 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Jan 22, 2011 3:36 pm
Posts: 10
Hello All,

My 67 Bassman AB165 circuit has suddenly started blowing fuses. If I turn it on and leave it on standby the fuse doesn't blow. If I switch it off of standby the power tube closest to the side of the amp starts to burn really bright and then the fuse blows. It will produce sound before the fuse blows. I have noticed the one of the wires that goes into the fuse holder looks a little burnt. I bought a set of new power tubes in hopes that would fix it, but it didn't.

Any tips on what to look at would be greatly appreciated.

Thanks!

Graham


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Thu Mar 10, 2011 6:56 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
You have a high-voltage problem with your B+ supply. Pull the chassis and take a look at the 470-ohm 1-watt resistors on the tube sockets of V5 and V6 (the power tubes). Examine these resistors for any signs of soot, cracks, or outright fractures. Replace as necessary. If those check okay the problem likely lies with the bias circuit itself -- either totally failed or failing.

Best of luck, HTH

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Thu Mar 10, 2011 7:12 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Jan 22, 2011 3:36 pm
Posts: 10
Thanks Arjay. I'll take a look at those resistors.


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Fri Mar 11, 2011 8:28 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Jan 22, 2011 3:36 pm
Posts: 10
OK I checked those resistors and the one on the tube that glows really bright before the fuse blows reads 780. The other one is at about 445. Seems like the 780 needs to be replaced?


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Fri Mar 11, 2011 9:07 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
The old "cart before the horse routine." The screen resistor may have gone way outta whack because the tube it's associated with shorted, drawing a wad of current before the fuse saved the day.

May want to hunt for a pair of known good 6L6GC to replace the present tubes. After changing out the 470-ohm resistor. My recent experience with JJ power and rectifier tubes is that they work just fine. Then, a few have had this irritating tendancy to go ballistic. At a drop of the hat.

Just a caveat. I haven't used JJ 6L6 or rectifiers in prolly a good two years, because of QA issues.

Not sure if your amp has "grid stopper" resistors (usually 1-1.5 K-ohm 1/2-1 watt resistors on the signal grid). You may want to look at those, too... in case they got spanked by the short.

Good luck with the fix-it! Keep us informed.


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Fri Mar 11, 2011 9:19 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Jan 22, 2011 3:36 pm
Posts: 10
I think I have the Tubes covered. The ones that were in the amp before this problem started seemed to be working fine, but just in case I bought a set of new ones from The Tube Amp Doctor.

Are the "grid stopper" resistors the ones that would go between pins 1 & 5 on the power tubes? Or would they run next to some of the capacitors?


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Sat Mar 12, 2011 12:52 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Yeah, the "grid stopper" resistors go between pins 1 and 5 of the 6L6GC. These resistors sometimes get cooked when the screen resistors (between 4 & 6) pop. Sometimes you get carbon tracks on the sockets after power tubes pop. So it's a good idea to clean the sockets, also after a meltdown.


http://www.thevintagesound.com/ffg/sche ... layout.gif


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Sat Mar 12, 2011 12:57 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
"Screen resistors", DOH!

(must've been all that Agent Orange I snorted back in the '60s)

Thanks for squaring that info away, B-Man!

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Sun Mar 13, 2011 4:01 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Jan 22, 2011 3:36 pm
Posts: 10
OK for an update.

I've replaced most of what I believe are the bias circuit resistors. I've replaced two 470 ohm's, two 1.5k ohm's, three 220k ohm's one 10k ohm (one of the 10k ohm was still reading with in tolerance) and one 15k ohm.

After all that it was still blowing fuses. I then remembered when I bought it the seller said I had to put the tubes back in the sockets in a specific way. One tube has to go in one side and the other tube has to go in the other side. The tubes are both International Servicemaster 6L6GC, but they look physically different and one is made in the USSR and the other in Yugoslavia. I found that now when I put the tubes back in as the seller said I should the amp seems to work fine again, but if I switch the tubes around or put my new tubes in the fuses blow.

Any suggestions on where I should go from here? I would like to make it so I can put my new tubes in,


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Sun Mar 13, 2011 5:15 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
If your V5 and V6 tube sockets are wired according to the schematic you should be able to heave those screwy Com-Bloc tubes and install your new TAD bottles in the correct manner (using the key-way provided in the center of each socket as a guide).

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Sun Mar 13, 2011 5:22 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Like Arjay said, I'd check the layout of your amp versus what Fender has for the AB165 (bottom page of link). You'll prolly see the difference right away (let's hope...)

Be sure both power tube sockets are aligned in the same direction. And the wiring is correct for each pin. The heaters are not shown. But the twisted green heater line should go to pins 2 & 7 of the 6L6GC's.

http://www.ampwares.com/schematics/bassman_ab165.pdf


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Sun Mar 13, 2011 5:32 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
BMW2002Ti wrote:
(let's hope...)


INDEED!

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Fri Mar 18, 2011 4:26 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Jan 22, 2011 3:36 pm
Posts: 10
Ok Another update.

The fuse blowing problem seems to be fixed. I can put either set of tubes in now and the fuse doesn't blow. Now I have a new problem. I don't seem have much volume any more. Before this all started this Amp was really loud. I never got it much over 6 with out worry of permanent hearing, and possibly brain, damage. Now I can turn it up all the way to 10 and it's just kinda quiet and muddy sounding.

All of the tubes, power, pre-amp & rectifier, lite up, but they don't get very bright.

Where should I start looking for this problem? I'm hoping you're not going to tell me my transformer is fried because I understand that's not a cheap fix.

Thanks!


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Fri Mar 18, 2011 5:50 pm
Offline
Aspiring Musician
Aspiring Musician

Joined: Fri Dec 12, 2008 5:32 pm
Posts: 329
Snagproof929@yahoo.com wrote:
Ok Another update.

The fuse blowing problem seems to be fixed. I can put either set of tubes in now and the fuse doesn't blow. Now I have a new problem. I don't seem have much volume any more. Before this all started this Amp was really loud. I never got it much over 6 with out worry of permanent hearing, and possibly brain, damage. Now I can turn it up all the way to 10 and it's just kinda quiet and muddy sounding.

All of the tubes, power, pre-amp & rectifier, lite up, but they don't get very bright.

Where should I start looking for this problem? I'm hoping you're not going to tell me my transformer is fried because I understand that's not a cheap fix.

Thanks!
So i am assuming you put the new set of tubes in it.Did you Bias this amp after you put the new tubes in?? If the bias is too cold you will not have enough volume. Is this on just one channel of the amp or is this on both channels.


Top
Profile
Post subject: Re: 1967 Fender Bassman AB165 blowing fuses
Posted: Sat Mar 19, 2011 3:46 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Jan 22, 2011 3:36 pm
Posts: 10
I have not re-biased the amp yet, but the problem seems to persist with both the new tubes and the old ones. I guess I should get a bias probe and do that first to see where that gets me.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 17 posts ]  Go to page 1, 2  Next
Go to page 1, 2  Next

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: