Fender Bassman 135 blowing fusss

[evil79]

http://www.thevintagesound.com/ffg/s..._135_schem.jpg

So I have a bassman 135 on the bench. Thought I fixed it. It was blowing fuses, and I saw carbon on two of the power tube sockets, so I replaced both. Replaced all screen resistors, disconnected hum pot and installed 2 100 ohm resistors on the pilot lamp for a fake center tap. Replaced old filter caps and power resistors in filter section. Converted tube matching to a bias pot. Changed PI section to blackface. Put new tubes in and biased them. It ran fine on my bench for a couple hours so I gave it back. The guy played one song and it blew a fuse and fried the screen resistor on one of the tube sockets that I didn't replace. So I replaced that one. I plugged it into the light bulb limiter and all was fine, except the tubes are running cold. The tubes seem fine. So what's going on? It only blew a fuse with signal applied when he had it. Also, what is the purpose of the 30k 20W and 2.7k 10W resistors?

[Leo_Gnardo]

what is the purpose of the 30k 20W and 2.7k 10W resistors?

2.7K is a voltage dropping resistor, part of the power supply string. 30K is a "bleeder" trying to keep some regulation of voltages in the preamp.

Maybe there was a carbon track you didn't see on the tube socket that gave you the last problem. Any original output tube sockets left? might as well complete the set, replace 'em all.

What's going on with the owner's speaker cab(s) and cable(s), they're all 100% perfect? No shorts, opens, intermittent connections, corroded jacks/plugs? Does he plug the speaker into the main output - not the ext output (if one speaker)?

Another fave spot for failures in these 70's amps: there's an automatic impedance selector switch on the ext. speaker jack. If the contacts on this switch aren't making good contact, the connection to the speaker can go open circuit & boom goes the amp. Similar if both speaker outputs are in use, other switch contact better be working well or else. Sometimes these contacts are corroded from age, or burnt up from handling excess current. If that's the case, tough time finding a new replacement switch; it may be best to wire the output jacks simply in parallel and adding a heavy duty toggle switch to select output impedance.

[evil79]

2.7K is a voltage dropping resistor, part of the power supply string. 30K is a "bleeder" trying to keep some regulation of voltages in the preamp.

Maybe there was a carbon track you didn't see on the tube socket that gave you the last problem. Any original output tube sockets left? might as well complete the set, replace 'em all.

What's going on with the owner's speaker cab(s) and cable(s), they're all 100% perfect? No shorts, opens, intermittent connections, corroded jacks/plugs? Does he plug the speaker into the main output - not the ext output (if one speaker)?

Another fave spot for failures in these 70's amps: there's an automatic impedance selector switch on the ext. speaker jack. If the contacts on this switch aren't making good contact, the connection to the speaker can go open circuit & boom goes the amp. Similar if both speaker outputs are in use, other switch contact better be working well or else. Sometimes these contacts are corroded from age, or burnt up from handling excess current. If that's the case, tough time finding a new replacement switch; it may be best to wire the output jacks simply in parallel and adding a heavy duty toggle switch to select output impedance.

I have one tube socket left and figured I might as well replace that one too. No carbon to be seen on that or the last one I replaced, but who knows. I asked him about his cabinets, and he tried several, so that doesn't help. It runs fine through two different cabs at my shop and fine through a dummy load. I should check the OT too. Will tubes be cool to the touch when amp is plugged into a light bulb limiter?

[evil79]

Also, you wouldn't think the diodes would have anything to do with it, right? They either work or don't? Correct? If one was faulty the amp wouldn't power up, right? Should I just put new ones in?

[Leo_Gnardo]

Will tubes be cool to the touch when amp is plugged into a light bulb limiter?

They sure won't heat up as much as they would with amp plugged straight into 120VAC.

I'd have a personal look at speakers & cables. Some people have dodgy cables & don't know it, sloppy connections inside the plug shells, like that. Inside cabs, sometimes wires wrapped around jack & speaker terminals, no solder. Push-on terminals are notorious for letting go - solder securely whenever you can. Heck in high school days the general "knowledge" was, you could use your worst guitar cable for speakers. Deafinitely not so! And in a Fender Deluxe I was looking at the other day, one of the lead-in wires from the speaker's terminals to the voice coil was barely hanging on by just one strand, about to let go any second. Dodgy speaker connections kills amps.

Diodes, what the hi voltage rectifiers? They don't work then short/open then mysteriously go back to working. Though they can loosen up at soldering connections. How's the bias department? All right & tight there?

[J M Fahey]

Agree that in general you can't solve a problem which is not present when you test the amp, but in this case I suggest you load the ampwith a Big F*ck*ng resistor and drive it to saturation and beyond for at least 2 hours (AC/DC or Ramones Hits are great for this, because they can be run very distorted and are still bearable, at the same time simulate a Rock Band way better than any synthetic test signal )

Personally I drop the load resistors in a bucket of water, not kidding, almost burnt my house once ..... plus they stand tons more power that way.

[evil79]

They sure won't heat up as much as they would with amp plugged straight into 120VAC.

I'd have a personal look at speakers & cables. Some people have dodgy cables & don't know it, sloppy connections inside the plug shells, like that. Inside cabs, sometimes wires wrapped around jack & speaker terminals, no solder. Push-on terminals are notorious for letting go - solder securely whenever you can. Heck in high school days the general "knowledge" was, you could use your worst guitar cable for speakers. Deafinitely not so! And in a Fender Deluxe I was looking at the other day, one of the lead-in wires from the speaker's terminals to the voice coil was barely hanging on by just one strand, about to let go any second. Dodgy speaker connections kills amps.

Yeah, I'm gonna have him bring his cab and speaker cable. Who knows what he's doing.

Diodes, what the hi voltage rectifiers? They don't work then short/open then mysteriously go back to working. Though they can loosen up at soldering connections. How's the bias department? All right & tight there?

Yes, rectifier diodes and maybe even bias diode. Bias was fine when I worked on it last. I need to check it out again. I think I'll just shotgun replace all 5 diodes.

[evil79]

Agree that in general you can't solve a problem which is not present when you test the amp, but in this case I suggest you load the ampwith a Big F*ck*ng resistor and drive it to saturation and beyond for at least 2 hours (AC/DC or Ramones Hits are great for this, because they can be run very distorted and are still bearable, at the same time simulate a Rock Band way better than any synthetic test signal )

Personally I drop the load resistors in a bucket of water, not kidding, almost burnt my house once ..... plus they stand tons more power that way.

The dummy load resistors? You dump them in water? Why? After they're hot? That sounds strange. Please elaborate.

[Justin Thomas]

Not quite what he meant... immerse the body of the resistor in water, not the leads or connections. And don't "dump" them in after they're hot. There are water and oil-cooled tubes, too. Usually for radio broadcasting. When you got something dissipating 2500W, air cooling/convection/fan just ain't gonna do it.

I just picked up some range top heating elements to use as dummy loads. Hafta see if I can make me some coffee with them and the Peavey Deuce...

Justin

[Chuck H]

Not quite what he meant... immerse the body of the resistor in water, not the leads or connections. And don't "dump" them in after they're hot. There are water and oil-cooled tubes, too. Usually for radio broadcasting. When you got something dissipating 2500W, air cooling/convection/fan just ain't gonna do it.

I just picked up some range top heating elements to use as dummy loads. Hafta see if I can make me some coffee with them and the Peavey Deuce...

Justin

As previously discussed here... Water heater elements work even better

[Chuck H]

I fully agree with Leo. The unit under test isn't the same as the unit in the customers hands. You'll need to see, first hand, what sort of shenanigans are going on.

On another note... Have you checked the bias supply? Rebuild it anyway.

[evil79]

TI fully agree with Leo. The unit under test isn't the same as the unit in the customers hands. You'll need to see, first hand, what sort of shenanigans are going on.

On another note... Have you checked the bias supply? Rebuild it anyway.

So put them in water before the amp is on? To keep it cool? Is that what y'all are saying?

[Justin Thomas]

But these were free... best of all! And I can hack them up for the right resistance. After all, "they're guitar amps, not NASA moon rockets!"

Justin

[Tom Phillips]

...The tubes seem fine. So what's going on? It only blew a fuse with signal applied when he had it. Also, what is the purpose of the 30k 20W and 2.7k 10W resistors?

A likely possibility is that one of the power tubes has an intermittent short. Hard to find and verify. I just put in a new set of power tubes and save the old set until it is verified that the problem is solved. On the average that is the fastest and least expensive (unless you count your labor time as $0) solution.

[evil79]

A likely possibility is that one of the power tubes has an intermittent short. Hard to find and verify. I just put in a new set of power tubes and save the old set until it is verified that the problem is solved. On the average that is the fastest and least expensive (unless you count your labor time as $0) solution.

Absolutely. I do/did the same. I have plenty of known working tubes which I desginate for this very reason. Once I know the amp is stable, I put in the new pair/quad that I'm selling the consumer. No use blowing a brand new quad of tubes if you can help it. And I checked several sets of tubes for this amp, because it's picky, and I even read that this amp is particular about the tubes it wants, so I was especially careful with it. I went all out and over repaired this thing because it cost the guy over $300, so I wanted to ensure that it was solid. Let me do a few more tests and I'll let y'all know what's up. If anyone has some insight into something I may have overlooked please let me know. My ego takes a hit when someone has to bring back an amp that I worked on. I'm sure y'all can relate. I hate doubting myself when I know I'm good at this. Ugh.