Showman impedence question

[0018G]

I've got a '65 Fender Showman, which was originally sold with a single 15" cab(which I don't have). It is currently set up with a '67 Bassman cab, which is 2-12" rig. The standard Showman is designed for an 8 ohm speaker load, if I'm not mistaken, and the Bassman cab is probably a 4 ohm setup. The rig has a tendency to cut out and crackle after it warms up. It spent a year in the repair shop chasing this problem, with no resolution. Am I right about the impedences? Could the problem be a simple impedence mismatch? If so, I'm probably lucky I haven't fried a transformer. I haven't used the thing in years becasue of this issue, and I'm hoping to set it right. Any thoughts?

 

[ggunn]

I've got a '65 Fender Showman, which was originally sold with a single 15" cab(which I don't have). It is currently set up with a '67 Bassman cab, which is 2-12" rig. The standard Showman is designed for an 8 ohm speaker load, if I'm not mistaken, and the Bassman cab is probably a 4 ohm setup. The rig has a tendency to cut out and crackle after it warms up. It spent a year in the repair shop chasing this problem, with no resolution. Am I right about the impedences? Could the problem be a simple impedence mismatch? If so, I'm probably lucky I haven't fried a transformer. I haven't used the thing in years becasue of this issue, and I'm hoping to set it right. Any thoughts?

Probably it's not the problem, but a (fairly) easy way to check would be to open the cab and disconnect one of the speakers.

 

[daav]

You should be abel to change the impedance by chaning the wiring from serial to parallel or vice versa right?

daav

 

[ggunn]

You should be abel to change the impedance by chaning the wiring from serial to parallel or vice versa right?

daav

Yes, but his choices are limited. Two 8 ohm speakers in parallel = 4 ohms. Two 8 ohm speakers in series = 16 ohms.

 

[boingoman]

It spent a year in the repair shop chasing this problem, with no resolution.

Time to get a new repair shop. The thing only has about 40 parts, it can't be that hard.

I'd suspect a bad solder joint that expands when it heats up. That's where I'd start. After that, maybe a grid resistor on the power tubes, or cathode resistor/bypass cap on the preamp tubes.

 

[0018G]

The shop is no longer in business.

I'll start with the solder joints(actually, I believe I'll start by draining the caps ). From there, I found an oscilloscope and someone who knows how to use it-he says there really ain't all that much in there, just make it act up and trace the signal until it gets wierd. Thanks guys-you have renewed my hope and enthusiam for returning this old war horse to service.

 

[boingoman]

If you are comfortable with draining the caps, go for it. I'd probably resolder the whole power supply just as general maintenance, since it's a '65.

After that, a good way to test for loose stuff is to fire it up, take a chopstick or some other wooden-type object, (not a pencil!!) and poke around, pressing on components and joints. You'll know if you poke something that's loose. Only use one hand, keep the other far away from the amp.

 

[Light]

Those old Fender transformers are pretty robust (or so I've read in some pretty reliable sources). My understanding is that they can usually take mismatches by a factor of 2 (half the impedance or twice the impedance) without blowing up. It is quite possible however that a impedance mismatch is your problem, and it certainly is not good for the transformers.

Before you open up the amp, try this: hook up a speaker cable to the cabinet (but not the amp), and test the resistance between the two conductors. Multiply that by 1.25 to get your impedance. If it is not close to 8 ohms (± an ohm or two), then try it with a different speaker cabinet. This is, of course, assuming that it actually is an 8 ohm amp. I'm not saying it isn't, but I looked it up in my book of Fender Schmatics, and I couldn't find the output impedance. If the problem is still there, THEN it is time to start worrying about other issues.


Light

"Cowards can never be moral."
M.K. Gandhi

 

[Light]

Oh, and if that doesn't deal with it, considering how old it is, you WILL want to replace any original electrolytic capacitors.


Light

"Cowards can never be moral."
M.K. Gandhi

 

[0018G]

Thanks for the tips, Light. I had it recapped and retubed when it started to act up(then retubed again), so those bases should hopefully be covered.

 

[ggunn]

When it gets warmed up and starts crackling, does it respond to whacking the side of it with your hand? My SVT was doing this, and it was a cold solder joint at the back of one of the preamp tube sockets.

 

[dementedchord]

seen alotof problems with amps (was a tech) but never the symptoms you've got and an output tranny.... FWIW... 75-80% of all problems come down to physical things (even when parts blow intermittent connections tend to be the source) relitively sure you've got a bad solder joint somewhere...

 

[0018G]

Found it! A connection had broken on one of the large caps on the bottom of the chassis, and it was making connection only part time. A little solder work and it seems to have cleared up the problem. Makes me wonder what I really got when I paid for that cap job several years ago. Thanks to everyone for their input.

Oh yeah, two of the largest caps on the bottom(on the tube side of the chassis) were blue, and three were orange-they all looked old-anybody know which might be original and which are replacements? I should probably replace them all now just to avoid problems, but I am curious.