Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hi all I found this forum and I wanted to post something about an amp I'm having some issues with.

First off, I can service tube gear and have access to a test bench its just that I don't have much familiarity with SS amps.

I have an '84 Fender Stage Lead 212 amp that is blowing the speakers fuse. I don't really use this amp much and wanted to sell it so of course I want to get it straightened out. Ir works and sounds good and will function on the bench for a short time with a test signal but then will blow the speaker fuse if you are pushing some power. It calls for a 4amp normal blow fuse and I must admit I've only tried 3 and 3.5 amp normal blow fuses.

The amp will run for a few minutes but then the output flat lines when the speaker fuse blows. other than getting the correct fuse in there, any idea what is up with this amp as I'm describing?

Quote from: gw on May 04, 2011, 06:45:48 PM
Hi all I found this forum and I wanted to post something about an amp I'm having some issues with.

First off, I can service tube gear and have access to a test bench its just that I don't have much familiarity with SS amps.

I have an '84 Fender Stage Lead 212 amp that is blowing the speakers fuse. I don't really use this amp much and wanted to sell it so of course I want to get it straightened out. Ir works and sounds good and will function on the bench for a short time with a test signal but then will blow the speaker fuse if you are pushing some power. It calls for a 4amp normal blow fuse and I must admit I've only tried 3 and 3.5 amp normal blow fuses.

The amp will run for a few minutes but then the output flat lines when the speaker fuse blows. other than getting the correct fuse in there, any idea what is up with this amp as I'm describing?

Start with the right fuse.  It's likely that Fender didn't give the fuse much wiggle room.  IE - it might need 3.8 amps when pushing the speaker hard.  If that blows then go from there.  It would also make sense while you are in there to check that your speakers are not blown/shorted out and also that they are of the right impedance and wired correctly.  I'm not sure what it was suppose to have stock, but if it had for instance two 8 ohm speakers wired in series and then someone rewired it in parallel, that could be causing your problem.

Yeah I'm looking for the right fuse. I looked at the speakers and they are wired in parallel so I'm guessing they  are two 8's wired for 4 ohms or two 4's wired for 2 ohms.

I was using a dummy load that the lowest I can set is 4 ohms and it still pops fuses. Like I said I don't know much about SS amps. Do they fuse the output to protect the power transistors?

I looked at this amp again tonight. The speakers are actually 16 ohms in parallel to get 8. I always thought SS amps used a low ohm speaker load.

I decided to live dangerously and tried the amp with a 5amp slo blo fuse that was available. The amp worked fine, and I ran it on the bench at a very healthy output for 1 hr w/no problems.

The amp is actually pretty cool for a clean sound. The box is even little deeper than a Twin.

Quote from: gw on May 06, 2011, 02:25:50 AM
I looked at this amp again tonight. The speakers are actually 16 ohms in parallel to get 8. I always thought SS amps used a low ohm speaker load.

I decided to live dangerously and tried the amp with a 5amp slo blo fuse that was available. The amp worked fine, and I ran it on the bench at a very healthy output for 1 hr w/no problems.

The amp is actually pretty cool for a clean sound. The box is even little deeper than a Twin.

SS amps don't always run low ohm speaker load, but many of them can.  Normal for an SS amp is 4 or 8 ohm, the latter being more common.  I'm pretty well convinced you just needed a larger fuse.