Troubleshooting Silvertone 1484 (Twin Twelve) - weak ouput

[eurekaiv]

Hi! New here to the forums. Been reading around a bit and so far its great. I'm sorta new to learning about amps/repair etc... I've fixed a few over the years but mainly by tracking down bad/cold solder joints. I've also built some fx and an 18watt Trinity SIII from a kit with great success so at this point just trying to learn a bit more about the circuits and troubleshooting which leads me to my questions...

I have two Silvertone 1484's in need of some reconditioning. One is working well but sounds rather dull compared to the other one. Unfortunately the "other one" is now no longer working properly. I've ordered tubes and capacitors (to replace the big red "Planets") in both amps but I don't believe that's what is wrong with the non-working amp. Right now everything appears to be normal, voltages are correct within a few volts for the most part (comparing to markings on the schematic here), but the output level is very very low. Like an amp with the master volume turned almost all the way down. I can even turn the volume all the way up/down and hear the amp distorting like crazy and cleaning up so I'm thinking it must be at the output stage. Reverb and trem are working as well. All my tubes are fine in the other amp so while a couple of them may be a bit weak, I'm pretty positive it's not a tube problem. I'm afraid it may be the output transformer but I've not had any luck trying to test it. I've been trying to duplicate measurements on both the working and non working amps, but so far everything is pretty much identical between the two except for the quiet sound part. Can anyone give me some ideas of other things to check and/or how I can test the output transformer?

Another question, the schematic shows 100uf/150v caps (which is what the Planets are marked as) but I measure 475v or so from the center of the output transformer to the first one. Is 150v a high enough rating? I ordered 100uf/350v to replace them after just glancing at the schematic and now I'm wondering if I should use them.

And finally, is there a source for the 20/10/5 cap can other then vibroworld.com which appears to be closed (despite a note saying they would be reopening months ago)? I don't think a Weber will fit and there's little to no room to build a "pack" inside the chassis.

Thanks!!!

[Chuck H]

If you havent tried it yet, start by plugging the non working amp into the other amps speakers. Then I would swap out the power tubes. Then I would swap out the preamp tubes. If nothing changes then you know it's the circuit and not the tubes or speakers.

Chuck

[eurekaiv]

If you havent tried it yet, start by plugging the non working amp into the other amps speakers. Then I would swap out the power tubes. Then I would swap out the preamp tubes. If nothing changes then you know it's the circuit and not the tubes or speakers.

Chuck

Hi Chuck, thanks for the reply. All the tubes from the "bad" amp work fine in the "good" amp and changing the speaker cabinet does nothing to alleviate the issue unfortunately.

[soundmasterg]

It is most likely a bad output transformer as those are very undersized stock and often blow. There is a way to actually test it to know for sure, but it requires some equipment that you probably don't have. You can replace with something larger and get more power out of the amp as it is, or you could replace with a larger OT, and bump the screen voltage up to close to the plate voltage and put in an adjustable bias supply with a voltage doubler on it to get a better adjustment range, and get probably close to 50 watts RMS power. Right now the amp is about 25 watts or so. Search around on ebay for a 1484 or 1485 that is a cheap project amp and has intact transformers, or order a new one from Mercury Magnetics. If it was me, I'd go with a Bassman head transformer replacement and bump the power up with the adjustable bias/voltage oubler setup. These amps can benefit from tweaking the preamp voltages up and going more towards Fender territory with the cathode resistors on the preamp too. The stock setup uses very low voltages on the preamp.

For the can cap, I put one of Zach's on from Vibroworld when he was still doing it. He's local to me, and I have no idea why he didn't start it back up after he moved it, but that option is gone. Weber's would work, but you would have to cut the hole in the chassis to be larger. You could also use a dual section cap with a discrete cap mounted somewhere inside, but again, you would have to enlarge the hole. The brown turn coupling caps in that amp are lousy caps and anything would be better than them too btw. I used a mix of M150's and 715P Orange Drops in my brother's.

If you study the schematic, you'll see that the power supply is arranged very strangely. It is two voltage doubler windings stacked on top of each other to get that 475v. The screen supply and preamp are tapped off after three stages to get around 340v. The caps are fine at where they are rated voltage-wise. With two doubler windings and 100uf caps, you essentially end up with a 25uf cap as the first stage whereas blackface Fenders essentially had a 70uf cap in the same place. This is one reason why the 1484 and 1485 don't have as good of a bass tone as most blackface Fenders. Another reason is the lousy and small output transformer. This small transformer and the low preamp voltages are another reason why the amp doesn't have much in the way of high frequencies either. You can spend the time and effort to change it and make the amp sound better, and I did this with my brother's 1484. It took me a year to iron out all the issues, but then I added Fender reverb and an EF86 and totally changed the thing. These are hard amps to work on, and while they can be made better, it is extremely frustrating because the layout is so poor. If it was me, I'd get the amp working with a larger output transformer and then sell it and get or build something easier to work on that sounds as good or better. But thats me. Hope I was able to help you out some.

Greg

[tboy]

Just a thought...

The standby switch in the 1484 works by shorting the phase inverter outputs together. A short in the switch or it's leads could produce the symptom you described.

[eurekaiv]

Thanks for the replies!

I'll try and check the switch this evening...

As for the output transformer, I wouldn't really want the amp to be more powerful then it is as I like the volume it produces. More volume and the original Jensens would almost certainly let go and it would be too loud to enjoy in the house. Would the Mercury be pretty much to the same specs as an original? That'd be cheaper then buying a broken one on eBay most likely...

FWIW... the amp was working ok until I took it apart to clean up a little. I dusted it, re-tensioned the tube sockets, cleaned the face plate up, and sprayed some contact cleaner in all the pots. When I put it back together it was no longer working.

[soundmasterg]

You must have knocked something loose or shorted something together. I'd pull it open and look really closely again.

Ther MM transformer is an exact clone, but as with all of their stuff, it is expensive.

Greg

[elixxrx66]

Troubleshooting Silvertone 1484 (Twin Twelve) - weak ouput

No question about it. It's your output transformer that's bit the dust. The output transformers on those amps are almost toy-like and are notorious for blowing when you push the amp too hard. I had one and the exact same thing happened with the same result i.e. the volume was weak and distorted. I ended up buying another one that had the output transformer replaced with a bigger one and I have no problem with cranking the volume on the amp. It also sounds a lot better. The tone is a lot fuller and nowhere near as brittle and thin sounding as the one I had with the original output transformer.

[eurekaiv]

Thanks for the reply...

I've pretty much come to the same conclusion and am now deciding when/if to buy a replacement for it. The good news is that I've re-capped my other twin twelve and got some used GE 6L6's for it and it sounds great now except that the reverb is just about dead. I always liked the sick spacey feedback type sound they made though I think I'm the only one.

[elixxrx66]

Troubleshooting Silvertone 1484 (Twin Twelve)

It's worth replacing. The one I have now sounds a lot better than the one I had with the original O.T. This amp is Detroit garage rock all the way i.e. The Gories. I know some people swear by the original O.T. though.. They say that's what gives the Twin Twelve it's unique tone but I found it to be very brittle and thin. Yeah I know what you mean about the reverb. I took mine out. It's utterly worthless. The tank was the size of a matchbook.

[Amp Kat]

I would test that transformer and make sure it's bad before I forked out some cash for a Mercury. Did you ohm out the primary from CT to each leg and see if it's shorted or open. Try RG's site at http://geofex.com/gtramps.htm and do his OT test. If theres two speaker jacks and your using the paralleled jack you will get those symptoms also. While I agree that the symptoms your getting sounds like the imfamous blowing of another small low wattage OT it doesn't make sense that it was working before and now it's not unless you powered it up with the volume wide open and no speaker attached to the speaker jack. But I would at least verify the validity of the OT before dwelling in the replacement waters.

[Twist]

Check R34. I've had several amps with that resistor opened up. And yes I've had some with blown output trannys.

[eurekaiv]

R34 is definitely not open, reading a little high in the circuit but I imagine if I pulled it it would read fine. Thanks for the suggestion though! I haven't had a chance to get in this amp in the last couple of weeks but I'm going to give the transformer a thorough test this weekend using the geofex page as a guide and see what I get.

[eurekaiv]

Swapped transformers around and that turns out to have been the problem. I've replaced the bad one with a Mercury and it sounds lovely.

Now that it's working again, there's another minor issue I'm having trouble tracking down. I'm getting a very low or quiet static/crackling sound that is consistent (but sorta intermittent in its pattern) with the amp playing (not in stand-by) and the volume all the way down. Probing yielded no change in the static that I could tell. Filter caps have not been changed as of yet, still waiting for Vibroworld to open back up so I can order a few drop ins, but I've done all the other power supply caps and the two cathode caps in the preamp. My other twin twelve does not have this problem though it seems to have a touch higher noise floor.

[Twist]

It's expensive but I use Deoxit Fader Lube in the 25ml 100% liquid dispenser. It has a small needle dispenser which is perfect for small tube sockets. That small bottle will pay for itself 50 times over. I put a small drop in each pin hole of each socket. Then I clean the pins on each tube with denatured alcohol. Reinstall the tubes and work them around in the socket.This almost always stops the intermittent static. If the noise still persist you may have noisy tubes or try replacing the plate resistors on the pre amp tubes. Also dont forget to clean and lube your pots.