Troubleshooting Silvertone 1484 (Twin Twelve) - weak ouput

[rf7]

That is possibly noisy plate resistors. You should pull preamp tubes to figure out what stage of the amp is making the noise. From Geofex: "Start at the front (preamp) end of the amp and pull a tube, listen, pull a tube, listen. When you find one where pulling it makes the crackling stop," then that is the stage to look into.

[eurekaiv]

Awesome... thanks for the tips guys. I'll go through it sometime this week hopefully and clean out the sockets as well as pull the tubes to check those resistors.

[Bear1484]

Troubleshooting Silvertone 1484

Hi,

New guy here with a 1484. Can someone please confirm the yellow secondary wire on the OT is 4 ohm, and ohm what the red/yellow secondary wire is.

Thanks!

Bear

[sterve]

Huge thanks to Twist. I had not seen this thread yet and assumed the OT as well. I was really bummed when the new one had no effect. I read this thread and checked R34 and found it to be wide open. 100k later, now she honks.
Now I'm looking for another one that actually has a OT problem because I have a spare.

[regoR]

Silvertone 1484 troubleshooting

This is an old thread but seems to exactly cover the problem I am trying to resolve. First difficulty is that the amp is in Mississauga Ont. Canada with my nephew - and I am in United Kingdom 3500 miles away!
Everyone seems to agree that the OT may be the cause of the faint output but nobody says why. Does it go shorted turns? If so, are there signs of burning or a bad smell maybe? I want to avoid buying a new OT and finding it was an O/C resistor or a duff standby switch.

Can anyone help me with the address of a supplier for the Mercury replacement and approx cost please? Better still is there anyone on the forum living in Ontario that could help?

Any help appreciated

RegoR

[abehensley]

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

can someone go into what it means to have a resistor "opened up" i got a 1484 at an auction for a whole 2 dollars because it has the same weak output problem. im trying to fix it but im new to working on amps.

[Chuck H]

Holy thread that can't die!!! This thing was started in 2008 and has been hijacked by noobs and resurrected four times! Once two mos. after it finished, again in 2009, again in 2010 and now again in 2014! This has to be some kind of record.

Abe (can I call you Abe?) An open resistor is a resistor which has failed for one or more of several possible reasons and no longer conducts at all. It would read infinity ohms on a meter. It isn't a known quantity with a finite cause. Two bucks is an excellent price for that amp in any condition obviously. If you don't know how to fix it you need to know that it can be very dangerous, even for a handy individual, to poke around unaware in a tube amp. Very possibly even with the power off and the cord unplugged. If you're a rank amateur electronically you shouldn't do it. The amp is almost certain to be in need of parts unrelated to it's failure as a matter of overdue maintenance. This can confuse repair efforts for someone that isn't skilled at amp repair. Fixing that amp with no experience, even with help here, is likely to be really hard and take a lot longer than it needs to. There's even the possibility that unfamiliarity with the work and procedures can cause new problems that confound the process and lead to a dead end. We've seen it many times. A good repair shop can diagnose the amp very quickly and give an estimate for repair and functional restoration with little possibility of any problems.

[olddawg]

can someone go into what it means to have a resistor "opened up" i got a 1484 at an auction for a whole 2 dollars because it has the same weak output problem. im trying to fix it but im new to working on amps.

When a resistor "opens up" it means that it has burned to an open circuit condition or become mechanically disconnected internally to an open circuit condition. An open circuit has no connection, like a switch in the open position. An open resistor will measure infinite resistance with an ohm meter out of circuit. Be aware that tube amplifiers can kill you if you do not possess basic trouble shooting skills.

[bob p]

On the subject of better late than never: I just like'd tboy's post on the standby switch. It's a great idea that's easy to overlook.

Holy thread that can't die!!! This thing was started in 2008 and has been hijacked by noobs and resurrected four times! Once two mos. after it finished, again in 2009, again in 2010 and now again in 2014! This has to be some kind of record.

It seems like a record, but it probably isn't. When I think about what a popular topic this is, it really doesn't surprise me that the thread would get resurrected over and over and over and over again. After all, the amps are all old and failing, and they've become popular again because a new generation of famous musicians are using them. The result is that a whole new generation of people are trying to rescue the amps from the salvage bin, and they're running into the same old problems. this doesn't surprise me at all.

The other thing that doesn't surprise me is that when the amps get resurrected, they always end up suffering the same fate. It seems that the OT always goes because someone tried hot-rodding the circuit instead of restoring it within the scope of it's hardware limitations.

So I guess my long-winded point is to say that if the thread keeps getting resurrected, that's telling us that there needs to be a Twin Twelve Restoration FAQ somewhere.

[bob p]

On the subject of "open" resistors or other methods of failure for old resistors:

The Twin Twelve is old enough that it's going to have carbon composition (CC) resistors everywhere. CC don't immediately fail by "opening up" or burning an open spot into the circuit so that no current flows. Their normal failure mode is one of thermal instability. As CC resistors grow old, time and heat take their toll, causing their resistance value to drift upwards. As the resistance value drifts upwards the voltages decrease to inappropriately low levels downstream. As the resistor heats up it enters a vicious cycle of more heat causing more damage, further increasing the resistor's impedance over time, and further degrading the circuit's performance. The death knell for the resistor is when it becomes so hot that it cracks from the heat. When it cracks, the resistor material fractures and there's no longer a connection internally. That's what people refer to as "opened up." It's the end point for a thermal failure mode.

In my experience, opening up is a late (final) manifestation of resistor failure, not an early manifestation of the problem. If someone pays close attention they'll notice much earlier manifestations of resistor failure, such as the upward drift in resistance value. It's not at all uncommon for old heat-damaged CC resistors to drift upward enough in value to make an amp sound weak.

Although I'm in agreement that the OT is a frequent point of failure on these amps, I'll also throw you the idea that the amp can also sound weak and thin for a number of other reasons because any part can fail. This makes the prospect of fixing an old amp decidedly more complicated than many people would like to think. The conventional wisdom says "just recap it, everything will be fine." Although everyone seems to have a good appreciation about how old caps can fail and break amps, it seems that fewer people talk about resistor drift and how this can adversely effect an amp's performance, making it sound weak.

As an example, here's a story about how I restored a 50-year old HiFi tube amp (aka the "Please Please Me" amp) so that I could listen to a 50-year old Beatles' record on a 50-year old amp on the 50th anniversary of the record's release. The take-home point is that the 50-year old amp had perfectly functioning 50-year old capacitors, which was not at all what I would have expected. It turned out that it was the 50-year old CC resistors that were the reason for the circuit's failure:

http://music-electronics-forum.com/t32615/

My point is to echo what Chuck said earlier. You made a great score for $2, but fixing 50 year old amps properly is never easy -- it is often deceptively complicated. The prospect of fixing an old amp like this often requires a fair amount of diagnostic skill. Chances are that there are any number of things that could be wrong inside of your amp, and fixing it properly will require addressing all of them. If the project sounds more complicated than you're comfortable tackling on your own, there's no shame in getting someone to help you. At the $2 starting point, you can even afford to pay for professional help if you need to.

Enjoy the Twin Twelve. I love mine.

[soundmasterg]

The resistors used in these amps were pretty crappy 20% tolerance carbon comps...I have a 1484 in the garage awaiting repair. It got the last of the Vibroworld 20/10/5 custom can caps that Zack made before he went out to tech for Robert Cray and then never reopened Vibroworld. I recapped the rest of the amp's electrolytics, but there are some further problems elsewhere in the amp (most likely the resistors) and it gives garbled sounds. One of these days I will get back to it. Twin Twelves are cool amps...I modded the piss out of my brother's years ago. It wasn't a great candidate for a perfect restoration because it was missing the reverb tank and had been dented on the chassis pretty bad....mine however is a great candidate for restoration and the reverb and trem both work fine. Working on these amps in particular is kind of hard for a newbie because of the way it was put together and the tight quarters inside the chassis. Surprisingly my brother's amp is 30 watts RMS and it has not blown the OT yet. The stock amps are 25 watts RMS and do tend to blow the OT's often. They also can benefit from an adjustable bias modded with a voltage doubler as the stock bias is pretty hot.

greg