On the subject of bad output transformers (silvertone 1484/twin twelve content)

[Jay]

Posted this over on another forum as well but figured I'd try here too in case there is anyone with some experience working on these.

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?

I did notice that I get about 150k across the two 1MA volume pots on the non-working amp (all the way down) vs. 300k on the working amp.  So far that's the only thing that seems noticeably different between the two.

[øøøøøøø]

I would check the screen resistors.  Sometimes bad filter caps and bad cathode bypass caps can cause low output.  You would not necessarily notice any discrepancy in voltage.

It could also be the output transformer.  The easiest way to test that would be to take the known-good transformer from the working amp and put it in the non-working amp (or vice-versa) to see what happens.

There are also ways to test if a transformer is good by putting a known voltage (I usually use 120VAC from the wall) onto the primary and measuring the secondary voltage to see if it's stepping it down at the proper ratio.  Google that for more details.

[Jay]

Swapping transformers is an option of last resort...  ever seen the inside of one of these things? —total mess.  Changing the filter cap isn't much of an option either as the ONLY place that has the right cap can is closed for good (vibroworld.com) and there is little to no room inside the chassis to place a new bundle.  Is R35 and R36 the screen grid resistor?  I also don't see anything that I would identify as a cathode bypass cap or a screen resistor.  Is the screen resistor R35 and R36 in the schem linked to in the first post?

[øøøøøøø]

R35 and R36... those are the ones.

C1, C17  are cathode bypass caps that jump right out at me.

You don't need to use a can-type capacitor.  You can use discrete caps instead, if you can find a place to mount them in the cabinet.  Electronics-grade silicone is good and reversible for mounting them to the chassis. 

What are the values on the cap can?  Is it 20 10 5?

Tubesandmore.com sells a 20/10/10.  That would probably work just fine.  Might even make the amp slightly quieter, although it might tighten up the bass just a smidgen as well.  You may or may not like this, or you may not even notice.  My bet is that a 5uf difference will be quite subtle.

I'd be looking at C26-C29 as well.

Best of luck.

[Jay]

R35 and 36 are ok.  I was only looking for bypass caps in the power section since I thought the problem was manifesting there somehow but I'll check the preamp tube bypass caps.

I've looked at AES for caps (I bought replacements for C26-C30—the big red Planet caps in the pic—from them already) but most of the multisections don't stand a chance of fitting and the ones that do are way off value wise.  I need a 1" width and the closest they have in that size is a 20/20 and a 50/50/50.  I might be able to use the 20/20 and add a 10/450 inside the case I suppose but finding a place to bundle three in this thing is going to be tricky. I also don't want to cut the chassis open and risk doing anymore damage. Here's a pic if you've not seen the guts of one before...  total spider web.  Thanks for the help btw...

Click for a huge ginormous up close version.

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you don't necessarily have to bundle the three together. 

If you are really short on space and really want to get tricky, you can cut open the original multi-section cap and scrape the guts out and load it with fresh discrete caps.  Seen it done.  Usually there's an easier way.