Silvertone model 1419 Widow Maker Amp - Installing isolation transformer

[DrGonz78]

I got this Silvertone 1419 amp that needs a good 1:1 isolation transformer installed. Strange that I have been doing this repair work for a while now and this is just my first time dealing with one of these things. I have sourced a Triad N-68X as a potential candidate to install in this amp. I am pretty sure that this should do the job but is there anything I might be overlooking? After installing the new transformer I realize there is still some changes that will need to be done to the circuit to make it safe. I do not have the 1419 schematic but the 1420 looks to be nearly identical. So first thing to get started on is to get a good transformer.

Triad-N-68X.pdf
Silvertone_5XL-14203.pdf

[DrGonz78]

I also have a few small 12v 1A transformers laying around that might prove useful for a heater supply. Would need to regulate it a bit but I think it could work. Might add too much weight but I could completely redesign the amp if I went that route. Could even ditch the rectifier tube and go solid state rectification. Also can I get rid of the other inter-stage transformer if I redesign the amp?

[52 Bill]

Also can I get rid of the other inter-stage transformer if I redesign the amp?

The small second transformer is an isolation transformer that separates the ac line ground from the input audio ground. So in theory the input ground is already ac isolated.

In normal widow-makers, the 12 volt filament is also in series with the rectifier and the output tube filaments.

[DrGonz78]

So now back to this thing that has been sitting waiting for me to get around to it. I have the Triad N-68X isolation transformer and I want to ditch the T2 transformer to gain ample enough room to mount the isolation transformer. I have read around for a bit on this and gathered two proposed ways of wiring in the isolation transformer. I intend to involve a full bridge rectifier as I read that it helps keep the Triad N-68X from running hot. I am posting two different ways that the heater filaments could be wired up after this conversion and just looking for some advice on this matter. Also, please note that the tube rectifier (half-wave) will still be installed and I have left it off of this drawing at this time.


With the half-wave bridge installed it will resemble this power supply in this schematic for the Pepco amp. This wiring shows
https://music-electronics-forum.com/...3&d=1374983142

[DrGonz78]

I am curious how others members of MEF have approached the heater filament wiring on these amps. Do I leave the T2 transformer in place? Or if I remove it what is the best wiring method for the filaments?

[olddawg]

What an odd design. Honestly I would just use an isolation transformer and be done with it. Btw.. is this amp yours or a customers? There is nothing magical about these amps imho. If it’s your own and you are changing transformer, rectifiers, etc.. you could just rebuild it into something else you might like better... like a Champ.

[DrGonz78]

This is an amp I have from my uncle to fix up and then we will sell it. He is very patient about some of these amps and allows me to take my time. In fact this amp represents more a learning lesson for modifying a hot chassis designed amp. The chassis is not the most heavy duty and the cab is pretty much crap too. So mounting the isolation PT in the cab doesn't seem like a good idea. I could keep T2 transformer on the amp and mount the isolation tranny inside the chassis, but then I would think it would not get good air circulation.

[Roboburger]

Funny enough, I'm doing the similar gag here. a 1420 with a similar schematic. What did you end up doing? I was gonna do what OldDawg suggested.

[DrGonz78]

I installed a 1:1 transformer. Note that some 1:1 transformers put out a higher secondary than 120v mains. So I reversed the transformer so it would not step up my B+ voltage. I installed a full bridge rectifier and that went into the half wave rectifier. The voltages were fine and it passes signal but the amp is way noisy. I put it on the back burner for while.

I decided that I am going to install a 6v filament transformer. For the power tube I will install a 6CU5 and keep the 12au6 in the preamp. So basically it becomes a normal tube amp. Then I am gifting the amp to my cousin for his first tube amp.

[Jazz P Bass]

I have a 'No Name' amp on the bench at the moment. (similar to a Wards Airline GDR-8611A)
Installed a N68X.
The 12AU6 is really amplifying the 60 Hz ripple. (as it did before the 1:1 was installed.)
Filter capacitors are new.
Very annoying to say the least.
Applying a signal directly in to the output tube results in zero hum.
Not too sure if a bridge rectifier would help or not (supposedly it helps the 1:1 transformer).

[DrGonz78]

I don't like working on these series filament pieces of crap amps. So isolate the preamp tube on a "real" amp, simply pull the tube from the socket. Viola!! If I ever do anything to a widow maker type amp again it will be to make it un-widow made.

[DrGonz78]

So I have installed a 12.6vct heater supply for this amp, running a 12cu5 and the 12au6 tubes in the amp. It is much quieter now in that when I turn the volume down it is 98% improvement of hum. Problem now is that when I turn up the volume it is a hum machine, just like Jazz said about the amp he has there. If I pull the 12au6 tube it is silent. The AC ripple measures 0.9vac on my fluke meter and my meter measures this very well. I tacked in another 100uf to measure AC ripple and to check if the hum went down. Adding the extra cap brings AC ripple down 0.4vac but it actually hums more with the extra capacitance added. I don't have another 12au6 to try and I have searched through my tube stash to no avail. I will check my tube stash later tonight to see if I can locate a 12ba6 tube.

Apparently the 12ba6 is a close enough replacement to a 12au6 and is lower MU so that might be a good thing. I have 3 old resistors still connected to the 12au6 pin circuit points. One is a 330k connected to the plate(pin 5), a 2.2meg connected to the screen(pin 6), and the 15meg grid leak resistors. I don't have any new resistors of those values to replace those ones yet. One question that I have is how would I reduce the gain of the 12au6? I think that the high MU gain of these 12au6 tubes is just too much for this amp.

Edit: BTW I have a full bridge rectifier for the DC B+ supply. I have about 125vdc on the plate of the 12cu5. I have the 1:1 transformer installed where it actually knocks down VAC being fed to the bridge rectifier so after rectification it is not sky high.

[Jazz P Bass]

A new 12AU6 solved the hum problem.
Don't overlook C2/ screen capacitor as that has an awful lot to do with the hum rejection (and gain)
I saw a schematic (which particular one is eluding me at the moment) that had an adjustable capacitor in that position.

[DrGonz78]

Good to know thanks for posting Jazz

[DrGonz78]

Well I eventually got the 12.6v filament transformer and started off using 12au6 & 12cu5 for the tube compliments. The voltage on the filaments was 13.9v with the tubes installed. I learned how to drop the voltage to 12.6 using a pair of 1 ohm resistors on the secondaries.

Replacing the 12au6 did not change my nasty hum so I drilled the hole bigger and installed a 9 pin socket. It did change the hum but only because it was lower gain using a 12ax7 tube.

I opted to remove the AC wiring going to the cheesy pot switch and relocated the fuse to a better place. This got rid of the nasty hum when I was using 1/2 of the 12ax7 stage. Then I wired up the second half of the 12ax7 tube stage only to have a nasty hum again. Not to mention the speaker had the worst voice coil rub I have ever heard. Sounded like a fuzz pedal on the low notes. So I changed the speaker and got it sounding pretty good.

So I opted to wire the 12ax7 in parallel and this proved to be an excellent decision. This added a bit more gain and the hum did not return. In fact the amp is very quiet and it is tough to tell that you have it on. I even had to mount the filament transformer in a not so ideal location and it surprises me there is barely any filament hum.

This simple amp taught many lessons while trying initially just to make it safe to use. Every time I made changes it painted me in a corner and I had to figure out new solutions to the problem. If I ever run across another widow maker piece of junk then I know what I can do to it.