Adding an isolation transformer to an older amp

[Steve A.]

I've never added an isolation transformer to an older amp myself so I am asking the amp techs here to post suggestions and tips on how to do it, with sources of parts if at all possible and explaining how to make a standalone box as well as adding the isolation transformer internally.

I will be moderating this thread and deleting posts that do not help explain how it is done and possibly editing others for brevity. The thread will be closed once the procedure has been explained.

Thanks in advance for your help in this!

Steve Ahola

P.S. I started this thread because one in the Guitar Amp section got really weird so I had to close it:

How do I make this 'widowmaker' safe?

The original question had to do with this schematic for a Magnatone amp:

http://music-electronics-forum.com/a...7-scarlett.pdf

[Chuck H]

As far as I know it's simply a 1:1 transformer in line with the AC mains before any circuit. In this scenario you need to account for ALL the current the amp will draw (filaments as well as amplifiers) when choosing a transformer spec. I would also add an additional fuse and a proper chassis ground for "in amp" installation. The drawback to using a separate unit for plugging an amp into is that there's no way to add a chassis ground without modification to the amp that will be plugged into it.

[bluto]

I added one to an old Western Electric series filament amp I converted into a sort-of 5C1 Champ and as was said it is basically wired like the power xfmr in any tube amp that had one, the schematic is below. FWIW it might be worth considering using a tube reverb unit PT instead with a SS bridge rectifier and 6 volt tubes, Weber sells the xfmr for $25...I ended up going that route as I was not happy with the sound or hum level of the series filament setup.

[Tech. Tom]

Please forgive my ignorance, but what are you attempting to isolate and for what reason? Most of the repair I do is television repair. When I was working on CRT sets sometimes I would have a heater/cathode short and I could save the set with an isolation transformer.

[Steve A.]

The drawback to using a separate unit for plugging an amp into is that there's no way to add a chassis ground without modification to the amp that will be plugged into it.

Good point- with a separate unit you would need to rewire the amp with a 3 prong cord with the ground connected to the chassis. And any connections from the neutral line to chassis would need to be removed (see attached schematic.)
As for total current draw it could be measured with a clamp-on ammeter and then increased appropriately (+50%? +100%?).

I started this thread because one in the Guitar Amp section got really weird so I had to close it:

How do I make this 'widowmaker' safe?

The original question had to do with this schematic for a Magnatone amp:

http://music-electronics-forum.com/a...7-scarlett.pdf

Steve Ahola

[Steve A.]

Please forgive my ignorance, but what are you attempting to isolate and for what reason? Most of the repair I do is television repair. When I was working on CRT sets sometimes I would have a heater/cathode short and I could save the set with an isolation transformer.

I started this thread because one in the Guitar Amp section got derailed so I had to close it:

http://music-electronics-forum.com/t33233/

The original question had to do with this schematic for a Magnatone amp:

http://music-electronics-forum.com/t...7-scarlett.pdf

[Alex R]

As I understand it (can't see the schemo for some reason) the basic problem with the amp is the heater circuit, which wires all the heaters of all the tubes in the amp in series, and connects them direct to the mains. This was common in old tube radios in the UK. Amps like this use less familiar tubes with high heater voltages like 90v and so on. The danger is obvious in a guitar amp I think. Well, obvious to most, gurus excepted.

An isolation transformer is a 1:1 transformer capable of handling the current draw of the amp. Placed in the supply line where you'd put a step-up. It isolates the amp from the mains entirely so that you get the same power in the mains supply but no ground reference, which means the mains isn't trying to get to ground through your body, rendering it safe to touch.

edited to add - yes, looked at the OP schemo and that's what it is, a mains powered series heater 'widowmaker'. The 5OL6 power tube is a 6L6 with a 50v heater, and so on. Thus soundguruman's suggestion, still on display on the other thread, that the heaters be lifted from ground and connected to two virtual ground resistors, would not make the amp safe. Neither would his suggestion that the chassis be grounded. I strongly feel that these dangerous suggestions, which could lead directly to a death by electric shock, should be removed from the forum. IMO so should soundguruman. Who knows when he is going to post another such deadly suggestion, secure as he always is in the certainty of his being right and everyone else wrong? Accepting that he is wrong is not his strong point is it? He hasn't withdrawn the dangerous suggestions, and even went so far as to state that he was being defamed (or something) by those who pointed out the wild irresponsibility of his posts, and even to threaten legal action!

The man is obviously a deluded fool, that I suppose we have to tolerate, but his arrogant and potentially deadly refusal to accept that his suggestion was wrong should lead any responsible forum admin to ban him instantly and remove the offending posts.

[Tech. Tom]

I didn't realize when I joined this forum that I would be so entertained. Thanks Alex.

[Alex R]

I didn't realize when I joined this forum that I would be so entertained. Thanks Alex.

it's just attention-seeking

[Steve A.]

Post #1 by Gaz 05-27-2013, 05:40 PM

I've read about transformerless 'widowmaker' amps before, but this is my first experience with an amp which has no power transformer. My friend who owns the amp complained of a buzz, and I said I'd take a look at it.

The amp in question is a Magnatone "Starlet" 107 (schematic below).



My first question is about the .05/170K combo that separates the ground from earth. What does it's function? This particular model I have does not have the 170K resistor.

Before I had this amp, my friend told me that a shop added a 3-prong cord. From my research it seems like it was the wrong thing to do, and it turns out the added earth connection to the chassis via the 3rd prong was the cause of the hum.

I think I just need to add this isolation transformer, then I can add the earth connection again. Is that correct?
Is there anything else I need to do? What about the mysterious .05/170K?

I also plan on adding a fuse to the primary of the iso transformer.

Thanks for any clarification on this process.

Gaz


Post #8 by Gaz 07-10-2013, 08:20 PM from original thread

Hi, Just wanted to post an update. I installed the isolation transformer, and simply removed the .05 cap, and connected the V- directly to the chassis. I did not make any other changes, and the amp is very quiet now. There is also a proper safety ground now with its own dedicated ground lug.oldgoodfornothinghusbandmaker.jpg

Here's a picture of my handy-work. Not exactly how I would have ideally done it, but the transformer was way too big and heavy to mount on the inside or outside of the tiny flimsy steel chassis.



I also added a primary fuse you can see in the photo, but am not sure how to calculate the proper value. Any ideas?



Submitted by Steve A. 5/25/14

[Steve A.]

Post #9 by g-one 7/10/2013:

The rear panel says "117 volts 21 watts", this would be power consumption of the amp, (not power output for this design ). A 1/2 amp fuse would be good for around 60 watts, a 1/4 amp would be good for about 30. Use slow-blow fuses. If you can get them, I would try the 1/4 amp with volume at loudest setting you will be using. If it blows, try the 1/2 amp.
I don't think you will find any sizes in between. If you can't get the 1/4 to try, the 1/2 amp should be fine.

Post #10 by enzo 7/10/2013

You know, I think I'd just use a 1A fast and never look back. The slow blows are used mainly to cover starting surge. The 1A fast is large enough to handle that, but small enough to pop if something goes pfffft. And the blow time of a 1/2 slow and a 1A fast, I don't think are all that different on something like this.

But I don't disagree with g-one either.

Post #13 by tedmich 5/10/14

this might help [ED. NOTE: this is a great link which explains how it is done]
Isolation transformer upgrade for old guitar amps

or this
Remember the Alamo - Telecaster Guitar Forum

[ED. NOTE: Here is a cool schematic from that thread based on the Scarlett 107]




Post #21 by bluto 5/19/14

from the Wikipedia article on series filament AA5 radios...

Many early examples of the 'All-American Five' posed a shock hazard to users. Lacking a mains transformer, the chassis of the AA5 radio was directly connected to one side of the mains electric supply. The hazard was made worse because the on/off switch was often in the wire of the mains supply which was connected to the chassis, meaning that the chassis could be "hot" when the set was either 'on' or 'off', depending on which way the plug was inserted in the power outlet. Many power plugs had two identical pins, and could be plugged in either way round. The metal chassis securing screws were sometimes accessible from the outside of the Bakelite or wood case, and there were many examples of owners receiving a shock by making contact with these screws while handling a set.

Here is the schematic of the Magnatone Scarlett 107 amp in JPG format:

[Steve A.]

edited to add - yes, looked at the OP schemo and that's what it is, a mains powered series heater 'widowmaker'. The 5OL6 power tube is a 6L6 with a 50v heater, and so on. Thus soundguruman's suggestion, still on display on the other thread, that the heaters be lifted from ground and connected to two virtual ground resistors, would not make the amp safe. Neither would his suggestion that the chassis be grounded. I strongly feel that these dangerous suggestions, which could lead directly to a death by electric shock, should be removed from the forum.

I went back and "pruned" that thread since it had gotten hopelessly sidetracked... derailed might be a better term.

IMO so should soundguruman [be removed from the forum].

I've always left decisions like that up to tboy but we haven't heard from him for awhile. I suppose if two other mods stepped forward the 3 of us could take a vote on whether or not to remove SGM from the forum. (I'm just the moderator of the "Fun With Computer" forum.)

[tedmich]

I've always left decisions like that up to tboy but we haven't heard from him for awhile. I suppose if two other mods stepped forward the 3 of us could take a vote on whether or not to remove SGM from the forum. (I'm just the moderator of the "Fun With Computer" forum.)

tboy hasn't posted here in 156 days

[Chuck H]

tboy hasn't posted here in 156 days

I think he and Steve Conner ran off together

Since tboy is in charge and Steve is admin there's a serious shortage of site police on hand right now it seems.

I tracked Steve to some recent fun stuff via some public facebook stuff. So I guess he's either abandoned the site or just taking an extended break from it. I'm more worried about tboy.

[tedmich]

maybe someone should call Steve M. or drop by and check on him in Utah
Steve Morrison at Website Informer