Iso transformer for Lectrolab R100C

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Iso transformer for Lectrolab R100C

Postby VinZenT on Tue Jun 20, 2017 10:19 am

Hi dear community,

I'm a new member on vintageamps forum.

I recently bought a Lectrolab R100C. Great tone, I love it.
Unfortunately, after some research, it appears this amp is a typical "widow maker".
As a matter of fact, the heaters are feeded directly from the AC line, there is also a "death cap".

Installing a 3 prong cord and a fuse is easy and will be done soon.
Installing an isolation transformer is a bit more complex for me, althought I built 2 amps and modified a couple more. I'm not totally sure of myself and, according this can kill me, I prefer to ask for more informations and help.
This Lectrolab already has a small power transformer and adding an iso trans on top of it is new for me.

You'll find a picture showing the amp's internals and another one showing the schematics (home made and not completely finished - the guitar input and preamp zone- but the more interesting part is done) :
Image

I also included some general picture of this amp, for information and posterity :
Image
Image
Image

My questions are :
- Should I install the iso trans (N-68X) before the existing power transformer ?
- Should I install the iso trans after the existing power transformer, or in parallel ?
- Before, after or in parallel, what would be the wiring ? I have troubles to see the wiring to isolate the heaters ...
As you can see, I need help to clear my mind coz I get confused after thinking too much about this .... :Doh:

Any help would be fantastic, then I could continue to play Blues and also record with that awesone amp.
I thank you in advance for that.

Vince
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Re: Iso transformer for Lectrolab R100C

Postby stratele52 on Sat Jul 01, 2017 10:03 am

I'll put isolating transformer between power cord and tubes's heaters.
Power cord go to primary and secondary go to heaters
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Re: Iso transformer for Lectrolab R100C

Postby Dean Jr. on Sat Jul 01, 2017 4:50 pm

IF there is a 'power transformer' there, then there is no need for an isolation transformer. The reason why one would use an isolation tranformer in an 'American Five' circuit is because some of these designs do NOT have a power transformer. IF there is a 'power transformer' in the map, that transformer is an isolation transformer. IT does not change the votlage....it merely isolates the incoming AC from the secondary because the primary and secondary windings are not physically connected. Their electrical connection is through inductance.
The true 'widow maker' amps have no transformer through which the AC runs before being sent to the circuit. That is the problem form a safety point of view.
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Re: Iso transformer for Lectrolab R100C

Postby stratele52 on Sat Jul 01, 2017 5:17 pm

Dean Jr. wrote:IF there is a 'power transformer' there, then there is no need for an isolation transformer..


If you look carefully at the circuit you will not say that. This is not a "regular" PT. :dumass:

Heater are feed by wall outlet voltage
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Re: Iso transformer for Lectrolab R100C

Postby Dean Jr. on Sat Jul 01, 2017 5:35 pm

http://schematicheaven.net/bargainbin/harmony_h303a.pdf

http://schematicheaven.net/bargainbin/kay500.pdf

stratele52.....and more importantly VinZenT, above are two schematics. the first one uses a transformer to isolate the AC wall voltage from the chassis. The second is a 'widow maker' in that there is no power transformer. IF an 'American Five' amp has a 'power transformer' then there is no need for an isolation transformer.
I will admit that I do NOT understand the schematic drawing above.....but there is a transformer there. The drawing is very confusing since the AC power in is NOT drawn in such a way that the primary and secondary wiring is obvious. Wired correctly, it will isolate the chassis from that AC power in. back at you......:dumass: indeed.......
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Re: Iso transformer for Lectrolab R100C

Postby stratele52 on Sat Jul 01, 2017 5:48 pm

the schematic you show are not same as the picture of the amp

Look at the picture of the amp ,

1- One side of the power cord ( from wall outlet) go to one side of tubes's heater .
Next side from the heaters is going to other side of AC power cord in series with a 5 watts resistor to on/off switch ; no insulation, this is dangerous.

2- Pt have four wires ; 2 for AC 120 volts input
2 for amp DC supply at output
3- Where do you see wires for tubes heaters ?
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Re: Iso transformer for Lectrolab R100C

Postby stratele52 on Sat Jul 01, 2017 6:35 pm

Sketch of what you can do ;

Sorry forum don't allow more pictures.... look here;

First sketch show your amp as built
Second sketch show with a isolating transformer

http://i859.photobucket.com/albums/ab15 ... j2j7kd.png
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Re: Iso transformer for Lectrolab R100C

Postby Dean Jr. on Sat Jul 01, 2017 8:47 pm

Dean Jr.
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