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Old 6th May 2011, 06:54 PM   #1
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Default Isolation transformation wiring project help

Second, I do not have any schematics whatsoever for my amp (Alamo 2525), and after many fruitless hours of web surfing, I don't think any exist.

The amp uses an old radio amp setup with a 35w4 diode rectifier, a 50c5 pentode output, and a 12au6 pentode preamp. I am attempting to install an isolation transformer to fix the grounding issue (the current was essentially being grounded into the chassis), and I have run into a problem with trying to counteract the half-wave rectification of the 35w4. The plan is to run the wall plug into the primary leads of the isolation transformer, and then run the secondary leads into a bridge rectifier before continuing into the tube rectifier. The bridge rectifier only gives us one lead which, currently, is intended to run directly into the tube rectifier. The problem is that I'm not sure where the lead to the power switch is supposed to come from. I don't know how to wire the on/off leads with all the leads from the transformer currently accounted for. I am using a Triad N68X isolation transformer, and here is the link to the article / schematic I have been using:

Fixing the half-wave rectifier problem
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Old 6th May 2011, 08:02 PM   #2
DF96 is offline DF96  England
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As the circuit was designed for half-wave why not leave it as it is, but using the isolation transformer for safety?
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Old 6th May 2011, 08:37 PM   #3
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You didn't read the full article. There's a picture telling you exactly the answer to your question. I've reattached it here.
Attached Images
File Type: jpg The-Plan.jpg (19.2 KB, 121 views)
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Old 6th May 2011, 09:23 PM   #4
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DF96: The isolation transformer wouldn't be able to handle the half-wave rectification because the 35w4 sends back the negative current to the transformer, which causes it the transformer to have a standing DC voltage and become saturated much quicker. By creating a full wave rectification, it takes out the negative current that is being sent back to the transformer.

Leadbelly: the problem with the on/off switch is that it is one of those volume/switch combo pots that you see on some older amps where all you do to turn on the amp is turn the volume knob. I can't include the pot before the transformer like the diagram says because the pot is connected to the rest of the amp circuit, leaving no output lead available to connect to the transformer. If I had a schematic, I could probably figure out some way to rewire the chassis, or see if there was another available solution, but I can't find one and the physical wiring of the chassis itself is very confusing and many of the components are old, faded, and unreadable (although still in working condition). If I used one secondary lead from the transformer to connect to the pot, and then one lead connect to both sides of the bridge, would that possibly work?

Also, I've looked a thousand times at the wiring and I'm slowly in the process of creating my own hand-drawn schematic, but I've never be able to find the source of the current that is somehow entering the chassis and causing 100+ volts to run through the chassis whenever the amp is on. Any suggestions on what/where I should look at, or another explanation as to why this is happening?
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Old 6th May 2011, 09:40 PM   #5
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Quote:
Originally Posted by Speedyz27 View Post
Leadbelly: the problem with the on/off switch is that it is one of those volume/switch combo pots that you see on some older amps where all you do to turn on the amp is turn the volume knob. I can't include the pot before the transformer like the diagram says because the pot is connected to the rest of the amp circuit, leaving no output lead available to connect to the transformer. If I had a schematic, I could probably figure out some way to rewire the chassis, or see if there was another available solution, but I can't find one and the physical wiring of the chassis itself is very confusing and many of the components are old, faded, and unreadable (although still in working condition). If I used one secondary lead from the transformer to connect to the pot, and then one lead connect to both sides of the bridge, would that possibly work?
No, I still think you're misdiagnosing. That pot should have 3 lugs for the variable resistance function and a separate set of lugs to do the on/off switching. All you need to do is figure out which leads are for the switch and which leads are for the variable resistance. All you need is 2 minutes with a multimeter.
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Old 6th May 2011, 09:44 PM   #6
DF96 is offline DF96  England
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The DC in the secondary will be balanced by DC in the primary. This is quite a different situation from applying a standing DC voltage to a transformer - in that case there is no balancing current in the other winding. Using an isolation transformer to power an AC/DC circuit is a standard servicing procedure, used for years. It provides both safety and the ability to connect test equipment.
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Old 6th May 2011, 09:46 PM   #7
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Quote:
Originally Posted by Speedyz27 View Post
Also, I've looked a thousand times at the wiring and I'm slowly in the process of creating my own hand-drawn schematic, but I've never be able to find the source of the current that is somehow entering the chassis and causing 100+ volts to run through the chassis whenever the amp is on. Any suggestions on what/where I should look at, or another explanation as to why this is happening?
I assume you are measuring that 100+ V with a multimeter? It's just the charge on the chassis through a small cap from the line connections. Standard in the old days. It's high impedance, not the dead short you are probably worried about. Still, don't use it until you get your mod sorted out to a proper, modern grounding system.
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Old 6th May 2011, 10:54 PM   #8
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Quote:
Originally Posted by leadbelly View Post
No, I still think you're misdiagnosing. That pot should have 3 lugs for the variable resistance function and a separate set of lugs to do the on/off switching. All you need to do is figure out which leads are for the switch and which leads are for the variable resistance. All you need is 2 minutes with a multimeter.
I've already located the two lugs for the on/off switch. They are situated on the circular back of the pot, where as the the variable resistance lugs are the three on the side of the pot. One of the on/off lugs was connected to the black (hot) wire from the wall, and the other has a wire leading to a small perf board with a capacitor and resistor, which then connects to two additional wires which lead to the filter cap and power tube separately. The variable lugs are also incorporated into the wiring, with one of them wired to the preamp tube, one wired to the power tube, and the other wired to the afore-mentioned perf board. I don't know how I would go about rewiring the power to allow for the circuit shown in the diagram.
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Old 6th May 2011, 11:05 PM   #9
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Quote:
Originally Posted by DF96 View Post
The DC in the secondary will be balanced by DC in the primary. This is quite a different situation from applying a standing DC voltage to a transformer - in that case there is no balancing current in the other winding. Using an isolation transformer to power an AC/DC circuit is a standard servicing procedure, used for years. It provides both safety and the ability to connect test equipment.
So, if I didn't need to worry about creating a full wave rectification, would I be able to attach one of the secondary leads from the transformer to the on/off lug on the pot and the other to the rectifier tube? I know that will cause the transformer to receive power as long as the amp is plugged in, but the main issues I'm trying to fix with the isolation transformer is the grounding problem and noise when the amp is turned on.
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Old 7th May 2011, 12:42 AM   #10
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Quote:
Originally Posted by Speedyz27 View Post
I've already located the two lugs for the on/off switch. They are situated on the circular back of the pot, where as the the variable resistance lugs are the three on the side of the pot. One of the on/off lugs was connected to the black (hot) wire from the wall, and the other has a wire leading to a small perf board with a capacitor and resistor, which then connects to two additional wires which lead to the filter cap and power tube separately. The variable lugs are also incorporated into the wiring, with one of them wired to the preamp tube, one wired to the power tube, and the other wired to the afore-mentioned perf board. I don't know how I would go about rewiring the power to allow for the circuit shown in the diagram.
Think of all the lines as wires. Think of everything coming before the tube rectifier as new wires. If you look at the at th symbol for the "on/off switch" on the drawing I attached, you will see that it has 2 black dots. Think of those 2 black dots as the 2 physical on/off lugs on the switch. Direction doesn't matter. You would connect one lug to the hot wire of your new 3 prong power cord. You would connect the other lug to the fuse with a wire as shown in the drawing.
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