isolation transformer install

[Randall]

I have a 60's Silvertone 1451 combo in the shop. It's not the guitar case amp. It is a widowmaker with no PT, so I want to install a Triode N-68X isolation tranny. Problem is on this one I am not sure exactly where the secondary should go. This post has the original wiring at the top and a modded drawing of an added iso tranny at the bottom:

http://music-electronics-forum.com/s...ad.php?t=16486

Is it wired correctly? I see one side of the secondary goes to pin 5 of the rectifier, it's the other side I am unclear on. And why are V2 and V3 filaments before the iso tranny?

Also, in the modded version he has all grounds common with no 68K/.05uF between them, is this a good idea?

[Enzo]

The simple solution is to wire the iso as if it were the wall socket. Your mains go to the primary of the iso, then the two secondary wires of the iso are connected to what used to be the power plug on the amp. Of course I mean electrically, not wires soldered to the plug prongs. So the amp circuit remains completely stock. If parts have faded or gone leaky, that is separate.

As to the guy's modified circuit, I see no reason at all to put the heaters on the unisolated side. it doesn't result in a hot chassis condition, but it does put mains wiring directly into the tube sockets. The only advantage I see doing it his way is the iso doesn't have to provide current for that heater string. But in my world we select an iso with sufficient current to start with and we don't have to worry.

It would be easy enough to make the iso work, then see how the amp plays with that original 68k ground coupler. It probably will be fine as is. You can always shunt that later if need be.

I think the question is are you trying to make this a safe working amp for a customer? Or are you putzing with your own amp looking to modify it?

[Randall]

I am definitely trying to make a safe working amp for a customer by adding an iso and filter caps and safety AC cord.

[Enzo]

Then in my shop, I would add the iso, wire it up, and leave the amp stock aside from necessary repairs.

If the customer wanted me to rewire it and change things around in there, we could discuss the extra labor costs.

[Randall]

"The simple solution is to wire the iso as if it were the wall socket. Your mains go to the primary of the iso, then the two secondary wires of the iso are connected to what used to be the power plug on the amp"

But that would mean the primary side of the iso is hot whenever the amp is plugged into the wall. I would think you'd rather put the iso after the switch and fuse?

[The Dude]

......But that would mean the primary side of the iso is hot whenever the amp is plugged into the wall. I would think you'd rather put the iso after the switch and fuse?

It really wouldn't hurt much to have it hot all the time. There won't be any current draw if the amp's off. That said, I agree with you and it shouldn't be a big deal to move the switch and fuse to the primary side.

[Enzo]

Randall, the example you linked to the guy had put the iso in between circuit elements for some reason. yes, I agree the power switch and fuse ought to be on the incoming side. I just described it the way I did to point out no special considerations need be made, just insert the iso between the wall and amp. That would work. If you want to dress it up a bit with the fuse and power switch, I figure you know how to do that.

I guess I just wanted us to avoid over-thinking it.

[Randall]

The iso came in finally, and now as I begin to wire it up I wonder about chassis ground. That 68K between neutral and ground is throwing me. Can I get an OK to connect mains ground to chassis?

[Enzo]

You can, but why not just leave it stock first? it may work fine, as the amp was designed with it in there. You can then tack a shorting wire across it to see later.

The original amp could not have ground to chassis, but now that it is isolated, you ground chassis to the powerline earth like any other amp.

[Randall]

Thank you Enzo, the iso is installed and the amp is up and very quiet. However, the output is very low, I measure it at about 1/4 watt. All resistors are high, but within 20%. I found the .01 interstage cap C4 to be leaking a few volts, so I replaced it. When checking voltages, which I will document in a second, I found the cathode of the power tube to be -1.2v, odd. Interestingly. jumpering past that 68K ground resistor causes the cathode to rise to +9.8v, but with no increase in output on the scope.

Other voltages are:

V1 plate 75v unjumpered, 82v jumpered
V1 screen 20.8v 21.5v

V2 plate 125v, 137v
V2 cathode -1.2V, 9.5v

Rectifier B+ 138v, 150v

My amp is not exact to this drawing, mine has no plate cap on the power tube, there is a ARCO 462 trimmer cap between pin 6 on V2 and B+ (left side of 2.2M R4 and right side of 4.7K R8). And there is an extra 820 ohm resistor between R3 and R4, and the power tube screen does not get fed directly after R8, but rather from the junction of R3 and R4.

[The Dude]

I'm not entirely sure exactly what your describing schematically. Can you draw what you have (the differences) on the schematic in the other thread and post it for us? If I read you right, there is a trimmer cap in between the resistors and the screen grid? Do I have that right? What is the voltage on the screen grid? That will settle it. My thought is, if there's a cap between screen voltage and the screen grid, there wouldn't be any DC screen voltage and, of course, the tube wouldn't conduct much. Are you sure it's a trimmer cap and not a trimmer resistor? Either that, or I totally misunderstand your description.

Oh, and P.S.- Did the amp work before you installed the isolation transformer?

[Enzo]

To measure the cathode of the power tube, you reference your meter to the bottom of the resistor, NOT chassis. That 68k and cap are in series with your meter if you do.

Half the circuit looks at chassis for ground, and the power amp half looks at its own common as "ground".

[Randall]

"If I read you right, there is a trimmer cap in between the resistors and the screen grid? Do I have that right? What is the voltage on the screen grid? That will settle it."

No, it is between pin 6 and B+. I have included my best attempt at my drawing, it's not really something I know how to do very well.

The voltage at the screen grid is 116v unjumpered and 128v jumpered. Are these ground resistor jumped/unjumped readings helpful or just noise?

[g1]

The voltage at the screen grid is 116v unjumpered and 128v jumpered. Are these ground resistor jumped/unjumped readings helpful or just noise?

See Enzo's post #12.
When you don't have the jumper, you need to use the neg. end of the filter caps for your black meter probe when checking any power tube voltages. That should give you proper cathode voltage measurement.
This is assuming the schematic is fairly close.

[Randall]

Thank you for explaining that. In that case, with the power caps neg. as ground I get plate = 135v, screen = 127v and cathode = 9.5v. Since these now all look good, would this point to a look back at the OT?

As to the question of whether it worked or not before the iso install, all I really did was switch it on and hear how noisy it was and say, "yup, time for some caps", so I don't really know how much power it was putting out.