Isolation transformer suggestions & info

[alange5]

I recently acquired a 1966 Sound Projects (Lectrolab) model T204.  I don't have a schematic, but the tube lineup is the same as this Gregory Mark V:



Unlike the Gregory, on my amp the heaters are in series with the mains, but there's also an isolation transformer feeding the rectifier, similar to this Kay 503A:



As I understand it, this is not ideal, but safer than nothing.  To minimize risk, I'd like to install an additional isolation transformer for the heaters and install a 3-prong cord with a fuse.


What kind of transformer should I buy and what VA rating?  AES has a cheap 1:1 transformer with a 35VA rating.  Newark has one with 50VA.  Since I'm only isolating the heaters, would the smaller transformer be ok?  Any other concerns I should be aware of?

[sluckey]

Since I'm only isolating the heaters, would the smaller transformer be ok?

Yes.

But, if the amp is truly wired similar to the Kay 503, I would not bother. The Kay is safe just as it is. The AC line is completely isolated from chassis ground so just add a three prong cord and connect the green to chassis.

Look closely at the AC line wiring. If neither wire connects to chassis, either directly, or through some component, you're good to go.

[alange5]

Look closely at the AC line wiring. If neither wire connects to chassis, either directly, or through some component, you're good to go.

It does have a .01 "death cap" from switch to ground, but I'd omit that with the 3-prong cord.


I guess my only real concern is a heater short (unlikely, but a concern nonetheless)... but with proper 3-prong wiring and a fuse, I should be safe?


Another potentially dangerous thing I noticed is the tube retainers are seated DIRECTLY under the heaters with nothing supporting them other than the tension on a right-angle bend as they protrude through the chassis - with only about 2mm distance between pin and retainer.  Fiddling with the retainers while the amp is turned on would almost certainly short a heater to the chassis.  I'll most likely insulate the ends.

[92Volts]

Since the circuit can be ground-referenced like any other if you install a 3-prong cord, I think you're as well protected against a heater-cathode or heater-chassis short as you'd ever be. Chassis should blow a fuse, cathodes might not (depends on cathode resistors), but shouldn't be connected to your guitar or anything. Just like other amps.

In fact, if you used an isolation transformer and tied one leg of it to ground, it would behave very similar to mains power anyways. The main point of isolation is to establish a safe/known ground for the chassis and audio circuit, which you already did. Heaters don't (necessarily) use ground for any functional/safety purpose, except in the case of a fault.

If there is no fuse in series with the heaters you should absolutely add one though.

[PRR]

There were some designs where B+ was transformer isolated but heaters were not.

This means you trust the thin heater insulation. There was some trouble with this in the first series miniature ACDC tube sets and the pinout was changed.

While a well-grounded(?) chassis "should" blow something before a user may be shocked, I still think it is a bad idea. I would go ahead and transformer isolate the whole thing. Cheaper than a funeral.

[jjasilli]

I'll chime-in in favor of sluckey. Assuming the layout matches the drawing, the heaters are already isolated from the chassis; & B+ is isolated by the tranny.


However, if the heaters are using the chassis as one leg of their supply, that needs to be changed to match the drawing.


You are further concerned about a heater short; but that can happen in any amp.  I agree with 92Volts that the solution to that is a fuse.  See Merlin's (Valve Wizard) site.

[alange5]

I may go ahead and do it just to be extra safe. 


As of right now, here's how I have it wired:



I powered it up and it actually sounds pretty cool.


If I purchase an iso transformer, how would I integrate it with the existing one? Is the wiring the same, but with the AC on the new primaries and heaters in series with the new secondaries?

[PRR]

> heater short; but that can happen in any amp

Heater short after a transformer just burns-up parts, replace them.

Heater short from the Power Line to cathode through small cathode resistor is about the same as sticking your finger in a power socket. You can get nearly 120V through your body. (Maybe less if the short is mid-string.) 120V through chest (hand to hand, or hand to foot) is very bad.