Need 3 Prong Power Conversion - But Heater On Primary Side of Iso Trans

[Jecarroll7]

Have a Custom Kraft Tremolo Deluxe that (schematic wise) seems to be a cross between a Kay 500 and a Kay 703.

It does have an isolation transformer, but this is the first amp I've run across where the tube heaters are wired to the primary side.

I'd like to do a 3 prong conversion on this, hopefully without replacing the Iso trans.

Any tips for this?

[g1]

If the heaters are connected to the primary, then it is not an isolation transformer. It is a power transformer for the high voltage, the heaters are still not isolated.
You still need an isolation transformer.

[nashvillebill]

I respectfully disagree with the need for an isolation transformer here.

My Lectrolab 200B was exactly like this, the B+ came off a power transformer but the filaments were in series off the 110V primary. The filaments are isolated from the chassis, there is little chance for the chassis to somehow get hot. And once the 3 prong plug is installed and the chassis grounded, that negates the issue of a hot chassis.

Having the filaments run off 110V is IMHO no different than having a desk lamp (from an electrical safety viewpoint). A desk lamp has 110 volts inside the bulb, with no isolation transformer--and often on a non-polarized 2 prong plug.

[Jazz P Bass]

Having the filaments run off 110V is IMHO no different than having a desk lamp (from an electrical safety viewpoint). A desk lamp has 110 volts inside the bulb, with no isolation transformer--and often on a non-polarized 2 prong plug.

Yeah, and often times the desk lamp is made of metal.
Go figure.
Post the schematic, if you could. (hopefully it was not posted before the archive meltdown)

[nashvillebill]

I was wondering why everything went to a standstill!

Anyhow, the fundamentals of the 3 prong conversion:
1) remove the "death cap"
2) make sure nothing else on the primary side ties to the chassis ground--use an ohmmeter to check between both leads of the primary to ground.
3) screw the incoming green wire to the chassis (assuming a US color code)
4) put the incoming black wire to the inside section of the fuse holder (you DO have a fuse?)
5) go from the other side of the fuse holder to the switch
6) the other side of the switch connects to one of the transformer's primaries (and one end of the series filament string)
7) the incoming white wire ties to the other primary (and the other end of the filament string).

Without plugging in the amp! check again for continuity between the plugs blades and the amp chassis. Only the round ground should show continuity.

[nashvillebill]

Oh, clarification on wiring to the fuse holder. Wire it so if somebody were to pull off the cap with the amp plugged in, the fuse end that would be exposed should not be energized with the cap off.

[Enzo]

Generally the fuse holders today have the fuse recessed so you cannot reach the end of the fuse with your finger, and if the fuse backs out enough so you can, it is no longer in cointact with the terminal in the bottom. That is a safety feature and the reason you can't buy the old style holders.

You don't need an iso for series string heaters. The problem with old amps that directly rectified the mains for B+ was that the chassis was referenced to the mains. But the heaters are not grounded to the chasis, so the mains connections to the heaters are no more dangerous than the other mains wiring in the amp, like fuse holder, power switch, etc. The power transformer for your high voltage covers the need for isolation.

[g1]

2) make sure nothing else on the primary side ties to the chassis ground--use an ohmmeter to check between both leads of the primary to ground.

Until this was dealt with I was not comfortable with the scenario. If it checks out, then you are right, having 120V on a tube is like having 120V on a light bulb.

[g1]

But the heaters are not grounded to the chasis, so the mains connections to the heaters are no more dangerous than the other mains wiring in the amp,

I was not aware this was always the case and thought the chassis could be connected to one side of the heaters in some cases?

[Dave Curtis, dB AudioTech]

Without plugging in the amp! check again for continuity between the plugs blades and the amp chassis. Only the round neutral should show continuity.

Just to clarify: the round prong on the plug is ground, not neutral.

[nashvillebill]

Just to clarify: the round prong on the plug is ground, not neutral.

Absolutely correct, sorry for the confusion, I was in a hurry to run out the door. I'll correct my original post in case somebody finds this thread later.

[Jecarroll7]

Thanks guys, good tips.

Not sure what the issue is but the site won't let me upload a schematic. It tries but nothing happens. Have tried twice today.

[Enzo]

g-one, sometimes one side of heaters is grounded, usuall on 6v like some old old Fenders. They wouldn't normally do that on a transformerless amp. Even the transformerless ones usually connected the chassis to the line through a good sized cap. They wouldn;t ground the heater string to chassis unless the chassis was already grounded to the rest of the power supply.

Having said that, I am sure there is some example out there to the contrary. But I've never seen one that way.

SOmehow this seems related, not sure why.

The very common - back when - 35W4 rectifier has a 35v heater for series string. But there is a tap on the heater for a 6v pilot lamp. It is across part of the heater creating a 6v drop for the lamp. Works pretty well, lights a #47 lamp without a 6v winding or series connection. Problem occurs when the bulb burns out or is missing. The tube not only supplies a 6v tap for the lamp, but it also depends on that bulb bing there. Without the bulb, the 35W4s tend to burn out early. The 6v lamp holder needed not to be grounded.

[Leo_Gnardo]

I respectfully disagree with the need for an isolation transformer here.
My Lectrolab 200B was exactly like this, the B+ came off a power transformer but the filaments were in series off the 110V primary. The filaments are isolated from the chassis, there is little chance for the chassis to somehow get hot. And once the 3 prong plug is installed and the chassis grounded, that negates the issue of a hot chassis.

Done the same with my Lectrolab 200B, now it's the bench test amp. Buzzes a bit more than I'd like but the price was right. (Zero). Simplified the input to a single jack to minimize buzz but it didn't help much.

Someday maybe I'll convert it to a more normal 6.3V filament setup (with appropriate tubes of course) & see if that doesn't knock down the buzz. When I get thru all the other amps awaiting service...

[trem]

Yeah, and often times the desk lamp is made of metal.
Go figure.
Post the schematic, if you could. (hopefully it was not posted before the archive meltdown)

Sorry if I am in the dark.....is the Ampix schematic page gone, or has it been moved.?
Thank You