Unknown PT tap

[TIMBO]

Hi guys, I have a PT that I can account for all the connections it has, but one has left me scratching my head.

It has one marked as "SH" on the secondary side. With the MM I get no continuity or resistance to any of the other taps (primary or secondary) or to the transformer itself.

When turned on I get the 350v per leg of the HT (marked 320) from the CT, so all is good there.

With MM I get a reading of 174v between the CT and the "SH" and I also get a reading of 128v between "SH" to the grounded transformer. 

I thought "SH" would be the EARTHSHIELD and have NO voltage on it. The transformer is not connected to the amp circuit as yet as this has got me a bit worried that it has an internal short, BUT it doesn't blow the fuse. 

[tubeswell]

'SH' = 'Shield' probably. In which case there should be no VAC on it. If there is, the PT is probably shorted as you surmise.


There is no reason a shield would have VAC on it if it wasn't connected to VAC. (And shields are designed not to be connected to VAC). Did you try connecting the SH to ground and switching the PT on with a dimmer?

[TIMBO]

Thanks tubeswell, I have had other PTs that have shields and never checked if there is voltage on then and just connected to ground as you do.
I assume that these shields are between the primary and secondary windings and may pick up induction between the two.

[jeff]

I don't see a 6.3V tap, this is just a guess, but any chance SH stands for Series Heaters or something... Heater. I know some tube equiptment would use a bunch of tubes with the heaters in series to add up to 120V(50V power tube+35V rect+ 12Vpre etc although that's usually AC/DC sets)??? don't know just a guess.

What'd it come out of? If it came off something with tubes in it I'd expect to see some way of heating those tubes. right??? dunno.

Be careful. If there's voltage between that tap and ct, grounding it may be very very bad.

 Did you check for continuity between CT and SH in the lowest range of your meter? I'd check it again on a higher range just to be sure. There may be continutity but more resistance than 200R. You wouldn't get a reading if that tap has 220R. Check again with meter in the 2K or 20K range to be sure.

[PRR]

With a sensitive meter, and no other load, it is likely that an "unconnected shield" will float-up toward the voltage of the nearest winding.

Try 0.1uFd from SH to case, then measure the voltage from SH to case.

[TIMBO]

Hi guys, I connected the SH to ground and no problems.

PRR, placed .1u as requested and measured .3v.

[PRR]

It's not connected to anything except stray capacitance. Ground it.

[TIMBO]