Philly Power Transformer

[William David Howard]

I built a light bulb test jig supplied through a Variac to protect my Philly's power supply while I get the PS/Amp checked out.  I notice that with the rectifiers removed and no load on the filament winding my 100W bulb glows dimly when the primary winding voltage is at 115VAC.  Primary current is 275mA.  I know there is always a small "magnetizing" or leakage current in a real transformer with no secondary load, but how much is too much for this one?  Does anyone have that info, or can you suggest a transformer shop with experience with EH Scott transformers where I can ask?  Thanks!

Dave Howard

[David C. Poland]

That is a big power transformer. May be ok. I seem to recall a bit of glow on the amp only without tubes for an AW-23 too.

You sure there is no load? All tubes out of the amp? The speaker and the receiver un-plugged?  The B+ winding voltage will read higher for lack of the designed load to stress the transformer a bit. Have you replaced the filter caps? They are still in the filament circuit although without the B+. If the receiver is plugged into the amp, and you have pulled the tubes, then the 8 or 9  dial lamps are still a load.

When I power up a high tube count set with all tubes including the rectifiers, the 100 watt bulb will get very bright. The first time i power up a Philharmonic or AW-23, I substitute a vintage floor lamp with the 3 way Mogul 300 watt lamp in the dim bulb tester.

[William David Howard]

Thanks for your help!  The PS/Amp chassis is empty of tubes and totally disconnected from the tuner and speaker.  The choke output leads are both disconnected from the filter caps.  The filter cap at the choke inputs is missing.  The transformer CT is isolated.  Anything else I'm missing?

I've since tried plugging an unloaded isolation transformer into my test jig.  It's a little bigger than the Philly's transformer, but it doesn't light the 100W bulb at all.

David C. Poland said:

That is a big power transformer. May be ok. I seem to recall a bit of glow on the amp only without tubes for an AW-23 too.

You sure there is no load? All tubes out of the amp? The speaker and the receiver un-plugged?  The B+ winding voltage will read higher for lack of the designed load to stress the transformer a bit. Have you replaced the filter caps? They are still in the filament circuit although without the B+. If the receiver is plugged into the amp, and you have pulled the tubes, then the 8 or 9  dial lamps are still a load.

When I power up a high tube count set with all tubes including the rectifiers, the 100 watt bulb will get very bright. The first time i power up a Philharmonic or AW-23, I substitute a vintage floor lamp with the 3 way Mogul 300 watt lamp in the dim bulb tester.

[mike hadley]

William,

Do you have an inductance meter that can measure the inductance of one of the larger inductance winding's,(ac input is good)

if you measure this winding then apply a short to another winding (i.e. HT output) on the transformer, there should be a large change in readings anywhere up to 10:1, if this does not happen then you can suspect the transformer has a short.

Mike

[William David Howard]

Good suggestion!  Thanks, Mike.  I don't think I have one here, but I probably can come up with something or borrow one next week. 

Dave

[mike hadley]

Another one to try is wind the transformer up on your variac and series lamp, and check the ht output windings, they should be roughly the same within 10v or so, or another check is with another transformer run the heater windings backwards, this will give you the ht and ac input voltages, allbeit a little high as there is no load.
My aw15 had a shorted turn on the ht secondary, caused by the radio being stored in a damp garden shed, cos me 100 gbp,
So I wish you luck as these things can get a little expensive.
Mike

[William David Howard]

OK, the HV secondary windings produce the same voltage either side to CT within a volt.  The filament windings are not as close, 3.10 and 3.40 VAC to their CT.  The rectifier heater winding is not tapped, but it produces exactly 5 VAC end-to-end (116VAC on the primary.)

If this transformer has a shorted turn, I suspect it is presently a high-resistance short, given that the primary is only drawing 275 mA.  Maybe a corrosion path is forming.

Where do EH Scott restorers get these transformers rewound?

Dave

[mike hadley]

run the transformer for a while with the series lamp, does it get hot?, mine did, in less than a couple of minutes, first a familiar hot insulation smell followed by a feint crackling.
I don't know where you can get your transformer rewound as I assume you are in the USA, and I'm in the uk.
On the plus side the materials that are used to wind transformers today are much better than what they used back then

[William David Howard]

Thanks, Mike.  I'll do a heat test later this evening.  Right now we're about to start one of our famous thunderstorms, so I'm disconnecting my radios and PC.

Dave

[David C. Poland]

I forgot to clarify, powering with all tubes after reaping, the lamp immediately goes rather bright and dims as the B+ voltage builds, and still glows until I throw a switch to bypass the floor lamp.

Before you decide your power transformer is bad, I like the suggestion with no tubes to see if it gets warm (or hot) after a few minutes with the Variac providing moderate AC. If stays cool, give it another half hour. If still cool, then advance the Variac towards full voltage. Keep it on the dim bulb tester for protection in the event it suddenly shorts. If it stays cool, then I would proceed with restoration. The Scott power transformers seem to be pretty ruggedly designed. 

Of all the Scotts i have had and restored, only ever encountered one bad power transformer. My experience many years ago with an AW-23 I had recapped - things seemed OK until first time powering it up. Advancing the Variac past  about 40 volts AC and blew the fuse. Determined the fuse began to glow at about 40 volts AC. Ultimately determined the B+ lead to the speaker field coil was resting on the point of a sheet metal screw with just enough low current leakage to cook the power transformer primary. Guess that explained why the amp had a 20 amp automobile fuse instead of 4 amps when I first acquired it. Guess it took that high a fuse to keep it playing for some previous owner until the transformer primary failed. I still have that AW-23  and it is an impressive sounding radio (Laureate Grande).

[William David Howard]

Thanks David.  I did know what you meant since you mentioned you had B+.  At power-up, the filament circuit has a pretty low resistance with all 30 tubes heating up. 

Our severe weather has abated, so I'm trying the heating test.  After 10 minutes the transformer is staying cool and the smoke hasn't escaped.

Dave