allwave fifteen audio driver transformer

[mike hadley]

Hi,

Can anyone please tell me the dc resistance of the windings between the first audio stage and the 56 triodes, one of the secondary windings has a slightly different resistance to the other,

as I have found bad tubes in this area, I want to see if all is well before I proceed,

Thanks

[Norman S Braithwaite]

I don't have the DC resistances for you but a slight difference in the DC resistance of the windings is normal.  The number of turns on each half of the secondary are matched but one half is wound outside of the other half.  As such, the outside half will be of greater diameter than the inside half and the DC resistance will of the outside half will be greater than the DC resistance of the inside half.

Norman

[mike hadley]

Thanks Norman,

The power supply transformer has these characteristics, the problem I have is the radio was stored in a very damp condition, causing some rust and flaking of the chrome, even some delamination of the sides of the tasman cabinet, damp has got in everywhere.

I had the main transformer develop problems because of this, and had to have it rewound, not cheap.

As I'm slowly working through the set I have already found the audio 57's in poor condition with one not working at all, checked on my tube tester confirmed this, not sure if this is due to age poor caps or a serious fault.

I would like to check if all is ok before I proceed as if there is a fault the transformer will have to come out.

Mike

[David C. Poland]

One thing to do with suspicion of damp history, is to pull the rectifier and most tubes. Power is up on reduced voltage - say 60 or 70 volts AC  using a Variac or perhaps a dim bulb tester (using a low wattage lamp bulb - say, 15 or 25 watts incandescent) and let it run for several hours. An old fuse may fail fast. If the fuse blows twice - then of course the transformer is likely bad. Otherwise with the light load, the transformer will warm up somewhat in a low stress environment and tend to drive out moisture.  Repeat a couple times. While being run like this you can take some secondary winding measurements to verify all windings are intact.  Then after a couple days, pull all tubes, then try full house voltage and repeat measurements.

By the same token -- When I complete a restoration, I first power up with most tubes but without rectifiers. Then with rectifiers and start with Variac at about 60 volts AC with a volt meter connected to monitor the B+ circuit after the first choke or speaker field coil. The B+ should rise to a moderate peak and then 

fall back a bit. Then after a few minutes, I advance the Variac to about 90 to 100 volts watching B+ on the volt meter - hoping to hear static or a station or try a hum test touching the first audio grid.

My first Scott AW-23 years ago - power transformer primary seemed OK until I applied AC - it shorted at about 40 volts AC. Discovered this problem after recapping.

[mike hadley]

Hi David,

Totally agree with the warm up process, the radio was working after a fashion, I reconed the speaker, and the power transformer went up, then after having the transformer repaired, the set ran long enough for me to be stunned by it's performance although the chassis was very "tappy", now there is nothing, that's when I found the dead tubes, the if strip local oscillator all appear to be ok, just waiting for some tubes to arrive here in the uk to sort things out.

Mike

[David C. Poland]

old tubes eventually do wear out from use.

[mike hadley]

Today I measured the transformer, primary is 3k4, secondary windings are 2k6 and 2k2, all are Dc resistances, the inductance meter I have gets upset by the high resistance of the windings,
So if all is ok the transformer looks like 1.4:1+1 which is believable for an interstage transformer.