Is this output transformer broken?

[JGP]

Hi, I have an Ampeg r-212r. I picked it up used, it's a reissue that I'm guessing is about 10 years old. I'm assuming that the OT should be center tapped on the input side. Measuring from center to each leg I'm getting 101.9 and 71.4 ohms. The windings are not grounded to the chassis and the amp seems to work otherwise. I'm not that experienced but the OT on my pro junior is balanced at about 90 ohms each leg to center. I don't have a schematic for the amp but this seems wrong. Is this OT useable? If so, is there a work around to compensate for this mismatch. Thanks for any help.

[John Phillips]

That would normally make me think it had a partial short (the usual mode of failure in OTs) on the lower-reading side, but not long ago I was surprised to discover an OT in a modern Fender amp that was 'mismatched' by nearly that much, and was perfectly OK.

There is always a mismatch in the DC resistance even if the turns numbers are exactly the same - the winding is put on from one end to the center tap, and then from the center tap to the other end, so the second half is wound over the first and must therefore use more wire length as its circumference is greater - so the DC resistance will measure higher. Normally - with most of the transformers I'm familiar with - the difference is in the order of 5% or 10% at the most, so a difference of 20% made me think the transformer was fried, but measuring both the input and output powers to it confirmed that it wasn't.

If the amp seems to work normally, it's almost certainly OK. Any short (even one turn) in the OT will produce a serious loss of power and probably obvious tube stress when it's cranked.

If it is damaged, there is no workaround though - the transformer is dead and will need replacing.

[JGP]

John,

Thanks for your reply. The amp seemed to be working properly, I just wasn't enjoying the sound, I am more used to el84 amps, currently own 2 and have sold another. Decided to do further testing, the voltage followed the resistance, 1 side lower than the other. But the idle current evened out for each 6l6. Guess that ohms law does work. Seems that the idle current was 52ma w/ power dissipation of about 20w. Adjusted the bias pot, went to 35ma and 14.1w for each 6l6. Amp sounds better now. Just curious, what is the accepted range for newer 6l6 tubes?

[John Phillips]

A 6L6GC can take up to 30W, so setting the bias at up to 70% of that (21W) is acceptable in a Class AB amp. In practice I don't think they sound good anywhere near that hot, and I base the settings on what a 6L6GB (22W rating) should be biased at, ie about 14-15W.

In other words, exactly what you've already done :).

This also gives much better tube life since the tubes are operating more conservatively, which IMO is the right reason for using higher-rated tubes, not to allow you to run the amp too hot ;).

Of course, if they are GBs (or equivalents eg the modern 5881-labeled tubes), 15W is plenty anyway...