Opinions on a 45216 OT on a 61 Bamdmaster?

[Redwood Bridge]

My Bandmaster has a 45216 (4ohm) OT rather than the spec'd 45217 (8ohm). Date codes and amp history makes me think its original. The amp has a single 12" tone ring cabinet. Given the price of over $1500 Canadian that Reverb is quoting for a used original working replacement 45216 and since this is my "go to" gigging amp would you suggest I keep using it and hope it never blows or should I preemptively swap it for a new repro?

[Redwood Bridge]

I should also mention that the amp does have an ext spkr jack so it could have been anticipated that 2 8 ohm cabinets would have been connected resulting in a 4 ohm load. Also, I am running TungSol 5881s biased at abput 16 watts and the speaker is currently a Celestion Vintage 30.

[pdf64]

The effective primary impedance of the OT will be about 8k. But as the inductance will only be intended to support 4k, the high power bass roll off will kick in at twice the normal frequency.
Consider that the Marshall JTM45 use a pair of 6L6 family beam pentodes with a 8k OT.

[Stevem]

Is your vintage 30 a 8 ohm or 16?

Anyway if it’s 8 ohms then this impedance difference is calling for like 30% less current out of the OT then if you had a 4 ohm load tacked onto the amp.
This makes the chances of popping the OT that same 30% less, of course you also have 30% less wattage output available out of the amp also!

[Redwood Bridge]

Thanks guys...thats in line with what my thoughts have been. I used to use 6L6GCs in the amp with no failure of the transformer so I think I am probably pretty safe with the lower powered 5881s (for which the amp was designed), and I like their earlier breakup. I have always found the amp to be loud enough in medium gigging venues and could always mike it if need be. I have toyed with the idea of pulling out the tubes and just applying an A/c voltage to one side of the transformer to see what I get out the other side which would let me be pretty certain as to the actual impedance ratio and thereby the primary impedance presented to the tubes. Or I could just leave the darn thing as is LOL.

[pdf64]

… I have toyed with the idea of pulling out the tubes and just applying an A/c voltage to one side of the transformer to see what I get out the other side …

If you do that, be sure to disconnect and tape off the primary CT of the OT whilst doing the test

I think OTs generally fail as collateral damage, rather than being the initial point of failure. With shorts / arcs inside or around the output valves being the issue that stresses the OT, eg due to excessive primary current being drawn by the short / arc.
With your amp, if the OT is intended for a 4ohm load but you’re running it with an 8ohm cab, high power levels will tend to put more stress on the screen grids than there should be. And screen grid shorts are a common output valve failure mode. So you may wish to consider increasing the value of the screen grid resistors, to mitigate that additional stress. eg 1k 2W.
There are various other mods you might consider, to protect those old transformers, eg HT fusing.

[Redwood Bridge]

Thanks...thats a very good idea (lifting the centre tap).