You briefly mentioned input transformer damage on V72s due to improper phantom powering. Would you please elaborate?

You briefly mentioned input transformer damage on V72s due to improper phantom powering. Would you please elaborate? What exactly is the damage and what causes it? i.e. hot patching, turn on/off transients, leakage current due to mis-matched resistors? I have a client that has eight that he wants to power and I want to do it right.

Phantom Powering

One of the biggest concerns with these historic microphone pre-amp input and output transformers is the phantom powering. Developed in the beginning of the 60s, and initially a sub-standard of powering microphones, this has now moved on to become the industry standard, applying 48V via two resistors to each leg of the modulation (pin 2/3) so there is no potential difference and using the shield as return path/0V.

Theoretically this shouldn't pose a problem for a none center taped transformer, but in practice there is always the capacitive property; primary to core or the Faraday shield, then also the primary windings to secondary windings etc. These will become charged if 48V DC is applied and the transformer (standard tube input x-former and tube mic outputs have ratios from 1 to 10 and even higher) acts like a ignition coil creating approx. 1 to 2KV!! on the transformers secondary. Mic pres with no damping resistor like the V76 (input transformer has 1 to 30 ratio) can even create up to 3KV!!!! this is enough for a millisecond energy burst that is powerful enough to burn microscopic holes into the insulation.

It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insolated with additional material like oilpaper, etc.

Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer. Some of them are already gone due to the cracking of PVC (like the good old M7) over the years.

DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and sometimes even make it worse due to resonance spikes and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.

If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constant of more than 5sec. This will take care of the problem posed by this energy burst, simply by having a slower time constant than the capacitive property of the transformer can charge. There is still a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, which had a maximum shelf life of approximately 25 years.

Hope that answers your question,

Best regards,

Oliver

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