Post by Johnkenn on May 23, 2017 17:57:04 GMT -6
There are plenty of tubes that have been used in U47 inspired mics, and clones. The unique thing about the 408 is that it can happily run off of 1/2 the heater voltage provided by the unmodified U47 circuit. Wiring two 408a tubes in parallel, with their heaters wired in series solves two problems with finding a suitable VF14 replacement. 1 - the 36V heater voltage is divided, so each tube gets roughly 18V to the heater which is close enough to the specified 20V to work just fine. 2 - The output of the tubes running in parallel provides enough current to sufficiently drive the large output transformer.
Another interesting point is that since the tubes are run in parallel, there is no specific need to match them in any way. In fact, two different brands can be mixed without issue, maybe even to tailor the sound to a finer degree (I have threatened to experiment but never found the time).
As for noise, I do not believe the problem lies solely in the tubes. Why? I have built a couple of mics that seemed to be very picky about which tubes they perform well with, only to find those very tubes perform just fine in another mic. I even swore off WE tubes early on, only to buy a box of 50 for a good price and find that pretty much every one I've pulled out worked fine in the mics I have in my possession still. I genuinely think the problem lies around the quality and type of the dropper resistor used to derive the heater voltage. The sand-bar type resistors were problematic most of the time while higher quality, sinked resistors seem to resolve the problem. Getting more contact between the resistor and the mic bell may help too, by drawing more heat out of the resistor (they get ridiculously hot). Certainly, in my experience I have found most tubes to work just fine in my mics, free of the crackling and pulsing being discussed here.
WRT noise floor, 408a isn't the quietest tube, and running 2 in parallel can compound the noise issue, but in my experience noise is pretty acceptable for most applications, and those where noise is a problem probably beg for a more modern mic design anyway. One of the reasons Neumann started using emphasis at the capsule and de-emphasis in the circuit was for the 6dB HF noise advantage. Vintage circuits will carry some noise penalty no matter what. Also, the 408a wasn't designed for microphone applications, which doesn't mean it can't work great, it just means you have to accept any variables that may find their way into the equation, and more relaxed QC is obviously one of those.
I'd also be glad to try someone else's noisy tubes to see how the translate from mic to mic. It may end up revealing some other important details.
If STAM is releasing a commercial product based on the dual 408a design I guarantee he is off Max's Christmas list.
Good to see you here, Shane...