trying not to exceed 6X4 ratings

[EETStudent]

My PT puts out about 360vac on each leg, and a 6X4 shouldnt exceed 325ish on each plate with a capacitor filter. So I rearranged it as a choke filter, where my datasheets tell me the 6X4 can handle around 400vac on each leg. I got A LOT of noise with this setup. I then changed back to solid state setup, still choke filtered, and no noise. Is this a bad tube or am I wrong somewhere else?

[Steve Conner]

A bad rectifier tube will usually either result in too low B+, or else it'll just arc internally, blowing your mains fuse or even your PT if you're unlucky. So no, I don't think your tube is bad.

For a choke filter to work properly, the choke must be bigger than a certain value, called the critical inductance. If you're below this value the B+ will shoot up to what it would have been with a capacitor input filter.

As for noise, you can expect the choke in a choke filter to buzz loudly and give off a strong hum field that will get into your guitar pickups. Much more so than in a capacitor input filter.

But if you're finding that the tube rectifier/choke filter combination makes your amp more electrically noisy than the silicon diode/choke filter setup, even without a guitar plugged in, I'd be worried that you might be on the way to a heater-cathode breakdown. It's the only factor that's different between the two setups.

Just put it back the way it was and it'll be fine. Tube maximum voltage ratings are negotiable If you're worried about the rectifier tube, make the first filter capacitor smaller.

Finally, if you're a real hardcore geek, you may want to check out the resonant choke filter...

[EETStudent]

Actually, I swapped the tube rectifier for the silicon diodes, but I still kept the choke filter. The only difference in the two setups is the rectifier type. Could a heater-cathode breakdown still be a possibility? Speaking of which, was it a heater-cathode breakdown?

[EETStudent]

Oh wait my bad Steve, I see you did recognize that the only thing i changed was the rectifier type. So, what is a heater-cathode breakdown?

[Bruce / Mission Amps]

For what it's worth, here is a well known (but recently forgotten for lack of use) secret using a 6X4 or EZ90s... use a 2 watt, 330-390 ohm resistor in each high voltage secondary lead to the rectifier and the thing will last a LONG time, even at 350vdc to 375vdc and +50ma to 75ma per leg.

[EETStudent]

you know Bruce I was thinking of doing just that! Seemed practical, just wasnt sure if it was recommended. I think Ill revert back to the capacitor filter setup and use resistors to accomodate the 6X4. Thanks!

[Bruce / Mission Amps]

you know Bruce I was thinking of doing just that! Seemed practical, just wasnt sure if it was recommended. I think Ill revert back to the capacitor filter setup and use resistors to accomodate the 6X4. Thanks!

Well, if you were as old as a few of us here.... you probably would have seen a bunch of these 6X4 type rectifiers actually in use.
With respect to Hi-Fi stuff, was common to put series resistance in the high voltage secondary leads to protect the rectifier diodes from abusive in rush current.
6X4s, by most of the manufacturer's specs, would like to see a few hundred ohms of DC resistance in each leg, before the tube to buffer that current inrush.
This total resistance, of course, would also include the DC resistance of the high voltage secondary winding, which is rarely really more then 75-150 ohms or so.