resistors as inrush limiters before full-wave diodes?

[brilliantblue]

Hi there! I've determined to eliminate the rectifier tube from a small amp project, going with two diodes for a full-wave rectifier. The original rectifier (6ca4) eats too much filament current for the transformer, and my attempt to use a 6x4 in its place resulted in too low a B+ voltage.

I'm wondering how to easily deal with the inrush voltage from diode rectification.

Can I effectively tame this by putting 5w+ resistors in each leg of the HV prior to rectification? And if so, what would be your suggested values/ratings?

Tube compliment = 2 6AQ5's, 1 12ax7, 1 12at7

Thanks for your time and help!

Jeff

[Jazz P Bass]

I don't see inrush as an issue.
That would more or less depend on the capacitance of the circuit.

Inrush.
The heaters don't care.
The plates & cathodes don't care either.

You will most certainly have a higher B+Vdc.
That may be more of an issue if it throws off the preamp.

[brilliantblue]

I'm sitting at 245v on the 6aq5s - voltage is fine though I've yet to install the preamp circuit. I was simply concerned about the startup voltage because it hits 370ish when I turn it on and then settles to 245 after a few seconds. Is this enough instant voltage that i need to worry about cathode stripping?

[Tom Phillips]

I'm sitting at 245v on the 6aq5s - voltage is fine though I've yet to install the preamp circuit. I was simply concerned about the startup voltage because it hits 370ish when I turn it on and then settles to 245 after a few seconds. Is this enough instant voltage that i need to worry about cathode stripping?

I'd say no worries about cathode stripping at all in this situation. However, That's quite a large voltage difference between the start up and the settled value. My thinking is that there is excessive resistance in the power supply somewhere or the PT in under rated for the amp's current requirements.

[Mick Bailey]

You could use an NTC inrush limiter. 6A 8 Ohm is a common value. You could drop the current rating and increase the resistance to tune to your particular needs. Warmup time to full voltage is around a minute and they give a really nice soft start - very close to a tube rectifier warmup. Resistance drops to just milliohms when hot - check spec sheets.

The device needs to run hot to operate correctly.

[brilliantblue]

I'd say no worries about cathode stripping at all in this situation. However, That's quite a large voltage difference between the start up and the settled value. My thinking is that there is excessive resistance in the power supply somewhere or the PT in under rated for the amp's current requirements.

It's a hammond transformer from an organ reverb amp. I've read from others repurposing them that they are "soft" in that their free voltage is much higher than the loaded voltage they were implemented with. This one was originally giving 300ish to two 6gw8a after the 6c4 rectifier.

[Enzo]

This is a small low power amp, none of these concerns apply to it. It is like worrying if you should put high speed tires and full racing suspension on your mom's sedan.

A typical 1N4007 rectifier diode has a 1 amp rating but a 30 amp surge rating. I don't think your circuit could draw that much nor your transformer supply it, so no inrush concerns are needed.