tube rectifier limiter resistors

[tubeswell]

Hi there fellow tube amp junkies

I am curious to find out if anyone here uses limiter resistors between the HT winding and the rectifier anodes? (I've never used any to-date, - although I do build mine with 1000V 1A diodes in there for protection, but having read that not putting in limiting resistors is a big no-no on the valve wizard site, I'm curious about contemporary amp building behaviour)

[6267]

I've only seen that on old radios...the resistors were fairly large. It seems inefficient to me, but I guess it would allow the following caps to be larger. It might also be to damp some kind of resonant circuit? I've seen people fix the frequency response of OPTs with resistors in one or the other primary lead, particularly for squarewave response in hifi amps. I'm not too deep into transformer theory yet, so I didn't quite get it.

[Enzo]

I don't think ther are any resonance issues with power transformer windings.

One web site might demand always putting these resistors in the circuit. How many commercial amps have been made over the last 60 years without them, but managed to work reliably all these years without them?

Resistors either drop voltage or reduce current, depending on what you are looking at. If I had to guess, I'd say those would reduce inrush current when the tube faces empty caps.

[Bruce / Mission Amps]

Resistors on the Hi-V secondary, used before the rectifier tube, is a fairly common thing found in old vintage audio amps.
Sort of like a current surge, shock absorber for the rectifier tube.
Don't use too big of R value or the B+ will suffer and they will get hot.

[Steve Conner]

Hi guys,

I think designers preferred to build the necessary resistance into the PT by just using a thinner gauge of wire for the HT winding. It eliminates two parts, saves money on wire, and may allow the whole PT to work out smaller.

The Mullard GZ34 datasheet says something like "If the resistance of the PT secondary is less than whatever-ohms, external resistors must be added..." The purpose is to keep the peak current under control and prevent early death of the cathode emission.

[mooreamps]

They absoluty would drop in-rush current for an empty cap, assuming the HV caps always discharge on power off.... .... However, in the Fender layout, those caps are always left charged, to help preserve cap life.... When you look at the data sheets for rec tubes, they will always spec a max value for the first filter cap after the rectifier tube. In a push/pull amp, you can get away with the published value. But for a SE design, and if you want to use a bigger filter cap for SE, then that would be reason to do in-rush current limiting.

-g

[clyde1]

What's the difference in SE and PP power supply design that would necessitate a resistor. Isn't a cap a cap? Thanks.

[Enzo]

The difference is that push pull output stages will cancel the B+_ ripple on the plate supply. SIngle ended amps can;t do that, so any B+ riple becomes hum in the output. One is then tempted to increase the size of the filter caps to squeeze the ripple down as far as possible. but this would stress the rectifier tube. So in that case the surge limiting resistors would be added to protect the rectifier tube from the increased cap size. The p-p output has smaller filtering requirements and so the resistors would not likely be necessary.

[R.G.]

They absoluty would drop in-rush current for an empty cap, assuming the HV caps always discharge on power off.

One good way to eliminate the inrush surge on power on is to put in an inrush limiting thermistor. These are available in all ratings specifically to limit inrush surges. Put in the primary side, they throttle the primary line current a bit until they heat up, limiting the inrush surge. When they get hot, the resistance drops to a very low value which does not impede AC line current much.

MOVs are very handy devices too. Transient protection MOVs can be put across the primary of the OT, where they do ...nothing... until a spike happens across the OT primary. When that happens, they switch to a conducting state in microseconds, eating the transient spike energy and preventing arcing of output tubes or sockets. Neat devices.

Both inrush thermistors and transient MOVs look like beefy ceramic disk caps, cost about $1-$2 each, and provide major protection for the money. Highly recommended.