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tmhajw 1st September 2009 06:20 PM

rectifier limiting resistors capacitor input
Hi There
Do people routinely use limiting resistors on the rectifier anodes for their capacitor input power supplies?! They seem to be occasionally mentioned as a good idea but I never see them on any schematics!
As a matter of fact I've fitted 2x 100r 12w resistors on my we91 300b p/s (5u4g) as the (Hammond) Tx had pretty negligible resistance. They do run very hot!! I'm wondering about taking them off to save cooking some of the capacitors in there...
Any advice welcome

HollowState 1st September 2009 07:04 PM

I never-ever use resistors in the plates of rectifier tubes. All they do is needlessly waste power as you have found out. As long as your 1st (input) capacitor is 50uFD or less, any self respecting 5U4 should feel no pain. The only time these resistors might help is in the case of a sickly Chicom tube or an OOT Russian tube. But in this case, get yourself a good NOS name brand American or European 5U4.

Gordy 1st September 2009 08:50 PM

Really? Surely the rectifier manufacturers knew their products well, and if their data sheets say add resistance... then add resistance.

ChrisA 1st September 2009 09:03 PM

The thing to use is not a resistor but a thermistor. Thermistors are made to limit in-rush current. They are actually temperature dependent resistors. At room temperature they are about 100 or 200 ohms and then after a few seconds of current flowing they drop to 1 or 2 ohms.

Most vacuum tube rectifiers must be used with smaller size caps. The reason for the smaller caps is to limit the total amount of in-rush current. Resistors can limit the current but they are only needed for a few seconds. Thermistors solve the problem. You can use a larger cap if you use a curent limitor of some kind or if might be worth it in some cases.

Limiting the start up current is not a bad idea. It is best to let the tubes heat up before raising the B+ voltage. The thermistor helps here too. They cost about $2.

You can place the thermistor either in series with the primary (limiting the primary curent has the effect of limiting the secondary current.) or you can place them on the secondary or in both places.

You do have to size them carefully. Almost every switching power supply uses current limiting thermistors.

Geek 1st September 2009 09:17 PM

Only time I use resistors is making SS replacements for tube rectifiers... calculate the resistor value to equal the voltage drop of the rectifier in question at the specified current draw.

12W resistors will get hot in normal operation. At full dissipation in an enclosed chassis, you can melt solder on them.


kevinkr 1st September 2009 11:03 PM

I try to design my power transformer HV secondaries to provide the minimum required resistance obviating the need for external resistors in most cases.
Adding a UF2007 in series with each plate helps a lot too, whilst keeping the nice soft rectification characteristics of the tube rectifier.

I tend to use a lot more 5AR4 than 5U4 because of significantly the lower forward drop under high current conditions. The 5U4 is marginally more rugged than an old school 5AR4 (Mullard, Tele, Philips) but a lot more so than most modern production I have encountered.

Ignore the minimum winding resistance spec for the 5AR4 at your peril, it may explain why so many of them fail in service, particularly the generally very marginally specified modern production.

Obviously not exceeding the maximum rated input capacitor value helps a lot too, but it is not the entire story. I have seen plenty of instances where tubes lived with 100uF input capacitors and others didn't with just 20uF input capacitors. The difference was winding resistance in many cases.

tmhajw 2nd September 2009 12:08 AM

Interesting opinions as ever! Thanks for your replies.
Yes I reckon the two resistors could certainly cook an egg...!
The thermistor idea sounds a promising way to turn down the heat... Which one would you suggest? I did look at the RS webpage (UK), the ones listed as 'inrush current supp' had good current (2 or 3A!) but their initial resistance seemed too low (2-25r) and I wasn't sure what current capabilities the 100r 'ntc thermistors' had...enough for a we91?? Help please! Thanks

tmhajw 2nd September 2009 12:14 AM

By the way the 5U4G I'm using at the moment is a nos Japanese one- no idea if its good/bad/indifferent... and the input capacitor is a 10uF.

kevinkr 2nd September 2009 12:35 AM


Originally Posted by tmhajw (
By the way the 5U4G I'm using at the moment is a nos Japanese one- no idea if its good/bad/indifferent... and the input capacitor is a 10uF.

I think you are all set, not sure why you think you need the resistor in this application? Have you measured the primary and secondary dcr to determine whether you have a problem?

I am not a fan of NTC on the high voltage secondaries, they aren't really designed for operation at voltages above 250Vrms and if they malfunction at the higher voltage some messy things can happen. (First hand experience)

Boris_The_Blade 2nd September 2009 12:47 AM

Use PSUDII to find the actual RMS current through the resistors, instead of the average expected rectifier current (it will be much higher than you expect), which will explain the heat they can create. Rectifier data sheets spec'd the minimum plate winding resistances so it was easy for the users to avoid exceeding the maximum peak plate current for a given winding voltage, load, etc. I would avoid exceeding that at all, especially for new production tubes.

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