Tube rectifier current limiting resistors?

[fyzygy]

In rectifier data sheets and over at Valvewizard, and elsewhere, it's pointed out that tube rectifiers require a minimum series resistance, in order to limit current. I don't see that resistance added in any 18watt schematics. The amount required would seem to vary according to DCR of the transformer windings, and HT voltage. Just wondering if anybody here bothers adding current-limiting resistors to their EZ81 power supplies? I get a bit lost in the math provided by Valvewizard, so any help understanding it would be appreciated.

[JMPGuitars]

First, ask why we would need that. What's the point? Current protection for the rectifier tube, correct?

Review the EZ81 datasheet and you'll see the max current the EZ81 can handle.

Some 18 Watt PTs also have datasheets, and you can see what the secondary current maximum output is. For example: https://www.hammfg.com/electronics/tran ... ic/290.pdf

The Hammond supplies 180mA maximum output on the PT secondary feeding the rectifier tube. I don't recall seeing anything claiming much more than that.

I checked a few EZ81 datasheets, and the plate current limiting value is typically 450mA to 500mA.

Thanks,
Josh

[fyzygy]

OK, thanks for the quick reply. If I understand correctly, given the modest demands of the 18watt circuit, the EZ81 is well within safe operating margins, even without the added series resistance. So then ... it's not a matter of max/peak/inrush current that puts a strain on rectifier tube, potentially shortening its life, which the series resistance would help prevent? Also, wouldn't the "sag" resistor in some guitar amp circuits serve a similar purpose? Or put the other way around, wouldn't current-limiting resistors in the anode or cathode legs of rectifier cause the power supply to sag? I suppose my question was motivated by comments I read online, suggesting that EZ81/6CA4 is unreliable. I'm working on converting an old console stereogram, which did have a series resistor installed, according to a handwritten note I found after I'd disassembled everything.

[JMPGuitars]

The EZ81 max current is at least around 2.5 times the max current produced by the transformer. Those are max levels. It's just not an issue.

The only EZ81 I ever had fail was a JJ. I don't trust any JJ tubes anymore anyway. I don't know where people are getting their ideas about tube reliability.

Anyway, I mostly build solid state rectifiers because they are generally more reliable and cheaper than any tube rectifiers.

PTs will usually have some sag regardless of the rest of the stuff you're contemplating. A sag resistor is usually used in place of a rectifier tube, I don't usually see a sag resistor with a tube rectifier. But you could do that if you wanted more sag for some reason.

Different tube rectifiers give different levels of sag, but so do different resistor values.

I've built some amps with both SS and Tube recs. The voltages were higher (as expected) on the SS rec. Tell me if you really think one sounds better than the other:


You also need to ask how much sag you actually want. It's a style dependent thing. Blues wants a little more sag, speed/metal/shredding want little to no sag.

Thanks,
Josh

[fyzygy]

Good to know, thanks.

If you swapped out an EZ81 for SS diodes, you'd get increased HT because of the lower forward voltage drop. But wouldn't you also get an increase in heater voltage because of the 1A @ 6.3v load reduction?

On the question of sag, that's only relevant for PP designs, not SE, is that correct?

[JMPGuitars]

Yes, I use Zener diodes to reduce B+ voltage. See the thread in my signature on the topic.

The rectifier heaters have a separate supply on any good PT.

Regarding your other question about sag: https://www.aikenamps.com/index.php/the ... on-class-a

Thanks,
Josh

[Bieworm]

I like SS rectifiers for their higher voltages. That enables me to build 6V6 amps with 36 watt power transformers and still yield my favourite 370 V B+. With the big power transformer I get big tone and still squishy enough.