valve rectifier series resistors

[chokesrule]

Hi, some double anode rectifier valves like the addition of series resistors, to bring the total resistance of the transformer/anode circuit to a certain value.....eg 5u4c likes a series anode resistance of 4.7k ohms.Why is this resistance necessary? Is it to protect against too much current? Is it the case :- the higher the resistance in the power line: the greater the loss of dynamics?

 

[boywonder]

Yes, without a minimun series R the rectifier will arc over on start-up when charging the empty filter caps, same reason the first cap size is limited and a function of the rectifier chosen. The series R increases the output impedance of the supply which can impact dynamics, and cause B+ sag, etc during transients.

 

[HollowState]

.....eg 5u4c likes a series anode resistance of 4.7k ohms.

First, 4700 ohms series resistance is waaay too high a value. Even 470 ohms is a lot. Second, if one needs a series anode resistance to protect the tube, then usually something else is wrong IMO. Either the tube is the wrong type for the task at hand, the input capacitance is too high or the cap is defective (leaky), or the transformer's seconday current capacity is too high.

In all my years of servicing and working with equipment, maybe only once did I see series anode resistors for the recitfier. And that's a big maybe. However, given the sorry state of today's inferior tubes, I can understand the use of these resistors. But if everything was done correctly, resistors should not be necessary.

 

[Rundmaus]

But if everything was done correctly, resistors should not be necessary.

Nearly all tube rectifier datasheets advise the use of series resistors, especially for cap input supplies. Usually there is a specified value for the total resistance needed in the plate circuit, part of it is already given by the transformer series resistance.

A state-of-the-art transformer will probably have a low series resistance, so resistors are not so unusual - given one sticks to the datasheet operating conditions and does not want to abuse the rectifiers.

Greetings,
Andreas

 

[Cookj]

Can't people read the datasheet?

 

[Cookj]

Paragraph : Minimum Limiting Resistance:

http://www.freewebs.com/valvewizard/fullwave.html

Sorry for the previous short reply - I'm feeling abit crabby

Regards

John

 

[HollowState]

The RCA Receiving Tube Manual, RCA HB-3 Handbook, Sylvania manual & GE Essential Characteristics all mention a "Total Effective Plate-Supply Impedance per Plate" value. With typical values all under 100 ohms for types 5U4, 5Y3, 5AU4/5V3A & 5V4. This value is easly met by the transformer secondary. No mention of external series resistors is found in these above publications. Smaller tubes like the 6CA4, 6X5 & 6X4 do require higher values of series resistance. But in the Amperex Tube Manual a series resistor of 200 ohms is recommended for the 5AR4 if not provided by the transformer.

 

[chokesrule]

I saw the value of 4,7kohms here

http://taooftubes.blogspot.com/2010/04/5u4c-russian-rectifier-tube-data.html

scroll down the page....

Google "5u4c datasheet" and no actual pdf link appears...tao of tubes is all I can find.....I am using the valve with no series resistance , it was finding the "valve wizard" site that got me to ask the question.

It seems to be running great.I just got worried that I had made a time bomb.If Hollow State thinks 4700ohms is way too high, then I think I might chose another tube ; one where I can actually get the data sheet and not have to read information which has past thru a secondary source. Thanks for the help.

 

[boywonder]

CR: Check out Duncan Amps TDSL pages. Query for a tube type, then look at the bottom of the page for numerous links to datasheets.

TDSL Tube search

 

[trondareo]

Here is the russian source (PDF) I got the data from, for writing the TAO of Tubes translation.
It is fully possible that the data is wrong, we will need to cross check from several sources.
I have some 5Ц4С tubes, but I have not had the chance to test them in practice yet.

 

[trondareo]

I have found this circuit (PDF) and this circuit using the 5Ц4С
I have also added a warning about the value on the TAO of Tubes post.

 

[prairiemystic]

It depends on the type of filter you use, the resistor is to limit peak currents in the rectifier. We always talk about average rectifier current but it's the peak values (@50/60Hz) that are not mentioned in the datasheets, affecting tube life. An L-C filter or a C-L-C filter is much easier on the rectifier tube than a C filter, especially with people using huge filter caps nowadays.

 

[minhaj]

hi, may be my input will not be useful, though I could not resist myself from speaking.

This is about rectifier perimeter/data, how you design your power supply and what you want the PS to do and load of the following circuit.

4700 ohms is way to high for most of the circumstances no matter what your data sheet/circuit says. I am sorry if I sound harse.

Regards

 

[chokesrule]

another 5u4c datasheet

hi I have found another datasheet

''GSTube.com''. Tubes, sockets etc. Parameters and characteristics 5z4c

it says the same as the previous link 4.7k..... maybe 5u4c just is not suitable for audio and safe?However , I am still using mine with 150r ohms anode circuit resistance provided by the centre tap transformer it seems to be happy.I emailed the lampizator who i basically copy and he said just add 100 ohms if you are worried....

 

[TG]

Oh c'mon.
5Ц4С's data just says that being fed with 2x500Vrms and loaded by 5uF capacitor and 4.7kOhm active (resistive) load this tube (average sample, with nominal heater voltage) will generate 122mA current through this load (i.e. will provide 0.122*4700=573.4V on this load).
It has nothing to do with series resistors, current limiting and all that stuff.

 

[Miles Prower]

It depends on the type of filter you use, the resistor is to limit peak currents in the rectifier. We always talk about average rectifier current but it's the peak values (@50/60Hz) that are not mentioned in the datasheets, affecting tube life.

This is not true. When I was designing a hollow state power supply, the spec sheet for the 5U4GB (the type I chose for this project because I had a PTX that overvolted badly with Si diodes, and for which the PTX seemed to have been designed) gives a peak repetitive current per plate (Isurge) of 1.0A, and a hot switching peak limit of 4.6A per plate. Given the need to ditch about 100V, from the plate characteristic, an Isurge= 0.8A did nicely. A 34uF (two 68uF / 300Vdc units in series with 100K equalization/bleed resistors) reservoir capacitor gave an Isurge quite close to that value. I used an LC LPF after that where L= 7.0H and C= 220uF. Design nominal DC was 350V; measured DC was 352V. The output o'scoped clean with any residual ripple under the noise floor. The only series resistance comes from the PTX itself.

The only mention of series resistance is in a section for operation with a C-input filter and hot switching. For the project I designed, if it had hot switching capability (which it doesn't) the series resistance comes to about 30R. Nowhere do I see mention of anything like 4K7 in series with the plate. Given the Isurges encountered with this type, the voltage drop would be:

(4700)(0.8)= 3760V

That's absurd.

An L-C filter or a C-L-C filter is much easier on the rectifier tube than a C filter, especially with people using huge filter caps nowadays.

You should never use a C filter with any hollow state diode, unless it's for very low currents and where ripple isn't a factor, like, ferinstance, a CRT HV power supply. Since it's almost always running at high frequencies and low currents, what do you care if there's ripple? You can't see flicker at a 30KHz rate anyway, and types like the 1B3GT don't need to source much more than one milliamp anyway. So a filter C= 470pF will do just fine here.

Back in "the day", they may have gotten away with it. I actually had a record player amp that used PP 6V6s, and a 5Y3GT with a 100uF capacitor connected across it. They were counting on a tech-illiterate public at large to not notice that replacing the 5Y3s every 4 -- 6 months was not normal. Besides, you could get replacements at just about any pharmacy, grocery store, convenience store, etc., so what did they care?

 

[quikie22]

Oh c'mon.
5Ц4С's data just says that being fed with 2x500Vrms and loaded by 5uF capacitor and 4.7kOhm active (resistive) load this tube (average sample, with nominal heater voltage) will generate 122mA current through this load (i.e. will provide 0.122*4700=573.4V on this load).
It has nothing to do with series resistors, current limiting and all that stuff.

+1

 

[trondareo]

Added the link chokesrule found, and a link to this discussion on my post at TAO of Tubes.

 

[prairiemystic]

I am talking about people using several hundred uF filter caps on a C-input filter with 5U4's. The power-up flash from cathode-stripping is impressive.

The 5U4-GB datasheet (pdf) is a bit confusing. Maximum ratings:
"Transient peak plate current per plate, maximum duration 0.2 second = 4.6 amperes"
"Steady-state peak plate current of 1.0 ampere maximum per plate." The graph shows 120V drop @ 1A... I call that a supernova

Steady-state, I'm not sure but I think the 5U4-GB is plate dissipation (thermal) limited and I thought thermionic emission limits the dynamic (8.3msec) repetitive peak current, affecting lifetime too.
For a capacitor-input, full-wave center-tapped rectifier, my texts show peak current is 6-10x average rectifier current depending on the power transformer's sec. resistance and filter cap size.
One can easily scope rectifier current by putting a ~10ohm resistor in series with the center-tap of the power transformer secondary (which usually goes to ground).

 

[richwalters]

Using a 5U4 with a massive C filter value is an unfair impedance match.....over 100:1. In fact I remember rectifier tubes at that time often being the first failure casualty even with considered soft low uF cap resevoir circuits.
Do what manufacturers did in the late 1960's, go to sand. Only way round it.

Those curious about series resistors in each rectifier anode leg, the value of the resistors should equal the DC resistance of each HT winding side. Or at least that's how it used to be applied.

richy