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chokesrule 31st March 2011 06:47 PM

valve rectifier series resistors
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 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 31st March 2011 07:07 PM

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 31st March 2011 08:08 PM


Originally Posted by chokesrule ( 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 31st March 2011 08:18 PM


Originally Posted by HollowState (
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.


Cookj 31st March 2011 08:38 PM

Can't people read the datasheet?

Cookj 31st March 2011 08:45 PM

Paragraph : Minimum Limiting Resistance:

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



HollowState 31st March 2011 09:18 PM

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 31st March 2011 10:40 PM

I saw the value of 4,7kohms here

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 31st March 2011 10:51 PM

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 2nd April 2011 10:30 PM

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.

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