Rectifier Flashover WAD Kits 34, 6550, 2A3

[FAQ Team]

Kit/Kat 34, Kit/Kat 6550 and 2A3PSE Rectifier Flashover - Fitting Balancing and Surge Resistors


Kit/Kat 34 and Kit/Kat 6550


Some owners have had problems with the valve rectifiers flashing over, usually at switch-on when surge is greatest. Being directly connected to the mains TX it would be possible that a subsequent valve failure could damage that too.

In these amps, parallel rectifiers are used to cope with current demand that cannot be handled by one rectifier. It's therefore important that both plates do an equal share of the work or one will be overloaded.

A look in Radio Designers Handbook gives notes for balancing resistors when using paralleled rectifiers, "When two units are connected in parallel it is also desirable to obtain equal sharing, and in such cases a resistance of 50 or 100 ohms should be connected in series with each plate, then the two units are connected in parallel."

By way of explanation; if one plate passes more current than the other its 47R resistor will create a larger voltage drop and thus the other plate will then have relatively more potential applied (as they have a common load) and will conduct more creating a see-saw balance arrangement.


Suitable resistors would be Welwyn 3W wirewound (such as RS 139-3425 about £5 for a pack of 5) added at pins 4 and 6 of the rectifiers as in the attached drawing.





2A3PSE


This amp used a single 5U4 rectifier and some owners have also had problems with flashover. The 5U4 has a maximum first cap of 32uF with 75ohms Rt. It is used in this amp with 100uF first cap so the assumption must be that the TX impedance is greater to allow this. It may be that things are a little near the limits for some valve samples. This can be fixed in a similar manner by fitting resistors at the anodes (pins 4 and 6 as above).

As an example, these resistors are shown in the original Mullard 5-20 circuit (to ensure minimum Rt) and 125R 6W are suggested for GZ34 with a first cap of 8uF.

For 2A3PSE, 47R would seem a sensible starting point. They will run hot so use 6W wirewounds mounted in free air.


All


Both applications will also act as surge resistors at switch-on. At switch-on the first cap is empty and causes a long, high drain (near short circuit) whilst it fills. Indeed, the impedance of the cap is very low and the only thing stopping the rectifier shorting the mains supply is the impedance of the mains TX and the valve itself.

Adding resistors increases this impedance. The initially larger current causes a correspondingly larger voltage drop across the resistors, so causing the cap to fill more slowly and reducing the surge.

A side effect in both applications will be a slightly lower HTv which may be useful in some areas if the mains is high.

It is worth noting that WAD will have tested at the design stage and many owners will have had no problems, perhaps due to well-matched sections in their rectifiers. It will therefore be up to the individual whether or not to fit resistors if they have not had problems but it is "good practice" in the case of balancing resistors.

Fitting an Inrush Current Limiter

Further to flashover and very particularly to the 6550 but applicable to all the WAD/WD amplifiers, if the mains fuse blows repeatedly on switch especially in high mains areas, this can be eliminated by fitting an NTC surge/inrush limiting resistor in series with either the live or neutral of the mains input. If you have the fused IEC variety then it can be wired in on long leads, suitably wrapped around the tags in this case as it gets hot, between the two tags on the IEC socket where you would normally have a a blue or brown wire link