I have an HMV Little Nipper (model A13B). I am yet to commence restoration but on inspection noticed that someone has replaced the 6X5GT tube with two diodes across the pins underneath the chassis. Not sure if any other alterations were done to allow this.

Question is do I leave as is (assuming diodes are good) or remove and replace with a good 6X5 tube. I know that the 6X5 is notorious for shorting and that is probably why this alteration was made but I have also read that the diodes can short too and a high watt resister of say 100ohms should be added to the B+ line to drop the current to be more like the 6x5 tube.

Any suggestions? I was told the radio was working with the diodes but I won't turn it on until all the electro caps and paper caps etc are replaced.

Regards

It is easy to see if the thing has been modified to accomodate the diodes. Check the circuit against the original. I would bet that they have used the socket pins not used by the rectifier plates of the 6X5, as terminals of convenience.

I would hope that they are 1N4007 or better as 400V diodes will die. 6X5 itself had certain design features in several version, that had (particularly in Zeniths in USA, which used a lot of them) a profound habit of developing a heater cathode short and destroying things. That may have indeed been why this mod. happened.

Not a good recommendation for a Valve designed for car radio's and it should have been a lot more robust. There are better.

6X5 is fairly efficient & will yield around 450VDC from 375VAC. However, I would expect a greater voltage from diodes and that can be predicted. and if the set is actually operational, which I doubt. it might be measurable.

The greatest risk lies in the fact that the Silicon diode will behave like a 5Y3 and produce a considerable surge voltage, until there is conduction of the heater valves. If the old caps are in there and do not have the standard 525V surge Voltage; There is every likelyhood of them "punching through".

This is what may have killed the set and the dropping resistance that is required to be added, is usless until the set is drawing current..

I would not bother to turn it on until after you have looked at the general condition of it and in particular the power supply, starting a the mains plug.

If you stay with silicon diodes I would not use filter caps below 500VDC

Marc

Not sure what diodes they have used because they are so tiny. How do you test such components?

Marc, would you suggest changing the set back to original specs or going with the diodes?

I have read that some people use more than one diode in series? Is this a good idea?

The diodes normally have numbers on them, albeit you need a magnifier.

Diodes either work or they don't. You can test them a variety of ways, many multimeters have a function for it..

An ohm meter with around 3V will also work. These only have conduction in one direction. These are not like valves & need high voltage.

It is your choice, but if you want the set original put the 6X5 back. Make sure R16 (55R) is not high wattage so that it will burn. One could also consider 1/2 Watt 100R resistors in series with the plate so that they also burn if it shorts,

Marc

Thanks Marc. The diodes test ok. Writing on them very difficult. Can make out a '400'.

I read on the US radio forum that an early 40's suggested fix was to place two small dial light bulbs in series with the plate. Is it correct to place them in series? Wouldn't there be too much current running through them?

This might be a good second opinion of what I said with Zeniths http://antiqueradios.com/forums/viewtopic.php?f=6&t=189197.

In that one there is a suggestion of 300 Ohms added series resistance to DC HT.

After paying attention to the circuits A13B should have a 5Y3..... A13A has the 6X5

A13A has only four valves two of which are philips and is reflexed. A13B has 5 Valves and is not reflexed.

The power supply backbias and the transformers are completely different.

There will be a winding for the 5Y3 filaments and this will be on pins 2 and 8 and B+ will be on one of those pins originally.

We better clear up what we have, before there is total confusion. And any further mis-information.

Having wrong valves in second hand sets, or in the wrong order, is not unusual and is another good reason for not hitting the "Go" button, without checking the set.

Don't like the globe idea as you have to get the right one, that is not easy. Even TV's were deliberately built with "fusistors" in them, i.e.resistors that on overload, self destructed.

Correctly chosen wattage, backbias resistors can make good fusistors.

Marc

Hi Marc

I have a circuit which I think you sent me in relation to another HMV Little Nipper for the HMV A 13B and that circuit definitely has the 6X5-GT tube. The tubes are a 6A8G, EBF35, 6v6GT and the 6X5.

That's an A13A not in any way the same as A31B

That should have an EL33 which has around half the grid bias of 6V6.

Marc