EBL31 Cathode to Heater Short

[John G8MWF]

This is my first Cathode to Heater Short that I have experienced and I wondered if there is any way to clear it?

[Leon Crampin]

The cathode of this valve when used in a short superhet will be well above chassis potential to give the high value of AGC delay needed in a low gain set. EBL31s are scarce in their genuine octal based versions, and re-based EBL1s won't fit some sets such as the A22.

I would make or obtain a 1:1 heater transformer to supply this valve and fit (say) a 100k resistor between heater and cathode. Zapping an otherwise good valve is too risky.

Leon.

[John G8MWF]

Originally Posted by Leon Crampin

The cathode of this valve when used in a short superhet will be well above chassis potential to give the high value of AGC delay needed in a low gain set. EBL31s are scarce in their genuine octal based versions, and re-based EBL1s won't fit some sets such as the A22.

I would make or obtain a 1:1 heater transformer to supply this valve and fit (say) a 100k resistor between heater and cathode. Zapping an otherwise good valve is too risky.

Leon.

Hello Leon - sounds like a plan to me.
What is the purpose of the 100k resistor?

Cheers - John

[Leon Crampin]

A heater cathode short is an "accidental" connection, so you don't know if it will be permanent, become resistive or clear altogether. If it should clear, the heater would be left floating with respect to the cathode, which may or may not cause problems such as heater emission to the cathode, or hum. Using a high value resistor satisifies the need to avoid a floating heater, but won't cause a partial heater short, depending on where the short circuit is in the heater loop within the cathode.

For the sake of a resistor, I'd fit it. Floating valve electrodes are never a good idea.

Leon.

[John G8MWF]

Thanks Leon - I will give that a try.

[merlinmaxwell]

One source of said transformer would be a TV CRT heater transformer, used for that very thing and (if needed) boost the volts for a tired tube. ANd would be in keeeping with the original at bit.

[turretslug]

The EBL31 is a bit of a greedy beggar though with its 1.5A heater current- almost as much as a KT88- so a 10VA capable transformer is needed whether 1:1 or mains isolating.

[John G8MWF]

I just noticed a old modification on the ELB31. G2 pin 6 has a 15K? resistor from the HT connected to it with a 0.5MFD to earth.
Presumably this was some kind of way to limit the G2 current and extend the valve life? Is it best to keep it in place or put it back as per the original diagram?

In addition to wax caps they are also using " Metal Tube" caps which show no signs of leakage - is it worth replacing them?

Pictures of the chassis enclosed.

Thank you.

[G6Tanuki]

Feeding G2 from the HT line via a series-resistor-and-capacitor-to-ground is quite a common thing - it does tend to reduce the total power-dissipation in the valve in cases where the main anode-HT-supply is on the high side.

Those wax-capacitors would, to me, be replace-on-sight. The 'metal tube' ones could _possibly_ be paper-in-oil, in which case they might still be OK - or [more likely] they are just the same waxed-paper design as the obviously-waxy ones - in which case replacement is highly recommended!

Let us know the branding on the outer metal casesof those capacitors and we can help you work out what's inside.

[Leon Crampin]

What set is it? Looks a bit like a Pye.

Leon.

[John G8MWF]

Originally Posted by G6Tanuki

Feeding G2 from the HT line via a series-resistor-and-capacitor-to-ground is quite a common thing - it does tend to reduce the total power-dissipation in the valve in cases where the main anode-HT-supply is on the high side.

Those wax-capacitors would, to me, be replace-on-sight. The 'metal tube' ones could _possibly_ be paper-in-oil, in which case they might still be OK - or [more likely] they are just the same waxed-paper design as the obviously-waxy ones - in which case replacement is highly recommended!

Let us know the branding on the outer metal cases of those capacitors and we can help you work out what's inside.

Sorry for the delay - this one looked innocent and then pulled more surprises.
The metal caps are Hunts - why they used metal and not wax types beats me.

The other "surprise" was the coil packs L3 - L5 and L8 - L10 are loose in the metal mounting bracket. They appear to have been locked in place with wax?
I was thinking of cleaning both surfaces with ISO and then using a hot glue gun to lock them in place - any other ideas?

Leon - YES it is a PYE P28.

Cheers all

[trobbins]

Originally Posted by Leon Crampin

Zapping an otherwise good valve is too risky.

I presume the zapping comment is related to connecting a current supply from cathode to heater with the aim of passing sufficient current through an offending metal whisker that may have slowly developed through the heater oxide coating, and hence melting the whisker until a high resistance path occurs?

Given that the valve has had a long service life, the fault scenario of a metal whisker developing would appear a high likelihood, as it is a known long-term failure mechanism, especially for a cathode that has elevated DCV from the heater.