Leak TL/10 dropper resistor running hot - UK Vintage Radio Repair and Restoration Discussion Forum

neville23 · 2nd Oct 2018, 5:00 pm

Garage find. All original. I replaced the four 0.1uF "Visconol -X" with polypropylene, except the input cap which has no voltage across it. No other replacements or changes made.

It works fine but the 1k Ohm dropper is gently smoking. It has about 85V across it. Nothing else is getting unexpectedly hot, and the h.t. measurements all seem reasonable.

I have measured nearly all the components and so far all are within 10%.

The cathode resistors on the output valves (220 Ohms each) show about 8.5V.

I have a diagram but without voltage readings.

Is this a known problem? Is there a preferred fix?

GrimJosef · 2nd Oct 2018, 7:26 pm

The output valves are running a bit hot (assuming the resistors actually are 220ohms). There should be 7.5V at the cathodes and 70V across the 1k (400V before, 330V after). Is the mains transformer primary voltage selector set for the highest possible mains voltage ?

The circuit I have, which is in a book, says the 1k should be 10W rated. 85V across 1k generates a little over 7W I think so a 'proper' 10W component should still be OK.

Cheers,

GJ

neville23 · 9th Oct 2018, 11:14 am

Thanks.
Yes the cathode resistors are still close to 220Ω. In fact, all the components seem to be high stability types.
I know old valves' characteristics drift, but I have never come across any that ran hot rather than cold; usually the emission from the cathodes drop.
It hasn't been used for at least 20 years, so I will leave it on for a bit and see if it settles down.
6W is managable for a 10W component, but there isn't much airflow.

boxdoctor · 9th Oct 2018, 3:33 pm

Its just possible that one of the output valves has a very slight leak to air. This will cause it to be "soft", and draw more current than normal due to ionization in the valve.
Possilby viewing the operating valves in a very low light will reveal this in the form of a hazy blue glow within the anode-cathode space.
Comparing the cathode voltages will also reveal the above fault, and shew any other dissimilarity or imbalance between the valves. Tony

2nd Oct 2018, 5:00 pm	#1
neville23 Diode Join Date: May 2011 Location: Taunton, Somerset, UK Posts: 6	Leak TL/10 dropper resistor running hot Garage find. All original. I replaced the four 0.1uF "Visconol -X" with polypropylene, except the input cap which has no voltage across it. No other replacements or changes made. It works fine but the 1k Ohm dropper is gently smoking. It has about 85V across it. Nothing else is getting unexpectedly hot, and the h.t. measurements all seem reasonable. I have measured nearly all the components and so far all are within 10%. The cathode resistors on the output valves (220 Ohms each) show about 8.5V. I have a diagram but without voltage readings. Is this a known problem? Is there a preferred fix?

2nd Oct 2018, 7:26 pm	#2
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Leak TL/10 dropper resistor running hot The output valves are running a bit hot (assuming the resistors actually are 220ohms). There should be 7.5V at the cathodes and 70V across the 1k (400V before, 330V after). Is the mains transformer primary voltage selector set for the highest possible mains voltage ? The circuit I have, which is in a book, says the 1k should be 10W rated. 85V across 1k generates a little over 7W I think so a 'proper' 10W component should still be OK. Cheers, GJ __________________ http://www.ampregen.com

9th Oct 2018, 11:14 am	#3
neville23 Diode Join Date: May 2011 Location: Taunton, Somerset, UK Posts: 6	Re: Leak TL/10 dropper resistor running hot Thanks. Yes the cathode resistors are still close to 220Ω. In fact, all the components seem to be high stability types. I know old valves' characteristics drift, but I have never come across any that ran hot rather than cold; usually the emission from the cathodes drop. It hasn't been used for at least 20 years, so I will leave it on for a bit and see if it settles down. 6W is managable for a 10W component, but there isn't much airflow.

9th Oct 2018, 3:33 pm	#4
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: Leak TL/10 dropper resistor running hot Its just possible that one of the output valves has a very slight leak to air. This will cause it to be "soft", and draw more current than normal due to ionization in the valve. Possilby viewing the operating valves in a very low light will reveal this in the form of a hazy blue glow within the anode-cathode space. Comparing the cathode voltages will also reveal the above fault, and shew any other dissimilarity or imbalance between the valves. Tony