Quad thermal runaway. - UK Vintage Radio Repair and Restoration Discussion Forum

sparkymike · 20th Oct 2019, 9:03 am

"If the valves really are gassy then keep an eye on them. The Quad II is vulnerable to thermal runaway with gassy valves. But since they're new I think it's more likely just to be fluorescence from the glass envelope, which is entirely harmless."

Quoted from an old post, can any protection be added to the Quads to disconnect any valve if the above occurs ? (Thermal switch ?)
I have had this happen to a couple of Quads in the past and luckily was on hand to switch them off.
Thanks for the link, makes interesting reading.
Mike.

GrimJosef · 20th Oct 2019, 1:18 pm

I think that was me.

People have designed protection circuits. These usually cut the power completely as running the Quad II with any of its valves disconnected isn't good for it. You could try sensing the control grid average voltage since the cause of any trouble is usually a rise in that (due either to gas in the valve or to leakage through a coupling capacitor). But any sensor would have to be very high-impedance to avoid upsetting the grid circuitry. An easier, if slightly less direct, check would be to monitor the DC component of the voltage across the output transformer windings in the KT66 cathode circuit. That's simply the product of the winding resistances and the valves' average cathode currents. The normal quiescent value is between 1V and 1.5V. Any more than 1.8V is a sign of trouble.

Cheers,

GJ

Craig Sawyers · 20th Oct 2019, 1:48 pm

The other way to do it is to sense screen grid current. That detects not only a gassy valve but can be used to detect an end of life valve too.

Audio Research did this in at least some of their power amps, see https://audioresearch.com/ar-package...alschempl-pdf/

Craig

20th Oct 2019, 9:03 am	#1
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Quad thermal runaway. "If the valves really are gassy then keep an eye on them. The Quad II is vulnerable to thermal runaway with gassy valves. But since they're new I think it's more likely just to be fluorescence from the glass envelope, which is entirely harmless." Quoted from an old post, can any protection be added to the Quads to disconnect any valve if the above occurs ? (Thermal switch ?) I have had this happen to a couple of Quads in the past and luckily was on hand to switch them off. Thanks for the link, makes interesting reading. Mike.

20th Oct 2019, 1:18 pm	#2
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Quad thermal runaway. I think that was me. People have designed protection circuits. These usually cut the power completely as running the Quad II with any of its valves disconnected isn't good for it. You could try sensing the control grid average voltage since the cause of any trouble is usually a rise in that (due either to gas in the valve or to leakage through a coupling capacitor). But any sensor would have to be very high-impedance to avoid upsetting the grid circuitry. An easier, if slightly less direct, check would be to monitor the DC component of the voltage across the output transformer windings in the KT66 cathode circuit. That's simply the product of the winding resistances and the valves' average cathode currents. The normal quiescent value is between 1V and 1.5V. Any more than 1.8V is a sign of trouble. Cheers, GJ __________________ http://www.ampregen.com

20th Oct 2019, 1:48 pm	#3
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,993	Re: Quad thermal runaway. The other way to do it is to sense screen grid current. That detects not only a gassy valve but can be used to detect an end of life valve too. Audio Research did this in at least some of their power amps, see https://audioresearch.com/ar-package...alschempl-pdf/ Craig Last edited by Craig Sawyers; 20th Oct 2019 at 2:05 pm.