1.4V battery valves: how low can I go? - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 17th Jan 2021, 6:10 pm

I've got a little single-valve regenerative shortwave receiver here - DF33 pentode, home-wound tuned-circuit, transformer-coupled to a WWII-surplus DLR5 headset. It's surprisingly effective at receiving 3.5 and 7MHz SSB.

Recently I noticed it was getting a bit twitchy to set the regeneration in the sweet-spot for resolving SSB; investigation revealed that the D-cell providing the LT was only delivering 0.7V off-load, which fell to 0.5V when it tried to power the DF33's filament.

I'm impressed that my little Rx still worked under such circumstances - but it led me to ponder just how-low you can go with directly-heated-filament supplies.

Bazz4CQJ · 17th Jan 2021, 6:19 pm

I think that would depend vary greatly on the condition of the valve, and perhaps, who made it. I recall the TRF I built using DF91's, as the batteries aged, it became pretty hard to see the filaments glow, even when they were still pulling in signals.

B

G6Tanuki · 17th Jan 2021, 7:12 pm

Yes in my experience the Dx91/96-series valves never really showed any visible filament-glow. My who-knows-what-it-had-been-subjected-to-before-I-got-it DF33 has the classic Mullard red spray-coating so I can't see the filament.

If it were in an application [such as a RF- or audio power-amplifier] where the cathode needed to emit tens of milliamps I'd be worried about running in what could be considered a cathode-emission-limited mode - but this little one-valver's HT consumption is only around a milliamp.

And if it does prove long-term-harmful, well, I've got a couple of spare unused Mullard DF33 in the attic, which will probably still be there when they cart me off to the cremmie.

paulsherwin · 17th Jan 2021, 8:55 pm

I think 0.9V was normally considered the minimum heater voltage for 1.4V valves. Obviously being directly heated the cathode will still produce some emission at quite low voltages.

17th Jan 2021, 6:10 pm	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	1.4V battery valves: how low can I go? I've got a little single-valve regenerative shortwave receiver here - DF33 pentode, home-wound tuned-circuit, transformer-coupled to a WWII-surplus DLR5 headset. It's surprisingly effective at receiving 3.5 and 7MHz SSB. Recently I noticed it was getting a bit twitchy to set the regeneration in the sweet-spot for resolving SSB; investigation revealed that the D-cell providing the LT was only delivering 0.7V off-load, which fell to 0.5V when it tried to power the DF33's filament. I'm impressed that my little Rx still worked under such circumstances - but it led me to ponder just how-low you can go with directly-heated-filament supplies.

17th Jan 2021, 6:19 pm	#2
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: 1.4V battery valves: how low can I go? I think that would depend vary greatly on the condition of the valve, and perhaps, who made it. I recall the TRF I built using DF91's, as the batteries aged, it became pretty hard to see the filaments glow, even when they were still pulling in signals. B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

17th Jan 2021, 7:12 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: 1.4V battery valves: how low can I go? Yes in my experience the Dx91/96-series valves never really showed any visible filament-glow. My who-knows-what-it-had-been-subjected-to-before-I-got-it DF33 has the classic Mullard red spray-coating so I can't see the filament. If it were in an application [such as a RF- or audio power-amplifier] where the cathode needed to emit tens of milliamps I'd be worried about running in what could be considered a cathode-emission-limited mode - but this little one-valver's HT consumption is only around a milliamp. And if it does prove long-term-harmful, well, I've got a couple of spare unused Mullard DF33 in the attic, which will probably still be there when they cart me off to the cremmie.

17th Jan 2021, 8:55 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,939	Re: 1.4V battery valves: how low can I go? I think 0.9V was normally considered the minimum heater voltage for 1.4V valves. Obviously being directly heated the cathode will still produce some emission at quite low voltages.