Using valves as a dropper substitute? - UK Vintage Radio Repair and Restoration Discussion Forum

Biggles · 9th Oct 2012, 7:58 pm

Hi all, has anyone tried this before? Maybe a bit radical but what about using a valve with perhaps low emission but an intact heater to replace an open circuit dropper section? Some series heater types like rectifiers or output pentodes could be used to drop maybe 40 or 50 volts at 150 0r 200 mA instead of a resistor. The heat generated will of course be the same but with the addition of a securely mounted valve base you can re-use otherwise worn out valves.

9th Oct 2012, 9:19 pm

Ah, the simplest ideas are the best. Now why didn't I think of that?

paulsherwin · 9th Oct 2012, 9:31 pm

This would certainly work, but as you say, the amount of heat dissipation would be exactly the same as a resistor, and a lot of physical modification would be needed. If you don't want to keep things original it would make more sense to convert to a capacitor or diode dropper, which would reduce the waste heat.

emeritus · 9th Oct 2012, 9:53 pm

That reminds me of a tale related by the instructor of a workshop training course that I once attended who had been in the radio and TV repair trade for years.

He said he had sometimes come across sets of US manufacture in which the only connections to some of the valves were the heaters. In others, the push-pull output stages used several valves in parallel, far more than was required by the speakers that they were driving.

He said that there had been a period where the set makers were in a "mine's bigger than yours" competition about how many valves their sets had, so the more the better, even if they didn't do anything useful. Or perhaps they did, by reducing the size of the mains dropper.

Herald1360 · 9th Oct 2012, 11:49 pm

Poor man's Barreter?

boxdoctor · 11th Oct 2012, 2:23 pm

One thing to consider if you are thinking of pressing a valve into service as a dropper, is that unlike a fixed dropper resistance, the initial turn-on resistance of the valve heater will be considerably lower than the operating resistance. This will exacerbate the current surge in the heater chain at switch-on, and may result in the premature failure of the weakest valve filament in the chain (Sod's law says this will be the most expensive/hard to come by one!).
At the least, it is likely to negate to some extent the benefit of any thermistor in the heater chain. Then there is the unlikely chance of a hearter that gives exactly or nearly the required voltage drop. -Not for me!

Leon Crampin · 11th Oct 2012, 5:51 pm

I remember as a lad repairing a pre-war Murphy AC/DC receiver which needed a dropper resistor. Part of my "fix" was to discard the U4020 rectifier and fit a U801 (octal base) which has an 80V heater. This enabled me to use the existing good parts of the dropper for normal operation at 240V input. The lower voltage taps were lost, but that is of no consequence.

I wouldn't dream of doing that now to a pre-war Murphy (or any other set) but I was rather pleased with my ingenuity at the time and it worked really well, with a slightly higher than normal HT voltage.

I did worse things than this - but learned a lot...

Leon.

kalee20 · 11th Oct 2012, 5:56 pm

Quote:

Originally Posted by boxdoctor

Then there is the unlikely chance of a hearter that gives exactly or nearly the required voltage drop.

I've always thought that heaters were pretty well controlled. Many valves are specified for either series or parallel operation. Too high resistance would result in overheating in the first case and underheating in the second, with the reverse for too low resistance.

Thus, manufacturers controlled them tightly. I'd be confident in a valve giving the specified voltage drop at the specified current.

boxdoctor · 12th Oct 2012, 1:35 pm

If you look carefully, Kalee, you will see I used the term "required" not "specified".
By required drop I mean the voltage that the replaced dropper resistance would have been required to drop, had not the substitution been made. Not the heater voltage of any specific valve. What in essence I mean, is that the chances of finding a valve with a rated heater voltage that will give the required drop within an acceptable tolerance is rather unlikely.I agree that valve heaters are generally manufactured to a fairly tight tolerance.

Radio Wrangler · 12th Oct 2012, 3:26 pm

Actually, it's an inverse Barretter

The normal valve heater has a positive tempco, and quite a strong one. Using a valve in place of a fixed resistor will give you a larger turn-on surge of current. It will probably reduce life expectancy a little.

Sorry

David

9th Oct 2012, 7:58 pm	#1
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Using valves as a dropper substitute? Hi all, has anyone tried this before? Maybe a bit radical but what about using a valve with perhaps low emission but an intact heater to replace an open circuit dropper section? Some series heater types like rectifiers or output pentodes could be used to drop maybe 40 or 50 volts at 150 0r 200 mA instead of a resistor. The heat generated will of course be the same but with the addition of a securely mounted valve base you can re-use otherwise worn out valves.

9th Oct 2012, 9:19 pm	#2
Guest Guest Posts: n/a	Re: Using valves as a dropper substitute? Ah, the simplest ideas are the best. Now why didn't I think of that?

9th Oct 2012, 9:31 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,967	Re: Using valves as a dropper substitute? This would certainly work, but as you say, the amount of heat dissipation would be exactly the same as a resistor, and a lot of physical modification would be needed. If you don't want to keep things original it would make more sense to convert to a capacitor or diode dropper, which would reduce the waste heat.

9th Oct 2012, 9:53 pm	#4
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,347	Re: Using valves as a dropper substitute? That reminds me of a tale related by the instructor of a workshop training course that I once attended who had been in the radio and TV repair trade for years. He said he had sometimes come across sets of US manufacture in which the only connections to some of the valves were the heaters. In others, the push-pull output stages used several valves in parallel, far more than was required by the speakers that they were driving. He said that there had been a period where the set makers were in a "mine's bigger than yours" competition about how many valves their sets had, so the more the better, even if they didn't do anything useful. Or perhaps they did, by reducing the size of the mains dropper.

9th Oct 2012, 11:49 pm	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Using valves as a dropper substitute? Poor man's Barreter? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

11th Oct 2012, 2:23 pm	#6
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: Using valves as a dropper substitute? One thing to consider if you are thinking of pressing a valve into service as a dropper, is that unlike a fixed dropper resistance, the initial turn-on resistance of the valve heater will be considerably lower than the operating resistance. This will exacerbate the current surge in the heater chain at switch-on, and may result in the premature failure of the weakest valve filament in the chain (Sod's law says this will be the most expensive/hard to come by one!). At the least, it is likely to negate to some extent the benefit of any thermistor in the heater chain. Then there is the unlikely chance of a hearter that gives exactly or nearly the required voltage drop. -Not for me! Last edited by boxdoctor; 11th Oct 2012 at 2:30 pm.

11th Oct 2012, 5:51 pm	#7
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Using valves as a dropper substitute? I remember as a lad repairing a pre-war Murphy AC/DC receiver which needed a dropper resistor. Part of my "fix" was to discard the U4020 rectifier and fit a U801 (octal base) which has an 80V heater. This enabled me to use the existing good parts of the dropper for normal operation at 240V input. The lower voltage taps were lost, but that is of no consequence. I wouldn't dream of doing that now to a pre-war Murphy (or any other set) but I was rather pleased with my ingenuity at the time and it worked really well, with a slightly higher than normal HT voltage. I did worse things than this - but learned a lot... Leon.

12th Oct 2012, 1:35 pm	#9
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: Using valves as a dropper substitute? If you look carefully, Kalee, you will see I used the term "required" not "specified". By required drop I mean the voltage that the replaced dropper resistance would have been required to drop, had not the substitution been made. Not the heater voltage of any specific valve. What in essence I mean, is that the chances of finding a valve with a rated heater voltage that will give the required drop within an acceptable tolerance is rather unlikely.I agree that valve heaters are generally manufactured to a fairly tight tolerance.

12th Oct 2012, 3:26 pm	#10
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Using valves as a dropper substitute? Actually, it's an inverse Barretter The normal valve heater has a positive tempco, and quite a strong one. Using a valve in place of a fixed resistor will give you a larger turn-on surge of current. It will probably reduce life expectancy a little. Sorry David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done