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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Dec 2015, 11:43 am   #1
short wave
Join Date: Mar 2013
Location: Leeds, West Yorkshire, UK.
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Default Valve heaters and dial lamps

After watching a youtube video I learned something quite new to me.
Most radio and TV forum members will be familiar with the idea of running valve heaters in series in particular the TV people (“U” series at 100mA and “P” series at 300mA) . The dial lamp can also be designed into the series circuit , being of the same current value as the valve heaters.

The video talks of tap in the 35Z5 rectifier diode. This is not a centre tap as you would think (for example used in 3S4 , 3V4 , 12AH8 , 12AU7 , 12AX7 to name a few ) but is a special one that you can add a suitable lamp (or resistor) in PARALLEL with it and not burn out the “other half” (or
valves in the rest of the string)... all new to me! (also,you?)

I will raise the following questions…

1) Has it been used in the UK or an American thing (“all American five”)
2) Has it been used in any other applications other than in TV /radio
3) Any other valves with an “off” centre tap ( found 35W4 also)
4) Is it just limited to diode/ rectifiers

link to video
(jump in at 7-40)

link to valve data

Other info here

regards S-W
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Old 13th Dec 2015, 1:50 pm   #2
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Default Re: Valve heaters and dial lamps

A nice write up of the PSU in the All American Five years radios. I think it was standard to put the lamp across that heater section, according to the write up earlier sets had a resistor as well as the lamp so that if/when the lamp burned out the rectifier did not take too much strain, Later sets did away with the resistor and damage to the rectifier woukd occur a short while after.
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Old 13th Dec 2015, 4:08 pm   #3
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Default Re: Valve heaters and dial lamps

Depending on your standpoint, an awful and cynical piece of penny-pinching and cheese-paring, or an effective and ingenious way of minimising component count. Personally, it offends the purist in me to stress an expensive component like a rectifier valve by over-running part of a filament when a known short-lived bulb inevitably failed, but it would have been to valve-makers' satisfaction....

Certainly, it keeps the radio going when the bulb fails, but an appropriate compensating NTC thermistor in parallel with the bulb to even things up would have been laughed (or budgeted) out of court.
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Old 13th Dec 2015, 5:56 pm   #4
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Default Re: Valve heaters and dial lamps

My felling is penny pinching, cheese pairing, but we don't know how cut throat the USA market was then and if a set cost a dollar more in the shop they would have lost sales.

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Old 13th Dec 2015, 7:37 pm   #5
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Default Re: Valve heaters and dial lamps

Yes, the American market was a lot cheaper (no BVA etc.) and much larger, so the odd penny (the same word on both sides of the pond) saved was worth it. The valve makers thought so too.
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Old 13th Dec 2015, 8:57 pm   #6
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Default Re: Valve heaters and dial lamps

I don't see the 'split' heater in "all-american five" radios as at all wrong: given the expected lifetime of valves (and radios) compared with the spending-power of the consumers-of-the-day it made *perfect* sense!

Some people get worryingly bogged-down in issues of designing-for-longevity; I rather focus on "how to bring the best technology to the most people at the lowest cost". The various "AA5" radios from the 1930s through to the 1960s met a market-need in a sensible way. Why design your radio to last ten years if the purchaser wants a new-style one in five?

From memory, in "The Setmakers" someone from the UK whose business was selling extension-loudspeakers who travelled to the US in the 1930s came back saying "there's no market for extension-loudspeakers in the US; they just buy another radio".

And for that I truly commend the US approach!
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Old 14th Dec 2015, 12:45 am   #7
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Default Re: Valve heaters and dial lamps

The whole "All American Five" idea was certainly ingenious. Whether it was for good or for evil depends on your point of view, I suppose. And the number of sets that have outlasted their design lifetime surely speaks for itself.
If I have seen further than others, it is because I was standing on a pile of failed experiments.
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Old 14th Dec 2015, 2:51 am   #8
Mr Moose
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Default Re: Valve heaters and dial lamps

Some American sets used tapped barretters (current regulator tubes) to supply the dial lamps.
One example is the 140R8 and part of a circuit (schematic) showing its use here (The 140R8 barretter is shown as a resistor.)
Yours, Richard
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Old 15th Dec 2015, 6:58 am   #9
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Default Re: Valve heaters and dial lamps

Here in the US, they were only used in radios and some "El Cheapo" amps here.
I have never seen one used in a TV set, (Excepting the combo sets, of course) at least in my 45+ years of repairing.(But they did tend to throw the odd designed set in once in a while too).
Since the filaments were series, in the AA5's, and cost was a concern, the bean counters wanted to save every where they could. So the 35Z5 and 35W4 were used as the most common rectifiers in the AA5 sets. Adding a power TX for safety and a fil. winding were out of the question, production cost wise, for most Mfgr's.

IIRC, there may have been other rectifiers also that used the split fil. , but none spring to mind immediately.
When the fil. in the pilot lamp died, an excessive amount of current flowed through the rectifier's remaining fil, and greatly shortened the life of the tube.
In fact I remember some tube mfgr's refused to accept defective new rectifiers of those types, unless you replaced the pilot lamp at the same time as the rect. tube!
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