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Author Topic: Why 5, 6.3 and 7 volt heaters?  (Read 694 times)

Posts: 1435

« on: August 05, 2006, 02:48:52 PM »

Anyone have accurate information on why tubes have heaters set for 6.3 volts instead of 6 volts?  If the extra .3 volts have an advantage, why are 12 volt heaters only 12 volts and not, lets say, 12.6 volts?

Why did the voltage get increased to 7 volts on some tubes, like ECC88 to PCC88?

Why did, do many rectifiers run on 5 volts?  

Why have rectifiers heaters running at a different voltage than the heater voltage of other tubes in the circuit?

Why were caps called condensers?


Posts: 1

« Reply #1 on: August 05, 2006, 03:13:13 PM »

I have been asking the same questions for years.  Your statement that tubes have 12v filaments is incorrect. They are standardized on 12.6 vac, that is for the common ones.  There probably is a practical reason why rectifiers are 5v and other can be 26 volts and up.  The word condensor probably came from the old jars used as capacitors which is name that refers to newer caps.  The rest is your guess.

Posts: 12665

« Reply #2 on: August 05, 2006, 03:16:26 PM »

I think some of the heater voltages are related to the available batteries in the early years. 6.3V tubes ran from early 6V cars which used 3-cell batteries. The 12V tubes are actually 12.6V and ran from the newer vehicles with 6-cell batteries.

Posts: 1531

« Reply #3 on: August 05, 2006, 03:17:18 PM »

The voltages are different for the very same reason engineers today design differences in products....
and/or there is a lack of standardization in lots of things. They were trying to accomplish certain things and they felt those voltages were "needed". Probably some of the early tube voltages were related to supply Battery voltages. Some tubes have indirectly heated cathodes, other have applicatons where filament voltage maybe determined by other power supply aspects. There is no one answer! The advancement of technology also mandated changes just as it does today.

The term condensers goes back to the early days of radio. If you look at a description of parts in a Spark Gap transmitter, there will be "hardware items" in there that have names most of us "modern" hams can't even identify!  I am sure you will get some more posts from people with specifics and details....

Just a personal view: it seems to me that the term "condenser" was the common term up until about the 1970's...after that "capacitor" seemed to take over as the more common term....

73,  K0ZN

Posts: 301


« Reply #4 on: August 05, 2006, 05:52:48 PM »

"Why were caps called condensers?"

Here is an excerpt from Wikipedia:

Early capacitors were also known as condensers, a term that is still occasionally used today. It was coined by Volta in 1782 (derived from the Italian condensatore), with reference to the device's ability to store a higher density of electric charge than a normal isolated conductor. Most non-English languages still use a word derived from "condensatore", like the French "condensateur", the German or Polish "Kondensator", or the Spanish "condensador".


Posts: 9304


« Reply #5 on: August 05, 2006, 06:23:58 PM »

6.3 and 12.6 volt heaters were based on the voltage of lead acid cells. Some tubes, like the 117L7, were based on power line voltage.

5 volt tubes were initially directly heated rectifiers. Since such tubes are always transformer operated and always required an independent isolated filament winding it wasn't necessary to match a common battery voltage. It was desirable to run the lowest reasonable voltage on them since the filament is effectively bucking AC from one plate and aiding AC from the other the other without a center tap, and the filament had to be very wide (low resistance) in order to have enough surface area for the required emission current. 5 wound up being a nice number, since the CT if used would be at 2.5 volts and it wasn't so low current would be too high.

There aren't many 7 volt tubes. Tubes like the 7H7 and 7L7 are 6.3 volt. The very few tubes that are 7 volt were made for series wired filaments in line operated devices. In those cases current was important, not voltage. I would guess whatever the current was to heat the cathode to the right temperature, that's what the voltage would be.

There was a combination of tubes developed for line operated radios that used three 12.6 volt tubes, one 35 volt tube, and one 50 volt tube. The filaments could run off of 120 volt power lines without a transformer.

73 Tom

Posts: 1422


« Reply #6 on: August 05, 2006, 06:27:16 PM »

Rectifiers such as the 5U4 have directly heated cathodes, that is, the heater is the cathode.  This means that the B+ potential appears on the heater.  If the rectifier had a 6.3- or 12.6-V filament, someone might be tempted to use a common filament transformer winding with the other tubes powered by the rectifier--resulting in the B+ being on their heaters.  In that case, current in those tubes would flow between their heaters and cathodes in parallel with the plate currents, and the parasitic currents would not be controlled by the tubes' grids.

Using the 5-V filament leads to using a isolated filament winding for the rectifier.

Posts: 671

« Reply #7 on: August 06, 2006, 03:00:24 PM »

Many old AM radios did not have a transformer. All of the fillaments were in series so they had to draw a similar current and add up to 110 volts so they would work with standard wall power. Many early Television designs did the same thing. Don WD8PTB

Posts: 478


« Reply #8 on: August 07, 2006, 08:09:57 AM »

...Many old AM radios did not have a transformer. All of the fillaments were in series so they had to draw a similar current .. Which reminds me why they used to earth the chassis!

Posts: 671

« Reply #9 on: August 07, 2006, 09:26:43 AM »

On transformerless radios and TV's you could sometimes eliminate interference and noise by turning the 2 prong power plug around. ( bothh prongs were the same width in "the old days". Don WD8PTB

Posts: 13005

« Reply #10 on: August 07, 2006, 12:26:37 PM »

The PCC88/ECC88 difference was due to the use of series-
connected filament strings.  There were a couple of standard
filament currents - perhaps 40ma?  Tubes that required
differing amounts of filament power all drew the same
current, but at different voltages.  This meant they could
all be connected in series across the line (with a dropping
resistor to make up any difference) which saved the price
of a filament transformer.  So the PCC88 (I think) had the
7V filament at 40mA for series connection, while the
ECC88 had the standard 6.3V (perhaps at 50ma?) for
sets with a filament transformer.

Some of the early tubes for portable radios, like the 1T4
and 1R5, ahd 1.4V filaments (or the 3V4 power output tube
that could run on 1.4V or 2.8V).  The carbon-zinc batteries
would use a single large cell (1.5V when new) for the
filaments and a stack of smaller cells to get the 22.5, 45
or 90V plate voltage.

Many of the older tubes types that start with "7" are
really 6.3V tubes in a "loctal" socket that was supposed
to hold the tube in the socket so it couldn't come
loose when bouncing around in mobile equipment like a
tank or an airplane.  (Note that American tube numbers
usually indicate the filament voltage in the first set
of digits.  European tube numbers indicate it with the
first letter - the "P" in the PCC88 indicates a standard
current for series connection if I remember correctly.)
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