34v heater on a 34GD5 tube ?

[petemoore]

http://oldradio.qrz.ru/tubes/foreign/01/34GD5.gif

  Frank has some white digits in black blocks, what are the white digits in the black blocks showing ?
  There is also a black circle with digits in it, what are the digits for.
   It makes more sense to simply ask the questions first than to guess what the answer is.
  Looks like the heater takes 34 V [strange ? not true ?] and the white letters in the black blocks are higher watt resistors or the voltage drop across the 110V ?
 

[amptramp]

Square blocks contain current in milliamps.  Circles are voltages, so this tube has a 34 volt 0.1 amp heater.  It also shows that the output transformer primary impedance should be 2500 ohms.

[drbob1]

The first number in a tube description is always the heater voltage. And 34v isn't too unusual, different equipment requires different voltages and it's relatively easy to pull 34v off of 110v for example. The better question might be: why did 6 and 12v become standards? And the answer is probably that it's relatively easy to build a battery to power the heaters on a 6 or 12v tube, but the later tubes, designed for TV or high output radio, don't have such limitations.

[PRR]

> Looks like the heater takes 34 V [strange ? not true ?]

It is part of a "5-tube 'AC/DC' radio".

"AC/DC" really means "too cheap to have a power transformer".

So the only source of power is 110V wall-juice. If you feed 6V heaters, you have to waste-off 20 times more power than you use.

Instead they made tubes with high-voltage heaters. Typically 35V rectifier, 50V or 35V power tube, 12V on audio, IF, and converter tubes. 12V+12V+12V+35/50V+35V =  106 or 121 Volts, close-enuff to 115V.

http://www.mif.pg.gda.pl/homepages/frank/sheets/106/3/34GD5A.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/3/34GD5A.pdf

Far more common series-heater power tubes are 50L6, 35C5, etc. 34GD5 appears to be a last-gasp improvement to stay ahead of the transistor radios which were about to eat the tubes' lunch.

> The first number in a tube description is always the heater voltage

In the most common _USA_ consumer type-number system, the first numbers is _nominal_ heater voltage. Especially at lower voltages 1 2 3, you need to check the datasheet to see if it is really 1.4V or 2.1V. "12" is often 12.6V. Also the "7" series are mostly good old 6.3V heaters (7V max).

When you get beyond common USA types, this is not true. Some transmitter tube numbers imply plate dissipation. Many many have arbitrary numbers, or sequential assignments such as the '01 thru '80 series. The more common European type designation uses letters for heater and function.

> why did 6 and 12v become standards?

This is quite clear, Early tubes were 2.1V because that is one lead-acid cell, readily available for incidental power and charged at the motor-car service shop or with your WindCharger. Early AC tubes were 2.5V, a convenient low voltage; some were 5V. But then came Car Radio. Suddenly everybody wanted to heat tubes in the car, preferably direct from the engine dynamo and starting battery (but the earliest car radios had their own batts). Thus 6.3V, the nominal voltage of a full-charge 3-cell lead-acid car battery. Since this was also fine for AC power, and the newest/best types were made suitable for cars, nearly everything new was 6.3V even on wall-power. Rectifiers hadn't changed much and need a dedicated filament winding, so stayed 5V.

Later larger and military systems had 12.6V batteries, or had reason to use lower current (thus higher voltage), and two triodes in the new 9-pin base had a spare pin, so the 6.3V/12.6V 12A_7 types appeared.

[gtudoran]

http://oldradio.qrz.ru/tubes/foreign/01/34GD5.gif

  Frank has some white digits in black blocks, what are the white digits in the black blocks showing ?
  There is also a black circle with digits in it, what are the digits for.
   It makes more sense to simply ask the questions first than to guess what the answer is.
  Looks like the heater takes 34 V [strange ? not true ?] and the white letters in the black blocks are higher watt resistors or the voltage drop across the 110V ?
 

And Ri=13k.... that would be a challenge

Best regards,
Gabriel Tudoran
Analog Sound

[PRR]

> Ri=13k.... that would be a challenge

13K is "plate resistance", "internal resistance" on many European datasheets.

Pentode amp design rarely cares about plate resistance, as long as it is "high", much higher than load impedance. 13K is indeed higher than the suggested 2.5K load.

If you thought R1 was INput resistance, no. It's a plain old negative-grid (>100Meg), a few pFd Miller and stray, and a max-500K grid resistor. Within the audio band, the input is darn-near 500K. a "normal power amp" input.