Why 5V filament

[mashaffer]

OK, I just have to know why on earth do rectifier tubes have 5V heaters and everything else is pretty much 6V except for those designed specifically for series heaters?

mike

[anatech]

Hi Mike,
Most tube designs came from a much earlier time. Many tubes had 2.0 and 2.5 V heaters designed for battery power. Your rectifiers are holdovers from those times. This doesn't matter since these parts have filaments rather than heaters. That means they also act as the cathode and are therefore at your raw B+ potential. You can not share these "heater" lines with any thing else anyway. Transformers designed to use these rectifiers will have your 5V winding(s) already.

-Chris

[Tom Bavis]

5V tubes were first used with 6V storage batteries. When it reached 5V, it was pretty well dead (a rheostat was used to drop the 6.3V of a freshly charged battery to 5.0). When RCA introduced its first AC tubes, they used multiples of 2.5V - so you had the 2.5V 57, 58, 55, 2A5, 2A3, 45, 46, 47 etc. The rectifiers had their own winding and needed more heater power so they used 5.0. The 10 and 50 used 7.5V - output tubes often had their own heater winding too. But Philco (the largest radio maker of the '30s) had other ideas - they made car radios which needed 6.3V tubes. They didn't want to use tubes from the enemy (RCA) and they decided to use the same tubes in their AC radios. Guess who won?

[7N7]

In more recent times, 5V has also been used. Have a look at the big RF power triode (TY4-400) in my avatar. This one needs 5V at 14A. When testing it, I found that an old switch-mode psu from a scrapped computer worked perfectly - and with very little noise too.

7N7

[316a]

Originally posted by 7N7
In more recent times, 5V has also been used. Have a look at the big RF power triode (TY4-400) in my avatar. This one needs 5V at 14A. When testing it, I found that an old switch-mode psu from a scrapped computer worked perfectly - and with very little noise too.

7N7

Hi 7N7 ,
Hope you are keeping well . I had not thought of using computer switch mode supplies , there is a pile of NOS Eimac VT127A sitting in the corner I'd like to use . Pity the filament is enclosed by the anode of those TY4-400 , those hungry thoriated valves are very impressive looking lit up . 826 in my avatar looked very impressive with the lights out , even more impressive when there is enough current to get the anode glowing cherry red

cheers

316A

[7N7]

Originally posted by 316a


Hi 7N7 ,
Hope you are keeping well . I had not thought of using computer switch mode supplies , there is a pile of NOS Eimac VT127A sitting in the corner I'd like to use . Pity the filament is enclosed by the anode of those TY4-400 , those hungry thoriated valves are very impressive looking lit up . 826 in my avatar looked very impressive with the lights out , even more impressive when there is enough current to get the anode glowing cherry red

cheers

316A

My dear chap, likewise.

I am surprised that you had not thought of that one: you were usually streets ahead of me on the ideas front! Ah yes VT127A, the "Pearl Harbour" radar valve. (It wasn't the valves' fault by the way...). Lovely. You might remember I had a collection of 826s - sold them to Gary Kaufmann - but they are 7.5v and yes, they look great. Pity they are so Class B; might make an eccentric input valve though - I know that Shishido used them - I have his load line. looks mad to me.

I have been drawing up a few ideas - email me for an exchange of views.

Regards to you and the family

7N7

[316a]

(off topic)
Unfortunately all those thoriated class B tx triodes need DC filaments . 3C24 is another one , very clean sounding and like the 826 , needs a ton of feedback , due to 32 ohms output impedence on the 8 ohm tap etc . Quite easy to drive , I used a 12B4A cathode follower direct coupled to the grid and ran into A2 . The grid provides the load for the 12B4a , only the 12B4A HT needs to be adjusted to set the bias . Choke input DC is the really the best way to go for DHT filaments , finding suitable chokes is another matter but those switchers are dirt cheap and plentiful at radio rallies I'll drop you a mail

cheers

316A

[coresta]

Hi 316A , do you have any idea to use your VT127A ? what would be the operating point ? Around the 100TH ?

[7N7]

Originally posted by 316a
(off topic)
Unfortunately all those thoriated class B tx triodes need DC filaments . 3C24 is another one , very clean sounding and like the 826 , needs a ton of feedback , due to 32 ohms output impedence on the 8 ohm tap etc . Quite easy to drive , I used a 12B4A cathode follower direct coupled to the grid and ran into A2 . The grid provides the load for the 12B4a , only the 12B4A HT needs to be adjusted to set the bias . Choke input DC is the really the best way to go for DHT filaments , finding suitable chokes is another matter but those switchers are dirt cheap and plentiful at radio rallies I'll drop you a mail

cheers

316A

Ha-ha! I always said you were a clever so-and-so!

I was laughing to myself the other day thinking about testing 4212Es...

7N7

[316a]

Originally posted by coresta
Hi 316A , do you have any idea to use your VT127A ? what would be the operating point ? Around the 100TH ?

(off topic )
No idea , I only got as far as lighting one up . From what I can tell the VT127A is like the 100TS which falls between the TL and TH . Ra around 8k mu 15 sort of thing I guess . 50 watt of filament and run at 2 watt as a driver Those 75TL look much nicer to play with

cheers

316A