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In Reply to: RE: Just outta curiosity posted by FenderLover on October 19, 2012 at 21:29:39
You have yto understand the history.
The earlier power tubes like the 2A3 used 2.5 volt filaments. Batteries were much lower voltage earlier on. The rectifiers were next to be developed and they simply employed twice the filament voltage for the power tubes.
The advent of 6.3 filament voltage parallels the development of automotive batteries. The early voltages were 6.3 volts and later expanded to 12.6 volts, in the 60's. Thus noting the filament voltages you can pretty much date the development of a particular tube.
Stu
Follow Ups:
" ...and they simply employed twice the filament voltage."
Thanks, Stu. One issue though. Why would you need a rectifier for a battery source? It's already DC. Right?
remember that I'm an old man
Early radios basically ran off of batteries and had A, B and C battery supplies. You can see this on old 5 tube radios in those huge shoebox cabinets. AC power came later and if you read the early ads, were primarily used for lighting only. I have pre 1920 radio magazines that advertise special adapters for your light socket to split the AC to a transformer and thus provide for your radio needs (no duplex outlets back in the early days).
Again the proliferation of battery radios and then the advent of AC meant that you had to rectify the B+ voltage to replace the B battery. You can see this on some military type radios and walkie talkies (B+ battery supplies).
The 6.3 volt filaments are not necessary for home use, but necessary for military application in their vehicles.
In fact there are a number of odd filament voltages for tubes. I have 115 volt filaments and you also have to realize that manufacturers eventually would string the filaments of the tubes in series to get a 115/120 volt draw and thus avoid the need for a transformer.
For example you could string two 50L6's in series with several 12AX7's in 6 volt configuration and get basically 120 volts so a filament transformer would not be necessary. Damper diodes were designed for this use, and you see it often in older TV sets.
Stu
have AC to step up the voltage with a transformer. Tube-type car radios used a vibrator which chopped up the DC into square-wave AC for the power transformer, then it was rectified back to DC.
Converting older battery powered equipment, to use an AC source. Yet, still utilizing the existing tubes.
Thanks!
Those GE 6CA7 are killer tubes. There is a trend to make EL34 guitar amps "cleaner" (I know, sacrligious), and use stomp boxes and/or programmable modelling kits to get close to the original "Mullard EL34" tone.
The GE's are much tougher tubes, than the original xf1 and xf2 Mullards.
Thanks, again.
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