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Why did manufacturers go to 5-volt rectifiers & separate windings on their PT's? As opposed to two 6.3-VAC heavier gauge wires --- one for the heaters and one for the rectifier?Anyhow, a nice site for voltage drop (sag).
Thanks!
Edits: 10/19/12Follow Ups:
Another reason for the second winding is the filament winding is connected to the cathode of the rectifier. That means you have some ripple noise and high voltage. You want the heater voltages closer to ground than the plate supply.
RCA did a lot of the early tube development and first used 5V for tubes that were made to be run on 6.3V batteries. Radios used filament rheostats which were adjusted as the battery voltage dropped, to get the last bit of power out of the battery before recharging. Lots of people didn't have AC power yet, so would have had to take the battery to a garage (or hook it up to the tractor!)
When they developed AC tubes, they used 2.5V for minimum hum. Since power tubes and rectifiers used a lot of heater power, they used higher voltages - 5V, 7.5V - they were run on separate heater windings anyway. Early higher power transmitting tubes were 10V. The multiples of 2.5V made it easy to get the voltages right - 2, 3, 4 times as many turns on the transformer.
Meanwhile, Philco was making car radios that ran on 6.3V, so naturally they used 6.3V tubes. They bought tubes from Sylvania, not the enemy (RCA). 6.3V tubes worked fine on AC as well, so they had fewer tubes to stock. The 84 and 6X4 rectifiers ran on 6.3V and had insulated cathodes so that there was no need for a separate heater winding - but were only good for 70 mA and were prone to shorting... a problem Zenith collectors know well. Lots of '30s Zenith sets died when a 6X5 shorted and took out the power transformer. Some got repaired with a different transformer and 5Y3 or 5U4.
I don't how many times I've replaced the power tranny and thought about the voltage question. Then, forgot to ask someone -- after the unit was working, again.
8^)
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
" ...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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