So don't just assume that heater voltage is safe if you expect 12V or 6.3.Īnother issue with mains droppers was if the heater chain went open circuit the ends of the chain were now at mains potential and that is why old Xmas tree lights were so dangerous. This means that a heater supply is 6.3 or 12V between heaters but can be 100V above ground. So sometimes you can float the heater chain, sometimes its ground referenced, and sometimes the 6.3 or 12V is lifted with another voltage to keep it above ground. The other thing is the way heaters are used sometimes means you have to elevate the heater supply eg on SRPP the top tube in the chain needs to be higher reference than the lower tube. One for the B+ and one for the heaters and one for the bias supply if you need one. You can use separate transformers for HT (B+) and heaters and even feed each supply with its own transformer. So dedicated mains isolation transformers are the thing to use. However the start up current (cold start) can be an issue more with DC supplies and regulators. So you should run the heaters from a dedicated supply ie even 12V DC is OK from a supply that can handle the running current. The reason is the fault and shock current is not limited and contact even by accident can and often is fatal. Never try to run any part of a circuit direct from the mains. The mains dropper was an old and dangerous idea. Never just assume, always check the specs. (Still makes a goof RF final, and a damn good audio final even though the spec sheet makes no mention of either alternative duties.) Since this type was originally a horizontal deflection final, it was also beefed up as screens grew larger and deflection angles increased. The 'B' here also doesn't mean controlled warm-up. The 'GTA will red plate badly if you try to operate at the same bias as the 'GTB. This was a third revision that increased both the PRV and I surge ratings so much that you can no longer use a 5U4G where a 5U4GB is required.įor 6BQ6GTA, the cathode is a good deal smaller, and the P D rating a good deal lower, than that of a 6BQ6GTB. The 'B' was added when the heaters were changed to controlled warm-up.Īs for the 5U4GB, that 'B' doesn't mean controlled warm-up, and the type was never intended for series string heaters. Another revision was the addition of control grid radiator wings for +v gk operation. Further revisions included an increase in plate dissipation from 3.5W to 5.0W since this was frequently seen in TV vertical deflection duty, and as screens grew in size, vertical deflection amps needed more output. (6AQ5s began showing up as TV vertical deflection finals in B&W sets that didn't include a PTX.) That may or may not be the case.įor an Octal like the 6SN7, the first revision was the "glass tubular" bottle (GT added). In the case of the 6AQ5, the 'A' was indeed added since the heater was changed. A tube gets a letter if, for example, a different grade of glass is used, or the shape of the bottle is changed. That change isn't even necessarily electronic. The additional letter indicates some revision made to the type in question.
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