I was looking at 7f7 tubes on eBay and came across some head scratchers of adapters. The obvious one is to use 7f7 instead of 6sl7. But how about 7f7 for 6sd7?
https://www.ebay.com/itm/387567242346
Pretty sure it is actually for a 7a7 and the seller made a typo in the title. But then I came across this one :
https://www.ebay.com/itm/205077894462
7f7 instead of 25z5? A 6v twin triode for a 25v double diode? It’s printed right in the adapter. The same seller is also selling a 7f7 to 25v6. What am I missing?
https://www.ebay.com/itm/387567242346
Pretty sure it is actually for a 7a7 and the seller made a typo in the title. But then I came across this one :
https://www.ebay.com/itm/205077894462
7f7 instead of 25z5? A 6v twin triode for a 25v double diode? It’s printed right in the adapter. The same seller is also selling a 7f7 to 25v6. What am I missing?
Well, if I had to guess, at one time there was a commonly needed sub for 25Z5 or equiv and the closest was a 7F7 or equiv. Remember they are both 300mA heater tubes and you can turn a triode into a diode by shorting grid and cathode. The TV repairman needed to know when he could use it but then they did know what they were doing. I remember the days when he would come to your house and almost always had some part that would work for any brand or model that needed fixing.
I remember Broskie talking about what a good diode a 300b would make lol. I guess I’m confused about the heater voltage. Going from a 25v tube to a 6v sounds bad. Does tying grid to cathode really increase the voltage demand that much?
During World War II, no consumer radios were sold in the US and most tubes were not available to the public due to military use. Many weird substitutions were made to get a radio working. Later in the war, some tubes were released (overruns or not meeting mil specs) marked "M-R" - (maintenance and repair only). My brother has a Sears Supertone guitar amp entirely filled with "M-R" tubes - live entertainment must have had a priority during WWII.
Tom--Live entertainment on the home front was indeed at a premium! Also, from late 1942 to early 1944, a recording ban was implemented by the American Federation of Musicians, so live music was the only music you could hear. ;-)
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