Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on January 12, 2015, 02:53:04 am
-
In many circuits Fender used both tubes
but looking to datasheets the two tubes seems to be equivalent
where is the scam :w2:
K
-
google "12ax7 vs 7025": get responses like: In the old days the 12AX7 and 7025 were different quality tubes. 12AX7 is the U.S. and 7025 is the industrial name. 7025 is higher in gain and a bit brighter then the 12AX7. It’s also more heavy duty so it can take a beating. Nowadays tubes are not made this specific so the name mean almost nothing except on old tubes. A Russian 12AX7 is the same as a Russian 7025.
-
A patent (US2677782-A 2,867,032 ) for a helically wound heater was filed in 1950 by Gehrke, Huntington, and Granger, assigned to Sylvania. The patent describes the chief benefit of this invention to be longer tube life, although it would also later be considered to be an improvement in minimizing heater supply related noise in the tube’s output.
The first audio tube produced with this technology was the 12AY7 in 1953. Of course, the 12AX7 predates the invention by three years. The 7025 and 12AX7A were 12AX7’s with the helically wound heater incorporated for its lower noise benefits. There may have been other low noise technologies incorporated as well, but this is the only one I’ve been able to find so far. EDIT: the ad below also mentions "a sturdy cage assembly featuring short, stuff stem leads, oversized side-rods, and micas of special design to reduce noise and microphonics."
I imagine the new heater was more expensive to produce, so the 7025 and 12AX7A would have been sold at a premium. To maintain a competitive price, all manufacturers probably elected to sell both a 7025 and a 12AX7 until manufacturing costs became low enough that they could use the same heater for both.
The 7025 appeared in 1958, and the 12AX7A in 1960.
Here's an add from 1958 touting the 7025's benefits:
What else? I'm sure there is more to it....
(http://i.imgur.com/f4Xk11D.png)
-
If a 7025 is a butterfly in flight, what's a 12AU7? A garden slug? LOL.
-
Many Thanks JJasilli & Terminalgs
@ Eleventeen :laugh:
Franco
-
> 12AX7A in 1960.
Non-issue. This "A", like most in that era, just means "controlled heater warmup" for use in series-string with assorted tubes (TV sets). We rarely find series-heaters in guitar amps. When we do, it is usually a string of the "same" types, so un-even warming is not an issue. And I suspect today's factories don't control warm-up (except they probably use the same heater-stuff throughout a range of types).
Yes, the 12AX7 has WWII roots, gunnery computers. It is a 6F5 prototype, made 2-in-1, in a mini bottle. It never had audio aspirations.... who could possibly need that much gain in that small space? And the gain is horrid with transformer coupling. Well, we did find audio uses for 12AX7. Nobody complained about the hum. But the trick-heater (thanks for the patent citation) seemed appropriate. OTOH there were millions of sockets known to work fine with old-style 12AX7, some mission-critical (advanced gunnery computers) where nobody wanted to take chances on new tricks. Also by this time 12AX7 was a competitive or mass-contracted product, you couldn't up the price. OTOH you could introduce a "new" tube in the new Industrial (4-digit) series, add a spot-check for hum, perhaps incorporate better micas, claim "upgraded 12AX7", and charge a higher price for a while. Apparently at some point Leo thought the 7025 was better in some sockets, though I think later he didn't really care (which does suggest the improvements spread to both parts).
6EU7 (~~1960) is a 12AX7/7025 prototype but with heater-pins moved well away from audio pins. From heater there is a gap and then a cathode (least likely to absorb hum). It avoids the ugly 4+5/9 spread of 6V 12AX7 heater wiring. It was popular in some hi-fi and PA work, but not all designers adopted it. If facing a 6EU7 hole, you can rip the wires off and re-wire to take 12AX7/7025.
-
Sonotone self-promotion, 1963.
Their "12AX7" claims much of the benefits touted for 7025.
It is not clear to me that Sonotone had their own tube factory (but not impossible), suggesting they were contracting with an established bottle-works to get self-branded tubes of certain specification.
-
Thanks for the interesting explanation PRR
Franco