Tube Experts: What's the difference ??

[slider313]

http://photobucket.com/albums/b376/slider313/Picture030.jpgHere we have an RCA 6V6GTA along side a RCA 7408. The shot of the getter rings shows the 7408's are larger. That's the only difference I could see. Tube spec. sheets show they are electronically identical. Anyone care to chime in? Are there any differences between the two?http://photobucket.com/albums/b376/slider313/Picture037.jpg

[Bruce O'D]

Only difference I'm aware of is that the 7408 is a MIL-SPEC version of the 6V6GT. In general MIL-SPEC required tighter tolerances, wider operating temperature range, higher duty cycles, increased reliability.

[phsyconoodler]

The data sheets say it has different no-signal characteristics than a 6v6.They say they are identical otherwise.Obviously it was designed for something inparticular but the data sheets don't say specifically.
If you can get them cheap they might be a great low-cost sub for a 6v6.

[antik]

RCA promoted the 7408 as being better for high fidelity applications. Not sure why though. Maybe because it has slightly lower distortion.

[Old Tele man]

...the MIL-SPEC tubes had exactly the same operational specifications (rp, mu, gm, etc.) as their commercial counterparts...the only differences were in environmental (vibration, shock, temperature, altitude, ruggedness) and reliability (operation lifetimes, initial failure rates, closeness to 'bogie' specs).

[slider313]

RCA promoted the 7408 as being better for high fidelity applications. Not sure why though. Maybe because it has slightly lower distortion.

For that matter I have a set of Sylvania's 7408's which look exactly like their 6V6GTA counterparts.http://photobucket.com/albums/b376/slider313/Picture035.jpg

[rockon1]

Looks like the one on the right in the first pic has a double mica spacer on the bottom. Bob

[Trout]

7408's were common in mid-priced Harmon Kardon Hi Fidelity receivers and some mid priced amplifiers. Strombeg also used them in a few models consoles, But Stromberg in most cases had their tubes re-labeled as such.

They were intended for audio applications so using them as a sub for a 6V6GTA in a guitar is fine. Of course as far as actual tone, some brands were better than others, but that is always the case with most tubes anyway.

When you do find them, very often they cost more than most 6V6 family tubes as they were less plentiful.

Trout

[slider313]

Looks like the one on the right in the first pic has a double mica spacer on the bottom. Bob

Good eye. I didn't catch that. The tube on the left is the 6V6 GTA.
By the way, these (the 7408's) sound fantastic in my original '65 Deluxe Reverb.

[Old Tele man]

...like I said, the MIL-SPEC tubes are often much "closer" in tolerances to the 'bogie' spec values (less distortion?) and are much more rugged (double mica, etc.).

[Blue Strat]

7408 is actually listed as an "industrial" number (whatever that means), NOT a military number. I've seen tons of JAN (military) 6V6GTs but don't remember seeing any RCA military 7408s. They always seem to be in commercial boxes and without the JAN labeling.

[John Phillips]

Probably the reason they were recommended for hi-fi is that the ruggedized construction should make them less prone to microphonics, which are a source of distortion.

But (mild) microphonics are part of what makes tube guitar amps sound good - as usual beware of importing values and concepts from hi-fi and assuming they'll be improvements for guitar! :) The goals are quite different...

Actually it probably doesn't make that much difference for power tubes, and since they're more rugged they should probably last longer in combos.

[Jeff West]

Yes, 7408s are a bit mysterious. They appeared at the end of the '50s from multiple makers. I agree with Mike that mil spec is a red herring in this case, they weren't designated mil spec. Although an industrial number, unlike many other industrial numbers inc for ex the 5871 which was explicitly designated as a "mobile-rated" and more mechanically rugged and electrically forgiving (e.g., at low filament voltage) 6V6, no manufacturer lit ever claimed increased durability, ruggedness or any other special standards or distinctions for 7408, with the single exception (sometimes) of "special controls on zero-bias characteristics" as also quoted by Sibley. Ratings and operating characteristics were absolutely identical to 6V6GTA.

GE had 7408s obviously made by them that do differ a tiny amt from their 6V6GTAs of the same year- an extra cutout flap in the grey plates IIRC. Like Slider said, Sylvania labelled ones that looked exactly like some of their 6V6GTAs, and so did RCA- some RCA grey plate 7408s look completely identical to RCA made 6V6GTAs. Whether they were specially selected or differed internally I couldn't say, but they sound the same to me. But be aware that even as early as 1960-61, many RCA branded 7408s were actually made by other U.S. manufacturers, in particular there are a lot that are clearly the distinctive looking clear glass shiny black plate Tung-Sol 6V6GTAs. Also plenty of top getter GE-made branded and sold by RCA, and I think I've seen Sylvanias too, esp later.

Looking at some old RCA and GE price lists, 7408s did consistently retail for significantly more than 6V6GTAs, about 25% more in 1964 and up to 50% more in 1966. But that may reflect other factors than actual internal differences!


My parents had a 1959 Knight stereo amp that ran GE 7408s branded for Knight stock. The manual specifies they should be replaced only by 7408s, but doesn't say why!

Maybe someone here has additional specific info about them.

Jeff

[mc5nrg]

Could it be savvy marketing?

[Jeff West]

If that was it, I'd say it wasn't wildly successful.