7581A Tubes

a DIY, modified Mullard 5-20 monoblock design

Postby EWBrown » Wed May 04, 2005 12:56 pm

Looks like they are a very close equivalent to 6L6GCs, specs here:

http://www.mif.pg.gda.pl/homepages/fran ... /7581A.pdf

/ed B in NH
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Postby erichayes » Wed May 04, 2005 11:43 pm

Hi All,

The 7581 is a tube I, quite frankly, can't figure out the rationale of origin. The 7027, which was a 6L6GC with higher plate voltage and dissipation spec's. was introduced in 1958. The 7581, introduced in 1960, was virtually identical to the 6L6GC, but with the dissipation of the 7027.

The 7581 was intended as an industrial/military tube, so I assume that it was a 6L6GC that went through an extra couple of quality control hoops.

I have several musician friends who are adamant about the sonic superiority of the 7581 over the 6L6, and would kill to get a pair or quartet. The fact that Antique Electronics has a "call" order for them would seem to bear that out. Go figure...
Eric in the Jefferson State
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6AR6 / 6098 / 6384 / 6889

Postby EWBrown » Thu May 05, 2005 5:42 am

I've been wondering how well Tung-Sol 6AR6s, 6098s or the elusive Bendix "Red Bank" 6384s would work in audio service. They look externally similar to a 5881, but have a totally different weird pinout, and much higher ratings, and the 6384 was made to be used in early guided missiles, they are as tough as a glass tube can be. ceramic base, ceramic interior spacers, the filament is threaded in a ceramic block. THis results in a very slow warm up time, but also makes it resistant to 300Gs and higher. The published specs don't go into amplifier service, just pulse, DC regulation and servo driver ratings.

To use these tubes on the IKE board, would require either some PCB surgery, or hard-wiring an octal socket using the appropriate pinout corrections.

I have a Bunch of 6AR6s and 6098s and a few prized and hard to find 6384s. Been looking for 6889s, but they seem to be made of unobtanium...

I figure these would be great for guitar amp service, as they are tough enough to take the bumps and bruises of being hauled around.

/ed B in NH
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Postby Uncle Ned » Tue May 10, 2005 1:25 am

erichayes wrote:Hi All,The 7581 is a tube I, quite frankly, can't figure out the rationale of origin. The 7027, which was a 6L6GC with higher plate voltage and dissipation spec's. was introduced in 1958. The 7581, introduced in 1960, was virtually identical to the 6L6GC, but with the dissipation of the 7027.



7581 is just a 6L6-GC that had a brown base so hams and commercial transmitters could run the piss out of them without the bases falling apart from the heat.
See: http://www.triodeel.com/ham1.gif and http://www.triodeel.com/ham2.gif
Note that as they were intended to be used, they'd only run at max dissipation for short periods of time (ie: when the mic key was down), not 100% duty cycle.

The ratings that RCA gave 7027 and 7027A were a joke from day one, no one who had to deal with these tubes on a regular basis ever took the RCA claims seriously. That's why the NOS RCA tubes are hard to find, they weren't that popular and burnt out quickly. Try running a GE 6L6-GC, and "black plate" RCA 6L6-GC and 7027 at 35W dissipation. You'll see what I mean.
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