7984

[gianis]

Does anyone knows something about 7984 tube? is good for audio? Thanks

[SY]

High perveance, low screen voltage rating, decent plate dissipation- I'd say you've got a good candidate output tube for screen drive.

[rsumperl]

Anyone else playing with 7984's? I bought some from eBay and have been doing some experimenting. I'm using 500V on the plate and 225V on the screen. I set idle plate current to 37.5ma/tube. Screen current doesn't seem to vary and stays at 1ma. I'm using a 3300 P-P Edcor. Grid 1 voltage is -40.4V for 37.5ma. I'm feeding with the board from a Dynaco Mark III with feedback hooked up. First impressions are really positive.

[w5jag]

How are the 7984's working out for you?

I ran across a sleeve of 8156's in my stash, and that got me thinking of 7984's. I'm wanting something 2E26 or 6146 like, and compact, without the inconvenience of plate caps.

I'll probably use them single ended, though.

win W5JAG

[rsumperl]

I have used 7984's in P-P. I used a 3.3K transformer w/500v on the plate and 225v on the screen.

[smoking-amp]

Very strange pinout on that 7984. The plate pins are doubly surrounded by cathode pins. What were they thinking? A recipe for disaster!

Curve and specs look close to a 12JN6, I think I'll stick to the $1 versions. (the 35 Watt spec is for ICAS, ie intermittant usage, only 20 Watt continous, which the 12JN6 easily matches in audio rating 17.5 x 1.2 => 21 Watt)

[tubelab.com]

What were they thinking? A recipe for disaster!

Actually it's a recipe for success.....in the tubes target use, a class C RF amp.

The cathode is RF ground, the plate uses 3 pins to reduce parasitic inductance, and the grids are on the opposite side of the tube. A tube, especially a pentode does not exhibit a 180 degree phase shift between grid and plate at VHF. Oscillation and instability is the prime concern here.

The tube was designed for VHF land mobile radio use where all the incumbent tubes were push pull. It was succesfully used in GE and Motorola VHF (136 to 174 MHz) mobile radios for several years.

I have a box full of these but never tried them. There is just too many cheap sweep tubes that look more appealing.

[smoking-amp]

That's fine for RF stability, but according to the 750 V B+ spec for the 7984 there could be 1500 V across the 1/8 inch gap occuring in two different places on the socket. The 6L6GC is only rated for 500 V B+ and has almost a 50% larger gap between octal pins with only one such adjacent pin threat. Yet they still manage to burn up sockets regularly.

The 8150/8149 tubes look to be a remake of the 7984. Note the improved pin layout for safety.