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7581 vs 7591

ZR9B

ZR9B

Well-Known Member
What would the difference between the two be as far as " will it work well"
I bought a Fisher 500b and the guy pulled the good tubes and put some HK 7581 in there. He said he never powered it up since he got it years ago....who knows ....thanks
 

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Look up "tube data sheets". They will show you what you want to know.

A 7581 is more like a 6L6GC or 5881 and is a different tube altogether so I wouldn't power it up unless you know the correct pin out and if they will bias up ok.
 
I did find what I was looking for....I should have looked be for I asked....I'm learning.
 
Me too! Ya gotta learn something new every day! :thumbsup:

:music:

I think that person that kept the 7591's, must have known what he's got or what the're worth. I'd make him give 'em back as he obviously mis lead you probably knowing the 7581 would not work and possibly cause damage. :dunno:
 
The pinout is also completely different, so either it won't work at all, or someone has rewired the sockets. Making one of those work in place of a 7591 also means changes to the bias circuit, and since the 7581 wants a lot more drive you're not going to get rated power out of it without reworking the phase inverter for more drive. You'd really have to see what was done to this thing. At one point it wasn't that uncommon to see 7591 amps re-wired to take a 6L6 family tube. You can make one work but its usually not going to perform as well as the original setup.

So nutshell, not a drop-in.
 
ZR9B said:
What would the difference between the two be as far as " will it work well"
I bought a Fisher 500b and the guy pulled the good tubes and put some HK 7581 in there. He said he never powered it up since he got it years ago....who knows ....thanks
Click to expand...
Gadget is on the money in that there was a point when the 7591 were unavailable and so in desperation some converted their units to use 5881 or Russian 6p3 (?) / 6l6
Now the 7591 are available again but in reality they are just a poor replacement for the originals, which if are good testing used are probably a better choice. So you need to check to be sure or disaster could happen if you put the wrong tube in the unit.
 
The current replacements really aren't that bad. Maybe not quite up to the originals but certainly better than a 6L6 family tube in that spot.
 
Can anyone tell by this pic if it's been rewired?
 

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The 1K resistor between the coupling cap and tube socket should tell the tale. If its wired stock, it will land on pin 6. if its rewired for a 6L6 family tube, it will land on pin 5.
 
The new Tungsol 7591A is an exact re-edition of the vintage one. Look and sound.
Like many others, I highly recommend it.
 
I like the Westinghouse 7591, with pink base. Most of my Westinghouse 7591 still tests like new after 60 years. One of the secrets was the metallurgy in constructing the plate, which is a sandwich of different materials. For awhile Westinghouse supplied all the 7591 to other tube makers since the others could not duplicate the design correctly. It is something to ponder that GE, RCA, Sylvania, Tung sol, etc. needed a few years of design and engineering before manufacturing their own. And if i am not mistaken only GE and Sylvania made their own 7591.
 
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