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Trouble concerning JJ 7591

trainbuftony

trainbuftony

Electron Herder
I am working on a Scott 340B, I decided to pull the original tubes and replace them with a quad of brand new JJ 7591s which I had on the shelf.I installed them for the purposes of testing. I slowly brought the voltage up on the Variac at about 110v (on the Variac dial) one of the grids went from -21v to 0. I switched the tubes side for side and got the same thing. Grid to cathode short. Every thing else comes up normally. The can caps are working and the coupling caps already got replaced. I have read here and elsewhere thst the JJ 7591 is not fantastic. My question is this, should I go ahead and drop in a known good set of vintage output tubes made by westinghouse? My problem is garbage for tubes right? I have a pair of good 7591 but I am leary about ruining one of them. They are not cheap. Of the tubes which were in the unit, 3 tested ok, but not great and another was shorted. When working on equippment this cool, I constantly second guess myself. Thanks in advance for your advice!
 
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I'd get some of the other 7591s which are made by newsensor. They are better than the JJs. Not sure if the EH's will fit in the Scott. But if not, I just get some of the tungsols.
 
EH tubes fit and work well if you're not using a case. The extend higher than the transformers. If you're using a case you can use the Tung Sols
 
I guess what I'm asking is: Is it normal for JJ 7591s to fail spectacularly? I have plenty of good old American made 7591s, not looking to buy any tubes. Should I just stuff some real, old school 7591A westinghouse tubes in it which came out of an organ? Is there some updating which needs to be done? Resistors are all within 10% of their value. Should I be worried about my vintage tubes?
 
Bias supply provides -21v at the grid the pots to adjust this all work. The problem is the JJ TUBES (I think) 1 shorted grid to cathode, then another shorted internally creating a spark show, and I was not able to see what was going on due to shutting of the unit in a hurry. Probably another grid to cathode short...
 
Train -- spectacular failures of the JJ 7591 is not uncommon at all -- however, did you install Screen Stability resistors?

Dave
 
I have not, what value do you recommend? Can I tie the screen voltage rail to the unused pin on the 7591 and simply run a 100 ohm resistor to the 2 screen connections?
 
Yes. Over on the Fisher Forum, there are a number of pics floating around showing how this can be done (much as you suggest). I mention this because it may be an arc that took out your tube, particularly if any of the can caps have been replaced, but always a good idea to install, whether you use JJ tubes or not.

Dave
 
The 100 ohm grid resistors and some used old stock westinghouse 7591 tubes did the trick. Sounds good and makes loads of power!
 
Train. Day late and $25.00 Short(accounting for inflation) AHHH WHAT THE HELL. I wrote the below and posted it before i saw your last post.

In the "COMMON FISHER PARTS" Sticky, HERE there is a drawing on how to install the screen stability resistors. See Improved procedure at bottom of post. Use 100ohm 1/4 watt. If you install the resistor from pin 1 to 4, you can daisy chain the supply rail to pin 1 all the way down.

If there is a wire between the tubes connecting pins 4 & 8 then btween the tubes, remove the part between the tubes, to separate them. The daisy chain wire on pin #1 does the same thing, and with the resistors between #1 and #4, it's a neater setup.

With a Scott, I would use the Tung-Sol's over the EH's mainly for the envelope size on a smaller chassis. The EH's won't have much room side to side, and they will stick out the top, should you want to use the case, the EH's won't work.
But if you are using Old Stock tubes go for it. Those old Westinghouse tubes are extremely tough. What killed the JJ wouldn't give the Westinghouse the time of day. Don't worry about old tubes. Once you get the screen stab resistors in you should be ok. Also Check the grid return resistors. If over 300K drop them to at least 270K. BUT IIRC the scott grids are around 220K or so. If thats the case, no need to replace, unless it's bad.


Steven Lafferty and Dave Gillespie did some comparison testing of 7591's including JJ's. Interesting reading. http://www.tronola.com/html/7591a_tubes.html
 
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