7591 tubes vs 7591 wafer base tubes

[keybank]

I was going to order the fisher x202b rebuild kit and give it a go myself. Looks like I may give it to my tech and have him rebuild it. will he be able to match these tubes or should I look for others?

If you're asking about whether he'll be able to match them, I can't speak to that, but if your question is whether the two tubes you bought will be matchable, that can't be determined until you get them in the amp. If their idle plate currents aren't too dissimilar, the answer is yes, by using resistors to alter grid voltage (i.e. C-, which controls plate current). Sometimes you have to go through a lot of tubes though to find two that match under working conditions.

 

[Patrice B]

How does one distinguish between the 7591 and 7591A as i have herad that the A version is of inferior quality? size? markings?

The A is in fact a somewhat improved version with innards oriented differently to prevent noise (correct me someone if I'm wrong).

I run a mix of A and originals in my Scott (even mixed brands).

 

[keybank]

You can have your tubes tested by someone using an Amplitrex AT-1000 tube tester and see how closely your tubes match.

Wb

Eek, $2,700 for one of those. That would be nice, but I don't know anybody with one myself, maybe the OP does.

All you really need to match tubes is a $15 DMM, anything that measures d.c. current down to milliamps (which most do). Accuracy is irrelevant, since the absolute level is meaningless to the match, only the push-pull pair's current relative to each other.

 

[skriefal]

I know "someone" (me!) with a Maximatcher who would be willing to test the tubes, but not sure if it'd be worth it to ship them both ways for this. Perhaps you can find someone local who'd be able to do the same.

 

[Isotonic]

You guys are painting a grim picture. You don't absolutely have to have a perfect matched set, matched with a matcher that replicates operating conditions. It is ideal to have triple matched set, like those sold by the better sellers, but people successfully use their tube amps with mutual conductance matching all the time, with no problems.

And incidentally, Green Girl (I am a satisfied customer), on Ebay has done me right with matched tubes every time. And there are several others whose seller records are outstanding.

Your best bet with the Scott is to have it modified to safely use current production EH 7591 tubes. I don't know how to do it, but I have seen it done, and it amounts to a couple of resistor changes. There are plenty of threads about it on the net and many on this forum.

While you are getting the mod done, have the electrolytics changed for safety. Get you some matched tubes and enjoy that Scott.

 

[buglegirl]

Hiya,

Not to start an argument.

but people successfully use their tube amps with mutual conductance matching all the time, with no problems.

I have to disagree but not in a bad way. Its close but not the same. However you can get the best of both worlds if you add a ammeter circuit to a good mutual conductance tester.

Of course someone is gonna show up and tell me that disagreement will be on curve traces.

Fun thing about this hobby. You can take it as far as you want.

Frannie

 

[scottfan]

Have one wafer/coin base 7591a. Unlike it's predecessors, there noticeable blue electron cloud engulfing the plate while the big base electron stream is hard to see. The glass envelope is also smaller in diameter but the getter flashing is also smaller.

From observation, wafer base 7591's fetch more $ than big base versions.

 

[Dave C]

My concern is that you bring your amy up to power on a variac and watch the voltages on the tubes so you don't smoke them. If you simply install the tubes and turn it on you stand the chance of making smoke signals. CYA!

 

[Tom Bavis]

BTW, Scott amps should be fine as-is with EH 7591 tubes, as they have a fairly low effective value of the grid resistor. Fishers should have the grid resistor value reduced from 330K to 180K or so... Heath used 470k (Eek!), well over the data sheet maximum of 300K.

 

[keybank]

Hiya,

I have to disagree but not in a bad way. Its close but not the same. However you can get the best of both worlds if you add a ammeter circuit to a good mutual conductance tester.

Frannie

That's helped me prematch many tubes, great idea. The TV-2/C is good for this, because it uses filtered d.c., has complete voltage variability on each element, and will run the plates to 250 volts.

 

[Bluelobster]

From what I've seen, the Westinghouse 7591 has a taller full base and the numbers on top. The earlier Sylvania 7591 & 7591A has a shorter full base and numbering on the side. It was later in the 1970's that the coin base tubes started showing up. Sylvania made the lions share of 7591 tubes, 7868 and 6GM5 tubes as well.

The oddballs from this family of tubes, (7591, 7868, 6GM5) is there is a Japanese 7591 made by Toshiba? and RCA had their own 7868 that I know of. Then you have the current production tubes.

If I come across my 7591 stash I'll post some pics.

I have two quads of GE 7898 and another two of GE coin base 7591a. All are acid etched with GE date/manufacture codes.