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7591 tubes vs 7591 wafer base tubes

rshadley

rshadley

Active Member
I recently won a fisher x202b without tubes and have been picking up some tubes for it. I just purchased 4 7591 tubes of epay and did not no that there were two version, standard base and wafer base. I have two wafer base 7591s and two regular base 7591s, will this be a problem if I use them in the fisher x202b? What are the differences between the two?
 
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rshadley said:
I recently won a fisher x202b without tubes and have been picking up some tubes for it. I just purchased 4 7591 tubes of epay and did not no that there were two version, standard base and wafer base. I have two wafer base 7591s and two regular base 7591s, will this be a problem if I use them in the fisher x202b? What are the differences between the two?
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The wafer or coin base is just an "improvement" of tube design. Some will claim that it also allows more heat dissipation from the tube also. The coin base tubes will often bring a little less money on flea bay because they do not look as original as the original type. Go ahead and use them. They are vintage and most likely made well by the tube tooling "of the day".
 
Difference is manufacturing date... wafer base is later. Who knows if there are other differences...
 
rshadley said:
Should I run the two wafer base on one channel or seperate them?
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It is entirely your choice. It may be advisable to use them as a pair since the manufacture date will most likely be the same or nearly the same. Can you tell us what brand and do they have any markings on them? I believe that in the US, Sylvania and Westinghouse were the manufacturers regardless of label. For instance, if it is labeled RCA, it was most likely made by Sylvania. You can tell by the font and location of the tube number etc. Sylvania "usually" marked the tubes on the side of the envelope. Westinghouse "usually" marked the top of the tube. And the Japanese made them too...
 
I have not received them yet, but the wafer base are one GE and one sylvania and the standard base are one RCA and one GE
 
My Fisher 800B came to me with one standard base and one coin base 7591 in each channel. It was already set up and working well that way, so I left it alone. There shouldn't be any significant internal difference, it's just cosmetic.

Easiest way to ID the true manufacturer of a tube is to look at the font and style of number etching. For instance, RCA marked their tubes with the tube number inside of an octagon, either on the side or top. Tung-Sol did the same, but their octagons tend to have lines of varying thickness and a short vertical line above the octagon. GE used stencil-style numbers, typically with the voltage above the rest of the code and a dot code below. Sylvania, Westinghouse and Amperex (US) tended to use very plain sans-serif numbers and letters for their codes. One of these days someone should start a "tube ID guide" thread.

If my memory isn't faulty, Westinghouse and Sylvania were the only US manufacturers of 7591s, with Sylvania only getting into the game with the 7591A. If they're non-A versions, they're probably all Westinghouse tubes rebranded for the other manufacturers.
 
How does one distinguish between the 7591 and 7591A as i have herad that the A version is of inferior quality? size? markings?
 
Always order matched pair tubes. This is essential for lowest distortion and having both tubes run at the same idle current.
 
Sam Cogley said:
My Fisher 800B came ...

If my memory isn't faulty, Westinghouse and Sylvania were the only US manufacturers of 7591s, with Sylvania only getting into the game with the 7591A. If they're non-A versions, they're probably all Westinghouse tubes rebranded for the other manufacturers.
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That is my understanding as well...
 
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 appreciate all the input but I have one last question. How does one determine if a set of tubes is matched?

Thanks
 
rshadley said:
How does one distinguish between the 7591 and 7591A as i have herad that the A version is of inferior quality? size? markings?
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The A shouldn't be inferior to the original 7591. Some later eastern bloc variants may have issues.

Distinguishing between them is easy. 7591s say "7591" on the tube. As say "7591A."
 
rshadley said:
I appreciate all the input but I have one last question. How does one determine if a set of tubes is matched?

Thanks
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It's probably not what you want to hear, but the short answer is, you can't, at least not without making measurements during operation in your amp.

True matching has to be done in your amp, or in a specialized matching device (NOT a tube tester) that can duplicate the conditions in your amp. Even the expensive devices like the MaxiMatcher only offer a few set options for element voltages. There's another recent thread in here about 6L6 conversions where this topic just got some air time, and that might be of help to you.

p.s. eBay-style matching, usually meaning saying the percentage meter on some Hickok-derived military tester that uses pulsating d.c. on the grid and barely over 100 volts on the plate means nothing. (I see your seller didn't use the term "matched," to his credit).
 
Rs,
Your best bet is to assume that any ePrey seller offering matched tubes will be shipping you anything but matched tubes! Tube construction, date codes, and similar markings are absolutely meaningless with regard to matching.

The upside is that the X202B has separate bias and DC balance controls. The downside is that Fisher elected to employ a common cathode resistor, one per channel, for each pair of output tubes. There is no easy, or safe, way for you to measure tube current (bias) with this arrangement without sophisticated equipment.

The good news is that you can remedy this by rewiring for four separate cathode resistors, one for each tube, that'll allow you to easily bias and measure current for mismatched output tubes.

There's always a chance that you might end up with a hot tube that won't bias down to the others, or a cold tube that won't bias up. It happens, but you'll probably be OK.

Anyway, this is an easy fix which requires no electronic theory to accomplish, just a little and patience mechanical skill.

Rich
 
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?
 
keybank said:
It's probably not what you want to hear, but the short answer is, you can't, at least not without making measurements during operation in your amp.

True matching has to be done in your amp, or in a specialized matching device (NOT a tube tester) that can duplicate the conditions in your amp. Even the expensive devices like the MaxiMatcher only offer a few set options for element voltages. There's another recent thread in here about 6L6 conversions where this topic just got some air time, and that might be of help to you.

p.s. eBay-style matching, usually meaning saying the percentage meter on some Hickok-derived military tester that uses pulsating d.c. on the grid and barely over 100 volts on the plate means nothing. (I see your seller didn't use the term "matched," to his credit).
Click to expand...

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

Wb
 
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