Comments for page: 7591A Tube Comparisons

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Posted by Dave Wax December 23, 2012 - 02:03 am
Steve, thanks for your quick response. No, I haven't checked the getters in the tubes - will do that tomorrow. Didn't even think about the getter in each tube. I have checked the output stage bias for each tube - it is close to the -17 volts shown in the schematic (fixed bias).

This 60-year old amplifier uses solid-state rectifier diodes! I'm amazed. The downside of course is that there is no warm-up delay, as you would get with a vacuum-tube rectifier. I really don't want to modify the amp. I put my RCA 7591A output tubes back into the amplifier, and it's now working like a champ with no problems, and definitely no arcing in the output tubes. These tubes are rated for 500 volts max, and do not seem to have trouble with the initial start-up voltage. The 12AX7's that are further down the RC filtering chain have never shown any problems with high start-up voltage. I have to assume the designer(s) knew what they were doing. My main concern is how well the filter caps are holding up, but so far so good.

Posted by Steve L. December 22, 2012 - 03:29 pm
Hi Dave W., Thank you for your kind comments. Sorry that you experienced problems with your EH tubes. A question about the loss of vacuum: Has the getter in the EH7591s turned white or otherwise lost its shiny silver color? That would be consistent with loss of vacuum. There are other phenomena which can result in arcing in a vacuum tube. I trust that you have checked the output stage bias voltages.

One other question: By any chance have you substituted a 5U4 (or solid-state rectifier) for the GZ34/5AR4 rectifier? The GZ34 has a slower warmup, which should minimize the surge in plate voltage by allowing the heaters to warmup before full B+ is developed.

I'm going to alert David Gillespie about your post, as he might have more to say about the arcing.

Posted by Dave Wax December 22, 2012 - 02:20 pm
Hi Steve. I just stumbled across your post and found it fascinating and very informative. I have been searching for discussions concerning the quality of new 7591A tubes vs the older tubes.

What prompted my search is trouble I have recently experienced with a quad matched set of EH-7591A tubes, which I bought a year ago for use in my Sherwood Model S-5500II Stereo Amplifier. I bought these to replace my 2 matched pairs of RCA 7591A tubes, because I was concerned with wearing them out. The Bakelite bases are already crumbling, and I want to preserve them as long as possible.

The LH-7591A tubes have worked fine for the past year, providing high quality sound on my old stereo system, which I bought new in 1962. Then about a month ago, when I turned it on, there was a pop from the speakers and the system went quiet. Assuming an old electrolytic cap had gone south, I opened up the amp, but found no problem with the caps. But two of the LH-7591A tubes started to arc when the high voltage to the plates was initially applied! I couldn't believe it! One-year old tubes were apparently leaking enough to allow arcing. The high voltage initially goes to 490 volts before the heaters warm up enough to allow the tubes to draw enough current to bring the output tube plate voltage down to about 420 volts. These voltages are within spec for 7591A, so apparently the Electro-Harmonix tubes are failing after not much use by losing their vacuum. Needless to say, I have reinstalled my trusty RCA tubes, but worry about what to replace them with when they eventually wear out. I am very happy to see from your testing that the RCA's provide superior power handling performance over the new tubes.

BTW, I found the EH-7591A's are much larger than my RCA's, but I was able to fit them in. I thought they sounded as good as the RCA's, but I don't want to spend $20 a tube when they'll only last a year. Does anyone have an alternative new tube to suggest using? Thanks for the great comments on this board. I'm now sad but informed about the short-comings of these new tubes.

Posted by Steve L. November 11, 2012 - 06:50 am
Hi Herbert, Thank you for your kind words and for your measurements of bias voltages. By the way, variation in tube bias requirements is one reason why simple testers like the classic Hickoks cannot provide accurate Gm figures.

Posted by Herbert^Siebler, Germany November 11, 2012 - 05:38 am
You did a very good work!
My own meassurement in an Epiphone EA16-RVT amplifier shows, that the EH-tubes need a BIAS voltage of -18,9 V compared to -20.5 V with nos tubes for same zero signal current.

Posted by Steve L. September 07, 2012 - 08:21 pm
Hi Mark, I appreciate your comment!


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