Copyright © 2010-2011 by Stephen H. Lafferty. All rights reserved. Rev. 4
Recently I needed to replace some 7591A vacuum tubes in two Eico amplifiers: the ST-70a and the ST-40. My choices were either to use vintage tubes of classic manufacture from eBay or to use one of the new manufacture brands available.
The problem with the available classic 7591’s is that ones described as NOS are very expensive and cannot be verified as actually being NOS. In fact, I have received so-called NOS tubes from reputable companies which upon being removed from the box, had heavy dust on the top of the tube! The problem with used power tubes is that even higher quality tube testers such as Hickok do not actually test the performance of the tube at high power, so they are unreliable at indicating the true quality of the tube.
While the new manufacture brands of power tubes such as the 6L6GC have always been available and are used with some confidence, new manufacture of the 7591A was a long time coming. When it finally did come, the tubes which appeared at first were not exact replacements. Even today, there are significant physical and electrical differences between the new tubes and the classics. Physically, the new tubes are larger. Electrically, they do not seem to be able to pull as much current at low plate voltage, as the classics.
I decided to make some careful measurements of amplifier performance to compare the brands. Separately, David Gillespie did some testing on new 7591 brands using his custom-built power tube analyzer. His instrument applies an appropriate plate load and voltage. Grid drive is adjusted to swing the plate between saturation and cutoff. It measures the amount of AC power developed across the load. This is effectively, the maximum amount of power which the tube is capable of putting out.
Please note that the tests presented here are by no means definitive. Few samples were tested. It is simply one humble attempt to compare these tubes. Also, it may not be indicative of how these tubes would perform in your particular amplifier. On the other hand, without much other hard data out on the Web, it is offered as something, better than nothing. Please see the conclusions at the end for the bottom line. Comments are welcome.
Performance in the Eico ST-40 and ST-70
Some info about these amps:
Eico ST-40 integrated amp
Eico ST-70 integrated amp
Of the two amps, the ST-70 demands more peak current from the 7591’s. The most obvious difference between tubes had to do with how close to ground the plate could pull at current peaks. The NOS RCA could pull the plates down to just 50V while the JJ’s could only get it down to around 120V.
The Tubes Tested
Discussion of ST-40 Results
The strongest, RCA, shows an 11% advantage over the weakest, JJ, at high power. The JJ and EH were roughly similar for power output in this amp, though. The GM may be 5% ahead of the JJ but can’t quite match the RCA champ. The lower-level distortion tests indicate that the EH is not as linear as the JJ. Also, during testing, the EH showed a phenomenon which was also seen in the ST-70a: The distortion rises to a peak as level is increased. Then it drops substantially until it approaches clipping. Typically, distortion only increases with level.
Discussion of ST-70a Results
The strongest, RCA, shows a 13-14% advantage over the weakest, JJ, at high power. The EH has pulled ahead of the JJ in this amp, being only 4-5% behind the RCA. However, the EH has spoiled the ultra-low distortion performance of the amp at modest power output. Linearity of the JJ is somewhat worse than the NOS in this amp. The so-called NOS SYL is behind the EH in power output.
Performance in Dave’s Power Output Tester
The tubes which Dave tested were different units from the one’s I used:
The Tubes Dave Tested
Discussion of Dave’s Results
Once again the NOS tube wins the power race. The EH average is 11% lower and the JJ is 32% lower. The Hickok gm numbers show only 65% of nominal for the EH and 93% of nominal for the JJ’s. One should bear in mind though, that tube tester gm figures are not very good indicators of actual power tube performance.
I would like to thank my good friend David Gillespie for the measurements and other information he contributed for this article.
Addendum #1 — Improved testing using class AB1 bias
Since this article was published, Dave has developed an enhancement of his method of testing output tube power. The basic approach is unchanged but now a bias current is added. This better represents the demands of class AB1 operation, whereas what was being tested before was essentially class-B operation. Most commercial tube audio power amps use class AB1 bias.
Addendum #2 — High accuracy Gm tests refute Hickok data. EH tubes partially vindicated!
It is well known that almost all commercial tube testers from the golden era give poor quality Gm measurements. For one thing, the grid drive levels used are generally far too high. Also, the plate voltage is pulsating DC, resulting in a blur of characteristics across a wide range of voltages.
To improve this situation, Dave has built a new tube tester, designated the TT-10, which operates tubes under controlled DC bias conditions, drives the grid with a reasonably low AC voltage and measures AC plate current. The results are accurate Gm values which can be compared directly to published databook standards.
Using the TT-10, the same 7591 tubes which were tested above, were retested. Here are the new Gm results, along with the original Hickok Gm figures and the original power output measurements:
As you can see, the Sylvania NOS tubes closely match the ideal databook Gm values, giving confidence in this approach. While the true Gm of the EH tubes is comparable to ideal, the Hickok reads them as less than half of normal. The true Gm of the JJ tubes is at least 80% of ideal, yet the Hickok reads them far lower. Interestingly, the Hickok reads the EH’s much lower than the JJ’s when, in fact, the true Gm of the EH’s is higher. So the Hickok tube tester cannot be trusted even for relative comparisons.
It is interesting to observe that output power tends to vary in the same direction as true Gm. However, this is not to say that it is proportional. Clearly, there is still a place for maximum output power testing, for power tubes.
In conclusion, these tests exonerate the Gm performance of the EH tubes, which had been called into question by the Hickok results. However, there still remains the fact that EH 7591’s showed higher distortion than others in actual amplifier operation. To summarize, we can say that:
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