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

• Self-biased pentode-connected output stage
• 20W per channel stereo
• 405VDC supply to output
• 56mA per tube idle current
• Primary output transformer impedance 8250ohms P-P.

Eico ST-70 integrated amp

• Fixed-biased pentode-connected output stage
• 35W per channel stereo
• 440VDC supply to output
• 38mA per tube idle current
• Primary output transformer impedance estimated 6500ohms P-P.
• Dynamic balance modification added (see ST-70a mods article)

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

• JJ-7591S - “JJ” New matched pair of  JJ Electronic brand
• EH-7591A - “EH” New matched pairs of Electro-Harmonix brand
• RCA-7591A - “RCA” Coin-base, low-hours matched pair of known NOS origin. These were purchased new by Dave in the old days.
• 7591GM - “GM” Rebased NOS matched pair of 6GM5’s. The 6GM5 has the same internal structure as the 7591A. Since believable and affordable NOS 6GM5’s are available from reputable vendors, it is attempt to achieve NOS performance from existing products.
• SYL-7591A - “SYL” Sylvania matched pair purchased on eBay. Was claimed to be NOS. Boxes matched tubes and all appeared new (FWIW).

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 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

• JJ-7591S - “JJ” New matched pair of  JJ Electronic brand
• EH-7591A - “EH” New matched pair of Electro-Harmonix brand
• SYL-7591A - “SYL” Coin-base, matched pair of known NOS origin. These were purchased new by Dave in the old days.

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.

Conclusions

• The new manufacture 7591’s fall short of true NOS by 11 to 32%.
• The JJ-7591S is substantially weaker than the EH-7591A. However, in a less demanding application like the ST-40, it might perform as well or better.
• The EH-7591A shows higher distortion and low tube tester gm. It is unclear whether the distortion is due to actual nonlinearity or reduced gm lowering feedback factor.
• If I had to choose between the JJ and the EH, it might be the JJ for the ST-40 and the EH for the ST-70a. The mods in the ST-70a increase feedback factor, allowing it to handle the issues with the EH better.
• Note that the large physical size of the EH might preclude its use in some equipment, such as the Scott LK-72.

Acknowledgment...

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.

The effect is to increase the average plate current, at which power is measured. Normally, fresh tubes have far more cathode emission than they need to sustain maximum plate current. That is required to protect the cathode. Such tubes should show about the same power output, with or without the AB1 bias. However, if cathode emission is weak, the additional DC bias current is more burden and power output could suffer. The new AB1 test results are shown at right, performed on the same tubes tested before. In addition, test results for four more samples of the EH7591A are included.

The new test method slightly increased the output power measured for the reference Sylvania NOS tubes and for the Electro-Harmonix samples, tested before. The minor increase is attributed to additional cathode heating provided by the higher plate current. The original two EH samples had shown over 20% difference in power output, so we were happy to test the additional four, to clarify their true performance. It turns out that the originals are at the extremes of the six tested. Average power output of the EH’s is 91% of the historical average of NOS tubes. Not a bad showing in terms of power output.

Unfortunately, the JJ-brand 7591S tubes didn’t fare well in the new test. Their original weaker performance became still weaker with the additional bias current, delivering only an average of 63% of the power that NOS tubes deliver. You might be tempted at this point to dismiss the JJ’s as a weak contender, next to the EH’s. However, recall from the previous testing, that the EH’s showed unusually low Gm in a Hickok tube tester. While tube tester readings should be regarded with suspicion, if low Gm is confirmed, it could explain why the EH’s showed higher distortion in amplifier operation. Bear in mind too, that only two JJ’s were tested. That isn’t much of a sample. Both Dave and I are working on new testers which will accurately measure Gm. Stay tuned for further developments...

Thanks to Dave Gillespie for the additional test data.

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:

1. There is no longer reason to think that the EH 7591’s have insufficient Gm.
2. The higher EH distortion is probably due to tube nonlinearities.
3. Hickok test results for these tubes are nearly worthless.

Reader Comments

Posted by Steve L.

January 04, 2012 - 05:08 pm

Hi Mike, Thank you for the kind words. We have considered offering a tube tester product. However, the market is small and there are one or two products out there which may do a decent job of it. There is no great secret to doing proper measurements on tubes. The problem is that all but a few of the testers from back in the day, used very crude test methods. For example, DC supplies were not filtered at all and grid AC levels were too high for meaningful results. Also, power tubes were tested at very low power levels. We just applied good engineering measurement practices to get accurate results. I guess that we hoped in the beginning, that we would find that classic tube tester results might correlate reasonably with the accurate measurements. Alas, that turned out not to be the case. Hence, we felt it was important to get some information (as crude as it is) out about the 7591's. Of course, the old adage about the actual operating equipment being the best tube tester, still applies. The measurements we present of actual amplifier performance, demonstrate that.

Posted by Mike

January 04, 2012 - 04:29 pm

Your data is MUCH appreciated simply because there is next to nothing on the 7591 tubes. If your tester engineering is so much superior to Hickok's why aren't you selling it? I've always understood a testers "GM" to be relevant and taken with a grain of salt. The 752a I own does fine for me.

Posted by Steve L.

October 25, 2011 - 12:47 pm

Hi Mo, Looking closely at EH's pictures of the 7868 and the 7591, it does indeed appear that they are different. In any case, our article is focused on tubes with the 7591 pinout and the 7868 would, of course, require rewiring. Nevertheless, I appreciate your bringing up the fact that EH uses a different structure in the 7868.

Posted by Mo

October 25, 2011 - 11:40 am

Steve- when you hold the EH7868 side by side with the EH7591 you can physically see that they are a different size internally. Unfortunately I can't attach a photo.

Posted by Steve L.

October 21, 2011 - 08:46 pm

Hi Mo, The original RCA 7868 was just the 7591 with a different pinout. Looking at the EH version of the 7868, there is no apparent difference in the internals, versus their 7591. What makes you think that the plate structure is different?

Posted by Mo

October 21, 2011 - 05:08 pm

Too bad you didn't include the EH-7868 in your test. These have a different plate structure than the EH-7591's. I use these in my Fisher 400 and they sound better to me than the EH-7591 that I'm running in a few other amps.

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