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.
- 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.
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:
- There is no longer reason to think that the EH 7591’s have insufficient Gm.
- The higher EH distortion is probably due to tube nonlinearities.
- Hickok test results for these tubes are nearly worthless.
Addendum #3 — Evaluation of the new Tung-Sol 7591A, including real-world and lab test results
In earlier sections of this article, we looked at new-manufacture, 7591 tube offerings under the JJ Electronic and Electro-Harmonix (EH) brands. Note that New Sensor Corp. owns at least the following brands: EH, Tung-Sol, Sovtek, Genalex, Mullard and (in North America) Svetlana.
Within the past few years, a new 7591A tube has been released under the Tung-Sol brand. You can see it at right next to a classic, Westinghouse, Eico-branded 7591A. For the first time in the modern era, we have newly-made 7591 tubes which actually look like the original! Even the internal construction looks reasonably similar. This comes as a relief to folks who have had trouble fitting other new 7591s into their equipment. For example, it’s been reported that the case doesn’t fit with previous new tubes in the H.H. Scott LK-72 and 299D and the Fisher X-200 models. There should be no problem with the new Tung-Sol tube, assuming that the minor increase in height seen above isn’t an issue. Aside from solving the fit problem, vintage audio collectors are no-doubt pleased that the new 7591A doesn’t look out of place in their equipment.
How Well Does It Perform?
As we saw above, the EH 7591A and, to a lesser extent, the JJ 7591S can perform adequately but they didn’t quite match original NOS in terms of output power. First, a few caveats about that statement:
- We can only afford to test a small sample of tubes, so this is not “scientific” and should be taken as an indication, rather than hard fact.
- It’s been a long time since we tested the EH and JJ products, so they might have changed. (If only we could afford to buy more tubes...)
- Our vacuum tube power testing is an evolving art and methods have improved over the years. (Check out the companion article about this.)
Performance in the Eico ST-70a
To get a real-world look at the new 7591A, we ran three pairs of them in the same amplifier and against the same RCA and Sylvania tubes that were used above, 7 years ago. Only the power amp of the ST-70a was used, as before. DC bias, DC balance and AC balance were adjusted for each tube pair. Details of the ST-70a are covered near the beginning of this article and in the ST-70a mods article. In the new table below, we’re seeing output power higher by about 3% for the RCA and 7.6% for the Sylvania, relative to the original readings. The primary filter capacitor was replaced recently, which could account for part of the difference. Distortion of the new tube at half output voltage is equal to the RCA, so linearity is excellent.
More detailed (but less reader-friendly) data is available by clicking here. In any case, we need to be careful about comparing this table to the previous one above, by using performance relative to the RCA tubes. Previously, the JJ7591S produced 89% of the RCA power and the EH had 96% (1kHz, 1% distortion). Output power results in the new table show the Tung-Sol at 92% of the RCA, so it’s higher than the JJ but lower than the EH tube. The advantage of a real-world test is that it shows the actual results you get in a real amplifier. The disadvantage is that it’s just a point-sample. A different amplifier, with different biasing and different power supplies, might perform quite differently. Before we close the book on output power performance, let’s look at the lab power and vacuum tube analyzer tests.
Laboratory Power Performance
Power testing in the lab was performed in a manner similar to the method used by Dave Gillespie, except this approach uses standard lab equipment instead of a custom-built vacuum tube power tester. I’m grateful to Dave for sharing the techniques used. The test procedure is feasible for serious hobbyists and it’s covered in detail with a companion article here. The power test operates the tube in a relay socket with a load resistor to represent the output transformer primary. We drive the tube grid with a sine wave source to cause maximum plate voltage swing. The peak-to-peak value of the swing gives us the maximum power that one side of a push-pull pair can deliver. Test results from eight new Tung-Sol 7591A tubes, along with a reference NOS RCA and four EH 7591A tubes are shown in the table below. More detailed (but less reader-friendly) data is available by clicking here.
You can see that the RCA tube is delivering slightly over the full power defined by the databook plate curves of the 7591A. It’s remarkable that the tube comes as close as it does to ideal. One of the Tung-Sol tubes, TS-7, was apparently defective in some way, as its output was only 82% of ideal versus the 97% average of the others. Discarding the bad one, I consider the 97% score to be very good, though not quite as good as the EH value of 99%. I guess one could spot them one bad egg. An occurrence of one count cannot be considered indicative of a trend. Still, if it’s included in the overall Tung-Sol power average, it pulls it down to 95%. In view of the fact that this new tube is so much smaller than it’s new-manufacture competition, I guess a slightly lower output power result can be accepted. I’m just thrilled that they finally produced one that’s physically similar to the original. By the way, no correlation between output power and Gm was found among the Tung-Sol tubes.
Gm Performance in a Vacuum Tube Analyzer
Measurements of Gm were performed with a newly designed vacuum tube analyzer, designated as the VTA-101, seen at right. This is the first time that this instrument has been mentioned publicly, so you can consider this a sneak preview of the upcoming article which will cover it in detail. The design attempts to achieve something like “HP quality.” [Referring to the instrument company.] Accuracy and stability were paramount.
In the table above, the term, “True Gm” is used as a column header to distinguish these readings from those given by many of the vintage tube testers. Very few were actually capable of measuring accurate Gm, as illustrated in Addendum #2 of this article. Anyway, the Gm of the Tung-Sol tubes varied from 91 to 98% of the ideal value, averaging about 94%. I consider this a very good showing, although the EH tubes averaged 99.9%. Small signal Gm isn’t a very critical parameter for power tubes.
Summary of the Tung-Sol 7591A
From the limited sample analyzed it was found that:
- It delivered 92% of “NOS” * power in an Eico ST-70a amplifier, versus 89% for JJ and 96% for EH.
- Lab power tests showed 97% of ideal power, excluding the impaired unit or 95% with it included, versus 99% for EH.
- Gm averaged about 94% of ideal versus 99.9% for EH.
- Medium level distortion in the ST-70a was 0.031%, exactly the same as the RCA “NOS”. The EH tubes weren’t retested for this but showed 0.11% in the earlier tests.
Overall, the Tung-Sol 7591A performed pretty well in these tests. The weaker power output in the amplifier is balanced somewhat by a very good showing in the lab power tests. Gm was fine, if not stellar, and distortion was excellent. One thing that should be taken into account with the power tests is, we really are looking at small variations which would not be audible. For example, 92% power represents a loss of only 0.4dB relative to 100%. To me, the advantages of the smaller size and having a tube that doesn’t look out of place in vintage equipment could far outweigh such minor variations among products. As far as the significance of the single impaired unit, it’s hard to judge but since the other seven tubes showed good performance, one would need a larger sample to take that seriously. Overall, I have to give this critter a big ’ol thumbs up!
I would like to thank Dave Gillespie for his sage advice and encouragement for this 7591 investigation, particularly in the area of vacuum tube power testing.
* The RCA and Sylvania tubes used in the ST-70a were originally NOS and have been lightly used. Since they are used, we refer to them as NOS in quotes.
May 02, 2017 - 07:26 am|
|Hi Thomas and thank you for the thoughtful posting. I would only say that some points are a bit overstated. As noted in the last addendum, the EH tubes' Gm readings, while 10% lower than NOS, are nonetheless quite respectable. We had been misled earlier by the gross inaccuracy of the Hickok tube tester. In terms of output power, the last lab tests showed the EH down only about 9% from NOS; again, a respectable showing. The original power tests in the Eico ST-70a were down only about 4% from NOS. |
Are the current productions of the 7591A "not really working?" I suppose one could say that the glass is not as full as it could be. But at 90% of NOS power, remember that we're haggling over less than 0.5dB here! I, for one, am very grateful that the Russkies (and others) have saved our vintage equipment from extinction. Finally, stay tuned for an upcoming addendum with coverage of the new Tung-Sol 7591A!
PS: Did you really mean to say, "...something the 6L6 could never do, which was to sound hifi"? I think my Eico HF-20 amps, on reasonably efficient speakers, sound terrific! They have the specs to prove it.
May 02, 2017 - 01:52 am|
|An article still generating lots of interest after years.|
Having some familiarity with USSR tube designs it's obvious the Reflektor 7591 are just reworked and rehoused tweaked versions of the Russian 6P3S, which sits somewhere between a slightly lower impedance more powerful 6V6 and a weak 6L6.
Being as the GM of the 6V6 is somewhere in the 4500 zone, and the 6L6 in the 5500-6000 area, that would give a perfect explanation of the poor EH tube gain, lower poor and higher distortion.
A reworked and respaced 6L6 with old fashioned grid winding practices would give this result.
This has got to come down to the technical abilities of the factory.
The jury is out on this, but let's say,so far they haven't managed to produce a high gain frame grid, high quality tube to date.
As Svetlana in St Petersburg stopped making audio tubes, (in my opinion the only factory capable of making high quality, as they proved with their EL34), it appears both Russian and Chinese factories now don't have the advanced technology that was available in the USA in the 1960s to make colour TVs.
The 7591A is an odd tube.
It seems to sit between the 7355 attempt to make a lower impedance, higher voltage 6L6, and the higher performance versions of the very last US made 6V6.
The 7355 was a failure, ending up making lower power and higher distortion that the 7591.
It was clear by mid 1950s the Philips/Mullard EL37/34 pentode was the tube to beat, because high gain lower impedance output stages were here to stay.
6L6 / 6V6 clones are fundamentally non linear, only working properly when about to melt.
The idea to stuff a 19W anode into a 6V6 size tube, with the cathode of a sweep tube, the heater of a 6L6, and the gain of a EL34, was bound to be tricky, but supposed to do something the 6L6 could never do, which was to sound hifi!
The fact is, reproducing 7591 in small quantities, after US companies like Sylvania & RCA could perfect mass production methods, is shown in this article as not really working.
Nobody should be suprised.
The pinnacle of valve techology was 50 years ago.
This is certainly why the only company ever to have copied the "double the gain - double the pain" 8417 was Toshiba!
April 20, 2017 - 07:24 am|
|Hi Tom, Thank you for your insightful post. I've also been concerned about whether current manufacture 7591s have gotten better since the article was first written. There is SO much more that could be done to cover the state of that classic tube. As a down payment on that and to address your question about JJ 7591s directly, we would be willing to evaluate your JJ 7591s, if you would be willing to loan them to us long enough to get them here and back. Incidentally, I tested two pairs of the JJ 7591s for the article and Dave tested another pair. So that was six tubes, rather than the two you mentioned. I agree though, that it would be better to test a more representative sample than our limited resources allow. We're just trying to make available the brief data we can get. |
October 01, 2015 - 05:48 am|
|I recall reading this when it first came out. I note that the last comment listed is from 2010. As we're now essentially 5 years later, I'm wondering if any further updates have been determined? I mention also that I believe there is now a 'reissue' of the Tung Sol version of the 7591, and I'm wondering how it would fit into the mix? Also, having built several amps that have used the 7591 tube (conversions from 7868), I've made a 'price' consideration and gone with the JJs....and honestly had no problems at all with them. This is not to dispute the initial results of this evaluation at all, but more to simply offer the possibility that perhaps either the quality of the JJ tube has gotten better, or that perhaps the initial sample of just the 2 tubes didn't really represent the JJ line as well as it should have. Just a thought....... Tom D. |
voice of reason||
January 23, 2015 - 01:08 pm|
|the Euro tubes, JJ and EH, and cheap junk replacements. vintage USA or Japan tubes are way better. no comparison. |
March 24, 2014 - 07:48 pm|
|Hi Mark W. -|
I own a very early Gibson GA-30RVT Invader amp with 7591 output tubes which sound fantastic as you mentioned. It is a first year (1962) oddball model (serial # 720123) & uses a GZ34 rectifier tube power supply (similar to an Epiphone EA-12RVT amp) instead of a full wave diode rectifier & OA2 voltage regulator tube as shown on the Gibson GA-30RVT schematic.
I am currently smoking the power transformer & can't seem to find the short. Also, believe its a 1000VCT tranny (430V output from the GZ34 rectifier), but I don't want to replace it & burn up a new one without knowing if something else is wrong with the amp.
Any suggestions ?
Mark the Amp-Shark||
December 06, 2013 - 06:32 pm|
|Hi Ya Dave- Thanks for your input on tubes. A lot of what you say makes sense, but I'll hold my opinion that some tubes can, in some (especially guitar amp?) circuits, sound so distinctive that they could have no viable replacements to ears preferring them - their sonic signature has nothing to do with amp or instrument settings, that's just how those tubes sound in whatever application. A lot of manufacturers were insistent on certain tubes in tone critical locations. I guess maybe as a more dynamic beast than a refined HiFi instrument, perhaps the average guitar amp shows more sensitivity to some tubes whereas maybe it's not so critical in a HiFi circuit (although I do know Telefunkens and MacIntosh go together). When it comes to power tubes, there is nothing around that sounds anything like an RCA blackplate 6L6 in a great guitar amp like any old Fender and many others - there is a reason of market demand some old tubes go for big dough, same as with many classic HiFi tubes, I presume - or could stereo amps be much less affected by inferior modern bottles?|
As far as the 7591 question, my concern is not so much with sound level output, I'm interested in breakup tone (typical guitar nut) as much as I am in its clean sound and dynamics. The 7591 has distinctive attack and "clean headroom" qualities which are unique to it, so I am expecting whatever I get there to be adequate at the very least. So I suppose I must decide on loudness and balls-out drive from a higher voltage, or do I go browner with a lower plate voltage and earlier breakup/lower headroom? Reckon I'll haveta build at least two amps, and take it from there (unless I were smart enough to make built-in switchable circuits - which I AIN'T). Basic idea is to have a 7591 pair putting out 10-15 watts, and guitar nuts will always take more if we can turn it up, and bigger numbers = greater dynamics, so logic dictates higher voltage and bigger trannies seems more fun -- stand back while I flip her on with this yardstick...
October 29, 2013 - 07:36 pm|
|Hi Mark --|
Everybody has their favorite tubes, but in production amp service, the preference can often be traced back to something that is measurable.
Regarding the sound of 12AX7s, my belief is that a particular sound is not so much generated from within a tube per se, but is a product of how a given tube interacts in a given circuit.
The 12AX7, like any other tube, can be shown to have significant variations from one manufacturer to the another, with the biggest variant being gain. As a result, when used in production equipment like a guitar amplifier -- where there are little or no corrective circuits in place to account for these differences -- one tube can come off as "hot", and others much weaker in performance. You see how this is accounted for in power tubes with the various grades they are given regarding how soon they breakup (or not).
With preamp tubes, such differences can cause different volume settings (for example) to achieve a given sound level, but using a different control position, has now likely changed the response of the circuit, and therefore, the character is the sound. The tube did cause the change, but only indirectly in this example.
You could try to account for such variations, but then the corrections themselves might spoil the very sound you were trying to emulate. Ultimately, with the very specific sound that players like, coupled with the types of circuit design they like, they will always be at the mercy of finding the particular tubes that will produce that particular sound they like in the particular equipment they use. That is about the long and the short of it.
As for voltages for 7591 tubes and overdrive sound, the first thing that must be determined is how much sound level you want before overdrive begins. 5 watts? OK. That establishes certain parameters of the design. 30 watts? Now you've changed everything. Voltages applied to power tubes have a huge affect on the power produced. Trying to make a 30 watt amp break up at 5 watts doesn't produce very good results. Using only 5 watts when you need 30 won't let you be heard no matter how hard you overdrive it!
Even guitar amps then have to be engineered to a deliver a defined end goal. Simply asking about voltages for a set of tubes, is like asking how big a motor do I need for my boat? By itself, that question cannot be answered! Are we talking a troller or an ocean liner? It makes a difference!
Start with step 1 by accurately defining your goals, and then let your project develop from there. Your projects will have the best chance of success in doing so!
Good luck with them!
Mark the Amp-Shark||
October 24, 2013 - 12:15 pm|
|THANKS for the help, Dave!|
It's nice to hear that an adaptation of something like a DeLuxe is not too difficult! I also agree it's wise to stick with a tried and true circuit, no doubt I have zero chops to design my own - LOL. I think I might start with a Premier Model 50 7591 circuit champ-style amp, I can get a schematic easily and there are lots of aficionados of these so it's probably pretty accessible for my needs. Then if I'm successful I might move up to your suggestion. I have been eyeballing the Angela Instruments Super Single Ended guitar amp, since I received their catalog back in '96 (typical procrastinator); your input makes me think I might possibly adapt this to 7591's without undue contortions. Thanks for the current 7591 situation info, I am aware they don't make em like they used to, and thankfully I have a small stash of originals; at any rate I am already a died-in-the-wool VINTAGE tube guy, I will always go out of my way to use old tubes, since they are what makes my old amps tick (not to mention I rarely/never hear anything that pleases my spoilt ears, after cutting my teeth on True Vintage Tone). I have never heard any modern tube equal, let alone excel, a good Old Tube. Yuck.
Hey Dave, lemme throw another curveball outta left field atcha:
My fave preamp tube is the Amperex Bugleboy 12AX7, it has the most wonderful "liquid" tone with a super-unique top end and sound in a good circuit; I wonder if anyone has attempted to capture this signature by creating special circuitry, in the absence of availability of this rare tube? I have a few of those salted away, but I wonder if someone has addressed this electronically...
And to pump you for additional opinion, any input concerning whether you'd suggest running a lower voltage to the 7591 for earlier/browner distortion, or what about going high voltage for maximum oomph and grind? I know either way should sound good, just splitting hairs...
Thanks to all for helping a newbie and tolerating a shade-tree hack who really has no business mucking around in unknown places! With the right advice I might be able to pull this insanity off.
BTW: Amp-Shark valve tidbit - has anyone heard the story about the 6550A being invented to power the monstrous Fender 400PS Bass Amp? Insane voltage + 6 6550's = 435 watts RMS, makes an SVT sound like a wimpy little brother, but takes THREE separate speaker cabs to tap the full power of its multi-wound O.T.. My ears ring to this day, DOH! It was funny to see the Chinese 6550's in glowing red and shedding molten displaced metallic particles in my amp taken in for service, the repairman had to call me on the phone so I could come down and witness the meltdown in person, very impressive.
I will always have a fondness for my brother's Harmon Cardon Citation IV, now THERE'S a clean high power stereo! On second thought, perhaps my tinnitus started there.
And THIS is what I want to "experiment" with? Stay tuned for obituary (hopefully not) to follow!
Keep On Rockin, TUBES RULE!!!
October 23, 2013 - 11:38 am|
|Hi Mark --|
You've got quite an ambitious project in mind! My comments will be centered on the easiest way to achieve your objectives, with the greatest chance of success.
Guitar amplifiers and high fidelity amplifiers share much in common, only differing in a couple of basic (but important) areas from a design standpoint. But one of the things they share is the use of high gain circuits. Understanding the nature of such circuits and their requirements with regards to physical layout, build quality, and component selection -- let alone the design theory alone are enough to sabotage many a project with excessive noise, hum, squeals, and general poor performance -- leading to much disappointment. I know, because that was the outcome of the first guitar amp I built as a young boy.
Since your primary goal is a sound you associate with a given tube coupled with a helping hand of learning, I think you have a much greater chance of achieving your goal by taking an existing unit that already matches the majority of your needs (number of channels, effects, etc.), and then convert the output stage to use your tube of choice.
The original 7591 family of tubes were pretty tough cookies, that could take a fair amount of abuse in guitar amp settings, and in the hifi arena as well. But understand that modern versions of it are not nearly so tough. Therefore, also understand that using the modern versions may not deliver the sound you remember -- but could be quite close. The biggest difference will be in power output produced, with the modern versions not up to the capabilities in guitar amp service that the original device could produce. In hifi amps, this is not nearly as crucial as it is in the guitar world, as over-drive at a given sound level has little meaning in the sound REproduction venue (where distortion is to be avoided at all cost).
I would suggest taking something like a Deluxe, and modifying it for use with 7591s. Using this approach, you would achieve at least the original power output level, the tube sockets -- while requiring re-wiring -- are the same as required for 7591s, the layout is already proven, the voltages are right in the neighborhood of what you need, and you could likely even use the same output transformer. The bias voltage would need to be adjusted, and the gain of the driver stage reduced somewhat to compensate for the higher Gm of the new output tubes, but these are all very doable things in an already established environment, with a high probability of successful outcome, along with the sound you are seeking.
As for the "easy to drive" question, all output tubes require a bias voltage to establish a correct quiescent operating current. Some tubes require more bias voltage ( 6L6 and 6550 for example), while others require less (6BQ5, 7591). For full power output to be developed, the drive signal must overcome this bias voltage to drive the tube to a saturated level. Since 7591 class tubes require less bias voltage, they also therefore require less drive voltage, making them easier to drive from the previous stages.
Finally, within any given tube class, some tubes will require more bias voltage (or less) than others, simply due to mechanical tolerances during manufacture of the tubes. Within a given tube class, those that require less bias voltage will achieve full power output sooner, or crunch sooner, where as those requiring a higher bias voltage will distort later. The 7591 is no different in this regard, since it too has manufacturing tolerances within the various examples of its manufacture.
I hope this helps!
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