7591 tubes illustration

Brief Comparisons
of 7591
Vacuum Tubes

Breaking News!—See addenda at the end

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 an 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).
Graphic table of 7591 tube performance in Eico ST-40
Graphic table of lower-level ST-40 Distortion

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.

Graphic table of 7591 tube performance in Eico ST-70A

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.
Graphic table of 7591 tube measurements by Dave

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. 7591 AB1 tube measurements by DaveThat 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:

7591 Tube Gm in TT-10 TesterAs 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 Dave Muth 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 ?

Posted by 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...

Posted by Dave G. 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!

Dave

Posted by 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!!!
'Shark Out

Posted by Dave G. 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!

Dave

Posted by Steve L. October 20, 2013 - 08:08 pm
Hi Mark, Thank you for your kind comments and interesting letter. My experience has solely been with hifi amps, so I'm not the best resource to address these things. But I will ask Dave if he can take a look at your questions. It's clear that you have a serious interest in the technical aspects of this craft and far be it from me to look down on the rock band end of things. The challenges on that end are in many ways just as interesting as the hifi end. Not to mention the fact that the performance market has benefited the tube hifi end in countless ways. Stay tuned.


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