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.
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!
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. |
October 20, 2013 - 05:28 pm|
|Howdy Steve and Fellow Tubers-|
My area of interest falls somewhat beyond the scope of yours, but for what it's worth I'd like to solicit any comments or advice.
I am a guitar guy and would like to make a homebrew "Fender Champ style" 3 valve amp (rectifier/preamp/power tube), using a 7591 output - does anyone have a schematic or layout to direct me toward, or perhaps another simple Single-Ended 7591 musical instrument type design - or any other ideas? Also, I am interested in adapting a traditional Push-Pull 6V6 (or even 6L6) pair design, modified to run 7591's. I realise the 7591 should likely have its properly chosen output transformer, I do not intend to use a 6V6 or 6L6 tranny - unless it could be used without sacrificing performance.
I have been wanting to do this for many years. I once owned a '66 Gibson Lancer GA35RVT which had a pair of 7591's, and it had the most unique crunchy distortion when cranked. Further conversation with a tube pro (Mr. Ken Fischer of Trainwreck Circuits, now deceased - R.I.P., Ken) further served to whet my appetite for pursuing this desire. Mr. Fischer told me the tone and distortion of the 7591 would ROCK a 6L6 - not a petty statement; that, combined with my crunchy Gibson experience, has had me on the lookout ever since, for another 7591 guitar amp - and since they ain't too common I'd like to try building my own - starting with a simple amp. My ultimate goal would be to modify a little-known classic Standel 50L12 design, to incorporate a 7591 PP pair.
I wonder if perhaps there are some common sense rules or typical design details to keep in mind in order to effect this mod? I am pretty much illiterate when it comes to electrical experience, I have no appreciable theory knowledge nor tech savvy, so my quest is a little over my head - although I am determined to try; at least I have plenty of soldering (parts replacement, kit building) and some troubleshooting experience (successfully blundered through a few amp repairs). I do grasp some elements of tech talk, and I understand the 7591 has its own special parameters. OF PARTICULAR INTEREST is the comment elsewhere in this discussion, that the 7591 was perhaps intended to be similar to EL84/7189 for "design compatibility", which makes me very curious whether I should be looking at an EL84 amp design (somewhat common for guitar), which I may more easily adapt to run 7591's. Also intriguing is the statement that the EL84/7591 types are "easy to drive" - does that mean quicker/more crunch? Do I need to be careful not to overdrive a 7591, meaning I need to keep the "drive level" lower than if I were driving a 6V6/6L6? Are the output tranny specs for 7591 closer to what the EL84 requires, as opposed to what the 6V6/6L6 tubes need?
I am nearly as interested in the CLEAN MUSICAL tone of the 7591, as I am in getting a good growl out of them, and I'd be amenable to any "HiFi" design I may be able to use or adapt for musical instrument reproduction purposes. I know we godawful guitar grinders are not always viewed in a respectable light by you high end stereo guys - so thanks for your humor and tolerance. The Standel was hands-down the most MUSICALLY RICH clean sounding amp I ever owned! I used to deal in old tube guitar amps, once I had a gifted Jazz player come over and I lined up half-a-dozen of my best vintage amps for his consideration; when he got to the Standel at the end, its full rich tone made all the others sound thin or otherwise lacking. I loved the Standel already before that session, but that protracted comparison elevated the Standel FAR above its common contenders in my ears! FYI: Bob Crooks - the Standel Man - was similar to Leo Fender, as he was not a musician and had his own peculiar ideas how an amp should sound. While I have owned a ton of Fenders and they were all great amps, non of them approahed the tonal richness of the Standel 50L12, nor that of a powerful Standel 80L15 I also once owned. Mr. Crooks had some weird designs, but they sounded great. PS - from what I know, these obscure Standel amps may not have had anything in common with the original issue (and currently reissued) Standel 25L15. Standel schematics are pretty much nonexistent; I drew my own layout and took pics of the50L12 chassis before I sold it.
THANKS in advance for anyone's consideration and response!
This has been a very interesting page to read, Thank You for all your efforts.
-Mark the Amp-Shark (ampshark at yahoo)
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