Comments for page: 7591A Tube Comparisons

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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.

Posted by Mark W. 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)

Posted by Steve L. October 19, 2013 - 10:36 am
Hi Craig, Thank you for your comments but I would be hesitant to accept a statement that EH 7591 tubes have long term issues without supporting data. Regarding the suggestion that the JJ 7591 tubes were mismatched: mine were bought from a well known reputable company as a matched pair, as were Dave's. While one could carp about how well they do the matching, that is typical of what people will buy. The fact that they performed okay in amplifier distortion tests confirms that they were decently matched.

Concerning the statement that the EH 7591 "is not a true 7591," I'm afraid that it isn't very useful, without specifying in what way it is different, other than its obvious physical size. Clearly, there are significant differences between all new manufacture 7591 tubes and the originals. However, the EH tube had 91% of the Gm and roughly the same bias voltage as NOS. It's performance in amplifiers was respectable, though somewhat higher in distortion. In the case of our modified Eico ST-70a, the extra distortion was effectively eliminated.

The EH tube wasn't the only one to test oddly in the Hickok tester. All of the 7591 tubes, including the NOS, tested low in it. Rather than blaming the tube, I would cite the crude test method used in the Hickok and almost all testers from back in the day. They apply low, unfiltered DC voltages, do not adjust bias properly and use untenably high excitation voltages. It's no wonder that their results are not only wrong but aren't even proportional to true readings.

I do appreciate your taking time to add to the discussion.

Posted by Dave G. October 18, 2013 - 08:44 pm
Hi Craig --
Yeah admittedly, we were dealing with a small sample number to be sure, and no doubt you have tremendous long term experience with modern 7591 class tubes to be sure.

However, I'm not quite sure what to make of the JJ 7591. I have JJ power tubes of another family class that are truly wonderful, but I've never found one of their 7591 types that impresses -- at least from the standpoint of power output. As you say however, the source may be the issue, with my own tubes arriving as a matched pair from the big box supply house out in the Southwest. As to how much they really burned in and culled out the weak ones may truly be a factor here. I bought the tubes purely for the test, as I don't really use the tubes myself.

Your comments about the long term performance of the EH version is interesting. I have good long term experience with the basic Russian 5881/6L6 offerings (which are generally pretty good), and which seem to hold up well over time. But based on your comments, maybe a controlled long term test of both the JJ and EH versions of the 7591 might be in order.

As for the performance of the EH tube in Hickok testers, there is no doubt that something must be different for the EH version to not test correctly, while the JJ versions generally do. Personally, I believe it is because the grid characteristics of the EH tube when operated at the low plate and screen voltages as found in Hickok testers -- which is the only place they are operated like that. On the other hand, my Gm tests had the tubes operating under the conditions as published by RCA, Westinghouse, and others, wherein both the EH and JJ versions of these tubes then performed very similarly, yet both still at a lower Gm than that of the NOS tubes tested. Granted, even the published operating point for the Gm figure quoted by manufacturers does not subject the tubes to the voltages they normally see in typical operation. But it's a heck of a lot closer, and well up onto the linear portion of the plate curves than the Hickok test point is. When both tubes are tested on the linear portion of their curves, then their Gm performance becomes very similar in deed. As a result, I more tend to look at this particular issue as an anomaly of the EH tubes when operated at very low quiescent current levels, rather than it being due to the EH versions being significantly different to the JJ and NOS offerings. Yet still, your observations are noted and appreciated.

These tubes were so popular in the final tube gear produced back in the day, and still popular today due to the resurgence of that gear, so thanks for weighing in with your considerable experience.

Dave


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