Comments for page: 7591A Tube Comparisons

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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.


Posted by Craig NOS Valves October 18, 2013 - 10:58 am
Really nice effort. I do have one question. Where did you source such a horribly mis-matched pair of JJ 7591's? I suggest you buy a good matching pair from some other source that does a good job weeding out the culls. I think your test results would change to some degree. The big problem with JJ 7591's is quality control. A well burned in and matched pair or quad would act very differently. The EH 7591 are horrible long term in an amplifier the test results and bias points after a few months of service change dramatically.
7591's are designed to be tested at low input signal levels for a reason......they are low signal level tubes. (Easy to drive). They were designed that way to allow the hi fi manufactures to keep the basic design of the EL84/7189 amplifier circuits and amplifier size relatively unchanged to implement more power.
The EH tube is absolutely not a true 7591 and it is absolutely why it fails the GM test in a vintage Hickok tube tester. The EH is not a horrible tube but the fact that it is not a true 7591 is pretty hard to dispute. That is why its in the larger EH/Reflector plant 6L6 bottle!

Cheers Craig

Posted by Fred Longworth - Classic Audio Repair, San Diego April 26, 2013 - 03:31 am
Stephen and Dave,

Thanks for this. Very illuminating. My associate (Jordan) and I were discussing 7591A's this very Thursday afternoon, faced with the need to retube a Sansui 1000A.

Posted by Dave Wax December 24, 2012 - 04:27 pm
Hi Dave G.,

Thanks for your great advice! I'm certainly going to put those resistors in place. The reason I mentioned the suppressor grid is because it is internally connected to the cathode and I thought its close proximity to the screen grid might promote the arcing. You are very correct about the operation of the screen grid near its max rating, and only being about 10 volts below the plate.

I live in Seattle, Washington, and our line voltage consistently runs about 120 VAC. I have a Variac transformer and I am going to use it to drop the line voltage to the amplifier down to 110 - 115 VAC range. I've already done some testing and have found dropping the line voltage to 110 VAC produces significantly lower B+ voltage. Combining the resistor mods and the Variac should provide a lot more reliability and hopefully extend the lifetime of the output tubes. Thanks again for your advise. Happy Holidays to you!

Posted by David Gillespie December 24, 2012 - 03:27 pm
Hi Dave --

In reviewing the schematic of your Sherwood, it has all the classic elements in place that promote output tube arcing. In high fidelity applications, all such event invariably occur between the screen grid and the cathode -- not at the plate as is commonly thought. Per my article, the elements that promote these events include:

1. The use of high Gm tubes, of which the 7868 tube certainly qualifies.

2. Operation of the screen grid near its maximum rated voltage. These tubes have a Design Maximum screen voltage rating of 440 vdc, and your unit -- operating on the typically lower AC line voltages of its day -- operates the screen grids at 415 volts. It likely powers them to even higher levels on today's AC line voltages.

3. Operating the plate and screens at very similar voltage levels. Considering the drop produced through the primary winding of your output transformers, these are likely within 10 volts or so of each other.

4. Fixed bias operation.

These elements are all accumulative, such that when they are all in place, the installation of screen stability resistors becomes mandatory. They are all clearly present in the design of your Sherwood.

That they were not included in the original design often speaks to the higher ESR power supply caps that were produced when this equipment was manufactured, so that the events were less likely to occur. On the other hand, some models (from Eico for example) were known to eat output tubes from the get-go by way of arcing.

When the older gear is upgraded with today's power supply components, the lower ESR values can really act as a trigger when the four elements are present, unfortunately making what was almost a rare event all too common.

As Steve mentioned, my recommendation in this case would be the installation of screen stability resistors (100 ohm, 1/4 watt is fine), which have been shown now to all but eliminate such occurrences.

I would also strongly recommend including a small 10 ohm 1/4 watt resistor in the cathode circuit of each output tube as well. Without any means to adjust the DC bias or balance on your unit, matched output tubes are a must. The inclusion of these resistors will not only allow you to check how well your output tubes are balanced among themselves, but also determine the actual current level they are operating at, which is important for the health of your tubes and the unit in general.

None of 7591/7868 class tubes manufactured today will produce the absolute performance of the original American products. But installation of the screen stability resistors and ensuring that the tubes are operating well within their design ratings will likely let today's tubes operate very acceptably in your unit.

I hope this helps!


Posted by Steve L. December 23, 2012 - 02:23 pm
Hi Dave W., (BTW, I only incorporate the initial, in case Dave Gillespie should join the thread.) Not having heard a response from him (yet), I will go ahead and mention the fact that he has seen a phenomenon of spontaneous arcing in pentodes/beam power tubes. In fact, it is covered in his article here:

He found that the solution was to add small stopper resistors in series with the screen grids. I later did some analysis and hypothesized that the phenomenon could be due to VHF oscillation resulting from the tube's interaction with stray capacitances and lead inductance. Unfortunately, attempts to produce the phenomenon in the lab were unsuccessful, so we couldn't investigate further. (On the bright side, it gave me ample fodder for teasing Dave about it endlessly :) I did find though, that such spontaneous arcing from RF parasitics had been documented as far back as the 1930's.

If you're curious about it, you could add 100ohm 1/2W resistors to the screens and see if the EH tubes still arc. I would be very interested in hearing the results, should you decide to experiment.

Posted by Dave Wax December 23, 2012 - 09:28 am
Hi Steve L. A correction to my last post: my amp is only 50 years old, not 60. I'm getting so old, I can't do simple arithmetic any more. Also, I shouldn't be surprised that solid-state rectifiers are used in the amplifier design - we started incorporating these devices into our vacuum-tube designs during the late 50's.

I checked the getter flashing on each of the LH7591A's and they all look good - no whitening. So my conjecture about leakage seems to be wrong. Maybe the suppressor grid is too close to the screen grid, causing arcing? I dunno. I do know that I don't have any problem with my RCA 7591A's. BTW, the max plate voltage rating for a 7591A is 550 volts, not 500 volts as stated in my last post. Thanks again for your help.

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