Hope you're right; I've heard some good things about other EH output tubes; but I'm going to give things some more time and see how they shake out. After all, they haven't really been out there long enough to call in a valid jury concerning "longevity."

With scuds and silkworms, "good longevity" used to be measured in days or even hours. Outgas, overheat, blastoff! BOOM!

Time will tell...

Larry

Well, technically, since I'm retired I guess I shouldn't be calling it a "vacation".

There will be much golf and adult beverages, but I'll try to check in now and then!

The new 7868's are also getting great reviews.

Honestly, I kinda-sorta think we need to be trying these new tubes, because if we don't buy them the manufacturers will stop making them, and I don't think we want to paint ourselves into that corner, if you get my drift. While it's not likely we'll run out of old stock tubes any time soon, they're getting pricier by the day, and you can get a quad of brand new ones for $70-$80.

I understand, and I agree; but if other guys are willing to do the testing, I prefer to stand by and see what shakes out. The role of "beta tester" (i.e. guinea pig) never appealed to me.

As far as 7868's are concerned, I have only the single 400 that uses them, and it's the linchpin of my synth lab, and I have to take the best possible care of it. To be frank, I'm more careful with it than I am with BB. And after that output transformer ordeal in 2009, I am going to be precious damned careful of what I plug into it.

Speaking of that, it's probably about time I restored those 400 and 800 overhaul threads and brought them back. Think I'll do just that....

Larry

I hear ya!



Sweet! I've got a few projects of my own I need to get to later this year.

Truthfully, I think the EH 7591 and 7868 tubes can be called "mature technology" now, pretty much.

I've used 'em in various things- Fishers, Scotts, Sherwoods Heathkits, and more- always remembering to not over-bias them (I keep 'em to a max of 30ma per tube, for safety), and they make plenty of power, and don't break. I think I've had one failure of an EH 7591, out of a few dozen I've used over the last 4 years. I've not had a single EH7868 give me a problem within that time, and I've done at least four or five sets of them.

No, that's not a big sample... but it also seems to jive with what I've seen from other repair techs. Recognize their limitations (i.e, don't drive them to the limit with idle current), and they work very well. They don't seem to care about plate voltage (I have run them on as high as 500v on the plates and 400v on the screens, no problem), so as long as you keep the idle current under control, AND be sure to use the RIGHT grid-bias resistors (I usually make SURE that the composite resistance of the bias resistors- the effective combined resistance of any series or parallel resistors, when there is more than one resistor in each tube's bias circuit- is NO HIGHER than 270K, per tube- I usually go for 220K), they run pretty much without any problems.

In short, except for the rare case where you need a very short bottle (the Heathkit AA100 comes to mind- that's a job for something like a coin-base Sylvania 7591- although if you're willing to run 'em with the lid off, it doesn't matter how tall they are), the EH tubes are what I'll specify most of the time. Unless someone can come up with a good set of old-stock tubes that actually reasonably match each other anymore, these new tubes are a better choice, IMHO...

Regards,
Gordon.

Larry, I know we may have mentioned this before here- but if you haven't done it already, I consider it "best practice" to put 10 ohm 1/4w resistors in-line with each output tube cathode lead (between cathode and ground), in these Fishers (and anything else that uses 7591s or 7868s). This will not only give a convenient bias-check point for each individual tube, but if you use a 1/4 watt resistor, it's likely the resistor will blow, before the output transformer. It's saved me once on the bench, from a "probe induced fault" between screen and grid on a 500C. POP, went the resistor. Swapped it out, and the amp was fine aferward. PHEW!

That was, of course, a special-case situation- but the same thing could happen (shorted tube), in use, with ANY output tube, with as close of grid-spacing as the 7868 or 7591 has, new or old production. With as hard as it is to find Fisher output transformers these days, and the number of char-broiled examples I've seen come across my bench, there's really no way I'm going to let a unit leave my bench WITHOUT those "safety resistors" installed...

Regards,
Gordon.

Hi Gordon, good to speak with you again. I agree with you, and those cathode safeties are in my 400 and 800. I took care of that when I overhauled them in 2009. I had to use 12 ohms instead of 10, because I had no 10-ohmers handy at the time, and I was under heavy time pressure in both cases. I would put such safeties (screen safeties too) into any output stage that uses tubes prone to develop grid emission, i.e., the ones with large cathodes and close-spaced control grids. The 7591 and 7868 might be the two most notorious, but they're not the only ones.

Yes, those Fisher OPTs are hard to come by. I have three pairs in stock (two for the 400, one for the 800), and I hope I'll never need them, let alone needing more. But I had to use up my third 400 pair in 2009, so I am always on the lookout.

Hope you're doing well and best regards!

Larry

I see, and I appreciate the insight. I prefer to keep output stages biased deep in the AB1 range anyway, for I don't like high idle current on principle. Class A was never my style; if I want to cook the tubes, I'll put them in the oven.

If these EH 7591 and 7868 tubes prefer to be run in deep AB1, that's fine with me. If I can come up with a suitable test jig that doesn't involve the 400 and 800 I have in daily service, then I'll try a set of those tubes. Meanwhile, I'll continue to watch and take notes...

Larry

Threads restored, all pix back (that's twice now; hopefully it won't have to be done again!)

Larry

I agree. The EH7591s & 7868s have been out for some time now and many are using them to great satisfaction. I've used EH7591s in my Sherwood 5000-II and Sansui 1000A with no problems and they sound very good. They're big so size can be an issue. My Fisher 400 has EH7868s with individual bias and are every bit as good as the EH7591s - their only draw back compared to OS 7868 is the large pin size. EH went to the stronger magnoval base whereas OS 76868s use the novar one. EHs stretch the socket pins so replacing them with OS is a pain - you either retension (if you can) or better, replace the sockets.

Here is an interesting comparison done on 7591s:
http://www.tronola.com/html/7591a_tubes.html

I'm no fan of the JJs having had a bad experience but others love them.

Thanks for the tip about the magnoval bases on the new 7868's. That only doubles my determination to stay with what I have, and my stash should be adequate as it stands.

Frankly, I don't care a counterfeit two cents whether others "love" the new offshore offerings or not; that's their business, and I attend to mine according to my own experiences, perception, and decisions.

Larry

There was a recent discussion at AK about those EH & JJ 7591 & Craig of NOS Valves says he has been using JJ's for years on his customers amps with a very low failure rate. He says you have to buy them from a tube dealer that tests them thoughly and weeds out the bad ones as he says none of the factories do. He is careful in biasing properly too and prefers them sonically to the EH ones. He get's his from Doug's Tubes.

Just thought i should bring that up as I like my JJ EL84's. Well made and sound decent.

Randy

When the manufacture of vacuum tubes began to ramp up after decades of inactivity, JJ, unfortunately, developed a well-deserved reputation for premature failures. Since, I assume, they didn't want to go bankrupt, they made an enormous effort to improve their quality control. They've been doing fine for quite a few years now, and I would no longer be concerned at all about using JJ's in my equipment.

7591 and similar tubes like 7868 pushed the envelope at the end of the tube era. Cheap enough to replace, and easy enough to measure at advanced levels made for a difference in sales. 7189/6BQ5 was also abused in Fisher/Scott//others.

When restoring, probably wise to bias down a notch. Current production replacements just won't stand such abuse.

As far as I'm concerned, it's ALWAYS wise to bias output tubes down a notch, regardless of what tubes you're using. Cooler output tubes are happier and longer-lived ones.

I bias everything to run as deep in the AB1 range as possible. Takes a little experimentation sometimes to get things just so, but I'm willing to take the time.

When I want to cook tubes, I'll put them in the oven.

Larry

That is why I modified the power supply section on my RCA Victor console - to save tubes.
Originally RCA ran the PP 6BQ5's with 340V on plates..... ugh.
Now their running nicely at 310V, with 11v cathode bias.

+1

L

The EH 7868 is a wonderful tube. I have been using these daily in 2 different amps around the house since about 2005 without any probelms- a home brew PP monoblock in the bedroom and in a Fisher 400 receiver. I've had a matched quad of EH-7591 that I've been using daily in several amps since about 2007 without any problems, too. This set is now being used in my Scott 340A receiver- fortunately they are short enough to clear the tall case of this unit.

It will take me a while to go back to the JJ 7591 after a few bad experiences in a row with 3 different matched quads from reputable suppliers. I look forward to hearing people's experiences with these in 12-24 months from now with today's manufacture. My experiences were from a few years back (~2007).

PM posted to the member who started the thread.

I sent you the "Fisher Doctor" and "MetalBone" upgrade documents.

They are a good place to begin, as they both deal with power supply issues and other matters discussed in the thread.

I myself have tried different "mods" discussed here and on AK.

To keep it simple, replacing a selenium rectifier, older high voltage caps, and protecting expensive output tubes all makes sense.

I have an "upgraded" Fisher 500C with EH tubes in it in my office, and I play it every day. Two years, no problems, cool (maybe 105 degrees) transformers, nice sound. I run two more in different rooms in my house with Fisher output tubes, same modifications, nice sound, no problems.

I'm going to sell my two Scott Amps, and two Sherwoods, as I like the Fisher units more. But that issue is like wine, it's all a matter of how you taste things.

Andy