7591 tubes

[Tube Radio]

Does any one know of a new manufacture 7591 tube that will work in a Scott LK-72a amp? I will not use JJ 7591 as it is a POS tube :thumbsdn: and the EH 7591 are to large in diameter. I don't care how tall the 7591 tubes are as I can run the amp outside its cabinet if need be. I do not want NOS unless that's the only way I can get tubes that fit. :wtf: Why is it so hard to make a quality tube these days?

[Tube Radio]

I found that Groove Tubes makes a 7591 tube. I'll attach a picture and let you experts tell me if it will fit in my Scott LK-72A?

[mpholland]

You might try a local electronics shop. I went to my local repair guy for 7591's for my Pilot and got good used vintage Westinghouses for 20 bucks each.

[Tube Radio]

I only know of one shop (TV repair) in my local area that had tubes at one time. Not sure if they have them any more.

[Tube Radio]

Even if I do find some how will I jnow if they are matched or not?

I did use JJ at first. I have had them about a week. This afternoon I replaced some resistors in the amp. I went to set bias and couldn't get it set right so I checked voltages on the control grid of the 7591 tubes. One had no volts so I put it in my Sencore Mighty Mite VI tube tester and showed a short on the control grid to cathode as with the tube in circuit I was showing sero ohms from control grid to ground. I could tap the tube and make the short go away, nut still showed questionable grid leakage. To prevent further damage I put the tubes back ion that came with the amp when I bought it. How anyone can make a complete POS tube and be happy about it is beyond me. The other three are still good, but I refuse to use them. I should have listened to the advice I was given about the JJ 7591 tube.

[mpholland]

If they have them, chances are they can test them, especially if they are going to sell them to you. It only takes a few minutes.

[Dave C]

I think you are being a little hard on the JJ's. I have a set and found the bias voltage needs to se set a bit higher, a volt or so. They sound real nice but seem to have some QC issues. Proceed with caution but give the JJ's a chance.
send the bad one back for exchange.

[DENNYDOG]

That Groove tubes 7591 was designed for a guitar amp. Keep your eyes open as they are plenty of decent original quads available on Ebay. I would stay clear of any of the new tubes as I've heard plenty of horror stories from both JJ and EH.

I brought up the same question as you in this thread. Check it out.

http://www.audiokarma.org/forums/showthread.php?t=144949&highlight=7591

[gogofast]

$20/tube of westinghouse brand is heck of a deal - i'd take 'em all. only way to really match them is to test them while in circuit. i've seen hickocks-matched tubes cause mis-match trouble in circuit. when using the new production tubes, you'd certainly need to change the bias level.

[Destructor]

I've had success with the JJ's in both a 299C and a 340B, same set of tubes switched from one to the other but I won't buy another set because of all the bad press I've read. I've settled for EH7591's in my 340B because they seem to have a bit more punch. The EH have worked fine. The JJ's are maybe 3 years old, seems I may have got a good batch.

[dshoaf]

I'd suggest you contact Jim McShane, with whom I've dealt on 7591s. He is aware of the issues with almost all of the current production tubes, tests everything he sells under load - not just with the old testers as many try to do.

I've purchased all my Eh7591s for the Mac tube amps from him with all positive results.

Cheers,

David

[Russellc]

I think you are being a little hard on the JJ's. I have a set and found the bias voltage needs to se set a bit higher, a volt or so. They sound real nice but seem to have some QC issues. Proceed with caution but give the JJ's a chance.
send the bad one back for exchange.

I dont think he is being the least bit hard on them. One of the biggest POS tubes being retailed. I'd give my Mcintosh MC 225 away before I would use them. Just my 2 cents.

Russellc

[Russellc]

I'd suggest you contact Jim McShane, with whom I've dealt on 7591s. He is aware of the issues with almost all of the current production tubes, tests everything he sells under load - not just with the old testers as many try to do.

I've purchased all my Eh7591s for the Mac tube amps from him with all positive results.

Cheers,

David


Of the new 7591 tubes the EH is the only one I personally would consider. I just find NOS and bite the bullet. Fortunately, the MC 225 is very easy on them, they seem to last forever in it.

Russellc

[NOSValves]

I really do not have the same experience as often reported with the JJ 7591. I've used hundreds with very little problems reported.

I really think many folks do not realize that tubes fail, some times prematurely, many times the defect that cause premature failure happens in shipping. This is almost always the case if a tube fails soon after being placed in service. Tube are very delicate devices. You know many a NOS tube back in the day failed also. They did not sell them at every drug store, Kmart, electronic shop and so on because there was no market.... The SS gear that took over the market place did it because of its reliability and head ache free service. It surely wasn't because of sonic benifits ;) .....especially with the early SS gear:tears:

If I was you I would contact who ever you purchased the quad of tubes from and request a matching replacement.

Also realize that no set negative voltage on the control grid of your LK72 is proper. You need to be able to test the current of each tube and adjust the control grid voltage to keep the tube in proper idle bias current state of 35 mA maximum per tube. This could be as low as -15 or as high as -22 on a LK72. Forget what a schematic might say as a reference voltage.

Craig

[fsjonsey]

I dont think he is being the least bit hard on them. One of the biggest POS tubes being retailed. I'd give my Mcintosh MC 225 away before I would use them. Just my 2 cents.

Russellc

Seconded! I've been through three matched quads of them on two separate amps, a Fisher X-100 and a Fisher 500b. At least one tube out of the quad would develop shorts within a few hours. I returned the bad tube and ordered a replacement. Within a week of casual use, the bias would begin to drift all over the place.

I switched to Eh 7591a's and haven't had a problem since.

[NOSValves]

What I'd like to know is where are you folks that are having all these problems buying these JJ 7591's from??

Craig

[FalconEddy]

Also realize that no set negative voltage on the control grid of your LK72 is proper. You need to be able to test the current of each tube and adjust the control grid voltage to keep the tube in proper idle bias current state of 35 mA maximum per tube. This could be as low as -15 or as high as -22 on a LK72. Forget what a schematic might say as a reference voltage.

Tube Radio,

This is exactly why I mentioned adding a full bias control with test points above the chassis to your LK-72A.

. . Falcon

[gogofast]

Tube Radio,

This is exactly why I mentioned adding a full bias control with test points above the chassis to your LK-72A.

. . Falcon

falcon,
i've always wondered about how to have that feature in any tube amp using different output tubes. how difficult is the mod - to bias each tube? most of my tube amps are push-pull and have cathode resistor for each channel.

[FalconEddy]

falcon,
i've always wondered about how to have that feature in any tube amp using different output tubes. how difficult is the mod - to bias each tube? most of my tube amps are push-pull and have cathode resistor for each channel.

Honestly, I do not have hands-on, or documented, information for these modifications to an Eico HF-87. Plus, I thought this amp did not have a DC balance pot for equalizing the current draw between those two EL34's.

However, I HAVE heard that this particular design that utilizes EL34's is REALLY pushing things to the max, and a KT-88 is a much better choice for this amp.

Some of the Fisher's are similar to some of the Scott's, so a full control bias mod would most likely be able to be incorporated.

I haven't even seen a schematic on the HF-87.

. . Falcon

[Russellc]

I really do not have the same experience as often reported with the JJ 7591. I've used hundreds with very little problems reported.

I really think many folks do not realize that tubes fail, some times prematurely, many times the defect that cause premature failure happens in shipping. This is almost always the case if a tube fails soon after being placed in service. Tube are very delicate devices. You know many a NOS tube back in the day failed also. They did not sell them at every drug store, Kmart, electronic shop and so on because there was no market.... The SS gear that took over the market place did it because of its reliability and head ache free service. It surely wasn't because of sonic benifits ;) .....especially with the early SS gear:tears:

If I was you I would contact who ever you purchased the quad of tubes from and request a matching replacement.

Also realize that no set negative voltage on the control grid of your LK72 is proper. You need to be able to test the current of each tube and adjust the control grid voltage to keep the tube in proper idle bias current state of 35 mA maximum per tube. This could be as low as -15 or as high as -22 on a LK72. Forget what a schematic might say as a reference voltage.

Craig

This indeed may be possible, but other tubes are also subjected to this. I read somewhere, maybe the Mcshane site, (sorry if mistaken) that for these tubes to be reliable a resistor change is necessary to make them reliable? Never had an old one go south.

Russellc

[Bruce221]

In this case for cost I would choose eh over jj nos 7591as are very high cost had a quad here for a fisher 202b amp-rcas cost me $200 bucks -this was before any current manufactur made em-bias up the EH slowly.