7591 questions

[Mike Stamper]

Having recently repaired my (restored/recapped) Sansui 1000A, I thought I would try some tube rolling. I had been using (4) new, matched EH 7591s. They sounded good, however, having read a recent article comparing old and new 7591s (an article which disparaged the new 7591s), I procured about 8 vintage 7591s - Westinghouse, RCA and Sylvania. These were sold as tested/good, and I confirmed this on my EICO 666. I have the 4 pot 1000A, so I can easily adjust and balance the idle cathode current individually for each of the four tubes. I usually set the bias to 28ma; this time I thought I would try 30ma.

One thing I noticed is that I was unable to get a stable bias current on some of the vintage tubes. I'd set the bias to 30ma, and then watch. Over time (on some of the tubes) the bias current would slowly but inevitably rise. Every time the current went up, I would crank up the bias. This would temporarily reduce the current. However, it would soon start rising as before. I put tubes that did this aside. Eventually, I found four vintage 7591s (2 Sylvania coin base, and two Westinghouse) that were stable even after 30 minutes.

My first question is: why is this happening with some of the tubes, but not all? Does this indicate a defect in the affected tubes? Would these tubes not exhibit this behavior in say, a cathode biased amp (the Sansui is fixed bias)? Or in a less-stressed 7591-based amp?

Next, after listening to the Sansui for a while, I'm not entirely sure the vintage tubes sound better than the EHs. The sound is clean with no obvious distortion. However, there is a kind of "dead" lifeless quality in the vintage tubes when compared to the EHs. It's not terrible, and it is perfectly listenable. I'm going to give the vintage tubes a bit more time. However, I may end up going back to the EHs. Just wondering if anyone else has noticed this.

 

[gogofast]

i think your old stock tubes are reaching the end of their lives. just because they test "good" on your tester (which does not apply much plate voltage when testing the tubes), that does not mean the tubes are strong.

basically, the problem with new production tubes is not much with sound quality but reliability. so they may as well sound better than worn out old production tubes.

 

[audiodon]

Grid bias runaway is common in vintage 7591 tubes.
It manifests itself in the tube not holding bias from turn-on through sustained use, as you've noted.
The EHs are quite good tubes when you have individual bias pots or can bias them in pairs. They can be hard to match up properly in a fixed bias situation with no pair or individual bias controls.
It does sound like your non-EHs may not be best of breed and may be tired.
The vintage 7591 is a funny tube.
It can look new and suffer grid bias runaway.
It can look like its on it's last legs with either brown or almost missing getter flash and obvious heat stress on the plates where they join together, yet the tube works fine and hold bias like a champ and will run trouble-free for years.

I can't think of any other output tubes with these same characteristics that show so obviously.

 

[redhotvalves]

I agree!

I sell some tubes from time to time and i always final test all tubes in a live amp checking for current draw stability.

 

[Tom Bavis]

You have some tubes with excessive grid leakage, which increases as the tubes warm up. The 7591 is probably more susceptible than most tubes because of its high transconductance (grid is very close to cathode) and because its designers allowed a rather high value of grid return resistor (max 300k, vs. 100K for 6V6/6L6, etc.) The EH tubes may have more grid leakage than the old stock ones did when new, and it's often recommended to reduce the grid resistor to 200K or less with them (1000A uses 200K).

 

[Mike Stamper]

I guess my experiment with vintage 7591s is over. Some of them were actually sold as "NOS"(!). I guess it's time to face facts and admit that there aren't that many decent used vintage 7591s left. I've also learned that tube testers, even "good" ones, don't test power tubes under actual operating conditions, so the results can't really be trusted.

I never tried the JJ/Tesla 7591s. Are these worth considering in my 1000A?

 

[apachef1]

JJ Tubes

I guess my experiment with vintage 7591s is over. Some of them were actually sold as "NOS"(!). I guess it's time to face facts and admit that there aren't that many decent used vintage 7591s left. I've also learned that tube testers, even "good" ones, don't test power tubes under actual operating conditions, so the results can't really be trusted.

I never tried the JJ/Tesla 7591s. Are these worth considering in my 1000A?

Stick with the EH tubes JJ have been very unreliable. Early tube failure very common with JJ
EH has had problems in the past but seem to have their QC in check

As far as tube testers-Unless the tube has a dead short or it life is completely spent a tube tester with give a clear reading
All a tube tester will tell you reliably that a tube is safe to use in a circuit

 

[mhardy6647]

Stick with the EH tubes JJ have been very unreliable. Early tube failure very common with JJ
EH has had problems in the past but seem to have their QC in check

As far as tube testers-Unless the tube has a dead short or it life is completely spent a tube tester with give a clear reading
All a tube tester will tell you reliably that a tube is safe to use in a circuit

Totally agree with the first statement - the EH tubes are just fine; the JJ's may be OK but why take the chance? (I say this as a guy with a couple of quads of the latter in the basement...)

As to the second - not quite sure what the poster's trying to say - I will say that the grid leakage issue mentioned for 7591s (and not uncommon for other power tubes, including - not surprisingly - the virtually identical electrically 7868) will show up if you cook the tube in, e.g., a Sencore Mighty-Mite. Been there, done that. The Mighty-Mite, despite being a lowly, unassuming emissions-type tester, does have a very good grid leakage test.

 

[Mike Stamper]

I have a Mighty-Mite as well. I think I'll try a leakage test on some of those troublesome vintage 7591s. And I guess I'll stay away from the JJ 7591s.

Thanks to all who commented.

 

[apachef1]

mighty mite

Totally agree with the first statement - the EH tubes are just fine; the JJ's may be OK but why take the chance? (I say this as a guy with a couple of quads of the latter in the basement...)

As to the second - not quite sure what the poster's trying to say - I will say that the grid leakage issue mentioned for 7591s (and not uncommon for other power tubes, including - not surprisingly - the virtually identical electrically 7868) will show up if you cook the tube in, e.g., a Sencore Mighty-Mite. Been there, done that. The Mighty-Mite, despite being a lowly, unassuming emissions-type tester, does have a very good grid leakage test.

I have a Mighty mite also and the grid leakage test is ok but not really a precursor to a tube problem but more of a search and find after a tube problem occurs. Even then the tube really needs to be in trouble.

I know when I have a "runaway tube" in my McIntosh amps by having to re-bias the tubes more frequently than once a month if needed.

 

[jaymanaa]

I've learned to stick with Westinghouse for vintage 7591s. It's a pretty crazy tube, but can sound really good with the right circuit. A lot of the old amps pushed them hard as it seems the watts race started with the 7591. Supposedly the whole power thing with the 7591 centered around special plate materials and coatings that dispersed the heat more evenly than other tubes. I think I've seen that it's true too. When a 7591 redplates, it does so over the entire plate instead of just in the middle like other tubes. A redplating 7591 is a sight to behold.:thumbsdn:

 

[Dave C]

I bought a quad of JJ 7591s for my 1000a a few years back. They lasted less than 18 months and took out a power resistor when 1 of them shorted. No more JJ for me although they did sound nice. I have vintage tubes installed now but I will say bias is hard to control. I generally check it a couple of times a year or then I touch the tubes and feel different heats from them. I think the market is wide open for a new 7591 tube. The problem with the EH is size, I hate to put a tube that close to the OPT. I have found however that a sheet of thin aluminum placed along side the OPT will really reduce the heat transfer into the OPT. I really need to pull my amp down again and check bias. My way is to run it about an hour before I even start and then get it as "safe" as I can. I seldom get a really close match.
This is quite different from my 300B SET amps. The Sophia tubes I have in them never move at all. I mean ZERO!

 

[Mike Stamper]

"A redplating 7591 is a sight to behold." I wonder how many Sansui 1000A 7591s were destroyed because no one thought to replace those aging, leaking 0.3uf coupling capacitors?

"a sheet of thin aluminum placed along side the OPT will really reduce the heat transfer" Thanks for the tip! I'll fabricate some pieces this weekend.

One last question: I put 10 ohm 1% 1/4 watt resistors in series between each 7591 cathode and ground. That allowed me to measure the bias current by measuring the voltage drop across the resistor. Is 10 ohms too much? Is it affecting power output?

 

[jaymanaa]

"A redplating 7591 is a sight to behold." I wonder how many Sansui 1000A 7591s were destroyed because no one thought to replace those aging, leaking 0.3uf coupling capacitors?

"a sheet of thin aluminum placed along side the OPT will really reduce the heat transfer" Thanks for the tip! I'll fabricate some pieces this weekend.

One last question: I put 10 ohm 1% 1/4 watt resistors in series between each 7591 cathode and ground. That allowed me to measure the bias current by measuring the voltage drop across the resistor. Is 10 ohms too much? Is it affecting power output?

Measure the voltage across the resistor and use ohms law to figure out how much power you are actually dissipating.

 

[Dave C]

One last question: I put 10 ohm 1% 1/4 watt resistors in series between each 7591 cathode and ground. That allowed me to measure the bias current by measuring the voltage drop across the resistor. Is 10 ohms too much? Is it affecting power output?

I did the same, it works out fine.

 

[kvflyer]

I've learned to stick with Westinghouse for vintage 7591s. It's a pretty crazy tube, but can sound really good with the right circuit. A lot of the old amps pushed them hard as it seems the watts race started with the 7591. Supposedly the whole power thing with the 7591 centered around special plate materials and coatings that dispersed the heat more evenly than other tubes. I think I've seen that it's true too. When a 7591 redplates, it does so over the entire plate instead of just in the middle like other tubes. A redplating 7591 is a sight to behold.:thumbsdn:

Hear, hear...

 

[drewbolce]

"A redplating 7591 is a sight to behold." I wonder how many Sansui 1000A 7591s were destroyed because no one thought to replace those aging, leaking 0.3uf coupling capacitors?

"a sheet of thin aluminum placed along side the OPT will really reduce the heat transfer" Thanks for the tip! I'll fabricate some pieces this weekend.

One last question: I put 10 ohm 1% 1/4 watt resistors in series between each 7591 cathode and ground. That allowed me to measure the bias current by measuring the voltage drop across the resistor. Is 10 ohms too much? Is it affecting power output?

I use 10 ohm 1/4 watt "flameproof" or metal resistors. These types behave more like fuses when you redplate a tube. My experience is that this in no way affect performance or power. This is the best way I have found to set bias in a fixed bias amp.

I remember not so fondly jumping for the variac control while a JJ 7591 redplated. I learned real quick what the 10 ohm "fusistors" really do. Finding output transformers is a PITA, replacing resistors not so much. I also learned that redplating is death for tubes, if you see the redplate the tube is junk.

There seems to be a commonly held misnomer about power tubes. Tube checkers do not fully test them. In particular they do not test for the ability to produce power, which is what we are most interested in. IMHO, the only way to test for power is to measure them in the target amp with a resistive load attached to the speaker outputs and a power/distortion meter (I use an ST1000a).

Also, most of the power tubes sold as NOS are not. As noted before appearance is not a reliable indicator of condition. Think about it, power tubes were commonly replaced. Most techs would put any tube that worked at all back in the tubebox. My experience with old tube collections is that the more often a tube type was replaced the less likely you are to find true NOS. Power tubes are at the top of this category.

 

[jaymanaa]

I use 10 ohm 1/4 watt "flameproof" or metal resistors. These types behave more like fuses when you redplate a tube. My experience is that this in no way affect performance or power. This is the best way I have found to set bias in a fixed bias amp.

I remember not so fondly jumping for the variac control while a JJ 7591 redplated. I learned real quick what the 10 ohm "fusistors" really do. Finding output transformers is a PITA, replacing resistors not so much. I also learned that redplating is death for tubes, if you see the redplate the tube is junk.

There seems to be a commonly held misnomer about power tubes. Tube checkers do not fully test them. In particular they do not test for the ability to produce power, which is what we are most interested in. IMHO, the only way to test for power is to measure them in the target amp with a resistive load attached to the speaker outputs and a power/distortion meter (I use an ST1000a).

Also, most of the power tubes sold as NOS are not. As noted before appearance is not a reliable indicator of condition. Think about it, power tubes were commonly replaced. Most techs would put any tube that worked at all back in the tubebox. My experience with old tube collections is that the more often a tube type was replaced the less likely you are to find true NOS. Power tubes are at the top of this category.

Excellent post. Back when I was carrying around tube caddies for my Dad, testers weren't held in nearly as high regard as they seem to be today. I honestly can't remember if he even owned one, and he had a heck of a well equipped shop with test jigs, etc.. He always said they were a tool for making money, more than a tool for repairing TVs. I think if you get one you can trust, and sort of get to know what it will or will not do, they are very handy, but in my shop, it takes me about three different testers to fully test a tube so that I would depend on it 90%. In circuit is where the real test is, and also where the matching should be done. Just my 2 pennies.

 

[jaymanaa]

That's the thing about matched quads of NOS tubes. IME it takes around 10 tubes to get a matched quad. Some sellers think if the brand, or date, or Mullard numbers are the same, they have a matched set.

I wish I could figure out why the old techs put bad tubes in new boxes. It sure has caused a lot of heartache to folks like us.:gigglemad

 

[FalconEddy]

That's the thing about matched quads of NOS tubes. IME it takes around 10 tubes to get a matched quad. Some sellers think if the brand, or date, or Mullard numbers are the same, they have a matched set.

I wish I could figure out why the old techs put bad tubes in new boxes. It sure has caused a lot of heartache to folks like us.:gigglemad

Because they were cheapskate pinheads! :yes:

Anyway, lifting the grounds on the 7591's by throwing in 10 ohm 1/4W resistors is a great idea; and 10 makes it easy to do the math. THEN. . . . maybe a couple of bias voltage adjust pots, and even test jacks above the chassis is an INCREDIBLE idea. This gives you complete control over all four 7591 tubes. No need to worry about matched sets ever again! I don't.

. . Falcon