With these tubes being new on the market, there has been quite a bit of buzz about them, from how they look, to their size, and how they sound -- all of which has received positive press. Of course, any long term dependability results will simply take time to prove out, but in the mean time, tests can still be run to see how they stack up against the original American pieces, and the other current production versions of this tube. Therefore, an order for a matched quad of these tubes was placed with Jim McShane, who quickly got them to my doorstep.
From the stand point of size and appearance, these tubes are for all intents and purposes virtually the same size as the original full base American 7591 (non A version) tube -- even though this new tube is identified as a 7591A. Therefore, this new tube is slightly taller than the button based A version of this tube as last produced in America, with that tube being slightly shorter than the non A American version as well.
The internal structure has detail differences as well, with the most notable difference being that the tooling holes in the plate are round in this new tube, versus square in the American versions. But don't just give these tubes a casual glance, or it would be quite easy to assume that they are an American product! A dead giveaway however is that the classic Tung Sol stop sign identifying the tube type appears to be painted onto the top of the tube, rather than etched.
Overall however, the size and appearance are very positive, with this new tube looking like it was actually born out of the original American device.
So onto some initial testing.
I have not run all the tests I plan to yet, as I wanted to get some initial findings out there to the community. Therefore, at this point, I have only been able to validate the match as provided by Jim, the power output capability, and simply how the tubes reacted to the test events. The results are significant.
From a standpoint of matching under quiescent (no signal) current draw conditions, 3 of the 4 tubes were matched within better than .5%, while the forth tube was still well matched, but deviated from the rest of the pack by 3.3%, which then set the overall balance achieved for the quad. By itself, this is very good matching, with the matching of the three tubes being beyond exceptional. With this level of quiescent matching -- particularly with high Gm tubes like the 7591 family of tubes represents -- you can expect the output tubes to be well balanced in equipment that offers limited or no ability to adjust individual output tube current draw.
Understand that while the tubes MAY be manufactured to a good tolerance level, the high degree of quiescent matching achieved in my quad is entirely due to the vender used. Therefore, the few extra pennies paid for a quality vender not only culls out the rejects, but also produces a superior match. I have no business connections with Jim, but when his service, culling, and matching are all considered, his reputation and value are well deserved.
With these being new tubes, and my sample so small, it is hard to establish what a norm is for their power capability. However, I can state that as judged against a nominal American device, the group as an average produced nearly 87% of the power that quality American devices can produce. On an "English" scale, this places these tubes as solidly GOOD as compared to what the American pieces can do. That scale has any tube producing 81% or better of what a nominal American piece can produce as being GOOD. Within the group, two of the tubes were dead on the 87% average, while one was weaker at 82%, and another being an over achiever at nearly 92%.
But that's not the whole story. There are details of the testing that show the very notable improvement that these tubes represent over the other current production 7591 tubes I've tested:
1. The bias setting is VERY stable in all four tubes. Other examples of non-American 7591s I've tested tend to be lazy in reaching a final resting point, or, will change with short term large signal (power output) excitation tests. The stability noted with these tubes was impressive.
2. The tubes performed very well with the heaters operating at 6.300 vdc. Many Russian tubes actually require a heater voltage that is about 5% high of spec to achieve a reasonable level of performance. With the Tung Sols, not only was the performance very good at the design center of 6.3 volts, but it only slightly improved with the heaters operating a +5%.
3. Most importantly, the power readings at maximum power output were quite stable!! This is huge, with a stability that is reminiscent of quality American pieces. Other examples of these tubes all too often either increase from a lower power level during testing due to internal heating of the tube, or actually sink in power, because the cathode simply cannot keep up. That these tubes -- all of them -- held a stable power output reading during testing represents a significant improvement over previous foreign efforts to manufacture this tube that I've tested, which should bode will for tube life. It speaks directly to the quality of cathode construction and activation, which is everything towards that end.
All in all then, these tubes are still somewhat short of the best American pieces produced. But in my opinion, they represent a major improvement over previous modern efforts to manufacture this tube. These tubes represent a significant effort to actually replicate the original device in terms of size, appearance, and performance. My testing is clearly and obviously limited to a four tube sample. But based on the results I'm seeing, versus that which I've all too often seen in the past, the results are very positive. I am very heartened by the development of this tube. I look forward to the experience that others can provide as well!
Happy tubing!
Dave
From the stand point of size and appearance, these tubes are for all intents and purposes virtually the same size as the original full base American 7591 (non A version) tube -- even though this new tube is identified as a 7591A. Therefore, this new tube is slightly taller than the button based A version of this tube as last produced in America, with that tube being slightly shorter than the non A American version as well.
The internal structure has detail differences as well, with the most notable difference being that the tooling holes in the plate are round in this new tube, versus square in the American versions. But don't just give these tubes a casual glance, or it would be quite easy to assume that they are an American product! A dead giveaway however is that the classic Tung Sol stop sign identifying the tube type appears to be painted onto the top of the tube, rather than etched.
Overall however, the size and appearance are very positive, with this new tube looking like it was actually born out of the original American device.
So onto some initial testing.
I have not run all the tests I plan to yet, as I wanted to get some initial findings out there to the community. Therefore, at this point, I have only been able to validate the match as provided by Jim, the power output capability, and simply how the tubes reacted to the test events. The results are significant.
From a standpoint of matching under quiescent (no signal) current draw conditions, 3 of the 4 tubes were matched within better than .5%, while the forth tube was still well matched, but deviated from the rest of the pack by 3.3%, which then set the overall balance achieved for the quad. By itself, this is very good matching, with the matching of the three tubes being beyond exceptional. With this level of quiescent matching -- particularly with high Gm tubes like the 7591 family of tubes represents -- you can expect the output tubes to be well balanced in equipment that offers limited or no ability to adjust individual output tube current draw.
Understand that while the tubes MAY be manufactured to a good tolerance level, the high degree of quiescent matching achieved in my quad is entirely due to the vender used. Therefore, the few extra pennies paid for a quality vender not only culls out the rejects, but also produces a superior match. I have no business connections with Jim, but when his service, culling, and matching are all considered, his reputation and value are well deserved.
With these being new tubes, and my sample so small, it is hard to establish what a norm is for their power capability. However, I can state that as judged against a nominal American device, the group as an average produced nearly 87% of the power that quality American devices can produce. On an "English" scale, this places these tubes as solidly GOOD as compared to what the American pieces can do. That scale has any tube producing 81% or better of what a nominal American piece can produce as being GOOD. Within the group, two of the tubes were dead on the 87% average, while one was weaker at 82%, and another being an over achiever at nearly 92%.
But that's not the whole story. There are details of the testing that show the very notable improvement that these tubes represent over the other current production 7591 tubes I've tested:
1. The bias setting is VERY stable in all four tubes. Other examples of non-American 7591s I've tested tend to be lazy in reaching a final resting point, or, will change with short term large signal (power output) excitation tests. The stability noted with these tubes was impressive.
2. The tubes performed very well with the heaters operating at 6.300 vdc. Many Russian tubes actually require a heater voltage that is about 5% high of spec to achieve a reasonable level of performance. With the Tung Sols, not only was the performance very good at the design center of 6.3 volts, but it only slightly improved with the heaters operating a +5%.
3. Most importantly, the power readings at maximum power output were quite stable!! This is huge, with a stability that is reminiscent of quality American pieces. Other examples of these tubes all too often either increase from a lower power level during testing due to internal heating of the tube, or actually sink in power, because the cathode simply cannot keep up. That these tubes -- all of them -- held a stable power output reading during testing represents a significant improvement over previous foreign efforts to manufacture this tube that I've tested, which should bode will for tube life. It speaks directly to the quality of cathode construction and activation, which is everything towards that end.
All in all then, these tubes are still somewhat short of the best American pieces produced. But in my opinion, they represent a major improvement over previous modern efforts to manufacture this tube. These tubes represent a significant effort to actually replicate the original device in terms of size, appearance, and performance. My testing is clearly and obviously limited to a four tube sample. But based on the results I'm seeing, versus that which I've all too often seen in the past, the results are very positive. I am very heartened by the development of this tube. I look forward to the experience that others can provide as well!
Happy tubing!
Dave
