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Caution about E-H 7591 replacements
I recently had an experience using the Electro-Harmonix 7591 tubes in a
McIntosh 1500 that I feel warrants a comment. I noticed on a couple of occasions that one of the 4 outputs tubes would suddenly overheat to the point of glowing bright red. I rushed to the receiver and turned it off, and of course immediately suspected bias. A complete examination of the bias pots and power supply shows no problem. Later, the same thing happend, except on a different tube. Now, I think "RF oscillation", and so I hook up instrumentation to the amp and await the next occurence. Result - no RF or oscillation, just tube current running away (normal 0.7 volts across the 15 ohm bias measurment resistor became 4.5 volts!, indicating a current of 300ma, where the normal bias level is 45ma). This time, I was fortunate enough to observe that the tube began getting red hot inside the plate first, then the plate followed. After closely examining the tube, I indentified the element overheating as the screen grid. So, I had a classic case of screen grid runaway, which means all of the tubes were sitting there like little volcanoes ready to erupt, and the choice of which one goes first was random. Examination of the tube specs and the schematic of the Mac 1500 shows that the screen is being operated off of a 450V supply, with just a 900 ohm dropping resistor and additional filter cap between it and the plate supply (at 460 volts). This is a very high screen voltage, and the 7591 tube data I have shows that the screen voltage should never exceed 400V. The receiver never exhibits this behavior on the old NOS 7591's, which just goes to show that a new tube can be designed as a match, but unless the exact same materials are used in the exact same geometry, Murphy can come and bite you. The solution? Screen stopper resistors don't "stop" it (no pun intended) until you reach a value of over 2000 ohms, which reduces the output power of the amp to 25W and increases distortion, measured on my own unit, from 0.1% to over 3.0%. So that's a no go. Another solution would be to downpower the screen voltage to below 400 volts, but I suspect that would have the same result of increased distortion and less power (the McIntosh unity-coupled circuit requires a lot of voltage swing to drive the output tubes, and I suspect that the high screen voltage was used to make the tubes easier to drive, and the hand selected tubes of the era could take it. Other McIntoshs that use 7591's, like the MC-225, have swapped screen drive (driven from the opposite plate), bootstrapping to increase the drive and cathode bias instead of the fixed bias in the 1500. Cathode bias would stop this dead, since it automatically limits the maximum current thru the tube). So, I've had no problem with these in MC 225's, but in a 1500, watch out!! |
#2
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"Me" wrote in message ... I recently had an experience using the Electro-Harmonix 7591 tubes in a McIntosh 1500 that I feel warrants a comment. ** I suspect the cause is just internal temperature ( due to high plate dissipation ) - at a high enough temp the ( imperfect) mica insulation from G2 to G1 becomes leaky and thermal runaway sets in. If the 7591 grids were supplied with negative bias through a lowish resistance (or cathode drive were used) this sort of problem is avoided. ............. Phil |
#3
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"Phil Allison" wrote in message u... "Me" wrote in message ... I recently had an experience using the Electro-Harmonix 7591 tubes in a McIntosh 1500 that I feel warrants a comment. ** I suspect the cause is just internal temperature ( due to high plate dissipation ) - at a high enough temp the ( imperfect) mica insulation from G2 to G1 becomes leaky and thermal runaway sets in. If the 7591 grids were supplied with negative bias through a lowish resistance (or cathode drive were used) this sort of problem is avoided. ............. Phil I think Phil is correct. I would suggest lowering the idle current to 30ma. The heat will go way down, and the Plate B+ will go up a little. I don't think the maximum power will drop very much, and your ears won't be able to hear the slight rise in distortion (open to debate). The tubes will then probably last for many years. If you are worried about the high screen voltage, an I am too, maybe try a suitably sized zener diode(s) in series with the screens from the high B+ to drop the idling screen voltage to something under 400 volts. If you do this, you will have to consider the zener power dissipation issues, and assume someone will run up both channels to full output with square wave drive from a audio generator. A once made a string of 5 watt zeners attached to a heatsink with thermally conductive epoxy. It worked well, but was UGLY! I run old style 7591's in a linear class AB1 RF circuit with 600 volts plate, and +375 screen grid, and 30 ma idle current. I had to regulate the screen grids. Before I did this the idle current would vary disproportionate with just a few volts change in line voltage. The screen voltage would rise to about 450 volts at 125 vac line voltage. I had hoped that the G1 fixed bias change would offset the G2 change with changing line voltage. It did not, and since I was running the tubes at max idle plate dissipation, I had no headroom. I too had a thermal run away with a old style 7591. They are never the same afterward. The lesson I learned was to not run G2 over 400 volts, and to regulate G1 and G2 such that they are immune to power line variation, and G2 was regulated up to 80 ma (for two 7591's). A little DC fan moving some air past the tubes is also useful. I used a Ti TL-431 adjustable shunt regulator for the g1 bias control. This is nice, and will also hold the bias should you have enough grid drive to extend into class Ab2 operation. I am curious about one other thing. Many pieces of tube gear designed in the 50's, 60's were expecting the AC line voltage to be 110 to 115 vac. It seems that the AC mains today in many locations is often between 120 to 130 vac. That means lots of vintage tube equipment is running everything high, and this needs to be taken into consideration. Besides the element voltages being high, and the electrolytic capacitors sometimes seeing DC beyond their ratings, we also sometimes end up with our precious filaments running at 7 volts or more instead of the nominal 6.3 vac. Thermal run away in tube equipment that was designed to run the output tubes at maximum dissipation is not uncommon, and after raising the line voltage up like it is these days, thermal runaway is very probable. Of course if running 100% stock is the goal, NOS 7591's should be used, and have a little air moving past the tubes. a variac with a AC volt meter attached would also be very desirable if not mandatory. Regards, Jim |
#4
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"Jim Candela" wrote in message . .. "Phil Allison" wrote in message u... "Me" wrote in message ... I recently had an experience using the Electro-Harmonix 7591 tubes in a McIntosh 1500 that I feel warrants a comment. ** I suspect the cause is just internal temperature ( due to high plate dissipation ) - at a high enough temp the ( imperfect) mica insulation from G2 to G1 becomes leaky and thermal runaway sets in. If the 7591 grids were supplied with negative bias through a lowish resistance (or cathode drive were used) this sort of problem is avoided. ............. Phil I think Phil is correct. I would suggest lowering the idle current to 30ma. The heat will go way down, and the Plate B+ will go up a little. I don't think the maximum power will drop very much.... I thought I would do a little more investigation. The thermal runaway due to Mica insulation intrigued me, so I took one of my NOS 7591's and compared it to one of the new E-H's on my Vacu-trace, while measuring actual screen current. Results as follows: In both case, plate was 380 volts, screen at plate potential and the tubes were biases to 70 ma (I wanted to put some thermal load on the tubes and try to spot a trend). E-H 7591 Screen current: NOS 7591screen current: 0 secs: 17.5 ma 6.5 ma 30 secs: 20 ma 6.8 ma 120 secs: 12.8 ma 6.9 ma 300 secs: 10.0 ma 6.8 ma As you can see, the NOS tube stayed stable, with fairly constant screen current flow. The E-H, on the other hand, immediately showed higher screen current (?exposed more to the electron flow?), and then screen current began to decrease, which I theorize is the screen beginning to emit electrons as it heated (positive current initially measured was current flow into the screen due to electron impingement....the only reason can think of for flow to decrease is that the screen began to emit as it heated and this screen -to-cathode flow (vs. the normal cathode -to-screen flow direction) reduced the apparent current inflow). This could also be a conduction effect across the mica to the screen spacers, of course, so maybe I haven't proven screen emission, but I'll try to think of a better test. No runaway, but the highest plate voltage I can get with the Vacu-trace is 380 volts, and I have no doubt that if I could push it to 450, I would get screen current going thru 0, then negative followed by a runaway. I may try that on my HV power supply soon. The E-H tube is usable within standard 7591 specs ( 400 V on the screen), but the Mac 1500 pushes it's tubes very hard at spec setup (460V plate, 450V screen) that they apparently can get away with on the NOS tubes. Interesting. able to hear the slight rise in distortion (open to debate). |
#5
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"Me" wrote in message ... "Jim Candela" wrote in message . .. "Phil Allison" wrote in message u... "Me" wrote in message ... I recently had an experience using the Electro-Harmonix 7591 tubes in a McIntosh 1500 that I feel warrants a comment. ** I suspect the cause is just internal temperature ( due to high plate dissipation ) - at a high enough temp the ( imperfect) mica insulation from G2 to G1 becomes leaky and thermal runaway sets in. If the 7591 grids were supplied with negative bias through a lowish resistance (or cathode drive were used) this sort of problem is avoided. ............. Phil I think Phil is correct. I would suggest lowering the idle current to 30ma. The heat will go way down, and the Plate B+ will go up a little. I don't think the maximum power will drop very much.... I thought I would do a little more investigation. The thermal runaway due to Mica insulation intrigued me, so I took one of my NOS 7591's and compared it to one of the new E-H's on my Vacu-trace, while measuring actual screen current. Results as follows: In both case, plate was 380 volts, screen at plate potential and the tubes were biases to 70 ma (I wanted to put some thermal load on the tubes and try to spot a trend). E-H 7591 Screen current: NOS 7591screen current: 0 secs: 17.5 ma 6.5 ma 30 secs: 20 ma 6.8 ma 120 secs: 12.8 ma 6.9 ma 300 secs: 10.0 ma 6.8 ma As you can see, the NOS tube stayed stable, with fairly constant screen current flow. The E-H, on the other hand, immediately showed higher screen current (?exposed more to the electron flow?), and then screen current began to decrease, which I theorize is the screen beginning to emit electrons as it heated (positive current initially measured was current flow into the screen due to electron impingement....the only reason can think of for flow to decrease is that the screen began to emit as it heated and this screen -to-cathode flow (vs. the normal cathode -to-screen flow direction) reduced the apparent current inflow). This could also be a conduction effect across the mica to the screen spacers, of course, so maybe I haven't proven screen emission, but I'll try to think of a better test. No runaway, but the highest plate voltage I can get with the Vacu-trace is 380 volts, and I have no doubt that if I could push it to 450, I would get screen current going thru 0, then negative followed by a runaway. I may try that on my HV power supply soon. The E-H tube is usable within standard 7591 specs ( 400 V on the screen), but the Mac 1500 pushes it's tubes very hard at spec setup (460V plate, 450V screen) that they apparently can get away with on the NOS tubes. Interesting. able to hear the slight rise in distortion (open to debate). Good test! I am curious if the grid bias required to get 70 ma was very different between the two tube types. I also wonder what the result would have been if your test would have inserted a few hundred ohms in series with G2 before it connects to the plate lead. As your test was described, I would have thought the screen grid would hog more current from the plate than it did. Very interesting stuff. This kind of reminds me of a issue with the Raytheon 4D32 tetrode. This is a pulse modulator tube used in WW2, and later as a class C RF amplifier in some ham radio transmitters built in the 1940's, and 1950's. One failure mode on these is the indirect cathode sheds little white flakes. These can be seen inside the glass envelope on heavily used tubes. The failure mode on this tube is primarily reverse G1 current, and eventual thermal run away. In class C RF service a bad tube will work fine for a while, but after a period of time (variable) the grid current will begin to decrease, and eventually go to zero. At this point the tube is still working, and output power is down a bit. It is like backing off the drive from class C to Ab1. Then the grid current reverses direction, and plate current overload soon follows. My theory here is that a small spec of oxide has impacted the first grid (G1), and eventually heats up a local hot spot such that the G1 becomes an emitter of electrons. I wonder if the EH 7591 has any visible symptoms of localized G2 heating, or is it all around? The RF parasitic idea you had is still a possibility. One easy thing to do is attach both leads of a neon lamp (say NE2) to each 7591 plate lead at the socket, and fold the neon bulb over till the glass is almost touching the chassis. Go ahead and power up the amp, and play some music. If a parasitic oscillation occurs, expect the lamps to flicker from time to time. The color of the neon is also a good indicator of frequency (poor mans spectrum analyzer). A amber glow occurs well into the HF region, and gradually becomes purple at VHF frequencies. I still think that cranking your Mac's line voltage down to nominal for a vintage Mac will help. I have some vintage tube equipment where I require only about 112 vac AC line to achieve 6.3 vac filament voltage. |
#6
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"Jim Candela" wrote in message ... Good test! I am curious if the grid bias required to get 70 ma was very different between the two tube types. I also wonder what the result would have been if your test would have inserted a few hundred ohms in series with G2 before it connects to the plate lead. As your test was described, I would have thought the screen grid would hog more current from the plate than it did. Very interesting stuff. This kind of reminds me of a issue with the Raytheon 4D32 tetrode. This is a pulse modulator tube used in WW2, and later as a class C RF amplifier in some ham radio transmitters built in the 1940's, and 1950's. One failure mode on these is the indirect cathode sheds little white flakes. These can be seen inside the glass envelope on heavily used tubes. The failure mode on this tube is primarily reverse G1 current, and eventual thermal run away. In class C RF service a bad tube will work fine for a while, but after a period of time (variable) the grid current will begin to decrease, and eventually go to zero. At this point the tube is still working, and output power is down a bit. It is like backing off the drive from class C to Ab1. Then the grid current reverses direction, and plate current overload soon follows. My theory here is that a small spec of oxide has impacted the first grid (G1), and eventually heats up a local hot spot such that the G1 becomes an emitter of electrons. I wonder if the EH 7591 has any visible symptoms of localized G2 heating, or is it all around? The RF parasitic idea you had is still a possibility. One easy thing to do is attach both leads of a neon lamp (say NE2) to each 7591 plate lead at the socket, and fold the neon bulb over till the glass is almost touching the chassis. Go ahead and power up the amp, and play some music. If a parasitic oscillation occurs, expect the lamps to flicker from time to time. The color of the neon is also a good indicator of frequency (poor mans spectrum analyzer). A amber glow occurs well into the HF region, and gradually becomes purple at VHF frequencies. I still think that cranking your Mac's line voltage down to nominal for a vintage Mac will help. I have some vintage tube equipment where I require only about 112 vac AC line to achieve 6.3 vac filament voltage. I may play with the neon lamp idea. I've about convinced myself that McIntosh pushed these things this hard to allow unity coupling without having to bootstrap the driver tube. The driver setup in the 1500 is absolutely "standard", which is what would be used with plate loaded P-P, but when you throw the unity-coupled winding in the cathode circuit, it makes the cathode voltage run up and down with the audio current, which requires G1 to swing much greater voltages to maintain drive to the tube than a normal P-P. I think, since this was a receiver and aimed more at affordability than Mac's usual stuff, they did it this way to save some parts and production cost. They drove the tube beyond spec because they could back then. Another interesting part of this to me, is that, seemingly, the E-H 7591, being physically larger, should dissipate heat more effectively. I'm an EE, and it'sbeen a while since I use thermodynamics, but an analysis of heat dissipation modes in these tubes would be interesting. It's obvious that some heat goes via conduction thru pins and mica, but the radiant heat loss from the plate is what I'm talking about. Does having the glass envelope physically closer to the plate help radiation cooling? If so, the old "little ones" would have an advantage, but the glass isn't close on a KT-88 and they can sure dissipate the heat. I'm also going to look at the line voltage end of things. Thanks! |
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