After reading some discussions on "red plate" baddness on output tubes - I thought I would post a picture.
This tube is running on my Tubelab Simple SE with the stock 560 ohm resistor.
I also took a temp reading - the plate where it is glowing is reading 341 deg F.
Thoughts?
This tube is running on my Tubelab Simple SE with the stock 560 ohm resistor.
I also took a temp reading - the plate where it is glowing is reading 341 deg F.
Thoughts?
Attachments
Yeah, that's about what my 6V6GTs look like. If you are after longevity, you should probably go to a colder bias resistor on those Chinese 6L6's.
It's difficult to get a good reading of the plates through the glass and know what to do with it. Many beam tubes direct the beam right at the seam of the two plate halves. The seam forms a wing that helps dissipate the heat. I've measured over 400^C on a big beam tube in the SimpleSE with no plate glow. My 6V6GTs don't have wings and the beam lands right in the middle of the plates. So those spots are hot, but the average reading of the plate is well below 400^C.
Here is an old pic:
Where are the red lines on those tubes?
It's difficult to get a good reading of the plates through the glass and know what to do with it. Many beam tubes direct the beam right at the seam of the two plate halves. The seam forms a wing that helps dissipate the heat. I've measured over 400^C on a big beam tube in the SimpleSE with no plate glow. My 6V6GTs don't have wings and the beam lands right in the middle of the plates. So those spots are hot, but the average reading of the plate is well below 400^C.
Here is an old pic:
Where are the red lines on those tubes?
I have been having the very same issues. I have a switch that I can change the value of r17 and r27. 560 ohm, 660 ohm, and 820 ohm. Unfortunately after some testing today even 820 has the biased too hot. I have purchased some kt88s. Problem solved lol. I will eventually build a 6 position switch and have even higher resistor values so that I can run my 6l6gcs also. 560ohms really had them glowing, 660 not as bad but still too much, 820 is pretty faint but still is visible. My b+ after the transformer, so testing on the blue wire of my opt I get a b+ of 450volts. Check my thread, Russ has helped me out a lot with some of the formula for figuring plate dissipation etc.
I'm listening to some 5992's in my Simple SE that have good stripes on them.
I tend to worry more about the current, than the overall dissipation, so as long as the current is not excessive, minor striping does not bother me. But I have a lot of tubes, so I have the luxury of being cavalier.
The worry seems to be that the newer tubes will outgas and degrade the vaccuum when the plate heats to incandescance. I don't know that there is any actual evidence of that.
If it bothers you, you can reduce the dissipation by increasing the value of the cathode bias resistor, or selecting a rectifier tube with a larger voltage drop (e.g. 5R4), or both.
Win W5JAG
I tend to worry more about the current, than the overall dissipation, so as long as the current is not excessive, minor striping does not bother me. But I have a lot of tubes, so I have the luxury of being cavalier.
The worry seems to be that the newer tubes will outgas and degrade the vaccuum when the plate heats to incandescance. I don't know that there is any actual evidence of that.
If it bothers you, you can reduce the dissipation by increasing the value of the cathode bias resistor, or selecting a rectifier tube with a larger voltage drop (e.g. 5R4), or both.
Win W5JAG
So all this talk about red plates and getters had me take a very close look at the quad of Philips/Sylvania JAN tubes I've been mentioning. One of the four tubes is particularly hotter than the rest and I have to bias its companion a bit hotter to balance the pair (matched quad my foot). So looking carefully at the getters, the hotter tubes' getters are slightly more transparent that the other pair. The hot tube in particular I can just see through the getter down to the mica if I shine the light just right. No rings in the getter, just a general fading.
There's nothing I can do about it...the bias supply has nothing more to give. I have many 100s of hours on these tubes, so I guess I can't complain. The hot tube is still really hot, so it appears that these tubes will get gassy before the cathodes give out. I have a quad of NOS "Mazda" tubes for this amp too that are much more sane as far as tube current. Maybe I'll set these JAN tubes aside since I like how they sound so much and the current price on them is getting a bit high....
There's nothing I can do about it...the bias supply has nothing more to give. I have many 100s of hours on these tubes, so I guess I can't complain. The hot tube is still really hot, so it appears that these tubes will get gassy before the cathodes give out. I have a quad of NOS "Mazda" tubes for this amp too that are much more sane as far as tube current. Maybe I'll set these JAN tubes aside since I like how they sound so much and the current price on them is getting a bit high....
Russ - The hot spot is where the two plate halves meet. Also the temp measurement was 340 F or 171 C!! I'm not even sure why it is glowing at that temp.
Nic6 - voltage & calculations are next on my list. I'm not sure that there is anything wrong the way it is.
Wavebourn - I plan to take a few measurements this evening.
Although killing these tubes would only put me out $18
Nic6 - voltage & calculations are next on my list. I'm not sure that there is anything wrong the way it is.
Wavebourn - I plan to take a few measurements this evening.
Although killing these tubes would only put me out $18
Ok here are the readings:
B+ 437v
cathode bias, 37v
That makes the cathode current 66ma. 400 volts drop across the tube - 26W tube dissipation. All these numbers meet spec (for an old RCA 6L6GC that is... god knows what the real numbers are on this Chinese version)
And boy it took a lot longer than I thought it would to ramp up B+. Thought for a minute there I broke something.
I'm just thinking that these tubes just glow... Wouldn't be good for an original RCA, but I don't have any of those now do I?
B+ 437v
cathode bias, 37v
That makes the cathode current 66ma. 400 volts drop across the tube - 26W tube dissipation. All these numbers meet spec (for an old RCA 6L6GC that is... god knows what the real numbers are on this Chinese version)
And boy it took a lot longer than I thought it would to ramp up B+. Thought for a minute there I broke something.
I'm just thinking that these tubes just glow... Wouldn't be good for an original RCA, but I don't have any of those now do I?
RCA tubes were like a charm... I once had a quad of original rusted metal 6L6s made for the Victory in WW-II, they were as close as twin brothers, no special matching were performed! I did not dare to use them, so sold to somebody of Japan on eBay.
...but I gave up using 6P3S tubes because they were gassy, and tolerances like gates of a barn. As soon I started using GU-50 I still feel a relief. By the way, Alligator project, and similar triode-pentode SE-pull designs were partly inspired by 6P3S that once run away thermally during a home concert. People kept saying that the amp sounded better, so I listened carefully and found that indeed it sounded better, though max power dropped down... So, SE-Pull was born: one triode and one modulated CCS loaded on P-P transformer.
...but I gave up using 6P3S tubes because they were gassy, and tolerances like gates of a barn. As soon I started using GU-50 I still feel a relief. By the way, Alligator project, and similar triode-pentode SE-pull designs were partly inspired by 6P3S that once run away thermally during a home concert. People kept saying that the amp sounded better, so I listened carefully and found that indeed it sounded better, though max power dropped down... So, SE-Pull was born: one triode and one modulated CCS loaded on P-P transformer.
Mr Strat,
I noticed that you are using Shuguang 6L6GC's from you photo. When I had first completed the Simple SE, I had four of the Shuguang's and one of them would get red. Your amp is fine. Can't say the same for the Shuguang's though. They don't make tubes like they used to.
If you like the 6L6 sound, try out a pair of the Philips-ECG 6BG6GA's (you'll need adapters). I've pushed them hard with no hint of red.
I noticed that you are using Shuguang 6L6GC's from you photo. When I had first completed the Simple SE, I had four of the Shuguang's and one of them would get red. Your amp is fine. Can't say the same for the Shuguang's though. They don't make tubes like they used to.
If you like the 6L6 sound, try out a pair of the Philips-ECG 6BG6GA's (you'll need adapters). I've pushed them hard with no hint of red.
Maximum Ug2 for 6P3S (6P3C) tube is 360-370V! If You have more, You need 6P3S-EV (6P3C-EB), or some from new production of 6L6GC.
You can try rise Rg2 from 470 Ohms to 1000-1500 Ohms, if You are satisfied with little less power. Maximum Pa (for 6P3S) is 22W, so you must fit bias to Pa0~14-15W, or less (with Ug2<360V, or Rg2>470!)!
You can try rise Rg2 from 470 Ohms to 1000-1500 Ohms, if You are satisfied with little less power. Maximum Pa (for 6P3S) is 22W, so you must fit bias to Pa0~14-15W, or less (with Ug2<360V, or Rg2>470!)!
I have brought this up before, but it looks like time to mention it again.
The original 6L6 was a metal cased tube that was advanced in its day (the 1930's). The 6L6G was the same guts installed in a glass bottle. Two improvements were made, the 6L6GA and the 6L6GB. Neither improvement increased the ratings.
All 6L6 types (even new production) have a drawback when operated at high plate voltages. The plate is pin 3 and the filament is pin 2. The filament is often grounded. It is possible especially in a guitar amp for an arc to jump from pin 3 to pin 2 in the tubes base or at the socket pins. This can blow up a lot of expensive parts, and yes I have seen it happen.
The 6L6GA or GB was adapted for use as a TV sweep tube and called the 6BG6G. The glass became straight sided and it was called the 6BG6GA. It was also adapted for transmitter use and called the 807. A 12 volt filament 807 is the 1625. The plate connection was moved to a cap on the top of the tube which allows for much higher plate voltages.
EVERY tube that I have mentioned so far has a 19 watt plate dissipation rating and a 270 volt screen grid voltage rating. These can not be used in a Simple SE with a B+ voltage over 340 volts or so, and that is violating the G2 spec. This includes the "normal" 6BG6G and 6GB6GA.
The 6L6GC has an upgraded plate and screen grid with higher ratings. The Tung Sol data sheet for the 6L6GC says 30 watts of plate dissipation and 450 volts on the screen in triode, 500 volts in UL or pentode. A properly manufactured 6L6GC should not glow in a Simple SE at 25 watts or so. Unfortunately some vendors don't always play by the rules, or have inconsistent quality. I have had good luck with the older Chinese Coke bottle shaped 6L6GC's but the newer ones and the straight sided 6L6GC's do glow at a lower power level. Some are better than others. There are many Soviet "6L6GC's" that are not really 6L6GC's at all. Some of these will glow brightly in a Simple SE.
In the quest for more power several "super 6L6GC's" were developed. The important one here is the 7027A. True 7027A's (not 7027's) are expensive. It is like a 6L6GC but it has a 35 watt plate and a 500 volt screen grid. There are some Russian 6L6GC types sold by New Sensor that have a large square plate structure that are capable of high dissipations as well.
As the vacuum tube era was dying several manufacturers (notably Sylvania) started consolidating their product lines and got real creative at stuffing the glass with just about anything that worked to fulfill contracts. The most outrageous is their 6B4GA which should be a DHT, but actually contained 6AV5 guts. Yes they stuck a pentode sweep tube in the place of a DHT.
They also stuck some 7027A guts inside some 6BG6GA bottles. These are actually 7027A's that will eat 35 watts and smile, inside a 6BG6GA bottle. They are NOT wired the same as a 6L6GC so an adapter is required to use them in a Simple SE, and they do have a plate cap on the top. The height of the adapter plus the tube plus the top cap is quite tall, but they will work quite well in a Simple SE. The standard 6BG6GA, even the ones with straight sided glass will not, they have 19 watt plates and will melt. The differences in the tubes are not obvious unless you have both in your hands. The special tubes have bigger plates.
The "special" 6BG6GA's are available from SND sales. I have installed a set in a friends guitar amp that ate 6L6GC's for lunch and they are happy after more than a year of abuse. I have not tried them in a Simple SE, but a few customers have with good reports. See their page (I have never ordered from them and don't have any knowledge of their business ethics).
http://www.vacuumtubes.com/6BG6.html
The original 6L6 was a metal cased tube that was advanced in its day (the 1930's). The 6L6G was the same guts installed in a glass bottle. Two improvements were made, the 6L6GA and the 6L6GB. Neither improvement increased the ratings.
All 6L6 types (even new production) have a drawback when operated at high plate voltages. The plate is pin 3 and the filament is pin 2. The filament is often grounded. It is possible especially in a guitar amp for an arc to jump from pin 3 to pin 2 in the tubes base or at the socket pins. This can blow up a lot of expensive parts, and yes I have seen it happen.
The 6L6GA or GB was adapted for use as a TV sweep tube and called the 6BG6G. The glass became straight sided and it was called the 6BG6GA. It was also adapted for transmitter use and called the 807. A 12 volt filament 807 is the 1625. The plate connection was moved to a cap on the top of the tube which allows for much higher plate voltages.
EVERY tube that I have mentioned so far has a 19 watt plate dissipation rating and a 270 volt screen grid voltage rating. These can not be used in a Simple SE with a B+ voltage over 340 volts or so, and that is violating the G2 spec. This includes the "normal" 6BG6G and 6GB6GA.
The 6L6GC has an upgraded plate and screen grid with higher ratings. The Tung Sol data sheet for the 6L6GC says 30 watts of plate dissipation and 450 volts on the screen in triode, 500 volts in UL or pentode. A properly manufactured 6L6GC should not glow in a Simple SE at 25 watts or so. Unfortunately some vendors don't always play by the rules, or have inconsistent quality. I have had good luck with the older Chinese Coke bottle shaped 6L6GC's but the newer ones and the straight sided 6L6GC's do glow at a lower power level. Some are better than others. There are many Soviet "6L6GC's" that are not really 6L6GC's at all. Some of these will glow brightly in a Simple SE.
In the quest for more power several "super 6L6GC's" were developed. The important one here is the 7027A. True 7027A's (not 7027's) are expensive. It is like a 6L6GC but it has a 35 watt plate and a 500 volt screen grid. There are some Russian 6L6GC types sold by New Sensor that have a large square plate structure that are capable of high dissipations as well.
As the vacuum tube era was dying several manufacturers (notably Sylvania) started consolidating their product lines and got real creative at stuffing the glass with just about anything that worked to fulfill contracts. The most outrageous is their 6B4GA which should be a DHT, but actually contained 6AV5 guts. Yes they stuck a pentode sweep tube in the place of a DHT.
They also stuck some 7027A guts inside some 6BG6GA bottles. These are actually 7027A's that will eat 35 watts and smile, inside a 6BG6GA bottle. They are NOT wired the same as a 6L6GC so an adapter is required to use them in a Simple SE, and they do have a plate cap on the top. The height of the adapter plus the tube plus the top cap is quite tall, but they will work quite well in a Simple SE. The standard 6BG6GA, even the ones with straight sided glass will not, they have 19 watt plates and will melt. The differences in the tubes are not obvious unless you have both in your hands. The special tubes have bigger plates.
The "special" 6BG6GA's are available from SND sales. I have installed a set in a friends guitar amp that ate 6L6GC's for lunch and they are happy after more than a year of abuse. I have not tried them in a Simple SE, but a few customers have with good reports. See their page (I have never ordered from them and don't have any knowledge of their business ethics).
http://www.vacuumtubes.com/6BG6.html
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