Cycline3 said:
Mmmm ... Methinks that there are also indications of misinterpretation? Let us see -
The following comment followed on that part of my post #6, where I said that I experienced glowing G2s with EL34s and subsequently used 6L6s (don't seem to be able to quote that here):
How do you reach that simplistic conclusion? (First sentence). Neither said by me, nor implied!
It can also have to do with the internal construction of the tube. As far as I could conclude at the time (having dissected the tube) the pentode construction could lead to more adjacent screen windings being fully exposed to spaces between control grid windings (thus drawing higher current there) in an "uncontrolled" relative grid alignment than would be the case with beam tubes. Whatever the reason, it happened and well within the maximum ratings of the relevant tubes. And going to beam tubes did make it better (after the proper checks and design considerations, thanks Cycline 3; I am an EE. I never said I simply swopped tubes.). I am not generalising; I state professional experiences of myself and one or two colleagues. Perhaps we had poor samples of Philips/Telefunken EL34s - who knows.
I agree. But others mentioned this. So why does a manufacturer call his EL34 a "beam tube" - does he not know what he is making? And the few 6CA7s I have seen were not beam tubes but pentodes; structural equivalents of EL34s - I handled a few from a Marantz 8 two weeks ago. Not a quibble with you, but a power pentode's characteristics (linearity etc.) can never be the same as that of a beam tube, even though quiescent working points might be identical.
Reduction of screen current with the normally used Rg2 of a few hundred ohms?? I am afraid the voltage drop as a result of these is too small to significantly affect screen dissipation, except in abnormal conditions as in guitar amps - see below. And distortion reduction is only significant for the EL34 at 1K Rg2. A great many successful amps exist without any g2 resistors in the power tubes, and I have data where high g2 resistors increased distortion. In UL this is associated with the output transformer characteristics; GEC often used RCs from anode - g2.
Good advice, as confirmed by others. But I do believe the design of guitar amps is a rather special case, not to be confused with general hi-fi design procedures. If ratings are exceeded etc. matters immediately go beyond the scope of normal design; that is the very reason why manufacturers give design guidelines. I am not at peace with propagating measures found to be necessary there as general practice for normal amplifier design (not saying Cycline 3 that you are doing that).
Correction - edit
I have just made a correction in the above reply to Cycline3, but those reading immediately after my posting may not have noticed.
After Cycline3s fourth quotation I inadvertantly said that he did not mention g2 oscillation prevention. That is of course not true, and I have now removed that.
My apologies.
I have just made a correction in the above reply to Cycline3, but those reading immediately after my posting may not have noticed.
After Cycline3s fourth quotation I inadvertantly said that he did not mention g2 oscillation prevention. That is of course not true, and I have now removed that.
My apologies.
General Electric made a 6CA7 beam tube which is supposed to be electrically equivalent to the EL34.. I would post a picture here, unfortunately if I have any left I have not been able to locate them. (This after seriously reorganizing my stash, and I will say I was surprised by what I did find. Now I am very organized and if I find one lurking out in the garage where I have not yet looked I will post a picture, but it is very unlikely 🙁 ..)
Incidentally not all/(many?) American companies made a beam power version of the 6CA7 - the Tungsol 6CA7 is a pentode.
I have not been able to find a single online scan of a GE data book showing their 6CA7 which is odd because the data books are still relatively common here.
Groove Tubes I understand is currently selling a reproduction of the GE 6CA7 beam power tetrode, there may be further details on their site.
Incidentally not all/(many?) American companies made a beam power version of the 6CA7 - the Tungsol 6CA7 is a pentode.
I have not been able to find a single online scan of a GE data book showing their 6CA7 which is odd because the data books are still relatively common here.
Groove Tubes I understand is currently selling a reproduction of the GE 6CA7 beam power tetrode, there may be further details on their site.
Still on EL34/6CA7s
Ah yech!! I so like sticky dust - at 33/91 degrees.
I went and rummaged around in the garage and found all of 8 old EL34s - all east! Two were from National marked EL34/6CA7 made in E. Germany. The rest were all Sovtek (only EL34) - all identical. Not saying much. I wished I noticed what make EL34s the mentioned Marantz 8 had, but it is back with its owner in another town. (He is an American, having brought it over here when he emigrated, thus it was still very much USA original. Those tubes were marked 6CA7 only.)
Ah yech!! I so like sticky dust - at 33/91 degrees.
I went and rummaged around in the garage and found all of 8 old EL34s - all east! Two were from National marked EL34/6CA7 made in E. Germany. The rest were all Sovtek (only EL34) - all identical. Not saying much. I wished I noticed what make EL34s the mentioned Marantz 8 had, but it is back with its owner in another town. (He is an American, having brought it over here when he emigrated, thus it was still very much USA original. Those tubes were marked 6CA7 only.)
Ok, one more secret: screen grid resistors are needed to sense a screen current peaks to drive an opto-compressor. I wanted to patent that, but it is too late: RoHS banned photo resistors so I donate this design solution to the community...
For 6L6 I use 820 Ohm resistors to a regulated G2 supply (+350V VS 375V plate supply). In parallel with them I connect an indicator LED, and opto-pair's LED, and 1K resistor in series. As soon as the amp starts clipping an indicator LED flashes indicating that the level is a bit high, also an opto-pair's led flashes on a photo resistor attenuating an input voltage level.
...this is the secret why my concert amps never distorted... Now because of RoHS they are unobtanium for consumers, and will live as prototypes forever...
For 6L6 I use 820 Ohm resistors to a regulated G2 supply (+350V VS 375V plate supply). In parallel with them I connect an indicator LED, and opto-pair's LED, and 1K resistor in series. As soon as the amp starts clipping an indicator LED flashes indicating that the level is a bit high, also an opto-pair's led flashes on a photo resistor attenuating an input voltage level.
...this is the secret why my concert amps never distorted... Now because of RoHS they are unobtanium for consumers, and will live as prototypes forever...
RoHS = Reduction of Hazardous Substances (Act) by the countries of the European Union. Basically if you want to sell electronics in those countries certain substances like lead, selenium, etc., are banned, except in industrial high rel and military applications where exemptions may be obtained.
Despite not exactly being the law of the land here it has had much the same effect here since the electronics industry is so international in nature.
BOT: (Badly off topic) 😀
The end result will probably be that most consumer electronics become much less reliable due to tin whiskering problems in ICs, and solder cracking with the new lead free solders. This I suspect unfortunately might result in even quicker trips to questionable offshore recycling centers and local landfills for the ubiquitous cell phone and mp3 players almost everyone has these days.. I hope someone will recognize the opportunity and fix these instead. Better still would be to build products with much longer life cycles that could be recycled to less demanding consumers as they age. (Sort of like a global freecycle?)
😀
Despite not exactly being the law of the land here it has had much the same effect here since the electronics industry is so international in nature.
BOT: (Badly off topic) 😀
The end result will probably be that most consumer electronics become much less reliable due to tin whiskering problems in ICs, and solder cracking with the new lead free solders. This I suspect unfortunately might result in even quicker trips to questionable offshore recycling centers and local landfills for the ubiquitous cell phone and mp3 players almost everyone has these days.. I hope someone will recognize the opportunity and fix these instead. Better still would be to build products with much longer life cycles that could be recycled to less demanding consumers as they age. (Sort of like a global freecycle?)
😀
BOT - To Kevin
Kevin,
See my comments toward bottom of this post over at AA regards lead free - good news
http://db.audioasylum.com/cgi/m.mpl...28&highlight=Lead+Free+Gingertube&r=&session=
Cheers,
Ian
Kevin,
See my comments toward bottom of this post over at AA regards lead free - good news
http://db.audioasylum.com/cgi/m.mpl...28&highlight=Lead+Free+Gingertube&r=&session=
Cheers,
Ian
Hi,
Well, this thread wont die...
I've got a grid problem with a pair of Quad II. I tried Sovtek, Saratov, and Gold Lion KT66.
All of them has the grid glowing...
Needless to say that the Gold Lion didn't stay long in there. I realized that what I was seeing in the tube wasn't good after reading a few very good thread about screen grids on this forum.
Just to make sure: the grid is the spring shape wire that
you see through the holes inside the tube structure, right?
I also tried Groovetube and JJ 6l6gc that I had sitting around: same thing!???!
Here is some measurements in one of the the amp:
V3
Anode is 352Volts
Cathode 29 Volts
Grid is 350Volts
Bias 59MA
V4
Anode is 353Volts
Cathode 29 Volts
Grid is 351Volts
Bias 60MA
the other one has pretty much the same values.
The Quad Grid is connected directly to B+.
You can see the circuit diagram here:
QUAD Diagram
Is there a way that I could add a grid resistor in this circuit as mention earlier in this thread? would it be useful?
It's kind of weird that both amp has the same behavior...
Any infos would be appreciated.
Thanks
Well, this thread wont die...
I've got a grid problem with a pair of Quad II. I tried Sovtek, Saratov, and Gold Lion KT66.
All of them has the grid glowing...
Needless to say that the Gold Lion didn't stay long in there. I realized that what I was seeing in the tube wasn't good after reading a few very good thread about screen grids on this forum.
Just to make sure: the grid is the spring shape wire that
you see through the holes inside the tube structure, right?
I also tried Groovetube and JJ 6l6gc that I had sitting around: same thing!???!
Here is some measurements in one of the the amp:
V3
Anode is 352Volts
Cathode 29 Volts
Grid is 350Volts
Bias 59MA
V4
Anode is 353Volts
Cathode 29 Volts
Grid is 351Volts
Bias 60MA
the other one has pretty much the same values.
The Quad Grid is connected directly to B+.
You can see the circuit diagram here:
QUAD Diagram
Is there a way that I could add a grid resistor in this circuit as mention earlier in this thread? would it be useful?
It's kind of weird that both amp has the same behavior...
Any infos would be appreciated.
Thanks
ixe13 said:
The cathode is a smallish, solid, cylindrical looking thing in the center of the tube. Surrounding that is a tight pitched sping wire (the control grid, G1). Around that is another spring which is the screen (G2). In beam power tubes the screen ought to have roughly the same pitch as the control grid, and it should be aligned with the control grid. Finally, if the tube is a "true" pentode it will have a third spring (the suppressor, G3) which is very loosely wound and furthest out. Beam tetrodes and kinkless tetrodes do not have the third grid - they have "beam forming plates" as the element before the plate.
This isn't the best picture I've seen, but it's the first one I found:
http://www.geocities.com/tube_theory/Pentode.htm
The cathode should appear a uniform color. It may be dull red or orange. You may see bits of the filament peeking out from the top and bottom. The filament may be bright orange, yellow, or white.
I've never seen the control grid glowing, and I can't imagine how it would. It does take a lot of heat, and typically has little heat radiators welded to the top of its support rods.
When the screen grid starts overheating, it seems to happen in localized spots. The majority of the screen will be dark, but a few (or many) tiny spots will glow red, orange, yellow, or bright white. All of these are bad. The more spots and the closer the color is to bright white, the worse the problem. Here's a photo showing some glowing spots on the screen grid. Click for full sized image. Sorry for the blurry focus.
You can see the hot cathode on the left most edge through the plate holes. You'll see small dark, evenly spaced lines across the cathode - that's the control grid. The bright white spots seen through the center of the plate holes is the screen which is burning to death. To the right of that can be seen the suppressor, which really can't ever be glowing unless the tube is wired up all wrong. On the very far left, through the lower hole you can see some bright, pale blue color. This is where the electrons are striking the inside of the plate. This color is perfectly normal, and common in Russian manufactured tubes. Even farther to the left of the plate holes you can see some rich, royal blue glow. This is from electrons escaping through the plate holes and striking the glass, causing a characteristic blue glow.
Thanks Ty_Bower,
This is a very informative reply.
So I was mistaking the cathode for the grid. I did read your post where this photo was displayed.
No wonder both amp and tube all had the same behavior. they're fine!!!! he he he!
So I can say all is ok then.
I had doubts cause, twice, I had runaway tubes with these amps.
I changed the power tubes sockets, and the coupling caps were only 450 volts I believe and that was a bit on the edge. So I replaced them with Murdof supreme (1200 Volts), ...yeah,I know, a bit overkilled...
I also reduce the bias by putting a 240 ohms cathode resistor and it's been over 6 months without problems with the 6L6GC.
I can say that I was very pleased with the Golden Lion. There was real noticable improvement in the sound compare to any of the other tubes I used so far.
A bit like pushing the loudness button on these '70's amps... Very nice
I've also read quite a few post about the simple SE and did learn quite a bit there also.
I bought the Simple SE board from George a few weeks ago and I'll start putting it together soon.
Thanks again for the very useful information you provided. I still have the KT66 tube that died on me and I think I'm gonna open it up and have a look and see!
Sheers!
Yves
This is a very informative reply.
So I was mistaking the cathode for the grid. I did read your post where this photo was displayed.
No wonder both amp and tube all had the same behavior. they're fine!!!! he he he!
So I can say all is ok then.
I had doubts cause, twice, I had runaway tubes with these amps.
I changed the power tubes sockets, and the coupling caps were only 450 volts I believe and that was a bit on the edge. So I replaced them with Murdof supreme (1200 Volts), ...yeah,I know, a bit overkilled...
I also reduce the bias by putting a 240 ohms cathode resistor and it's been over 6 months without problems with the 6L6GC.
I can say that I was very pleased with the Golden Lion. There was real noticable improvement in the sound compare to any of the other tubes I used so far.
A bit like pushing the loudness button on these '70's amps... Very nice
I've also read quite a few post about the simple SE and did learn quite a bit there also.
I bought the Simple SE board from George a few weeks ago and I'll start putting it together soon.
Thanks again for the very useful information you provided. I still have the KT66 tube that died on me and I think I'm gonna open it up and have a look and see!
Sheers!
Yves
Ah! Glad for you, Ixe13.
Going somewhat off-topic, but since the Quad II came up:
In the Quad II the power tubes are quite underpowered. In your case the voltages are rather high; you might consider going to the next mains tap on the selector if not already at 240V. Most Quads have this tendency; somewhere the calculations went astray - or they were based on every ancillary being used, but that still does not help. If able to increase the mains setting you can go back to the cathode resistor of 180 ohm. Also the heater voltage would have been higher than 6,3V, getting that down would be an advantage.
Still: Anode voltage of 350V minus cathode of 29V = 321V. Cathode current/tube is 29/180 = 80mA/tube, giving 26W. That is on maximum for KT66, but 6L6GC can take 30W. With the voltages you have now you would be able to get a goodly 18W out of the amplifier.
Perhaps one more piece of important advice (though unasked for as well as off-topic!): The cathode bypass capacitor in the original version was mounted right close to that HOT cathode resistor. Move it! ... or the resistor. All of the many Quad IIs I have refurbished had that capacitor fried. (With better quality small electrolytic caps available these days one can inclease the value with advantage - I use 1000uF-35V types.)
Regards
Going somewhat off-topic, but since the Quad II came up:
In the Quad II the power tubes are quite underpowered. In your case the voltages are rather high; you might consider going to the next mains tap on the selector if not already at 240V. Most Quads have this tendency; somewhere the calculations went astray - or they were based on every ancillary being used, but that still does not help. If able to increase the mains setting you can go back to the cathode resistor of 180 ohm. Also the heater voltage would have been higher than 6,3V, getting that down would be an advantage.
Still: Anode voltage of 350V minus cathode of 29V = 321V. Cathode current/tube is 29/180 = 80mA/tube, giving 26W. That is on maximum for KT66, but 6L6GC can take 30W. With the voltages you have now you would be able to get a goodly 18W out of the amplifier.
Perhaps one more piece of important advice (though unasked for as well as off-topic!): The cathode bypass capacitor in the original version was mounted right close to that HOT cathode resistor. Move it! ... or the resistor. All of the many Quad IIs I have refurbished had that capacitor fried. (With better quality small electrolytic caps available these days one can inclease the value with advantage - I use 1000uF-35V types.)
Regards
hi Johan,
Actually, I build these QUAD myself, one day I was bored and started to look around on EBAY
for electronic stuff and found these 2 QUAD chassis with the tags still on it.
So I got in my head the idea of building these amps from scratch by buying the
parts here and there. I took a basic 140 hours electronic course 30 years
ago, so I can read diagrams. The theory does help, but I didn't have any experiences
working in the field.
Everything went fine till I got to the Transformers. Didnt want to take a chance
with used tranfo so I got the wallet out and bought them from QUAD.
I still took a chance cause if something went wrong, well...
Wont do that type of move ever again, cause since then I know better, one can get
a pretty good tube amp build from scratch for half the price I end up paying for
these. But still, I dont regret it, i did it, and they're sounding fine now
and they're still much cheaper than the brand new ones that QUAD sell these days.
I'll bet the simple SE will sound as good as the QUAD, although different.
Back to topic, the new power transfo made by QUAD for the classic version doesn't have
the 3 "plugs" to fine tune the power input.
Here in Canada, it's 120 volts in summer and around 115 to 117 in winter.
I guess that the difference in winter is because people heat their homes mostly with electricity these days.
Right now it's -15 celcius outside and the main in my place is at 116 volts at the moment. So I'm
kind of stuck with what I have. That's why I had to drop the bias using a 240 ohms cathode resistor.
I mesure the bias using the 1 ohms resistor trick and a voltmeter.
When I used the 180 ohms cathode resistor, the cathode was at 27 volts, and the bias at around 70MA in summer.
I only wonder if the output transformer can take the 29 volts.
Thanx by the way for the advice about the cathode bypass capacitor.
I did excactly that: resistor and caps are far apart.
Now the Gold Lion are back on the amp. Nice nice nice 🙂
Yves
Actually, I build these QUAD myself, one day I was bored and started to look around on EBAY
for electronic stuff and found these 2 QUAD chassis with the tags still on it.
So I got in my head the idea of building these amps from scratch by buying the
parts here and there. I took a basic 140 hours electronic course 30 years
ago, so I can read diagrams. The theory does help, but I didn't have any experiences
working in the field.
Everything went fine till I got to the Transformers. Didnt want to take a chance
with used tranfo so I got the wallet out and bought them from QUAD.
I still took a chance cause if something went wrong, well...
Wont do that type of move ever again, cause since then I know better, one can get
a pretty good tube amp build from scratch for half the price I end up paying for
these. But still, I dont regret it, i did it, and they're sounding fine now
and they're still much cheaper than the brand new ones that QUAD sell these days.
I'll bet the simple SE will sound as good as the QUAD, although different.
Back to topic, the new power transfo made by QUAD for the classic version doesn't have
the 3 "plugs" to fine tune the power input.
Here in Canada, it's 120 volts in summer and around 115 to 117 in winter.
I guess that the difference in winter is because people heat their homes mostly with electricity these days.
Right now it's -15 celcius outside and the main in my place is at 116 volts at the moment. So I'm
kind of stuck with what I have. That's why I had to drop the bias using a 240 ohms cathode resistor.
I mesure the bias using the 1 ohms resistor trick and a voltmeter.
When I used the 180 ohms cathode resistor, the cathode was at 27 volts, and the bias at around 70MA in summer.
I only wonder if the output transformer can take the 29 volts.
Thanx by the way for the advice about the cathode bypass capacitor.
I did excactly that: resistor and caps are far apart.
Now the Gold Lion are back on the amp. Nice nice nice 🙂
Yves
Hi everybody,
First of all I want to say that I am completely new in this forum and very glad to find people talking about these matters. I am Spanish and my English could be not good or too basic, but I'll try my best.
Well, the thing is that I have a guitar amplifier of about 100wats, with the typical 4x6L6GC valves and some more 12AX7 in the preamp. Then I want to make some modifications in order to reduce power from 100w to 50w and to 25w, more or less, using a few switches. One of the ways I want to reduce from 100w to 50w is by either changing the four valves to triode mode or taking out from the circuit one of the 2 pairs. After that, and in order to leave the amp givin a maximum power of 25w, I will leave in triode mode the working pair.
The original circuit is rather classical; the four screen grids are connected to the power supply through each 680R resistor, and the plates - in parallel two and two - to the primary of the output transformer. The cathodes are directly connected to ground. In relation with the control grids, I don't think I need to say a lot about it.
Then, with this scenery I think the easiest way to get rid of a pair of valves is by disconnecting their respective cathodes from ground. At the moment, and as I don't want more than 50 or 60 watts and had no time to do the aforementioned, I took the two valves off and carefully kept inside my guitar case. But as this is not a professional solution, I had opened the amp with the decision to pass the two cathodes through a switch in order to do it properly.
The problem is that, in practice, and due to the PCB design I have to do it in a very drastic way since I have to destroy rather thick pieces of PCB tracks. Then I was thinking to do it by inserting the switch in the circuit of the screen grids. That is, making the option to connect them - by the switch - to either the original point or to ground, and always through their respective 680R resistor. Therefore cutting both the screens and the plates currents.
My only concern with this configuration is the control grids of these valves, and what is going to happen with them in a clipping situation, that is, when receiving a high positive signal that overcomes the fixed negative bias. Taking into account that now all the electronic current will not go beyond the cathode and the control grid, since the screen grid and plate are out of the game.
I know that the negative bias will increase according to the input signal increase due to the current from the grid to ground through the drainage resistance, but... will it be enough to stop the grid from being destroyed?
Then that is my concern and the reason for what I am seeking advice about it. Of course, any other idea about this and the other modifications shall be wellcome.
I know, for example, that the screen grid will melt in the case the plate lacks of positive voltage. But I am not sure if the same thing will happen with the control grid in the circumstances I was telling before.
Kind regards and thanks a lot for your help, in advance.
mabruk
First of all I want to say that I am completely new in this forum and very glad to find people talking about these matters. I am Spanish and my English could be not good or too basic, but I'll try my best.
Well, the thing is that I have a guitar amplifier of about 100wats, with the typical 4x6L6GC valves and some more 12AX7 in the preamp. Then I want to make some modifications in order to reduce power from 100w to 50w and to 25w, more or less, using a few switches. One of the ways I want to reduce from 100w to 50w is by either changing the four valves to triode mode or taking out from the circuit one of the 2 pairs. After that, and in order to leave the amp givin a maximum power of 25w, I will leave in triode mode the working pair.
The original circuit is rather classical; the four screen grids are connected to the power supply through each 680R resistor, and the plates - in parallel two and two - to the primary of the output transformer. The cathodes are directly connected to ground. In relation with the control grids, I don't think I need to say a lot about it.
Then, with this scenery I think the easiest way to get rid of a pair of valves is by disconnecting their respective cathodes from ground. At the moment, and as I don't want more than 50 or 60 watts and had no time to do the aforementioned, I took the two valves off and carefully kept inside my guitar case. But as this is not a professional solution, I had opened the amp with the decision to pass the two cathodes through a switch in order to do it properly.
The problem is that, in practice, and due to the PCB design I have to do it in a very drastic way since I have to destroy rather thick pieces of PCB tracks. Then I was thinking to do it by inserting the switch in the circuit of the screen grids. That is, making the option to connect them - by the switch - to either the original point or to ground, and always through their respective 680R resistor. Therefore cutting both the screens and the plates currents.
My only concern with this configuration is the control grids of these valves, and what is going to happen with them in a clipping situation, that is, when receiving a high positive signal that overcomes the fixed negative bias. Taking into account that now all the electronic current will not go beyond the cathode and the control grid, since the screen grid and plate are out of the game.
I know that the negative bias will increase according to the input signal increase due to the current from the grid to ground through the drainage resistance, but... will it be enough to stop the grid from being destroyed?
Then that is my concern and the reason for what I am seeking advice about it. Of course, any other idea about this and the other modifications shall be wellcome.
I know, for example, that the screen grid will melt in the case the plate lacks of positive voltage. But I am not sure if the same thing will happen with the control grid in the circumstances I was telling before.
Kind regards and thanks a lot for your help, in advance.
mabruk
Back to screen grid resistors...
This pertains to using pentodes / BPTs in 'pentode mode':
In theory, you would want the screen grid powered from a low impedance supply. The stiffness of this supply is what actually guarantees the pentode / BPT high internal impedance. This is fairly easy to see by looking at plate curves for a given Vg2. As long as it is constant and Va is over a certain minimum (also known as the knee), the curves are horizontal, indicating current source behaviour, i.e. high internal impedance.
At some point, as the plate current decreases with plate voltage decrease, the current in the screen grid rises. In high gm tubes this can approach a seizable percentage of the maximum plate current (30-50%!), whereas to the right of the knee, it would typically be around 5-10% of the plate current (sepending on tube and construction, the percentage is usually lower for BPTs). Since the screen grid is far smaller and less able to dissipate heat than the plate, it's obvious that operating the tube in this region will kill it in a very short time, by melting the screen grid (with attendant extra effects, for instance secondary emission and thermal runaway).
The 'usual' way to prevent this, or at least make the tubes far more robust in applications where this could happen, is to include a screen grid resistor between screen supply and screen grid. Since normally the screen grid current is almost a fixed percentage of the plate current, the voltage drop on the resistor can be predicted and the resistor chosen for minimal impact in normal operation. When the operating point finds itself left of the knee area, screen current will rise fast, so will the voltage drop on the screen resistor, and therefore screen current is limited. Also, the plate current will drop since it is highly dependent on the screen voltage. Depending on the value chosen, not only is it a form of negative feedback, it can also have a 'foldback' action where trying to keep the operating point in a dangerous area, results in a quick cascade of reduction of screen and plate currents, saving the tubes.
That being said, the dissipation on the screen resistor rises drastically, and often it is dimensioned not to survive this condition for more than a few seconds, if the required resistor for absolute reliability, impacts performance too much. The resistor will burn but the tube will be saved. In some cases, fuses are used as well.
More modern designs with solid-state regulation can implement over-current tresholds and fold-backs in various ways, including compression and indication of such a condition.
In power amps, sudden rise of screen current normally means the onset of output voltage clipping. Another possibility, especially with tubes that have top caps, is a loss of plate connection (i.e. cap fell off).
All of this of course does not pertain to taming parasitics - use of a resistor or small choke is a given there, so if there is screen voltage modulation with screen current due to this, it is something that is unavoidable, and the rest of the design must take it into account.
This pertains to using pentodes / BPTs in 'pentode mode':
In theory, you would want the screen grid powered from a low impedance supply. The stiffness of this supply is what actually guarantees the pentode / BPT high internal impedance. This is fairly easy to see by looking at plate curves for a given Vg2. As long as it is constant and Va is over a certain minimum (also known as the knee), the curves are horizontal, indicating current source behaviour, i.e. high internal impedance.
At some point, as the plate current decreases with plate voltage decrease, the current in the screen grid rises. In high gm tubes this can approach a seizable percentage of the maximum plate current (30-50%!), whereas to the right of the knee, it would typically be around 5-10% of the plate current (sepending on tube and construction, the percentage is usually lower for BPTs). Since the screen grid is far smaller and less able to dissipate heat than the plate, it's obvious that operating the tube in this region will kill it in a very short time, by melting the screen grid (with attendant extra effects, for instance secondary emission and thermal runaway).
The 'usual' way to prevent this, or at least make the tubes far more robust in applications where this could happen, is to include a screen grid resistor between screen supply and screen grid. Since normally the screen grid current is almost a fixed percentage of the plate current, the voltage drop on the resistor can be predicted and the resistor chosen for minimal impact in normal operation. When the operating point finds itself left of the knee area, screen current will rise fast, so will the voltage drop on the screen resistor, and therefore screen current is limited. Also, the plate current will drop since it is highly dependent on the screen voltage. Depending on the value chosen, not only is it a form of negative feedback, it can also have a 'foldback' action where trying to keep the operating point in a dangerous area, results in a quick cascade of reduction of screen and plate currents, saving the tubes.
That being said, the dissipation on the screen resistor rises drastically, and often it is dimensioned not to survive this condition for more than a few seconds, if the required resistor for absolute reliability, impacts performance too much. The resistor will burn but the tube will be saved. In some cases, fuses are used as well.
More modern designs with solid-state regulation can implement over-current tresholds and fold-backs in various ways, including compression and indication of such a condition.
In power amps, sudden rise of screen current normally means the onset of output voltage clipping. Another possibility, especially with tubes that have top caps, is a loss of plate connection (i.e. cap fell off).
All of this of course does not pertain to taming parasitics - use of a resistor or small choke is a given there, so if there is screen voltage modulation with screen current due to this, it is something that is unavoidable, and the rest of the design must take it into account.
Hello again,
Well, I was thinking a little more about the problem I posted before and realize that it could be very unlikely that the control grids would suffer any damage by cutting the screen and plate current since even in the case that they draw some current, it will always be very low given the large value of the grid drainage resistors - around 200k - beetwen the grid and the bias point.
Anyway, I would like to hear some opinions about this way of pulling valves out. That is, by connecting their screens to ground instead of cutting their cathodes off.
Thanks again,
mabruk
Well, I was thinking a little more about the problem I posted before and realize that it could be very unlikely that the control grids would suffer any damage by cutting the screen and plate current since even in the case that they draw some current, it will always be very low given the large value of the grid drainage resistors - around 200k - beetwen the grid and the bias point.
Anyway, I would like to hear some opinions about this way of pulling valves out. That is, by connecting their screens to ground instead of cutting their cathodes off.
Thanks again,
mabruk
I done some try on a 807 PP guitar amplifier. The best sound I get was with a stabilized screen voltage. The original design was made with the tubes at their limit with 300V UG2, but in order to insure a decent life for the tubes, I stepped down this boltage to 250 V and to be sure (guitar amplifiers are a crazy thing), I put a small fuse resistor between the cathodes and the ground.
The macimum constant output power didn't changed much, but the dynamic is definitely better with a stabilized G2 voltage.
For Hifi amplifiers, to stabilize the G2 would be an overkill because such an amplifer will normaly not go into saturation, and you can design it to use an almost constant G2 current.
I also considere to put a small fusible resistor around 1 ohm between the cathodes and the ground circuit like a good practice. It can save you a power transformer in case of catastrophic tube failure.
The macimum constant output power didn't changed much, but the dynamic is definitely better with a stabilized G2 voltage.
For Hifi amplifiers, to stabilize the G2 would be an overkill because such an amplifer will normaly not go into saturation, and you can design it to use an almost constant G2 current.
I also considere to put a small fusible resistor around 1 ohm between the cathodes and the ground circuit like a good practice. It can save you a power transformer in case of catastrophic tube failure.
Two matters:
With a beam tube the screen current can vary by a considerable quantity, so operation through a resistor will change sound quality.
Degree of protection is often in inverse relationship to complexity/cost. I would pefer the 'fusible' resistor, if this could be done with any degree of exactness, but with the proviso that the cathode is not thereafter open. With that one runs into the danger of exceeding the maximum allowable cathode-heater spec. Probably not going to happen often, but the solution is also simple: Put another resistor across the fusable one, so that the heater-cathode potential cannot exceed a safe voltage but the tube will still be in cut-off, say at a bias of 60V or whatever.
With a beam tube the screen current can vary by a considerable quantity, so operation through a resistor will change sound quality.
Degree of protection is often in inverse relationship to complexity/cost. I would pefer the 'fusible' resistor, if this could be done with any degree of exactness, but with the proviso that the cathode is not thereafter open. With that one runs into the danger of exceeding the maximum allowable cathode-heater spec. Probably not going to happen often, but the solution is also simple: Put another resistor across the fusable one, so that the heater-cathode potential cannot exceed a safe voltage but the tube will still be in cut-off, say at a bias of 60V or whatever.
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