It limits peak screen current, a good thing for reliability. The higher the B+, the more they're needed. May help prevent high-frequency oscillations. And they limit the damage if a tube shorts... resistors are cheap! They may cause a very small loss of maximum power, a little more distortion, but it's worth it in an instrument amp.
Hi Stew,
I have no idea what companies like Fender used them for (very little science to guitar amps), but in Hi-Fi amps they are there solely to prevent parasitic oscillations.
Mullard, the original maker of the EL34, used 1/2 watt 47 ohms consistantly. Make sure they are carbon composition, or other non-inductive type.
Joel
I have no idea what companies like Fender used them for (very little science to guitar amps), but in Hi-Fi amps they are there solely to prevent parasitic oscillations.
Mullard, the original maker of the EL34, used 1/2 watt 47 ohms consistantly. Make sure they are carbon composition, or other non-inductive type.
Joel
There probably was very little science in guitar amp designs. And a lot of emperical design. Anyone who has spent some time probing around inside them, or watching the tubes, while driving the amp to extreme clipping knows what the resistor is for. It keeps the screen grid from melting.
A guitar amp is often subjected to conditions that your typical stereo would never see, like continuous clipping for extended periods of time. In a hard clip the output tube can see the plate voltage way below the screen voltage for 50% of the time (the bottom half of a square wave). During this time the screen current must be limited, or it will glow red.
I can't say for sure, since I wasn't there, but the resistor value (and connection) was probably a compromize between power, tube life, and sound quality. It is my experience that some (not all) 6BQ5/EL-84's are particularly prone to glowing grid wires, followed soon by sparking out, and tube death.
A guitar amp is often subjected to conditions that your typical stereo would never see, like continuous clipping for extended periods of time. In a hard clip the output tube can see the plate voltage way below the screen voltage for 50% of the time (the bottom half of a square wave). During this time the screen current must be limited, or it will glow red.
I can't say for sure, since I wasn't there, but the resistor value (and connection) was probably a compromize between power, tube life, and sound quality. It is my experience that some (not all) 6BQ5/EL-84's are particularly prone to glowing grid wires, followed soon by sparking out, and tube death.
I can echo the observation of Tubelab regarding red screens, in my case with EL34s. Not to discourage their use, but after some experience there I went to beam tubes.
Regarding the value of these resistors (and apart from guitar amps where certain desired effects come in), I fear they are often piously put there .... but, as resistors are cheap.... Only, especially with beam tubes, the screen current can vary quite substantially during operation and the value must be kept low, say not much above 220 ohm. As voltage reducers, i.e. where the screen voltage must be substantially lower than anode voltage for whatever reason, a voltage divider (wasteful of energy) or better, a voltage regulator should rather be used.
The manufacturer's recommendation for EL34 for lowest distortion is a 1K.ohm screen resistor. I have never seen why so high, but they confirm with graphs, so I do not argue (typical load 6.6 K, anode-anode in PP).
Regards.
Regarding the value of these resistors (and apart from guitar amps where certain desired effects come in), I fear they are often piously put there .... but, as resistors are cheap.... Only, especially with beam tubes, the screen current can vary quite substantially during operation and the value must be kept low, say not much above 220 ohm. As voltage reducers, i.e. where the screen voltage must be substantially lower than anode voltage for whatever reason, a voltage divider (wasteful of energy) or better, a voltage regulator should rather be used.
The manufacturer's recommendation for EL34 for lowest distortion is a 1K.ohm screen resistor. I have never seen why so high, but they confirm with graphs, so I do not argue (typical load 6.6 K, anode-anode in PP).
Regards.
There is an obvious difference between a guitar amp and a HiFi amp. A HiFi amp is designed for low distortion. The screen grid supply needs to be stiff, and is often regulated.
A guitar amp is often designed to have a distortion "signature", the amps "sound". The manufacturers use several tricks to get this "sound". Poor power supply regulation "sag", screen circuit resistance, and "special" (undersized) output transformer design.
A guitar amp is often designed to have a distortion "signature", the amps "sound". The manufacturers use several tricks to get this "sound". Poor power supply regulation "sag", screen circuit resistance, and "special" (undersized) output transformer design.
Screen grid tubes are usually unreliable at high power without screen resistors. Small wirewound resistors do just fine as screen resistors, as long as they are not excessive in physical size. Around 500 ohms is a good general purpose value for screen resistors, especially in an EL34 based design. I've frequently seen 100 watts from a pair of EL34 tubes, so power loss with screen resistors is not a very real concern.
tubelab.com hit the nail on the head. Not being able to do anything other than carry a musical instrument I was awe struck when I had the chance to listen to different brands of guitar amplifiers. The Marshall and Mesa had vast differences even with similar circuit designs. Not to mention Fender and other amplifiers each with their sonic signature. Vastly different than any HiFi amplifier. Each seemed to have a special sound for a special purpose? I liked the different output stages, the 6L6 or EL34 choice each had a different sound.
Each manufacturer do financial calulation when designing an amplifier. The most expensive marshal is costing no more as 100$ to manufacturate, inclusive the manual and the tape on the box. So they have to compromize.
Peavey have very cheap main transformer, so half of the saturation you can hear at high volume is the saturation inside the main transformer and not inside the amplifier. It is why peavy is a good amp for jazz, but not for blues or rock. It is also why, in most case, the main transformer in a such amp, is the first component that will smell when playing at high volume.
Fender and mashall have better main transformer. I don't know for marshal, but with fender, if the output transformer have a good design, they are small. And they will smell quickly if you are playing ofen with high volume.
At the other hand, what I like with Fender is at they have a very good design of all parts of the amplifier. They compromize on the cost of the components, but not much with the design. As exemple, their preamps have a lot of gain and it is very easy to sature the whole preamp from the first stage to the last stage.
All the commercial guitar amps I know have a very poor design in regard to the driver stage. No one is using a driver transformer when it is the best solution to obtain a low output impedance with a good symmetry. The raison is at a good driver transformer cost too much money.
Guitar amp are using class B for the output stage, and in class B, the grid of the tube can be positive. It mean at it will use current, and we need a driver that can drive that current. The RMS value of that current is certainly below the watt for an EL34, but can be a few watts in peak. In consequence, the driver of a guitar amp determine most of the sounding of the final stage.
I have try to stabilize the g2 in one of my amp using 2x807 at the output. The result was 2 time more dynamic when clipping, but the whole tubes was becoming orange, so I went back to a more traditional and less expensive alimentation. I would not recommand even to try it if you don't know exactly what you are doing.
A last word about the EL34. Its a tube that is manufactured both as a classic pentode and as a screen grid tube. So I beleive at variant manufactured as screen grid tube would be better for a guitar amp. It is what a good friend to me tell me. I don't know more and can not ask him more on that subject, because it just desapear and leave the country with his wife it was 2 yeras ago.
Wikipedia on http://en.wikipedia.org/wiki/Beam_tetrode tell this:
"Interestingly, many tubes that are described as pentodes actually turn out to be beam tetrodes. The ubiquitous Mullard EL34 (6CA7) although manufactured by Mullard as a pentode was also produced by many manufacturers around the world, either as a beam tetrode or as a true pentode.
Even Philips/Mullard themselves were not immune. Several examples of Mullard marked ECL82s (a signal triode and low power pentode intended for single ended operation) have turned out to be triode/beam tetrodes."
Peavey have very cheap main transformer, so half of the saturation you can hear at high volume is the saturation inside the main transformer and not inside the amplifier. It is why peavy is a good amp for jazz, but not for blues or rock. It is also why, in most case, the main transformer in a such amp, is the first component that will smell when playing at high volume.
Fender and mashall have better main transformer. I don't know for marshal, but with fender, if the output transformer have a good design, they are small. And they will smell quickly if you are playing ofen with high volume.
At the other hand, what I like with Fender is at they have a very good design of all parts of the amplifier. They compromize on the cost of the components, but not much with the design. As exemple, their preamps have a lot of gain and it is very easy to sature the whole preamp from the first stage to the last stage.
All the commercial guitar amps I know have a very poor design in regard to the driver stage. No one is using a driver transformer when it is the best solution to obtain a low output impedance with a good symmetry. The raison is at a good driver transformer cost too much money.
Guitar amp are using class B for the output stage, and in class B, the grid of the tube can be positive. It mean at it will use current, and we need a driver that can drive that current. The RMS value of that current is certainly below the watt for an EL34, but can be a few watts in peak. In consequence, the driver of a guitar amp determine most of the sounding of the final stage.
I have try to stabilize the g2 in one of my amp using 2x807 at the output. The result was 2 time more dynamic when clipping, but the whole tubes was becoming orange, so I went back to a more traditional and less expensive alimentation. I would not recommand even to try it if you don't know exactly what you are doing.
A last word about the EL34. Its a tube that is manufactured both as a classic pentode and as a screen grid tube. So I beleive at variant manufactured as screen grid tube would be better for a guitar amp. It is what a good friend to me tell me. I don't know more and can not ask him more on that subject, because it just desapear and leave the country with his wife it was 2 yeras ago.
Wikipedia on http://en.wikipedia.org/wiki/Beam_tetrode tell this:
"Interestingly, many tubes that are described as pentodes actually turn out to be beam tetrodes. The ubiquitous Mullard EL34 (6CA7) although manufactured by Mullard as a pentode was also produced by many manufacturers around the world, either as a beam tetrode or as a true pentode.
Even Philips/Mullard themselves were not immune. Several examples of Mullard marked ECL82s (a signal triode and low power pentode intended for single ended operation) have turned out to be triode/beam tetrodes."
I used 100 ohm resistors in series with the screens right at the sockets in all of my EL34 based amplifiers, whether triode or UL connected, this was mainly to prevent vhf parasitic oscillations.. It did also help at clipping in UL to limit the screen current. I never built any pentode connected amplifiers commercially.
Kevin
Kevin
I have graphs somewhere that show that the distortion for p.p. EL34s in UL with 1K screen resistors is noticably lower at maximum output than for very much lower values. I will have difficulty in finding them though - boxes, boxes, boxes.....This is the only tube for which I have seen this.
D_F,
One is never too old to learn (hopefully!) The EL34 as a beam tube? I admit that I have seen them described as such but always regarded that as a rough generalisation. The construction is significantly different, as are the screen characteristics, so one wonders why they did it under the same designation. You are not talking about some of the firms from the east (and north) that merely save by planting the same construction inside a glass bulb? Equating KT66 and 6L6GC I can understand (with an effort, because replacing 6L6s by original KT66s can land one in trouble with heater current), but a pentode with a beam tube? I am lazy; is there somewhere on the net where I can read about this, maybe see a picture of the innards?
Regarding guitar amps I can agree with you; I had to oblige someone once with a design. They are part and parcel of the sound required and I had to do some pretty strange things electronically speaking but the guy ended up very satisfied. (It was quite an adventure to convert to electronics what he wanted soundwise.) In that sense one cannot qualify them as hi-fi, but I also do not "look down" on them if that is what they are required to do. One can say that the guitarist uses say a Fender to produce his particular flavour of sound, but when he replays a guitar CD at home he will use a hi-fi amplifier.
Regards.
D_F,
One is never too old to learn (hopefully!) The EL34 as a beam tube? I admit that I have seen them described as such but always regarded that as a rough generalisation. The construction is significantly different, as are the screen characteristics, so one wonders why they did it under the same designation. You are not talking about some of the firms from the east (and north) that merely save by planting the same construction inside a glass bulb? Equating KT66 and 6L6GC I can understand (with an effort, because replacing 6L6s by original KT66s can land one in trouble with heater current), but a pentode with a beam tube? I am lazy; is there somewhere on the net where I can read about this, maybe see a picture of the innards?
Regarding guitar amps I can agree with you; I had to oblige someone once with a design. They are part and parcel of the sound required and I had to do some pretty strange things electronically speaking but the guy ended up very satisfied. (It was quite an adventure to convert to electronics what he wanted soundwise.) In that sense one cannot qualify them as hi-fi, but I also do not "look down" on them if that is what they are required to do. One can say that the guitarist uses say a Fender to produce his particular flavour of sound, but when he replays a guitar CD at home he will use a hi-fi amplifier.
Regards.
Me again.
Dominique_free, I read right over the web-site you quoted and only opened it after my previous post. It does not appear to give exact details of construction, but set me a-thinking. I have seen ECL82 or ECL86 (pentode section) as beam tubes, but only by virue of beam-forming plates round the G3s. The suppressor grids were still there; I did not bother to investigate whether G1 and G2 were actually of equal pitch and aligned. This was a long time ago, but as above as I recall. Is this what might be meant by pentodes as beam tubes, or are the grids actually aligned?
It might be interesting to note that sensitivity is not always down with beam tubes. The formidable 8417s need a signal of only 17Vp to drive a pair to 100W. (They have a gm of 23000 umho.)
Regards.
Dominique_free, I read right over the web-site you quoted and only opened it after my previous post. It does not appear to give exact details of construction, but set me a-thinking. I have seen ECL82 or ECL86 (pentode section) as beam tubes, but only by virue of beam-forming plates round the G3s. The suppressor grids were still there; I did not bother to investigate whether G1 and G2 were actually of equal pitch and aligned. This was a long time ago, but as above as I recall. Is this what might be meant by pentodes as beam tubes, or are the grids actually aligned?
It might be interesting to note that sensitivity is not always down with beam tubes. The formidable 8417s need a signal of only 17Vp to drive a pair to 100W. (They have a gm of 23000 umho.)
Regards.
Besides limiting screen current, these resistors provide a form of negative feedback. They're sorta similar to unbypassed cathode resistors in that the AC signal current through them produces an AC voltage at the screen that is 'negative.' It doesn't surprise me at all that measured distortion is reduced with larger value resistors. In my experience the size does affect the sound.
I also remember seeing PP circuits (I think in old tube manuals) with one resistor shared by both tubes. Just like a shared cathode resistor, I think these tend to improve balance.
-- Dave
I also remember seeing PP circuits (I think in old tube manuals) with one resistor shared by both tubes. Just like a shared cathode resistor, I think these tend to improve balance.
-- Dave
The tetrode is an evolution of a triode, the problem is at the g2 take a lot of current at low Ua. The pentode is an evolution of the tetrode that supprim this effect but give a tube with an extremly high output impedance.
The beam tetrode is in fact an evolution of the pentode. It is a pentode where the g2 concentrate the electron beam. As every pentode, it have 5 electrodes. The goal was to archieve a tube with as much of the advantages of the triode (low output impedance) and of the pentode (high sensibility). As it is the g2 that is specialy formed to archieve the beam effect, you have to break the tube to see if it is a pentode or a beam tube. In Europa, we call a such tube both as beam tetrode or beam pentode.
Now, an EL34 have those 5 electrodes on the socket when a 6L6 or 807 have only 4 of them on the socket. But that doesn't mean at a 6L6 or 807 have only 4 electrodes inside the tube. They have 5, the only difference is at the g3, the suppressor, is internly wired to the cathode with a 6L6, when you have to wire it externly with an EL34.
I have found this page: http://www.jogis-roehrenbude.de/EL34-Story/EL34-Story.htm
On the first page, we can see at when philips/mullard was manufacturing this tube as pentode, the siemens version was a beam tube.
On the second page, we see at even philips have made some beam EL34 under the name of ECG EL34, and at siemens manufactured some EL34 as pentode. The zenith EL34 from sylvania (philips) was a beam tube, as the sylvania EL34 made in USA.
The beam tetrode is in fact an evolution of the pentode. It is a pentode where the g2 concentrate the electron beam. As every pentode, it have 5 electrodes. The goal was to archieve a tube with as much of the advantages of the triode (low output impedance) and of the pentode (high sensibility). As it is the g2 that is specialy formed to archieve the beam effect, you have to break the tube to see if it is a pentode or a beam tube. In Europa, we call a such tube both as beam tetrode or beam pentode.
Now, an EL34 have those 5 electrodes on the socket when a 6L6 or 807 have only 4 of them on the socket. But that doesn't mean at a 6L6 or 807 have only 4 electrodes inside the tube. They have 5, the only difference is at the g3, the suppressor, is internly wired to the cathode with a 6L6, when you have to wire it externly with an EL34.
I have found this page: http://www.jogis-roehrenbude.de/EL34-Story/EL34-Story.htm
On the first page, we can see at when philips/mullard was manufacturing this tube as pentode, the siemens version was a beam tube.
On the second page, we see at even philips have made some beam EL34 under the name of ECG EL34, and at siemens manufactured some EL34 as pentode. The zenith EL34 from sylvania (philips) was a beam tube, as the sylvania EL34 made in USA.
Dominique_free said:
It is not exact. The wires of the g1 and the g2 are aligned and the g3 is specialy formed. See http://www.jogis-roehrenbude.de/EL34-Story/Innenaufbau/Innenaufbau.htm
I found a very good reading on the screen grid: http://www.webace.com.au/~electron/tubes/screens.htm
On the screen grid resistor, it write:
"What we want is for the Screen Grids to be at a DC potential sufficiently high enough to attract and accelerate electrons towards the Plates but, to maximise power output, not to collect and divert them to earth through the B+ supply.
Clearly there will be a particular value of Screen Grid Stopper Resistor that will provide optimum balance between the conventional "short-circuited" Screen Grid configuration and an arrangement whereby the Screen Grids are suitably loaded.
The optimum value will clearly be variable depending upon the particular circuit configuration and operating voltages.
However, as a rule of thumb, and noting the advice of Philips Miniwatt to instal a value of Grid #2 resistor of between 500 to 1000 ohms in each Grid #2 supply lead, we can assume that a value of 50% of the Plate to Plate primary load impedance is an approximate ideal for the Screen to Screen loading.
This will result in a grid stopper resistor value of:
500 ohms per Screen Grid when the transformer primary load impedance is 2,000 ohms Plate to Plate
1000 ohms per Screen Grid when the primary transformer primary load impedanceis 4,000 ohms Plate to Plate
2000 ohms per Screen Grid when the primary transformer primary load impedanceis 8,000 ohms Plate to Plate
For other values of Plate to Plate load, calculate on the basis that each Screen Grid resistor should be 25% of the transformer Plate to Plate primary load impedance
In all cases, pursuant to Philips Miniwatt advice, the Screen Grid resistor is "non-decoupled" - ie is unbypassed.
This resistor must be installed directly to the Grid #2 pin of the tube socket and be preferably non-inductive.
The Screen resistors must have sufficient heat rating to operate safely and reliably without distress."
On the screen grid resistor, it write:
"What we want is for the Screen Grids to be at a DC potential sufficiently high enough to attract and accelerate electrons towards the Plates but, to maximise power output, not to collect and divert them to earth through the B+ supply.
Clearly there will be a particular value of Screen Grid Stopper Resistor that will provide optimum balance between the conventional "short-circuited" Screen Grid configuration and an arrangement whereby the Screen Grids are suitably loaded.
The optimum value will clearly be variable depending upon the particular circuit configuration and operating voltages.
However, as a rule of thumb, and noting the advice of Philips Miniwatt to instal a value of Grid #2 resistor of between 500 to 1000 ohms in each Grid #2 supply lead, we can assume that a value of 50% of the Plate to Plate primary load impedance is an approximate ideal for the Screen to Screen loading.
This will result in a grid stopper resistor value of:
500 ohms per Screen Grid when the transformer primary load impedance is 2,000 ohms Plate to Plate
1000 ohms per Screen Grid when the primary transformer primary load impedanceis 4,000 ohms Plate to Plate
2000 ohms per Screen Grid when the primary transformer primary load impedanceis 8,000 ohms Plate to Plate
For other values of Plate to Plate load, calculate on the basis that each Screen Grid resistor should be 25% of the transformer Plate to Plate primary load impedance
In all cases, pursuant to Philips Miniwatt advice, the Screen Grid resistor is "non-decoupled" - ie is unbypassed.
This resistor must be installed directly to the Grid #2 pin of the tube socket and be preferably non-inductive.
The Screen resistors must have sufficient heat rating to operate safely and reliably without distress."
Dominique_free,
Many thanks for pointing my nose at those websites. I have not seen so many EL34s in my life! I hunted for a cut-open view though, so that one could see whether the grids were in fact aligned, which is the main advantage of the 6L6 type. I will call it that, because as often happens a device is called by its most obvious feature but not the most important one. You had to correct yourself for what I also often do - concentrate on the screen alignment feature. I did seem to notice beam-forming electrodes in some of the many pictures, but in all where it was descernable a G3 was evident. The question remains whether beam confining electrodes were merely added to the existing EL34 topology, in which case the advantage would be secondary.
We used quite a munber of EL34s in my days at the CSIR (see my CV), mostly European manufacture but the construction was always the classic one. Though they were comparatively sensitive I went off EL34s because I had glowing spots on G2 when I did not want them. Cutting the tubes open, this appeared to have been in places where several consecutive screen windings were quite open to the cathode (i.e. not shielded in any place by a G1 winding), but that was difficult to see with any certainty. Using beam tunes - OK then, grid-aligned types - I never had the problem. I only encountered 6CA7s once, but they were the same construction as the classic EL34.
Thanks once again,
Johan
Many thanks for pointing my nose at those websites. I have not seen so many EL34s in my life! I hunted for a cut-open view though, so that one could see whether the grids were in fact aligned, which is the main advantage of the 6L6 type. I will call it that, because as often happens a device is called by its most obvious feature but not the most important one. You had to correct yourself for what I also often do - concentrate on the screen alignment feature. I did seem to notice beam-forming electrodes in some of the many pictures, but in all where it was descernable a G3 was evident. The question remains whether beam confining electrodes were merely added to the existing EL34 topology, in which case the advantage would be secondary.
We used quite a munber of EL34s in my days at the CSIR (see my CV), mostly European manufacture but the construction was always the classic one. Though they were comparatively sensitive I went off EL34s because I had glowing spots on G2 when I did not want them. Cutting the tubes open, this appeared to have been in places where several consecutive screen windings were quite open to the cathode (i.e. not shielded in any place by a G1 winding), but that was difficult to see with any certainty. Using beam tunes - OK then, grid-aligned types - I never had the problem. I only encountered 6CA7s once, but they were the same construction as the classic EL34.
Thanks once again,
Johan
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