Hi there.......the plate voltage (anode) must never drop below a voltage level that creates excessive screen (g2) dissipation in tetrode mode. With a signal, the screen current will rise because it comes from another source unconnected with the plate. The specified screen current of 1.6mA must not go higher othewise the tube is ruined.
This g2 voltage must be decoupled with a cap i.e, a low impedance to audio signals otherwise high distortion will result and is best stabilised; usually fed via a stabiliser tube or other combination.
I recommend reading the article by Goron Rankin; Wavelength Audio, the article appeared in the fall of 1992 "Single 807 Amplifier". A cure for triode fever......Others in the forum will know better links as my location has a very slow access to the www.
Hope this helps.
rich
This g2 voltage must be decoupled with a cap i.e, a low impedance to audio signals otherwise high distortion will result and is best stabilised; usually fed via a stabiliser tube or other combination.
I recommend reading the article by Goron Rankin; Wavelength Audio, the article appeared in the fall of 1992 "Single 807 Amplifier". A cure for triode fever......Others in the forum will know better links as my location has a very slow access to the www.
Hope this helps.
rich
> 807 STC data and not understand
Rich notes a common problem in pentode use (more so in tetrodes; pentodes mostly eliminate it), but that's not what STC is trying to say here.
A conventional pentode has a large and pretty constant screen grid (G2) current. For a conventional pentode with 50mA plate current, we'd expect 5mA or 10mA screen current.
The 807/6L6 has aligned grids, putting G2 in the shadows of the G1 wires. This gives much lower G2 current. Ideally nearly zero! BUT with the main G2 current hidden, the "small" effects of grid alignment and secondary emission pull in opposite ways. So when plate current is 50mA, screen current is about 1.6mA, very low for a 50mA plate current. But small variations in tube and signal can give more or less current. I know in 6L6, the G2 current can even flow backward when secondary emission is high.
If you take the 1.6mA value literally, without knowing that it is very variable, you might try to get your 200V screen supply from the 500V plate supply. (500V-200V)/1.6mA= 187K G2 resistor. BUT if G2 current wanted to go to 2mA, then G2 voltage would sag bad, to 125V, and peak plate current would be too low for full power. Or if G2 current fell to 0.5mA (as it can), then G2 voltage soars to 400V, which may be more than 807 G2 rating and will urge too much plate current.
So on 807/6L6, when G2 voltage is less than plate voltage, and you want to get G2 voltage from plate voltage, you can't just use a dropping resistor. Use a 2-resistor voltage divider. Ideally this will flow at least 10 times the nominal G2 current. 16mA for this case of 1.6mA G2 current. That's another 8 watts of heat. If you have other stages that can eat 200V, you can use them as part of the voltage divider. These days you can use a big MOSFET, though you need a bleeder resistor to handle at least a couple mA of reverse G2 current. Or use a 140V AC winding to make some solid 200VDC. (A neat trick is to find a condition where G2 voltage is half of plate voltage, and use a voltage-doubler to generate both supplies on one winding.)
Rich notes a common problem in pentode use (more so in tetrodes; pentodes mostly eliminate it), but that's not what STC is trying to say here.
A conventional pentode has a large and pretty constant screen grid (G2) current. For a conventional pentode with 50mA plate current, we'd expect 5mA or 10mA screen current.
The 807/6L6 has aligned grids, putting G2 in the shadows of the G1 wires. This gives much lower G2 current. Ideally nearly zero! BUT with the main G2 current hidden, the "small" effects of grid alignment and secondary emission pull in opposite ways. So when plate current is 50mA, screen current is about 1.6mA, very low for a 50mA plate current. But small variations in tube and signal can give more or less current. I know in 6L6, the G2 current can even flow backward when secondary emission is high.
If you take the 1.6mA value literally, without knowing that it is very variable, you might try to get your 200V screen supply from the 500V plate supply. (500V-200V)/1.6mA= 187K G2 resistor. BUT if G2 current wanted to go to 2mA, then G2 voltage would sag bad, to 125V, and peak plate current would be too low for full power. Or if G2 current fell to 0.5mA (as it can), then G2 voltage soars to 400V, which may be more than 807 G2 rating and will urge too much plate current.
So on 807/6L6, when G2 voltage is less than plate voltage, and you want to get G2 voltage from plate voltage, you can't just use a dropping resistor. Use a 2-resistor voltage divider. Ideally this will flow at least 10 times the nominal G2 current. 16mA for this case of 1.6mA G2 current. That's another 8 watts of heat. If you have other stages that can eat 200V, you can use them as part of the voltage divider. These days you can use a big MOSFET, though you need a bleeder resistor to handle at least a couple mA of reverse G2 current. Or use a 140V AC winding to make some solid 200VDC. (A neat trick is to find a condition where G2 voltage is half of plate voltage, and use a voltage-doubler to generate both supplies on one winding.)
What to do about G2?
It is my opinion that the issue differs between pentodes and tetrodes. Generally, the point is whether one should devote much attention to regulating or otherwise dealing with particular g2 requirements, or not.
Well, I personally am of the opinion that g2 voltage should be as close to a voltage as possible, when it comes to tetrode/pentode configurations. The more different the a and g2 potential is, the more current will be drawn across the tube.
When it comes to regulating and otherwise dealing with g2, I leave it to the immagination of the beholder (incorrect expression, obviously). What I want to say, take a look at my RH amps (site link included with the message). It can be noticed that I did not take particular care to that issue: most spice simulations do not show the need for bypassing or anything, since there are no differences between unbaypassed, bypassed -- or for that matter separate B+ source...
Furthermore, to my opinion bypassing is not always a good idea, since you must take the RC filter effect derived from that into account (need for a large C, especially if the R is large).
Maybe when you make amps where a and g2 are very close, where the tube is working in class A exclusively (SE amps) etc. all these issues do not come forward as that important. Who knows, when you build PP amps with high anode voltage, large potential difference between a and g2, eventually AB2 mode... maybe than additional care given to g2 power supply (regulation, separate supply, etc.) is necessary and will give adequate rewards.
Furthermore, one should think of the g2 in a similar fashion as of the g1. You can control a tetrode/pentode tube using the g2 just as well as you can do it (but not as easily) by using g1. Finally, I would like to remind everyone interested that there are amps where tetrodes/pentodes are applied in a particular configuration where the signal is "injected" thru g2 and not g1, giving the tube a specific "modus operandi". I have not tried that, but it does interest me, obviously.
Regards to all, particularly Yugo... when are you going to build the RH34?
Aleksandar
It is my opinion that the issue differs between pentodes and tetrodes. Generally, the point is whether one should devote much attention to regulating or otherwise dealing with particular g2 requirements, or not.
Well, I personally am of the opinion that g2 voltage should be as close to a voltage as possible, when it comes to tetrode/pentode configurations. The more different the a and g2 potential is, the more current will be drawn across the tube.
When it comes to regulating and otherwise dealing with g2, I leave it to the immagination of the beholder (incorrect expression, obviously). What I want to say, take a look at my RH amps (site link included with the message). It can be noticed that I did not take particular care to that issue: most spice simulations do not show the need for bypassing or anything, since there are no differences between unbaypassed, bypassed -- or for that matter separate B+ source...
Furthermore, to my opinion bypassing is not always a good idea, since you must take the RC filter effect derived from that into account (need for a large C, especially if the R is large).
Maybe when you make amps where a and g2 are very close, where the tube is working in class A exclusively (SE amps) etc. all these issues do not come forward as that important. Who knows, when you build PP amps with high anode voltage, large potential difference between a and g2, eventually AB2 mode... maybe than additional care given to g2 power supply (regulation, separate supply, etc.) is necessary and will give adequate rewards.
Furthermore, one should think of the g2 in a similar fashion as of the g1. You can control a tetrode/pentode tube using the g2 just as well as you can do it (but not as easily) by using g1. Finally, I would like to remind everyone interested that there are amps where tetrodes/pentodes are applied in a particular configuration where the signal is "injected" thru g2 and not g1, giving the tube a specific "modus operandi". I have not tried that, but it does interest me, obviously.
Regards to all, particularly Yugo... when are you going to build the RH34?
Aleksandar
Re: What to do about G2?
Alexe,
According to your statements and ,hope, extensive spice simulations you've done,I do not doubt that there is no difference as you said beetween bypassed or not bypassed.As I said in another thread I have not yet compared two cases with/without bypassing and done extensive listening test,but having experience with guitar amps ,in almost every Fender I"ve had opened g2's are bypassed and come after the choke or another R and thus have more regulated nature.These are indeed PP amps and as you mentioned it CAN be exclusive carecteristic for SE amps that they don't expose those isssues so strongly!!
Regarding your last chapter,I am aware of the all benefits that enhanced mode of pentode's operation can bring.Here is another racing horse: (I will try to attach.smth.)
Currently I am doing a 2a3 SE amp and after finishing it I will get my hands on your racing horse.BTW is that horse's peedigree from old Yugoslavian's stable or maybe a new Serbian one?
Regards,
Yugovitz
Alex Kitic said:
Alexe,
According to your statements and ,hope, extensive spice simulations you've done,I do not doubt that there is no difference as you said beetween bypassed or not bypassed.As I said in another thread I have not yet compared two cases with/without bypassing and done extensive listening test,but having experience with guitar amps ,in almost every Fender I"ve had opened g2's are bypassed and come after the choke or another R and thus have more regulated nature.These are indeed PP amps and as you mentioned it CAN be exclusive carecteristic for SE amps that they don't expose those isssues so strongly!!
Regarding your last chapter,I am aware of the all benefits that enhanced mode of pentode's operation can bring.Here is another racing horse: (I will try to attach.smth.)
Currently I am doing a 2a3 SE amp and after finishing it I will get my hands on your racing horse.BTW is that horse's peedigree from old Yugoslavian's stable or maybe a new Serbian one?
Regards,
Yugovitz
Bad presentation
I know that it is very bad sketch but my intention was to ilustrate Alex's comments on different ways of driving 807:as grounded grid like this and in enhanced mode ,screen drive.The first is more complicated because you need very low Z for driving the cathode (thus trafo is oblligatorilly or a CF) and the screen drive is more appopriate.Some people claim that once you hear GG 807 you'll forget all the other ways of driving it.
Druze Kiticu where are you?What would you say about this circuit!
It is using the same lamps,pedestrian if you want, as your's RH amp and employing partial feedbeck as an option.I must admit to you that you gained my symphaties just from the beggining with your fresh ideas and enthousiasm and because you named all your fruits RH!!!!You got me temptated with RH 34 so I am going to abandone for a while my 2a3 project and get my hands on RH 34 project.But all this after the old new year,or if you put in an another way,how do you call it a serbian New Year ,wasn't that?
What a nice forum is this and what a miracle is the net!It is nice to see all you guys from my beloved country Ex Ex Yu again in my herd ,together exchanging ideas,knowledge....,from Vardara to Triglava at one place,you, and here is drug Milan and drug choky is joining us...
Every time I test some new project I always use (as a vynil fan) two recordings:One is from Jugoton and another from PGP RTB with two version of my favorite song "Hej Sloveni" !!!! Am I getting too nostalgic......?What can I do ,the old school....
Regards,
Drug Yugovitz
I know that it is very bad sketch but my intention was to ilustrate Alex's comments on different ways of driving 807:as grounded grid like this and in enhanced mode ,screen drive.The first is more complicated because you need very low Z for driving the cathode (thus trafo is oblligatorilly or a CF) and the screen drive is more appopriate.Some people claim that once you hear GG 807 you'll forget all the other ways of driving it.
Druze Kiticu where are you?What would you say about this circuit!
It is using the same lamps,pedestrian if you want, as your's RH amp and employing partial feedbeck as an option.I must admit to you that you gained my symphaties just from the beggining with your fresh ideas and enthousiasm and because you named all your fruits RH!!!!You got me temptated with RH 34 so I am going to abandone for a while my 2a3 project and get my hands on RH 34 project.But all this after the old new year,or if you put in an another way,how do you call it a serbian New Year ,wasn't that?
What a nice forum is this and what a miracle is the net!It is nice to see all you guys from my beloved country Ex Ex Yu again in my herd ,together exchanging ideas,knowledge....,from Vardara to Triglava at one place,you, and here is drug Milan and drug choky is joining us...
Every time I test some new project I always use (as a vynil fan) two recordings:One is from Jugoton and another from PGP RTB with two version of my favorite song "Hej Sloveni" !!!! Am I getting too nostalgic......?What can I do ,the old school....
Regards,
Drug Yugovitz
I'm here!
Yugo (and others...)
Sorry, I recall having seen an info that there was a reply -- but I actually forgot about it since I've been very busy -- both at work and doing some RH stuff.
Well, I've just "downloaded" the 807 screen driven amp, and cannot say what do I think about it right now. I do have some ideas about doing the screen drive thing, but I'm not sure whether everything would work as planned.
So, you will have to wait a little for that one...
The RH6550 triode version is completed and works very well indeed... a pentode version with a little more power awaits to be tried... and another RH amp is also waiting to show what can it do.
Since you are working on a 2A3 amp, which is almost the same as if you were working on a 6B4G amp -- you could e-mail me: yes, the thingy is disabled, but you can find my address on my web site, which you already know. My 6B4G amp is great, and maybe my expertise in DHT could help you with your project?
When it comes to screen drive as on the schematic vs. RH807 -- I'm inclined to believe that it is difficult to make a better amp with the 807 than the RH version...
without false modesty. The rest is in the "ear" of the "listener".
Regards,
Aleksandar
Yugo (and others...)
Sorry, I recall having seen an info that there was a reply -- but I actually forgot about it since I've been very busy -- both at work and doing some RH stuff.
Well, I've just "downloaded" the 807 screen driven amp, and cannot say what do I think about it right now. I do have some ideas about doing the screen drive thing, but I'm not sure whether everything would work as planned.
So, you will have to wait a little for that one...
The RH6550 triode version is completed and works very well indeed... a pentode version with a little more power awaits to be tried... and another RH amp is also waiting to show what can it do.
Since you are working on a 2A3 amp, which is almost the same as if you were working on a 6B4G amp -- you could e-mail me: yes, the thingy is disabled, but you can find my address on my web site, which you already know. My 6B4G amp is great, and maybe my expertise in DHT could help you with your project?
When it comes to screen drive as on the schematic vs. RH807 -- I'm inclined to believe that it is difficult to make a better amp with the 807 than the RH version...
without false modesty. The rest is in the "ear" of the "listener".Regards,
Aleksandar
Yugo (and others)
I will check the screen driven schematics since it does interest me directly. When it comes to your interesting schematics -- I must say it is very hard "to read" since it is poor quality graphics... don't you have some schematics drawing software at hand?
Besides... did you try to e-mail me?
Tonight we celebrate the coming of the "serbian new year" -- not that it means much to me, but actually evokes different feelings. To make it seem "very serbian" one should celebrate by listening to the famous Bregovic song "Kalashnikov"... I dislike both the weapon and the song They were obviously not meant for people like me.
RH -- the horse is from an independent extraterrestrial stable: I live in Serbia, but in the morning, when I wake up, I sip coffee watching CNN... in the evening I watch Discovery or National Geographic -- if I am not listening to music or enjoying some movie via DVD... I found out that there was an assasination attempt on the president of Serbia by watching CNN in the morning. Therefore, it is very difficult to ascertain where do I live. And most certainly, I do not intend to stay here much longer... Or should that be a since I was born here and have some rights to live like a real person and human being in my fatherland?! But, that way off topic.
I will check the screen driven schematics since it does interest me directly. When it comes to your interesting schematics -- I must say it is very hard "to read" since it is poor quality graphics... don't you have some schematics drawing software at hand?
Besides... did you try to e-mail me?
Tonight we celebrate the coming of the "serbian new year" -- not that it means much to me, but actually evokes different feelings. To make it seem "very serbian" one should celebrate by listening to the famous Bregovic song "Kalashnikov"... I dislike both the weapon and the song
They were obviously not meant for people like me.RH -- the horse is from an independent extraterrestrial stable: I live in Serbia, but in the morning, when I wake up, I sip coffee watching CNN... in the evening I watch Discovery or National Geographic -- if I am not listening to music or enjoying some movie via DVD... I found out that there was an assasination attempt on the president of Serbia by watching CNN in the morning. Therefore, it is very difficult to ascertain where do I live. And most certainly, I do not intend to stay here much longer...
Or should that be a since I was born here and have some rights to live like a real person and human being in my fatherland?! But, that way off topic.
mail received
Hi ...
Mail received -- info useful. Try e-mailing me more... let's have a chat about your amps... and what can be done in the RH direction?
Mais certainment, mon cher ami! De gouts...
And, to all others:
this thread has stalled! Is it possible that no other propositions or soultions for an amp with 807 are available from forum members? Has anyone tried building (constructing) the RH807?
Regards,
Aleksandar
Hi ...
Mail received -- info useful. Try e-mailing me more... let's have a chat about your amps... and what can be done in the RH direction?
Mais certainment, mon cher ami! De gouts...
And, to all others:
this thread has stalled! Is it possible that no other propositions or soultions for an amp with 807 are available from forum members? Has anyone tried building (constructing) the RH807?
Regards,
Aleksandar
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.