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Old 7th January 2005, 05:03 PM   #1
alejo is offline alejo  Argentina
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Default Help in SE 807 amp

Hello, I read 807 STC data and not understand; see lines in red.

Please, need graphic exaple....
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Old 7th January 2005, 06:06 PM   #2
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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
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Old 9th January 2005, 02:59 AM   #3
PRR is offline PRR  United States
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> 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.)
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Old 9th January 2005, 06:45 PM   #4
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Default 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
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Old 10th January 2005, 10:42 PM   #5
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Default Re: What to do about G2?

Quote:
Originally posted by Alex Kitic
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
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
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Old 11th January 2005, 05:13 AM   #6
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Default Another try....

Here it comes...

Regards,
Drug Yugovitz
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Old 11th January 2005, 11:13 AM   #7
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That is the worst hand cad I have ever seen.
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Old 11th January 2005, 11:31 AM   #8
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Quote:
That is the worst hand cad I have ever seen.

I agree......presentation.......a stately tube amp deserves better treatment.

rich
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Old 11th January 2005, 04:47 PM   #9
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Default 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
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Old 11th January 2005, 04:49 PM   #10
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Default Screen drive

Quite better sketch...

Regards,
Yugovitz
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