Yes. Learn to use plate curves. Select regime, and you will see first grid bias you have for such regime. This bias voltage is roughly amplitude of signal on input of output stage, unless you want to go to A2 or AB2 region (direct coupling required, like interstage transformer or cathode/source follower).
Wavebourn,
From my calculations for 30ma at 300 volts I would have a bias point of -4 volts. I'm planning to use an interstage transformer that has a slight step up of 12% So I would need to have a 3.5 volt input to get close to the 4 volts needed to drive the tubes. I have a pair of 25 watt pentode output transformers. They are from a Sansui amp and were made by Tango.
If I run four tubes in push pull parallel that should give me about 24 watts. I was thinking of not applying any voltage to the 2nd grid. And since four tubes in PPP will give me an output impedance of 4k the OPT will work for a 4ohm and an 8ohm load instead of the 8 & 16 from the original 7189 amp.
Is it true that since the interstage transformers are capable of being exposed to DC HT I won't need coupling caps? The idea of an amp without coupling caps or feedback sounds really cool. Of course I may need to add some feedback once the amp is built.
This is the first amp I've ever designed so any help would be greatly appreciated.
Kevin
From my calculations for 30ma at 300 volts I would have a bias point of -4 volts. I'm planning to use an interstage transformer that has a slight step up of 12% So I would need to have a 3.5 volt input to get close to the 4 volts needed to drive the tubes. I have a pair of 25 watt pentode output transformers. They are from a Sansui amp and were made by Tango.
If I run four tubes in push pull parallel that should give me about 24 watts. I was thinking of not applying any voltage to the 2nd grid. And since four tubes in PPP will give me an output impedance of 4k the OPT will work for a 4ohm and an 8ohm load instead of the 8 & 16 from the original 7189 amp.
Is it true that since the interstage transformers are capable of being exposed to DC HT I won't need coupling caps? The idea of an amp without coupling caps or feedback sounds really cool. Of course I may need to add some feedback once the amp is built.
This is the first amp I've ever designed so any help would be greatly appreciated.
Kevin
A first, I don't understand what you mean without any voltage on 2nd grid
Second, all depends on construction of the interstage transformer. Most of them were designed to work with DC bias, but some of them were not.
Second, all depends on construction of the interstage transformer. Most of them were designed to work with DC bias, but some of them were not.
It is possible strictly speaking, to turn the tube into right handed triode, something like 6N7. But can first grid tolerate current needed to drive it such a way?
you have to connect that 2nd grid to someother element, say g1 and g2 together or g2 and g3 together tied to plate....to leave any element hanging not connected to anything is unheard of.....
Yes. And if to ground g2 and g3 (how can be understood "don't apply any voltage to g2") you will get right handed triode (that works in region of positive voltages on the firs grid). 6N7 is an example of such tube.
g2 controls plate current, not the plate voltage, this is the nature of pentodes...
so having no voltage at g2, who knows? i have never operated tubes like that, so imho plate current will be very small, you may even need positive voltage to get plate current....
so having no voltage at g2, who knows? i have never operated tubes like that, so imho plate current will be very small, you may even need positive voltage to get plate current....
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OK,
Obviously I've planned something stupid here. But the reason is that g2 on the 6P15P is kinda fragile to begin with. And as noted here in the earlier posts is limited to 150 volts/ 1.5 watts of dissipation. So I was suggesting not applying any voltage to it at all. Is this not possible? In an earlier post someone said to apply less than 150 volts to grid 2. This would be safer although at the expense of power. So Tony you are saying that I must apply some voltage to g2? I was not aware nor do I understand how g2 controls plate current. I want to lower output impedance to 4k to bring the load capabilities in line with modern speakers. Hence PPP. And I want about 6 watts per tube. Maybe it would be better to lower the B+ instead?
And I was also not aware that there is DC bias. I'll have to research that. Attached is a file with the schematic that includes the interstage transformers I want to use. Can anyone tell me if it is DC bias from the schematic?
Obviously I've planned something stupid here. But the reason is that g2 on the 6P15P is kinda fragile to begin with. And as noted here in the earlier posts is limited to 150 volts/ 1.5 watts of dissipation. So I was suggesting not applying any voltage to it at all. Is this not possible? In an earlier post someone said to apply less than 150 volts to grid 2. This would be safer although at the expense of power. So Tony you are saying that I must apply some voltage to g2? I was not aware nor do I understand how g2 controls plate current. I want to lower output impedance to 4k to bring the load capabilities in line with modern speakers. Hence PPP. And I want about 6 watts per tube. Maybe it would be better to lower the B+ instead?
And I was also not aware that there is DC bias. I'll have to research that. Attached is a file with the schematic that includes the interstage transformers I want to use. Can anyone tell me if it is DC bias from the schematic?
Also, if g2 controls plate current how do I figure out how much voltage to apply to g2 for a specific current flow? I was under the impression that current flow was set by cathode resistor. Obviously its back to Morgan Jones for more research on pentodes.
The reason I was suggesting not applying voltage to g2 was to bring the power down to levels the output transformers can handle < 25 watts.
The reason I was suggesting not applying voltage to g2 was to bring the power down to levels the output transformers can handle < 25 watts.
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Output Power is not directly determined by G2 Voltage. This just increases internal resistance.
There is a nice Slide Show from the ETF from Tom Schlangen which shows you how to connect the G2.
www.kaponk.com/~yanyong/ETF06TS.pdf
Apart from that, the search function and literature are your friends 🙂
There is a nice Slide Show from the ETF from Tom Schlangen which shows you how to connect the G2.
www.kaponk.com/~yanyong/ETF06TS.pdf
Apart from that, the search function and literature are your friends 🙂
g2 voltage has almost the same effect on a pentode as anode voltage has for a triode. The current flow in a pentode is set by the combination of g1 and g2 voltages, just as for a triode it is set by the combination of g1 and anode voltages. More reading needed!
i can not see where your problem is if you want to operate the 6P15P in ppp, this is entirely possible, although ultralinear mode is out out the equation imho......operate them in pentode mode with 150volts at G2 and you are good to go...
as far as your attached scheme goes, the only change you need to do is make a 150volt supply for g2....
the reason that the 6V6 g2 goes direct to B+ in your scheme is that it can be so connected......not so with the 6P15P
i bet that with lower g2, the 6p15p will be easier to drive because of the 150 volt grid...
you can read tube databooks downloadable from the net, they contain good information....
search here for the postings of PRR, i learned a lot from him, there are a lot other posters equally good if not even better, but PRR's postings are very easy to understand....
as far as your attached scheme goes, the only change you need to do is make a 150volt supply for g2....
the reason that the 6V6 g2 goes direct to B+ in your scheme is that it can be so connected......not so with the 6P15P
i bet that with lower g2, the 6p15p will be easier to drive because of the 150 volt grid...
you can read tube databooks downloadable from the net, they contain good information....
search here for the postings of PRR, i learned a lot from him, there are a lot other posters equally good if not even better, but PRR's postings are very easy to understand....
just make yourself a good 150 volt supply for g2....
there are many ways you can do that...
I suppose you could triode strap them; gm is pretty good so maybe the anode resistance (as a triode) might not be crippling. Unfortunately the data I have does not give mu (g1-g2) But in parallel push-pull I daresay you'd be OK.
Paul
Paul
Just give it a 150 Volt regulated voltage. Well since you have so many of em, give them a little more and see if they can cope. But no matter what you do, dont leave G2 or G3 disconnected.
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Yes, it will need positive voltage, hence "Right Handed Triode" name. I did not try 6P15P in right handed triode mode, but in left handed triode mode (g2 and g3 to anode) it is very linear.
I'm not surprised at this as the pentode curves are lovely - as good as EL822. Any idea of mu and Ra in triode connection?
Paul
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