Hi!
I have made the attached amplifier. Yet only one channel.
I get only a few watts from this amp. I think this is becouse the amp doasn't have enough gain.
Maybe I should use another one 6H8C, for more voltage gain?
The output xformer: Raa=2.5k, secondary is 4ohm.
The quiescent current through the output tubes is about 30mA.
I read somewhere on the net, that I can get about 30W from a push-pull triode-connected PL509.
Can I expect this?
I have made the attached amplifier. Yet only one channel.
I get only a few watts from this amp. I think this is becouse the amp doasn't have enough gain.
Maybe I should use another one 6H8C, for more voltage gain?
The output xformer: Raa=2.5k, secondary is 4ohm.
The quiescent current through the output tubes is about 30mA.
I read somewhere on the net, that I can get about 30W from a push-pull triode-connected PL509.
Can I expect this?
Attachments
The grid voltage on your cathode followers is 0 volt. (Connected to GND with a 470k resistor) That is not a good working point for a CF. I think you should elevate the grid voltage to about 80V.
Thanks for the fast reply, jane!
Tomorrow I will try soldering an 1.2M resistor between the grin and the plate.
Tomorrow I will try soldering an 1.2M resistor between the grin and the plate.
You might want to think about carefully chosing the operating point of the input stage (maybe using a +B drop resistor and cap for it) which may make it possible to direct couple the followers.
Can the 6H8C drive properly a PL509, if the PL509's grid is connected to the 6H8C's anode? (of course, through the appropiate R-C network...)
If it can drive poperly, then I can exchange the cathode-follower with a "plate-follower" which can amplify voltage.
What do you think of it?
If it can drive poperly, then I can exchange the cathode-follower with a "plate-follower" which can amplify voltage.
What do you think of it?
SY: The datasheet "says", that the maximum voltage is 275V on the G2. I have on it just a few volts lower than B+, so it's about 295-298.
Oops, it's beyond the tube's maximum limit 🙂
I gonna try a bigger resistor.
Oops, it's beyond the tube's maximum limit 🙂
I gonna try a bigger resistor.
Now there's an 1K resistor between the G2 grid and the plate. And on the G2 there's still too much voltage. Shall I try about 10k?
I haven't seen any tube-amp schematics, where such big resistor was in that position...
I haven't seen any tube-amp schematics, where such big resistor was in that position...
Danko, you aren't going to drop significant voltage with the configuration you have in mind. Use a 100ohm resistor and be happy, a PL509 is a very robust tube and don't mind a bit more voltage on the screen grid.
The problem on screen grids isn't voltage, is power dissipation. You have a class A amp (for what I see), so max. dissipation is at quiescent state. Put a DMM in series with the screen resistor and measure screen current on quiescent state. Now measure screen voltage, multiply those values by each other and see what's your current screen quiescent dissipation, and then compare it with the maximum Pg2 ratings.
Usually sweep and RF tubes like this (and the 6146 comes to mind) are specified at very low screen voltages, but they can handle serious voltage when trioded, at least that's my experience.
If you are under 300V, I wouldn't be too worried, that's pretty low 😉
The problem on screen grids isn't voltage, is power dissipation. You have a class A amp (for what I see), so max. dissipation is at quiescent state. Put a DMM in series with the screen resistor and measure screen current on quiescent state. Now measure screen voltage, multiply those values by each other and see what's your current screen quiescent dissipation, and then compare it with the maximum Pg2 ratings.
Usually sweep and RF tubes like this (and the 6146 comes to mind) are specified at very low screen voltages, but they can handle serious voltage when trioded, at least that's my experience.
If you are under 300V, I wouldn't be too worried, that's pretty low 😉
The screen grid dissipacion is about 1W.
The datasheet says, that the max. screen grid dissipacion is 9W.
So it's OK.
Thanks! 🙂
The datasheet says, that the max. screen grid dissipacion is 9W.
So it's OK.
Thanks! 🙂
I think you could run them up to 400V in triode mode without problems. Good luck for your amp! 😉
Running the screens at or near the spec'ed limit is a sure path to early failure. The numbers on the spec sheets were not pulled out of thin air.
The low Vg2 rating is one major reason that PL509 is not a very good choice for triode operation- you need to have the B+ limited to a pretty low value. So to get any sort of power at all, the anode-to-anode load must be low. But that increases distortion...
There are some fancy schemes where you could possibly level shift the DC component of the B+, but now you're talking about expense, complication, and some fairly unexplored territory.
The low Vg2 rating is one major reason that PL509 is not a very good choice for triode operation- you need to have the B+ limited to a pretty low value. So to get any sort of power at all, the anode-to-anode load must be low. But that increases distortion...
There are some fancy schemes where you could possibly level shift the DC component of the B+, but now you're talking about expense, complication, and some fairly unexplored territory.
Sy, can I make the following "trick" ? -> I make with a separadet supply, with about 150-200V, and I connect thoose 100 Ohm resistors to this smaller voltage, so I can raise the B+.
See the attached image, I thickened the part, i'm talking about.
But... I think, connecting a pentode in this way, isn't a triode mode. It's pentode mode. Am I right?
See the attached image, I thickened the part, i'm talking about.
But... I think, connecting a pentode in this way, isn't a triode mode. It's pentode mode. Am I right?
Attachments
That's pentode mode, and quite a bad implementation.
Stuart, I talk for experience. I've seen and heard some amps with trioded EL509 with B+ up to 350/380V that are running fine with Ei tubes for years 😉 But I agree, that's not a good idea to use this tube for triode operation. The 6550 has similar output power and much more suited to this kind of thing. But you know, when you can have a box of EL509 for 50 euro, you don't ask 😀
Stuart, I talk for experience. I've seen and heard some amps with trioded EL509 with B+ up to 350/380V that are running fine with Ei tubes for years 😉 But I agree, that's not a good idea to use this tube for triode operation. The 6550 has similar output power and much more suited to this kind of thing. But you know, when you can have a box of EL509 for 50 euro, you don't ask 😀
Why is this implementation bad? Becouse of the fact, that it's pentode mode, or the way I make the screen-grid-voltage?
Pentode mode is a good idea, but you need to provide stable screen voltage from a low source impedance- a regulator is ideal. And you will need to use loop feedback from the transformer secondary because the source impedance and gain of pentodes is too high.
On the bright side, this allows you to raise plate voltage and get considerably more power.
On the bright side, this allows you to raise plate voltage and get considerably more power.
Well, surely you can start to bypass the center node of the voltage divider for the g2 to ground with a cap (47uF will do). But if you ask me how would I do a screen supply for a pentode amp, I'll say I would regulate it, maybe using tubes if I feel rich and sand stuff if I feel cheap. An LM317 + IRF820/840 can supply all the current a single channel needs. And this way you can use up to 600V on the plates... yum yum 😀
But big feedback would be necessary.
So you have to decide: do you want
1) better sonics: change tube type. Even a EL34 will perform better, and EL34 triode PP have a wonderful sound.
2) more power and avoid purchasing other tubes: go pentode mode, but plan NFB...
But big feedback would be necessary.
So you have to decide: do you want
1) better sonics: change tube type. Even a EL34 will perform better, and EL34 triode PP have a wonderful sound.
2) more power and avoid purchasing other tubes: go pentode mode, but plan NFB...
And what's your opinion about the pentode's sound? I read on the Net, that the pentode's sound is like solid state's sound.
Or, I shouldn't care with it, just biuld, and hear?
Or, I shouldn't care with it, just biuld, and hear?
Piece of advice: stop believing everything you read on the 'Net.😉
One day of experimentation is worth years of sifting through unsubstantiated opinions trying to determine what's true.
One day of experimentation is worth years of sifting through unsubstantiated opinions trying to determine what's true.
I was playing around with tubes, and resistors, and I could increase the gain of the driver stage.
I exchanged the firs 6H8C tube to a 6H9C, this has higher mu.
I also changed the 47k plate-resistors to 150k.
SY: Thanks your advice! 🙂
I could "produce" about 15W to a 8 ohm load, which is I think OK, from a 4ohm secondary impedance output transformer.
But... The distortion is still too high, I have to figure out why.
I will post pictures from the distortion, maybe you can tell me what's wrong 🙂
I exchanged the firs 6H8C tube to a 6H9C, this has higher mu.
I also changed the 47k plate-resistors to 150k.
SY: Thanks your advice! 🙂
I could "produce" about 15W to a 8 ohm load, which is I think OK, from a 4ohm secondary impedance output transformer.
But... The distortion is still too high, I have to figure out why.
I will post pictures from the distortion, maybe you can tell me what's wrong 🙂
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