So currently I am working on a EL34 25-35W push-pull amp, HT around 280-320V
(I don't wanna use a PPP EL84, maybe 6L6 PP is also possible).
For this project I still have a transformer laying around that is a perfect candidate for this.
The transformer only has a single HV output plus a 6.3V heater tap.
Since I have some current (power) "left" on the heater tap, I thought that it would be a nice idea to use a tube rectifier as well.
The DC bias point of the EL34 is going to be around 35-45mA
So I think the EZ81 (6CA4) or maybe even a 6AX5GT would suit this just fine with some tweaks and tricks.
After some tinkering I came up with the following idea.
Some kind of hybrid.
To prevent inrush current, I think I will add a NTC somewhere as well.
Besides just sharing the idea, do you guys have any further thoughts?
Obviously the bridge rectifier will see a uneven load at one side, but I don't really think that's a huge deal.
I am also a little confused about the maximum output current of the EZ81?
The datasheet says 150mA, so for a push-pull EL34 pair that will be just fine.
Maaaybe I can even add the G2 to it, but that's going to be very close?
However, as far as I understand, this is without the ripple current?
Obviously care will be taken to prevent noise and other things on the heaters, but that's something for later.
(I don't wanna use a PPP EL84, maybe 6L6 PP is also possible).
For this project I still have a transformer laying around that is a perfect candidate for this.
The transformer only has a single HV output plus a 6.3V heater tap.
Since I have some current (power) "left" on the heater tap, I thought that it would be a nice idea to use a tube rectifier as well.
The DC bias point of the EL34 is going to be around 35-45mA
So I think the EZ81 (6CA4) or maybe even a 6AX5GT would suit this just fine with some tweaks and tricks.
After some tinkering I came up with the following idea.
Some kind of hybrid.
To prevent inrush current, I think I will add a NTC somewhere as well.
Besides just sharing the idea, do you guys have any further thoughts?
Obviously the bridge rectifier will see a uneven load at one side, but I don't really think that's a huge deal.
I am also a little confused about the maximum output current of the EZ81?
The datasheet says 150mA, so for a push-pull EL34 pair that will be just fine.
Maaaybe I can even add the G2 to it, but that's going to be very close?
However, as far as I understand, this is without the ripple current?
Obviously care will be taken to prevent noise and other things on the heaters, but that's something for later.
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If G2 voltage comes up before the anode voltage, and EL34s are warm, you'll have a dangerous condition for the G2s.
All good fortune,
Chris
All good fortune,
Chris
How much spare 6.3 VAC is available? If a 6BY5 dual damper can be supported, all the B+ can go through it and Chris' point is no longer a matter of concern. Construct a hybrid bridge with snubbed UF5408s on the ground side and the dual damper on the "hot" side.
BTW, of the "common" power O/P types suited to the desired 25+ W. yield, the 7591 (at 800 mA.) has the smallest heater draw. The EL34/6CA7 draws 1.5 A. and the 6L6 draws 900 mA. Another advantage of the 7591 is the ease with which it's driven. Small signal circuitry suited to the 6BQ5/EL84 and 6V6 is just fine.
BTW, of the "common" power O/P types suited to the desired 25+ W. yield, the 7591 (at 800 mA.) has the smallest heater draw. The EL34/6CA7 draws 1.5 A. and the 6L6 draws 900 mA. Another advantage of the 7591 is the ease with which it's driven. Small signal circuitry suited to the 6BQ5/EL84 and 6V6 is just fine.
True, forgot about that!
Although not really a show stopper, just some kind of delay circuitry will fix this issue.
I am not 100% sure yet.
Originally this transformer was used for a PPP EL84 amplifier and had a bunch of preamp sections in front of it. I think six 12ax7 were used. I am going to use it for something more audio related.
However, these older transformers were mostly a little over engineered, so I might have a bit of reserve left. Since four EL84 use up around 3A, I already have plenty to spare!!
The main reason to go for an EZ81, is that it's readily available.
Putting two in parallel is also an option, or just having the power tubes fed by it (including G2), while the rest goes through the solid state rectifier.
Haha, well, that kind of defeats the whole purpose.
Anyway, one other option would be finding a indirectly heated 5V rectifier.
But that wouldn't be my first pick
In addition, the plate supply voltage will be lower than the screen supply. That makes the screens' job a challenging task even under normal operating conditions.
Best regards!
According to the JJ datasheet, the EZ81 can handle between 160-150mA (250-350V).
So say an average of around 155mA in total for 300V.
That should be enough for a pair of EL34 with a bias point around 40-45mA
So say an average of around 155mA in total for 300V.
That should be enough for a pair of EL34 with a bias point around 40-45mA
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I think that's very obvious.
The challenge is to make it work with some kind of tube rectifier.
At least just for the power tubes.
Very interesting, I can get used EZ81's here for around 8 bucks, or JJ's new for around 10.
So the price difference is not a big deal.
I am building this for someone else who doesn't really live next door.
This person is also not very knowledgeable, so ideally I would like to find a solution with parts that are easy to get.
I guess it's a bit cheating, but this is also still an option.
Let the EZ81 do the majority of the work, and just add the rest of the current by the bridge rectifier via a resistor.
(please don't mind the details, this is just a rough idea/schematic)
I have to admit that the 6BY5 is still most appealing.
Just only not the most practical, .... such a dilemma.... 😀
Let the EZ81 do the majority of the work, and just add the rest of the current by the bridge rectifier via a resistor.
(please don't mind the details, this is just a rough idea/schematic)
I have to admit that the 6BY5 is still most appealing.
Just only not the most practical, .... such a dilemma.... 😀
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True, although the EZ81 is not doing a lot of rectification anymore. (none basically)😀
In fact, this way we don't even need to add the resistor, since the peak current is going to be very small. The rectifier just basically sees a constant DC load (with a tiny bit of ripple)
In this case we can even use just a single tube diode (although I don't know if there are any 150mA ones available)
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Very interesting tube!
The 6AX4GTB is also nice, although max ratings a little conflicting.
(but a much sexier bulb! )
I mean these are also not new tubes obviously, but they seem to be more available than the 6BY5.
In fact, some are as cheap as 2-4 bucks, which is a nice bonus.
edit: max voltage of the EY82 is maybe a little low?
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Perhaps, max input 2x300Veff.
If you want more, the EY81, but it has the cathode on a top connector.
Mona
In my experience these TV rectifier and/or flyback ballast diodes are very forgiving in terms of voltage capabilities. As they're cheap enough, especially the series heated P variants, you're encouraged to try.
Best regards!
Best regards!