Hello
I was busy in my garage and found a sketch again of a 20 Watt tube poweramp. I redraw it and made a photo of it to show here, comments welcome.
The idea is to make a simple power amp with the tubes, chokes and transformers i have here at hand. Have a lot of EF86's penthodes, four EL36 power penthode with anode cap on top.(are slightly smaller then the well known EL34)
Input stage is simple of in triode operating penthodes, second EF86 is for phase reversal.
The virtues of the single ended endtube design is the simplicity, and the rather low impedance, one kathode drives the output transformer and one anode. This low impedance gives a chance of using a simple transformer of about 500 ohms The transformers i have here are with a single winding, this is better for high frequency coupling to the speaker. But on the contrary it needs an output cap to the transformer, otherwise the core is saturated due to dc current.
Each EL36 is in triode and top and bottom tube has to be biased with its own negative supply, automatic bias with a resistor is not easy (nor possible?) with series endtubes.
Well, comments welcome!
I was busy in my garage and found a sketch again of a 20 Watt tube poweramp. I redraw it and made a photo of it to show here, comments welcome.
The idea is to make a simple power amp with the tubes, chokes and transformers i have here at hand. Have a lot of EF86's penthodes, four EL36 power penthode with anode cap on top.(are slightly smaller then the well known EL34)
Input stage is simple of in triode operating penthodes, second EF86 is for phase reversal.
The virtues of the single ended endtube design is the simplicity, and the rather low impedance, one kathode drives the output transformer and one anode. This low impedance gives a chance of using a simple transformer of about 500 ohms The transformers i have here are with a single winding, this is better for high frequency coupling to the speaker. But on the contrary it needs an output cap to the transformer, otherwise the core is saturated due to dc current.
Each EL36 is in triode and top and bottom tube has to be biased with its own negative supply, automatic bias with a resistor is not easy (nor possible?) with series endtubes.
Well, comments welcome!
An externally hosted image should be here but it was not working when we last tested it.
Hello Tubee,
interesting circuit, but for better performance, the upper tube should get about 3...4 times the driving swing the lower tubes gets. (Unloaded mu of trioded EL36 is about 5 times.)
The easiest method to get such a drive ratio would be to follow up the concertina with a differential stage, or replace it by a LTP splitter, with one of the both plate resistors split into two serialized ones giving the desired ratio from the tap between them.
If of interest to you, you can find the triode strapped plate curves for EL36
here at my site.
Regards, Tom
interesting circuit, but for better performance, the upper tube should get about 3...4 times the driving swing the lower tubes gets. (Unloaded mu of trioded EL36 is about 5 times.)
The easiest method to get such a drive ratio would be to follow up the concertina with a differential stage, or replace it by a LTP splitter, with one of the both plate resistors split into two serialized ones giving the desired ratio from the tap between them.
If of interest to you, you can find the triode strapped plate curves for EL36
here at my site.
Regards, Tom
Thanks for replys Tubes4e4 and ilimzn
Interesting thought yes the different driving of the endtubes, didn't thought of that! I have somewhere a good phase inverter schematic which is able to drive unequal, i think it's a paraphase, not sure how it's called. Have to get into tubes again a little, did a lot of other stuff lately.
The stated 20 watt is a little overated yes, realised it when i closed the post. A single EL34 in class A in triode delivers about 8 Watt, so this circuit max 16 watt.
The coupling cap on the end can be a computer grade lytic, or a black gate (expensive!) First find some stuff out of a box and see if the schematic works. The end transformers did i find in big trashbin at street, two identicals.
Interesting thought yes the different driving of the endtubes, didn't thought of that! I have somewhere a good phase inverter schematic which is able to drive unequal, i think it's a paraphase, not sure how it's called. Have to get into tubes again a little, did a lot of other stuff lately.
The stated 20 watt is a little overated yes, realised it when i closed the post. A single EL34 in class A in triode delivers about 8 Watt, so this circuit max 16 watt.
The coupling cap on the end can be a computer grade lytic, or a black gate (expensive!) First find some stuff out of a box and see if the schematic works. The end transformers did i find in big trashbin at street, two identicals.
Yes and of course the same methods as used in OTL to achieve balance can be used, e.g Futterman or other methods. BTW the method suggetsed in post 2, (changing the gain of the signal to the lower tube dosen't work as the gain of the lower tube changes with different load, therefore static gain balancing only work for one load impedance value, the Futterman method achieve full balance regardless of output impedance.
Regards Hans
I am glad with all these reactions here! I didn't know this out of my mind drawing had such famous names behind it like Futterman, and i realise now it's allmost an OTL, choke coupled though.
I came on the idea to put 2 tubes in series after i had build a single ended transistor JLH amp.
Agreed kathode follower is not regarded as best sounding (muff sounding?) but a cheap/moderate balanced output transformer spoils a lot if you ask me. Or you have to invest in special core solutions, major part of sound goes through the balanced core, with 1 winding transformer only bass and low-mid goes through the core. Single ended is ideal, core is slightly magnetised. In my drawing the core is not dc magnetised and thus it can give some cross-over distorsion, in special with a simple si-fe core transformer intended for mains AC.
What about a simple solution: vary the top and botton resistor in different value's in the second EF86, the upper larger, the lower smaller?
First i had another quick sketch with another kind of phase splitter, the Schmidt, a combination of a common kathode/common grid circuit. Schmidt IS a Long Tailed Pair, isn't it? Then each anode drives the endtube, and the anode resistors had to be matched to get (specific or equal) driving voltages. But it uses coupling caps extra which i don't like.
The first stage as drawn is known as the "kathodyne" stage.
I like the symmetry of it. The disadvantage of 2nd tube is that it drives the upper EL36 endtube with it's anode, and the lower endtube with it's low impedance kathode. When different driving voltages have to be realise, it sounds logic to me that the EF86 kathode would deliver a higher driving voltage swing, and the EF86 anode a lower voltage swing.
Here is more told about different phase splitters (dutch): http://www.triodedick.com/Bill_deel2.htm
This one is interesting too:
http://www.bonavolta.ch/hobby/en/audio/split.htm
How it all sounds i don't know. But i have all parts here, even a housing, it deserves a few evenings of work
but my car has to be repaired too. (yes the toyota )
I came on the idea to put 2 tubes in series after i had build a single ended transistor JLH amp.
Agreed kathode follower is not regarded as best sounding (muff sounding?) but a cheap/moderate balanced output transformer spoils a lot if you ask me. Or you have to invest in special core solutions, major part of sound goes through the balanced core, with 1 winding transformer only bass and low-mid goes through the core. Single ended is ideal, core is slightly magnetised. In my drawing the core is not dc magnetised and thus it can give some cross-over distorsion, in special with a simple si-fe core transformer intended for mains AC.
Tubes4e4 said:
What about a simple solution: vary the top and botton resistor in different value's in the second EF86, the upper larger, the lower smaller?
First i had another quick sketch with another kind of phase splitter, the Schmidt, a combination of a common kathode/common grid circuit. Schmidt IS a Long Tailed Pair, isn't it? Then each anode drives the endtube, and the anode resistors had to be matched to get (specific or equal) driving voltages. But it uses coupling caps extra which i don't like.
The first stage as drawn is known as the "kathodyne" stage.
I like the symmetry of it. The disadvantage of 2nd tube is that it drives the upper EL36 endtube with it's anode, and the lower endtube with it's low impedance kathode. When different driving voltages have to be realise, it sounds logic to me that the EF86 kathode would deliver a higher driving voltage swing, and the EF86 anode a lower voltage swing.
Here is more told about different phase splitters (dutch): http://www.triodedick.com/Bill_deel2.htm
This one is interesting too:
http://www.bonavolta.ch/hobby/en/audio/split.htm
How it all sounds i don't know. But i have all parts here, even a housing, it deserves a few evenings of work
but my car has to be repaired too. (yes the toyota )What you need is a phase splitter similar to that used in the class A JLH solid state amp, in principle.
Translated to tubes, it is somewhat different, although it looks almost the same. In your case the top tube works as a follower, while the bottom tube has voltage gain. Ideally, to provide some balance, you must provide the same gain as the bottom output tube, from the plate of your phase splitter, so that it provides the same voltage gain for the top half. There is some discussion about this method on the tubecad pages, it was a while back, you will have to look in the archives.
Another way to solve this problem is a well isolated interstage transformer. Each oposite phase winding has to be referenced to the appropriate output tube cathode. While at it, it IS possible to use cathode bias in this topology, with cathode resistors suitably bypassed. The problem is, you burn twice as much voltage for the biasing off your B+.
Finally, there are various OTL topologies, of which yours is actually one, so you may have a look at those. Of course, your amp will be MUCH more efficient with proper load matching. Another tecghnique you may want to look at is indeed using low voltage sweep tubes. The reason for this is the required B+ for this sort of topology, which makes itself a real problem for the phase splitter. Consider that a standard output transformer topology relies on the transformer to get as much voltage swing from the tube above the B+ as it gets below B+ - in an arrangement such as yours, the B+ has to be twice as high for the same end result. For an EL34 in triode, this may well mean some 800V and makes all sorts of other considerations much more problematic - for one thing, you will need to bootstrap the phase splitter tube, since it needs to provide hundreds of volts of clean swing on it's plate - in all probability, you will need to use something like an EL34 for it just to have the right maximum plate voltage spec. Using a sweep tube you may be able to lower this signifficantly, maybe even to one half, which brings you back into the realm of 'usual' tubes.
Another thing to remember is the Vh-k ratings of the top output tube. You will need a separate heater winding for it, with low capacitance towards ground, as the output of the amp has to drive this capacitance in parallel with the load.
The output autoformer MUST have the low impedance speaker output referenced to ground on one side, you risk high AC voltages on both speaker outputs.
Translated to tubes, it is somewhat different, although it looks almost the same. In your case the top tube works as a follower, while the bottom tube has voltage gain. Ideally, to provide some balance, you must provide the same gain as the bottom output tube, from the plate of your phase splitter, so that it provides the same voltage gain for the top half. There is some discussion about this method on the tubecad pages, it was a while back, you will have to look in the archives.
Another way to solve this problem is a well isolated interstage transformer. Each oposite phase winding has to be referenced to the appropriate output tube cathode. While at it, it IS possible to use cathode bias in this topology, with cathode resistors suitably bypassed. The problem is, you burn twice as much voltage for the biasing off your B+.
Finally, there are various OTL topologies, of which yours is actually one, so you may have a look at those. Of course, your amp will be MUCH more efficient with proper load matching. Another tecghnique you may want to look at is indeed using low voltage sweep tubes. The reason for this is the required B+ for this sort of topology, which makes itself a real problem for the phase splitter. Consider that a standard output transformer topology relies on the transformer to get as much voltage swing from the tube above the B+ as it gets below B+ - in an arrangement such as yours, the B+ has to be twice as high for the same end result. For an EL34 in triode, this may well mean some 800V and makes all sorts of other considerations much more problematic - for one thing, you will need to bootstrap the phase splitter tube, since it needs to provide hundreds of volts of clean swing on it's plate - in all probability, you will need to use something like an EL34 for it just to have the right maximum plate voltage spec. Using a sweep tube you may be able to lower this signifficantly, maybe even to one half, which brings you back into the realm of 'usual' tubes.
Another thing to remember is the Vh-k ratings of the top output tube. You will need a separate heater winding for it, with low capacitance towards ground, as the output of the amp has to drive this capacitance in parallel with the load.
The output autoformer MUST have the low impedance speaker output referenced to ground on one side, you risk high AC voltages on both speaker outputs.
Hi,
This a cathodyne with a twist:
The anode is "bootstrapped" to the output and the OPT to the B+.
Adding a resistor and a cap should allow for grounding the OPT.
As in any cathodyne/concertina, the voltages accross the cathode and anode resistors are equal.
AC speaking, the cathode load resistor returns to ground, as does the cathode of the lower output tube, while the anode load resistor returns to output as the cathode of the upper tube does.
So, each power tube in the "totem pole" receives the same voltage between respective grids and cathodes.
Yves.
This a cathodyne with a twist:
The anode is "bootstrapped" to the output and the OPT to the B+.
Adding a resistor and a cap should allow for grounding the OPT.
As in any cathodyne/concertina, the voltages accross the cathode and anode resistors are equal.
AC speaking, the cathode load resistor returns to ground, as does the cathode of the lower output tube, while the anode load resistor returns to output as the cathode of the upper tube does.
So, each power tube in the "totem pole" receives the same voltage between respective grids and cathodes.
Yves.
Well, this is one of those 'why didn't I think of that' moments
Thanks, Yves!
BTW, this gives me an idea. In theory, for higher mu output triodes, wouldn't you be able to connect the top of the plate resistor of the concertina, directly to the midpoint of the two output tubes, and then use a cap coupled autoformer to conenct the speaker? Of course, this only works if about half B+ is enough for the concertina to get the full required swing...
BTW2, how does the ECC83 cope with the 6AS7 Cmiller (I am assuming you have tried this...)
Thanks, Yves!
BTW, this gives me an idea. In theory, for higher mu output triodes, wouldn't you be able to connect the top of the plate resistor of the concertina, directly to the midpoint of the two output tubes, and then use a cap coupled autoformer to conenct the speaker? Of course, this only works if about half B+ is enough for the concertina to get the full required swing...
BTW2, how does the ECC83 cope with the 6AS7 Cmiller (I am assuming you have tried this...)
ilimzn said:
Thanks for reply ilimzn
I was waiting for the first one who comments on Vh to K! Usual this is max 100V. I have a SRPP preamp wherein the voltage difference is more then 100V (6N1P). I have only one Vh winding, a 2nd transformer must heat top tubes then.
Yes the automatic bias is simpler, but wastes voltage of PS. Not sure, but i have a EI core transformer of 120VA, 265V AC out, this could give in theory 370V max B+, regulated is better but gives also lower B+ The neg bias schem is more elaborate, but better to control/set. I am not in for the biggest power output, if it sounds good it is allright.
Good point about the autoformer, one pole of speaker on gnd.
Will look at some OTL's. I have an old dutch magazine of Audio & Techniek, www should offer some too.
ilimzn said:
I've not personally tested the idea, I've just some reports from "Idefixes" who posts here occasionally.
He says he likes the sound but did not make any measurement.
Incidently - good to know- he used a PS toroïd as OPT.
Anyway, the 6AS7 is not so difficult to drive looking at its lo Mu.
Cag is 8.6 pF, and assuming an effective Mu of, says 1.5, this gives a total of 20 pF, including 5.5 pF Cgk and this applies to the lower tube only, the top one being "plate grounded" does not suffer from Miller effect.
The output Z of the cathode side of the concertina cannot be higher than the cathode resistance itself, here 82K, this gives a f-3dB near 100Khz. So far so good !
The bootstrap at the anode side will undoubtly increase the concertina's output Z.
I bet it will be in the order of the anode resistor multiplied by the totem voltage gain, says 1.5 as above, 82K * 1.5 : 123K
But we just has to drive Gag (8.6 pF), not Gag * Mu, nor Cgk.
Only my guess, of course
Yves.
Appears that one can swap the grid drives to the outputs (referring to Yvesm/Idefixes bootstrapped schematic), just like in the Technics or Futterman OTL variation, to get cathode follower outputs:
http://www.tubecad.com/articles_2001/totem-pole_output_stage_psrr/page18.html
Needs more drive signal for the cathode follower outputs then, hence the LTP splitter-drive.
More totem pole/ SEPP here:
http://www.tubecad.com/april_may2001/page17.html
Don
http://www.tubecad.com/articles_2001/totem-pole_output_stage_psrr/page18.html
Needs more drive signal for the cathode follower outputs then, hence the LTP splitter-drive.
More totem pole/ SEPP here:
http://www.tubecad.com/april_may2001/page17.html
Don
Hello
Been away on this trhead for a while, but figured out in meantime what i am going to do.
As said i have some stuff to build it, EL36's EF86's, powertx, 2 autotx, caps, ps choke's and casing.
*for first stage the bootstrap circuit posted by Ives looks very good to me, but i want to use in triode putted EF86, have 4 noval holes in casing for it. EF86 in triode has a transconductance of about 2.2, fast enough for audio and for first experiments. But the resistor value's have to be recalculated then.
*As output stage EL36 as totem pole and with resistor on cathode for auto bias: simplicity/first experiments.
* Allmost OTL like with my autotransformers used and coupling cap.
I will make a drawing soon on linux, i saw a drawing tool included tubes!
Been away on this trhead for a while, but figured out in meantime what i am going to do.
As said i have some stuff to build it, EL36's EF86's, powertx, 2 autotx, caps, ps choke's and casing.
*for first stage the bootstrap circuit posted by Ives looks very good to me, but i want to use in triode putted EF86, have 4 noval holes in casing for it. EF86 in triode has a transconductance of about 2.2, fast enough for audio and for first experiments. But the resistor value's have to be recalculated then.
*As output stage EL36 as totem pole and with resistor on cathode for auto bias: simplicity/first experiments.
* Allmost OTL like with my autotransformers used and coupling cap.
I will make a drawing soon on linux, i saw a drawing tool included tubes!
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