The output transformer is ruled out as it cannot be changed. This is a requirement.
Run in class A, each tube will see 5K and will have (@110V/15 mA) approx 6K plate resistance and mu about 18. So the effective gain will be just over 8. Cathode follower is less than 1....
You have to pick a good op-amp. Just use the input transformer you have.
I don't use op-amps usually. The first op-amp that comes to my mind and that was made to drive 600R is the AD797. But there might be better ones. I am not familiar with this stuff.
Otherwise, as I said, forget input transformers and make a simple JFet SE stage + concertina splitter and RC couple to 12AU7. If you self bias the 12AU7 you won't need a DC coupled source follower (althogh this might give you more performance).
Then don't buy anyone. Just use an active stage. Life will be less complicated...
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Probably the LTP is a better choice if SS devices are going to be used. Using 2x ECC88 for the stereo amp to make SE + concertina will work fine with 250-270V supply (the same as the power tube). So no additional supply. Only need to be careful about cathode-to-heater voltage ratings.
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Why do you need a good one? This is clearly a novelty circuit (e.g. can I make an audio PA without any resistors, can I make an audio PA which fits into this tiny box I happen to have in my junk box?). Build it as a novelty if you wish, but don't expect hi-fi sound.Wigwam Jones said:
I get it - it won't sound good. All I am asking for is a recommendation for an input transformer that matches the ST-24 (which I cannot get here in the US) and doesn't cost a lot. That's it. If no one wants to answer because they don't think it's a good idea, that's fine. I will move on.
I don't understand how my request can be that unclear. All I can understand that is that you folks really really do not want me to build this circuit. Message delivered. Thanks.
If you can get these input transformers, why not build this circuit.
As long as you realize that it is best considered being a fun project; or a nice way to get into tube electronics when you are new into this.
Transformers are cheap, tube is cheap so it would not cost you much and you'll have the fun.
Not much can go wrong, but don't expect quality sound if that is what you're after.
As long as you realize that it is best considered being a fun project; or a nice way to get into tube electronics when you are new into this.
Transformers are cheap, tube is cheap so it would not cost you much and you'll have the fun.
Not much can go wrong, but don't expect quality sound if that is what you're after.
270V supply (a bit more if output transformer has high DC resistance).
2xECC88 + 2XECC82 for the stereo amp.
Input stage: 1/2 ECC88 per channel. Cathode resistor 470R, anode resistor 43K. It should settle around -2.2V bias and just over 4 mA current. Can try with or without cathode bypass capacitor (220-330 uF should do if bypass). Volume pot 100K.
Concertina splitter with the second ECC88 (again one triode per channel): grid DC coupled to the plate of the input stage. Same 270V supply. 22K resistor for both anode and cathode. 2x 0.1 uF coupling capacitors.
Power stage: 2x 220 grid resistors. Common 470R cathode resistor of suitable power rating (2W should be enough). Bypass 220-330 uF 25V.
Buy a decent supply transformer where you have two 6.3V secondaries. One is going to be used for the concertina only has its cathode will be about +90V above ground. It is a limiting factor with ECC88. This will have a reference to +90V. At the end of the supply filters you can put 150K +75K in series from 270V to ground. The 150K has one end connected to 270V and the 75K has one end connected to ground. In between you have the reference voltage for the 6.3V secondary. Bypass the 75K resistor with a small cap. It should work ok.
This is a starting point. Then you can tweak it if you need or improve if you think it is worth....
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Wigwam, have a look at what Edcor has to offer, perhaps something on this page could be pressed into service as an input transformer:
EDCOR - WSM Series Matching Transformers
Look at the WSM tri-filar series (1:1:1) or the WSM 1:1 with center tap. These are 150 ohm so will need something pretty robust to drive them, perhaps to start an iPod or similar.
They cost approximately $11 to $17 each. I would consider the 6N6P (Russian) as a better choice in this application due to lower plate resistance.
EDCOR - WSM Series Matching Transformers
Look at the WSM tri-filar series (1:1:1) or the WSM 1:1 with center tap. These are 150 ohm so will need something pretty robust to drive them, perhaps to start an iPod or similar.
They cost approximately $11 to $17 each. I would consider the 6N6P (Russian) as a better choice in this application due to lower plate resistance.
Hi,
Is there something I'm missing here ? Is the valve input
impedance very high ? or is there some conduction going
on that drags down the input impedance to a low level ?
If so what is that level ? If not what is the valve input
impedance the transformer is required to drive ?
rgds, sreten.
I still think it sucks and an ECL82 design a far better idea.
Is there something I'm missing here ? Is the valve input
impedance very high ? or is there some conduction going
on that drags down the input impedance to a low level ?
If so what is that level ? If not what is the valve input
impedance the transformer is required to drive ?
rgds, sreten.
I still think it sucks and an ECL82 design a far better idea.
The initial circuit doesn't have high input impedance because the triodes are driven into positive grid region. The more are driven the lower the impedance. The step-up transformer makes things even worse for the source because makes such impedance even lower and input capacitance is multiplied by n^2 (high frequency worse and worse).
I agree the ECL82 is a better choice. With the two signal triodes one can do both LTP and SE+concertina without many worries about cathode-to-heater voltage and the output pentodes in triode mode can provide 4W with 10K, 270-280V/22-24mA.
Guys, ur talking past what the OP has asked of you... not really nice.
One option here is to parallel more than one tube (they are duals).
You'd get more power, a lower Rp, and closer to a 1:1 ratio for the transformer, which in some ways makes the output transformer "better".
As far as the answer for an output transformer - you'd want to pick one with a proper ratio, it could be fairly small core size since you're only running 1watt or so... so a 5 watt core would be sufficient. A 10watt rated core (transformer) would be super as far as LF response (lots of core to not saturate).
I'd pick ANY transformer that is close to the suggested impedance ratios and build it.
Period.
Get a cheap transformer - could be a general purpose from Hammond or even a general purpose replacement type from a guitar supplier, or even one from a place like Antique Radio Supply.
Make the amp, make it work, do it cheap and then improve and work from there...
The input transformer is really going to be the most difficult part.
Here it will be necessary to determine how much voltage swing will be needed to drive the tubes into Class A2 (positive grid region) and then work back from whatever your source is. That will give you the voltage ratio. Then you need to look at the formula for converting voltage ratio to impedance ratio and pick a transformer.
Someone mentioned that a standard "600 ohm" transformer might suffice.
IN THE EVENT that that the required ratio is more or less 1:1 or 1:4, etc... those being more or less standard "600 ohm matching/isolation" types, THEN all you need to get the necessary current to DRIVE the tube properly would be another tube as a cathode follower, or even a JFET or a few in parallel (to get enough enough power).
This would make a very nice amp to build, simple, and functional.
Later you can experiment with the various ideas and see how they work out.
These guys are all elite circuit geeks - ignore their attitudes, go have fun and build the amp with whatever parts you can scrounge up. As I said build the FIRST ONE with the cheapest parts you can get. Used and random parts being ok...
GO FOR IT!!
One option here is to parallel more than one tube (they are duals).
You'd get more power, a lower Rp, and closer to a 1:1 ratio for the transformer, which in some ways makes the output transformer "better".
As far as the answer for an output transformer - you'd want to pick one with a proper ratio, it could be fairly small core size since you're only running 1watt or so... so a 5 watt core would be sufficient. A 10watt rated core (transformer) would be super as far as LF response (lots of core to not saturate).
I'd pick ANY transformer that is close to the suggested impedance ratios and build it.
Period.
Get a cheap transformer - could be a general purpose from Hammond or even a general purpose replacement type from a guitar supplier, or even one from a place like Antique Radio Supply.
Make the amp, make it work, do it cheap and then improve and work from there...
The input transformer is really going to be the most difficult part.
Here it will be necessary to determine how much voltage swing will be needed to drive the tubes into Class A2 (positive grid region) and then work back from whatever your source is. That will give you the voltage ratio. Then you need to look at the formula for converting voltage ratio to impedance ratio and pick a transformer.
Someone mentioned that a standard "600 ohm" transformer might suffice.
IN THE EVENT that that the required ratio is more or less 1:1 or 1:4, etc... those being more or less standard "600 ohm matching/isolation" types, THEN all you need to get the necessary current to DRIVE the tube properly would be another tube as a cathode follower, or even a JFET or a few in parallel (to get enough enough power).
This would make a very nice amp to build, simple, and functional.
Later you can experiment with the various ideas and see how they work out.
These guys are all elite circuit geeks - ignore their attitudes, go have fun and build the amp with whatever parts you can scrounge up. As I said build the FIRST ONE with the cheapest parts you can get. Used and random parts being ok...
GO FOR IT!!
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Sreten, is there ANYTHING you think does not suck??
Really!?!??
What's with folks here anyhow??
EXPLAIN to newbies. What's the point in being condescending or pedantic??
For EVERYONE here there is someone who knows more than you about something in
audio, and maybe on this topic too. Grrrr.
Anyhow, back to the original article, they use a "line operated" supply.
Please do not do that?? Please be sure to use a real transformer, preferably not the halfwave thing that likely will work, and not the fullwave version either, use a transformer with a full wave rectifier... the line operated idea is NOT SAFE. Just don't do it.
Really!?!??
What's with folks here anyhow??
EXPLAIN to newbies. What's the point in being condescending or pedantic??
For EVERYONE here there is someone who knows more than you about something in
audio, and maybe on this topic too. Grrrr.
Anyhow, back to the original article, they use a "line operated" supply.
Please do not do that?? Please be sure to use a real transformer, preferably not the halfwave thing that likely will work, and not the fullwave version either, use a transformer with a full wave rectifier... the line operated idea is NOT SAFE. Just don't do it.
Hi,
All the stuff I like and recommend.
In this case an ECL82 based amplifier.
Not hard to find a decent circuit.
rgds, sreten.
I accept I'm more for saying something is
poor, when it is, rather than pussyfooting.
An idea is not great, if there is a better one.
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Wigwam, if the others have not run you off I think the proposed design is a perfectly reasonable choice to experiment with - you'll learn a lot and you might discover it's not nearly as bad as the neighsayers would have you believe. Not hifi but it's simple, easy to build and can be tweaked later.
+1 100% agree. Those transformers are listed for sale online in Japan for $5-$7 - less if you order 100!
1K primary (dcr 57Ω) to 2K secondary center tapped (dcr 160Ω) Ratio 0.7:1
Maybe you can find a reasonable sub , or maybe the US Hashimoto distributor can get a pair thrown onto his next shipment for not too much?
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