Thanks again for taking the time to respond and offering an outline on how to design a 205d amp. Unfortunately your 'Reasonably simple' design is my... Nah-I don't think so, not with my lack of circuit design knowledge. Nevertheless I appreciate it.
NOS WE 205D tubes are ridiculously expensive, but I have a NOS pair that I bought about 15 years ago from a antique radio repair guy (rip). I always thought those round bulbs were beautiful and that they would look nice in an amp for my stereo setup. $100 each he said and I could'nt resist.
NOS WE 205D tubes are ridiculously expensive, but I have a NOS pair that I bought about 15 years ago from a antique radio repair guy (rip). I always thought those round bulbs were beautiful and that they would look nice in an amp for my stereo setup. $100 each he said and I could'nt resist.
Perhaps someone will design a single ended 205D amp for you.
But before you build something for the 205D, you need to apply filament voltage,
and see if they light up.
If one or both are dead, it will be expen$ive to replace.
You keep making me want to build another type 45 amplifier.
The stereo one I designed for work has been torn down, and rebuilt as another amplifier.
Good luck in your quest for a 205D amplifier.
One more thing, if I ever was to build a 205D amplifier, I would use Self Bias, not Fixed Bias, and not Fixed Adjustable Bias.
Just my preference.
I once finished another stereo type 45 amplifier at 12:00 midnight. It sounded so good that I stayed up until 2:00 listening to it, amazed that something so low powered could sound like that, and then I turned the amp off.
The next morning, when I turned the amp on, one 45 tube started screeching, and had a purple glow (went gassy). I did not have another 45 to replace it at that time.
I only had two days to rebuild the amp to show at VSAC (Vacuum State of the Art Conference)
I picked the 4-65 as the output tube (that re-design was a major project, even though I could not do an optimal design).
I had to settle for less performance. At least it looked good, but it did not sound nearly as good as the 45 amp.
But before you build something for the 205D, you need to apply filament voltage,
and see if they light up.
If one or both are dead, it will be expen$ive to replace.
You keep making me want to build another type 45 amplifier.
The stereo one I designed for work has been torn down, and rebuilt as another amplifier.
Good luck in your quest for a 205D amplifier.
One more thing, if I ever was to build a 205D amplifier, I would use Self Bias, not Fixed Bias, and not Fixed Adjustable Bias.
Just my preference.
I once finished another stereo type 45 amplifier at 12:00 midnight. It sounded so good that I stayed up until 2:00 listening to it, amazed that something so low powered could sound like that, and then I turned the amp off.
The next morning, when I turned the amp on, one 45 tube started screeching, and had a purple glow (went gassy). I did not have another 45 to replace it at that time.
I only had two days to rebuild the amp to show at VSAC (Vacuum State of the Art Conference)
I picked the 4-65 as the output tube (that re-design was a major project, even though I could not do an optimal design).
I had to settle for less performance. At least it looked good, but it did not sound nearly as good as the 45 amp.
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I'd start first with choosing an output tube and -principle your favorable with and that matches the speakers and the room size.
Afterwards, design a good and beefy driver/ input section with lots of PSU power and enough signal headroom around it.
And voila, thats your favorite amp that suits your room and speakers. Pretty easy. I prefer to design from scratch, because that gives the utmost freedom. No tweaking of known amps, no hidden problems. And always have that extra watt in spare that your speakers don't need, but that makes for powerfull sound.
I once designed my RE604 SET amps this way and they were really excellent.
One guy came up with his Sun Audio 300B to compare and this amp isn't easy to beat because it has direct Loftin white stages and best ingredients. We agreed that every one has his personal strengths and weaknesses, but I prefered the tone of the Telefunken tube in combination with the permalloy output trans. It was simply the more refined and magical amp design and that almost can beat everything, its just a question of the best speaker matching. Do the right spice, the right parts at the best positions and it will sing like a bird.
Afterwards, design a good and beefy driver/ input section with lots of PSU power and enough signal headroom around it.
And voila, thats your favorite amp that suits your room and speakers. Pretty easy. I prefer to design from scratch, because that gives the utmost freedom. No tweaking of known amps, no hidden problems. And always have that extra watt in spare that your speakers don't need, but that makes for powerfull sound.
I once designed my RE604 SET amps this way and they were really excellent.
One guy came up with his Sun Audio 300B to compare and this amp isn't easy to beat because it has direct Loftin white stages and best ingredients. We agreed that every one has his personal strengths and weaknesses, but I prefered the tone of the Telefunken tube in combination with the permalloy output trans. It was simply the more refined and magical amp design and that almost can beat everything, its just a question of the best speaker matching. Do the right spice, the right parts at the best positions and it will sing like a bird.
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I have to admit it, I live in a quiet home.
I do most of my listening to speakers that are within 2 or 3 feet (near field listening).
I usually do not listen to music real loud.
That really frees me to use low power amplifiers.
Build according to your wants and to your needs.
I do most of my listening to speakers that are within 2 or 3 feet (near field listening).
I usually do not listen to music real loud.
That really frees me to use low power amplifiers.
Build according to your wants and to your needs.
Perhaps the circuit could be modified to avert catastrophe? Keep the driver stage, replace the direct coupling of driver to 205D with an interstage transformer? Lower 205D B+ to 375V and apply cathode bias to achieve Vgk=22V (750R @ 29mA)?
Yes, it's no longer Loftin-White. Yes, the interstage transformer will cost $ and steal a little bandwidth. Yes, the 205D's cathode resistor will need a bypass capacitor, unless you use filament bias in which case Rk drops from 750R to 13R and you can probably do away with the bypass capacitor. Yes, the power supply will need a re-design - thank God for PSUD!
Most high eff. LOUDspeakers, and especially those we are talking about when using ultra-low power amps, haven't been designed for near field monitor auditioning.
We are talking VOTT here, front and backloaded bass reflex speakers in the range of approx 100dB. Those are the construction principles when high eff. is on the specification list.
A WE 728B falls on the downside with its 93dB and I bet, when working in the closed housing (monitor situation) of the 757A, even will fall more apart from being a LOUDspeaker. With every dB lower, the power output has to become much higher.
So this couldn't be a thread about small monitor speakers for near field listening situations. The opposite is the truth. In fact, we are talking here of big installations for bigger room environments.
My listening distance to the speakers is 16 feet in a 380 square feet damped room. Anything less and the sound becomes fractured by the sheer dimensions of the speakers. With the right distance, the speakers can create listening situations which feel like a music band plays right in the room incl. goosebumps and an real physical presence never auditioned with small speakers. Thats the difference between small speakers and those, which use 15'' or more inch big and heavy woofers in a loaded bass reflex cabinet.
Btw, the same effect applies on every big speaker cabinet for electric guitars. Take an old tubed Fender 100W amp with a big, two or four pieces of 15'' JBL alnico speakers and play it in your room and one will see (and feel) what physical magic I'm writing about.
An interstage is a nice idea, but its quite costly. This little accesoires will ad a nice $1K on the amplifier bill alone. If one wants to enjoy good quality.
We are talking VOTT here, front and backloaded bass reflex speakers in the range of approx 100dB. Those are the construction principles when high eff. is on the specification list.
A WE 728B falls on the downside with its 93dB and I bet, when working in the closed housing (monitor situation) of the 757A, even will fall more apart from being a LOUDspeaker. With every dB lower, the power output has to become much higher.
So this couldn't be a thread about small monitor speakers for near field listening situations. The opposite is the truth. In fact, we are talking here of big installations for bigger room environments.
My listening distance to the speakers is 16 feet in a 380 square feet damped room. Anything less and the sound becomes fractured by the sheer dimensions of the speakers. With the right distance, the speakers can create listening situations which feel like a music band plays right in the room incl. goosebumps and an real physical presence never auditioned with small speakers. Thats the difference between small speakers and those, which use 15'' or more inch big and heavy woofers in a loaded bass reflex cabinet.
Btw, the same effect applies on every big speaker cabinet for electric guitars. Take an old tubed Fender 100W amp with a big, two or four pieces of 15'' JBL alnico speakers and play it in your room and one will see (and feel) what physical magic I'm writing about.
An interstage is a nice idea, but its quite costly. This little accesoires will ad a nice $1K on the amplifier bill alone. If one wants to enjoy good quality.
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Of course.
Voice of the Theatre really needs a very large room, not near field, and not the average living room either.
Without a large enough distance to the listener, the woofer/mid, and compression driver sounds will not integrate properly.
And, without large room dimensions, there will not be low frequency standing waves to augment the low frequency tones out of the woofer/mid's reflex port.
The woofer/mid gets its mid frequency efficiency from the horn that is in front of the large cone.
The compression driver gets its efficiency from its horn too.
But without the bass standing waves from the room dimensions, the bass will be missing (low efficiency).
A bass reflex by itself is not as efficient as a true horn, but true bass horns are too big for most installations.
Makes me wonder how many people bought voice of the theater speakers. They did not have a proper listening space for them.
And they wondered why they did not like the sound of them, so they sold them.
And why is an expen$ive inter$tage transformer the only solution to replace DC coupling?
RC coupling is a valid way to interface a driver and an output stage.
(an arguable point for many).
If someone is worried about blocking distortion, the answer has at least two solutions:
Turn the volume down.
Build a more powerful amplifier.
At some point, every amplifier distorts. At some point, every speaker distorts. At some point, every ear distorts.
"For every engineering problem, there are 100 solutions, of which at least 3 will actually work; but only if they are implemented properly".
Voice of the Theatre really needs a very large room, not near field, and not the average living room either.
Without a large enough distance to the listener, the woofer/mid, and compression driver sounds will not integrate properly.
And, without large room dimensions, there will not be low frequency standing waves to augment the low frequency tones out of the woofer/mid's reflex port.
The woofer/mid gets its mid frequency efficiency from the horn that is in front of the large cone.
The compression driver gets its efficiency from its horn too.
But without the bass standing waves from the room dimensions, the bass will be missing (low efficiency).
A bass reflex by itself is not as efficient as a true horn, but true bass horns are too big for most installations.
Makes me wonder how many people bought voice of the theater speakers. They did not have a proper listening space for them.
And they wondered why they did not like the sound of them, so they sold them.
And why is an expen$ive inter$tage transformer the only solution to replace DC coupling?
RC coupling is a valid way to interface a driver and an output stage.
(an arguable point for many).
If someone is worried about blocking distortion, the answer has at least two solutions:
Turn the volume down.
Build a more powerful amplifier.
At some point, every amplifier distorts. At some point, every speaker distorts. At some point, every ear distorts.
"For every engineering problem, there are 100 solutions, of which at least 3 will actually work; but only if they are implemented properly".
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bondini,
As I already said, the amplifier schematic was marked 'Loftin White'.
But it is not a Loftin White circuit.
DC coupling is not the only characteristic of a Loftin White amplifier.
DC coupling is only one aspect of the circuit.
A Mazda eccentric circular mode engine is an internal combustion engine.
But it is not a Reciprocating Piston engine, such as most V8 internal combustion engines are.
They are different, but have one thing in common, they are both internal combustion engines.
Likewise, a McIntosh 275 push pull amplifier circuit, is not a Williamson push pull amplifier curcuit.
As I already said, the amplifier schematic was marked 'Loftin White'.
But it is not a Loftin White circuit.
DC coupling is not the only characteristic of a Loftin White amplifier.
DC coupling is only one aspect of the circuit.
A Mazda eccentric circular mode engine is an internal combustion engine.
But it is not a Reciprocating Piston engine, such as most V8 internal combustion engines are.
They are different, but have one thing in common, they are both internal combustion engines.
Likewise, a McIntosh 275 push pull amplifier circuit, is not a Williamson push pull amplifier curcuit.
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You're right, 6A3sUMMER
It ain't quite Loftin-White (and that ain't quite a rhyming couplet). And RC coupling is fine too (and I did forget to mention a coupling cap when I suggested an interstage transformer ..... 😱).
Perhaps I have grown too fond of what Terman called "Transformer Coupling with Shunt (Parallel) Feed"? From the design perspective, what I like about it is the potential to really drive the grid of the output valve, especially output triodes that tend to draw grid current as Vg swings positive .... and that may be handy if the designer is looking for a little headroom from a small output triode like the 205D. And there are some things I don't like - an interstage transformer costs more than RC coupling, you still need to buy a good coupling cap, the value of which has to calculated to optimise the low frequency response.
That said, I do enjoy the sound ... 🙂
It ain't quite Loftin-White (and that ain't quite a rhyming couplet). And RC coupling is fine too (and I did forget to mention a coupling cap when I suggested an interstage transformer ..... 😱).
Perhaps I have grown too fond of what Terman called "Transformer Coupling with Shunt (Parallel) Feed"? From the design perspective, what I like about it is the potential to really drive the grid of the output valve, especially output triodes that tend to draw grid current as Vg swings positive .... and that may be handy if the designer is looking for a little headroom from a small output triode like the 205D. And there are some things I don't like - an interstage transformer costs more than RC coupling, you still need to buy a good coupling cap, the value of which has to calculated to optimise the low frequency response.
That said, I do enjoy the sound ... 🙂
bondini,
In the past, I have designed and built quite a few amplifiers that used interstage transformers. Some were single ended, some were push pull.
I did that for 300B, 2A3, and some Beam Power tubes.
I also used a phase splitter auto transformer (no DC current, used a Cap to couple the signal to it, and connected the windings directly to the grids, and the "center tap" to ground.
The interstage, and auto transformer DCR work well with tubes that have to have very low grid resistors (rg). That way, a driver is not required to drive a low resistance rg.
I totally agree with your listing of the advantages of using interstage transformers.
After that, I went to RC coupling.
I have had the good fortune to get some 600V caps that were used in oscilloscope AC coupling circuits. They have metal casings, and glass seals (or glass-like seals at the end).
They sound as good as a number of several different manufacturing companies caps that are very highly rated by listeners, which I have also listened to.
Over the years, I have used Fixed Adjustable Bias, Battery Bias (in Series with the grid stopper to prevent battery current during normal signal levels), and Individual Self Bias.
I do have my preferences there too.
In the past, I have designed and built quite a few amplifiers that used interstage transformers. Some were single ended, some were push pull.
I did that for 300B, 2A3, and some Beam Power tubes.
I also used a phase splitter auto transformer (no DC current, used a Cap to couple the signal to it, and connected the windings directly to the grids, and the "center tap" to ground.
The interstage, and auto transformer DCR work well with tubes that have to have very low grid resistors (rg). That way, a driver is not required to drive a low resistance rg.
I totally agree with your listing of the advantages of using interstage transformers.
After that, I went to RC coupling.
I have had the good fortune to get some 600V caps that were used in oscilloscope AC coupling circuits. They have metal casings, and glass seals (or glass-like seals at the end).
They sound as good as a number of several different manufacturing companies caps that are very highly rated by listeners, which I have also listened to.
Over the years, I have used Fixed Adjustable Bias, Battery Bias (in Series with the grid stopper to prevent battery current during normal signal levels), and Individual Self Bias.
I do have my preferences there too.
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Here's a little more detail about those parallel feed drivers. WE used them, and because they had very smart designs, they gained my attention.
[FONT=Verdana, Helvetica, Arial, sans-serif]Western Electric 92A Parallel-Feed Driver[/FONT]
[FONT=Verdana, Helvetica, Arial, sans-serif]Western Electric - Rosetta Stone for Triodes
[/FONT]
[FONT=Verdana, Helvetica, Arial, sans-serif]Western Electric 92A Parallel-Feed Driver[/FONT]
[FONT=Verdana, Helvetica, Arial, sans-serif]Western Electric - Rosetta Stone for Triodes
[/FONT]
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