Since your speakers are easy to drive, why not build a 2a3 amp? Better sounding tube in my experience and easier to drive. You can also use the 6B4G or Russian 6C4C versions. Even a simple 6N1P sounds good into a 2a3. It's an easy build. To tell you the truth I've never been a fan of the 300b, despite its popularity. Give me a 2a3 any day - much nicer valve and cheaper too.
If you've heard what you like I think it's wise to start there - If you can get details from your friend's amp about the driver (C3g) connections and the voltages/currents everything is running at you should be able to get very close to what that circuit is (or even better) without having to spend the money on C3g's . There are plenty of other suitable tubes without the high price.
The only thing I'd recommend from the outset is to plan on building the circuit in a way that allows for experimentation.
If you're interested in reading and it turns out to be what your friend's amp is using, I'd also suggest searches on SRPP vs Mu Follower.
2¢
@Hearinspace
I will note that I'm not super well-versed in what different SET circuits sound like, as I haven't heard many. I'm not convinced that this is the best 300B amp out there, but it's one that's local to me and that I enjoyed listening to, so it seemed like a decent idea for a build. The more I study the photo, the more it looks very similar to the Sun Audio circuit, at least in the 300B stage. It also seems like corners were cut as the amp is advertised as being able to run a 300B or a PX25 with a toggle switch. If the transformer is indeed the FC-20s (which it appears to be based on the pinout), ISO doesn't even list the FC-20s as being able to run the PX25 (they list the larger, more expensive FC-30 and FC-40 as being able to). Maybe these things indicate a sloppy design? I'm not experienced enough to make that call.
Edit: also just identified the power transformer as the ISO MS-3716 (ISO lists the application as the 2a3 and the 45)
I will note that I'm not super well-versed in what different SET circuits sound like, as I haven't heard many. I'm not convinced that this is the best 300B amp out there, but it's one that's local to me and that I enjoyed listening to, so it seemed like a decent idea for a build. The more I study the photo, the more it looks very similar to the Sun Audio circuit, at least in the 300B stage. It also seems like corners were cut as the amp is advertised as being able to run a 300B or a PX25 with a toggle switch. If the transformer is indeed the FC-20s (which it appears to be based on the pinout), ISO doesn't even list the FC-20s as being able to run the PX25 (they list the larger, more expensive FC-30 and FC-40 as being able to). Maybe these things indicate a sloppy design? I'm not experienced enough to make that call.
Edit: also just identified the power transformer as the ISO MS-3716 (ISO lists the application as the 2a3 and the 45)
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Would you say that the 2a3 excels with a particular type of music? The thing I liked about the 300B was that it didn't seem to matter, everything just sounded quite good. I was considering both the 2a3 and the 45 before hearing my friend's 300B. I've also been reading through Sound Practices, which is definitely very pro-300B.
tighter bass and more detail throughout the range. Noticeable improvement but also a lot more $$
The choice of transformer whether power or output has to do with how the tube is to be operated, not the name of the tube.
So in the case of a power transformer , if it provides the voltages and currents needed from it , it's good.
Similarly the output transformer is chosen for the suitability of its characteristics to the operating parameters of the tube, not the type of tube.
In the case of an amp that is designed to run either this or that tube, it will depend on the tubes chosen and the priorities/ thinking behind the design.
A rough example for PX25 and 300B. : If you look at the data sheet for the KR PX25, they give an operating point of Vp=400V , Ip=63mA. So for sake of arguement, Let's take that as a place to start.
First off, I like to use Paul Joppa's rule of thumb (for choosing an impedance ) as it puts you in the ballpark and is easy to remember. That is RL = Eo/Io - 2.38rp , where RL is load resistance, Eo is operating voltage, Io is operating current and rp is the tube's internal resistance at that operating point.
For the KR PX25s given 400V / 63mA - 2.38 x 1150 Ohms, that's 6349 - 2737 = 3612 Ohms .
The data sheet for the Western Electric 300B gives two different loads for the same recommended operating condition , one of 3500 Ohms and the other of 5000. The lower gives more power, the higher less, but with different levels of distortion. The data sheet recommended load for the KR PX25 falls between the two given for the 300B at the same operating points.
So under those conditions, the ISO output transformer is suitable for both though it doesn't guarantee that the sound from either or both will be what makes you happy.
You might want a different operating point with a different load. Some prefer higher voltages with a lower current and others , the opposite, each opening the possibility of a better different load resistance. However, you only figure out which way you lean with experience, trying out various operating points and schemes.
I still think you should explore your friend's circuit. You introduced it to the thread so it means something to you. It may only be a place to start , but for now at least you know it works with your speakers and source and you know you like it. Of course it's just another point along that same old slippery slope !
🙂
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andyjevans,
I know that you are a well experienced 2A3 fan.
Have you ever heard any of the last two decades mono-plate "2A3" tubes?
('modern' mono-plate "2A3" with plate curves just like early 2A3 dual plate tubes)?
Comments please.
Thanks!
I know that you are a well experienced 2A3 fan.
Have you ever heard any of the last two decades mono-plate "2A3" tubes?
('modern' mono-plate "2A3" with plate curves just like early 2A3 dual plate tubes)?
Comments please.
Thanks!
Hearinspace,
Thanks for posting the WE 300B data sheet.
I have posted it in Tubes / Valves as well.
I like the list of different operating conditions . . .
Pick a combination of plate voltage, plate current, and bias voltage.
Then pick a plate load impedance, and see what output power you get, and the resultant 2nd and 3rd harmonic distortion you get.
The table gives a good series of tradeoffs for the designer.
It also is an example of other output tubes; the operating conditions, versus the results, which is a series of tradeoffs; just as it is for the 300B.
Thanks for posting the WE 300B data sheet.
I have posted it in Tubes / Valves as well.
I like the list of different operating conditions . . .
Pick a combination of plate voltage, plate current, and bias voltage.
Then pick a plate load impedance, and see what output power you get, and the resultant 2nd and 3rd harmonic distortion you get.
The table gives a good series of tradeoffs for the designer.
It also is an example of other output tubes; the operating conditions, versus the results, which is a series of tradeoffs; just as it is for the 300B.
No - I just used EH 2a3 tubes. No idea what other 2a3s sound like. I found the sound more focussed with better tone than the 300b, which tended to sound a bit fat and warm by comparison. 300b did sound a bit better with a 5K primary. I still preferred the 2a3. I listen to classical, jazz and opera. Lots of voices and acoustic instruments.
I no longer use 2a3 or 300b. I use high gain outputs like EL12 and EL6 with low gain inputs including DHTs.
andyjevans,
Thanks!
The Electro Harmonix 2A3 tubes I have are Mono Plate tubes.
It is nice to know about the changes over time people make in their sound systems;
And it adds so much when we know what kinds of music they like.
I think you, and many others would like this CD:
Walton "Belshazzar's Feast" NAXOS 8.555869
It is really good.
Track 5, Thus in Babylon is so Spacious, Dynamic, and the sound Collapses into Black.
Duke Ellington was once asked what kind of music he liked . . .
He said: "Good Music"!
Thanks!
The Electro Harmonix 2A3 tubes I have are Mono Plate tubes.
It is nice to know about the changes over time people make in their sound systems;
And it adds so much when we know what kinds of music they like.
I think you, and many others would like this CD:
Walton "Belshazzar's Feast" NAXOS 8.555869
It is really good.
Track 5, Thus in Babylon is so Spacious, Dynamic, and the sound Collapses into Black.
Duke Ellington was once asked what kind of music he liked . . .
He said: "Good Music"!
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Duke Ellington was quite a wit. He was once asked how he had managed to keep a big band together for so many years. He replied..
"Every successful bandleader has to have a gimmick. I have a gimmick with my musicians. I pay them money"
"Every successful bandleader has to have a gimmick. I have a gimmick with my musicians. I pay them money"
When choosing the static operating point for the 300b I first look at the harmonic distribution because I noticed that the best sound is when the difference between the harmonics is about -10dB. For my amplifier I chose Ua-k=405v Ia=85mA and Z 5K transformer
I just got the CD yesterday, and I have to say that, while I have never been a fan, I truly enjoyed it, the instruments are very clear, separated. Great advice, thank you!
popa marius,
Thanks for telling us that you prefer the sound with the 3rd harmonic to be about 10dB less than the 2nd harmonic.
I never before heard anybody tell of their preferred power relationship between the 2nd and 3rd harmonics.
I have noticed that a lot of Single Ended DHT amplifiers have about 15dB less 3rd harmonic distortion versus the 2nd harmonic distortion.
But when the driver tube's 2nd harmonic distortion partially cancels out the DHT output tube's 2nd harmonic distortion;
That makes the 2nd harmonic distortion only be 10dB more than the 3rd harmonic distortion.
Thanks for telling us that you prefer the sound with the 3rd harmonic to be about 10dB less than the 2nd harmonic.
I never before heard anybody tell of their preferred power relationship between the 2nd and 3rd harmonics.
I have noticed that a lot of Single Ended DHT amplifiers have about 15dB less 3rd harmonic distortion versus the 2nd harmonic distortion.
But when the driver tube's 2nd harmonic distortion partially cancels out the DHT output tube's 2nd harmonic distortion;
That makes the 2nd harmonic distortion only be 10dB more than the 3rd harmonic distortion.
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