The OPT's I used were custom wound and cost around 550 EUR the pair. It took some searching for and they probably cost more today.
300b is not cheap but if you have the space and the right loudspeakers and source.. it is really special. I would rather build a cheap 2a3 amplifier than a cheap 300b, because I know it will sound a LOT better.
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So you didn't look at the evidence of this amp performing well, because you think YouTube is clickbait, and then tell me "But if you can't be bothered I understand"?
Seriously is this a language issue or are people on this forum just this rude and nasty to everyone?
I don't watch youtube videos. Sorry. I also avoid flash photography. Please respect this.
6SN7 cascode is a well known circuit. It's advantages and shortcomings are also well known. One of the main reasons for manufacturers inserting two triodes in the same envelope was to do a cascode. This goes back to the days of the first pentodes and tetrodes. The pentode was 'patented' by Philips. There had to be some alternatives to this monopoly - and the cascode is very much like a pentode. This is why we have the xSN7 series of valves/tubes.
Cascodes were used in many oscilloscope circuits going way back to the 1950's at least as well (for example). I even used 6SN7 in cascode on 300b some years before you posted your schematic. It worked, yes, but was not a great performer for me. Sorry. Then I did a lot more reading and began to understand much more about valves/tubes.
I mean no disrespect to you or anyone else on this board whatsoever. I have no doubt your amplifier works. I am not trying to being nasty.
If you want to make it perform better then you can try the MOSFET follower. You will still have your voltage swing but you will suddenly power your 300b with a more powerful low impedance source. This is also a cost-effective and unobtrusive solution. Many here have done it and the posts on it are numerous with glowing results.
But if you wish to not do this, it is your choice. This is all I mean. No disrespect intended at all.
6SN7 cascode is a well known circuit. It's advantages and shortcomings are also well known. One of the main reasons for manufacturers inserting two triodes in the same envelope was to do a cascode. This goes back to the days of the first pentodes and tetrodes. The pentode was 'patented' by Philips. There had to be some alternatives to this monopoly - and the cascode is very much like a pentode. This is why we have the xSN7 series of valves/tubes.
Cascodes were used in many oscilloscope circuits going way back to the 1950's at least as well (for example). I even used 6SN7 in cascode on 300b some years before you posted your schematic. It worked, yes, but was not a great performer for me. Sorry. Then I did a lot more reading and began to understand much more about valves/tubes.
I mean no disrespect to you or anyone else on this board whatsoever. I have no doubt your amplifier works. I am not trying to being nasty.
If you want to make it perform better then you can try the MOSFET follower. You will still have your voltage swing but you will suddenly power your 300b with a more powerful low impedance source. This is also a cost-effective and unobtrusive solution. Many here have done it and the posts on it are numerous with glowing results.
But if you wish to not do this, it is your choice. This is all I mean. No disrespect intended at all.
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This is an interesting schematic:
https://www.bartola.co.uk/valves/2020/07/25/300b-se-amp-47-driver/
I never built it because I do not have any 47 tubes.
This one is closer to what I have built:
https://www.bartola.co.uk/valves/300b-se-amplifier/
You will find that most here with experience design their own schematics. They don't always post them. Ale Moglia is very approachable. We exchanged emails more than once and he was very kind. I don't even use his boards either.
There is a lot in his designs to admire. For example, fixed bias of the 300b which allows you to get away from the cathode RC network. Sure, you will need a negative rail for this, but the constant current sink loaded MOSFET follower is a great performer.
Building amps like this does not need to be expensive, but it is indeed a lot more work. That is what i mean when I say 'there is no free lunch'.
Ian
I don’t understand the sentiment that a 6SN7 isn’t beefy enough to drive a 300B. The tube was designed to be driven from a high gain pentode. The Angela 91 is almost a direct copy of the original WE circuit and uses a 6SJ7.
Actually the "well known circuit" is the 6SN7 direct coupled CASCADE version (JC Morrison/Sun Audio). I honestly believe you are confusing the two things and are repeating what you have read online about that other circuit. I agree that one doesn't work, I tried it first and is what prompted me to find another solution.
I will also agree if the CASCODE isn't tuned correctly, they don't work well in this application. That it true for anything used to drive a 300B.
Changing the cathode resistor on the tail of the cascode just 150ohms turned the amp from a dog to a great performer. I also feel using an LED for the bias helped a lot in making this work well. I can easily see someone who didn't find the solution I did might feel a 6SN7 CASCODE won't work well. If I had built an amp of this design and never experimented with different bias points on the CASCODE, I likely would feel the same way. Properly implemented a CASCODE works great, poorly implemented they don't. I point all this out in the video, but that appears to be a moot issue, first it was because the video posted is click bait, now it's flash photography...
The 300B tube was designed to be driven by a high gain pentode (Which is what a cascode emulates) and the 300B was made decades before the MOSFETs, which you claim are needed for a 300B to work, even existed. The one I built here makes right at 8W of power at under 1% distortion using a 5K OT. Your claim here that a CASCODE 6SN7 can't drive a 300B tube is simply nonsense given there is proof shown it works fine.
I never claimed this is the be all end all design, but you post here this can't possibly work. That this design is garbage and not worth building. Then comments like "But if you can't be bothered I understand." are very rude and condescending to anyone. But then you aren't this first poster in this same thread to behave like this towards me, so I shouldn't be shocked. That attitude appears to be the norm on this forum.
I am currently helping a friend thru this mod via long distance. I find Stephe’s videos well done and useful for a tube newb. They help someone who hasn’t done this sort of thing before. The modifications seem straightforward and well explained. Nothing fancy, just making a lack luster circuit better.
Only one little thing I would ask for: Better parts descriptions on the BOM. That would be a big help for the inexperienced.
Only one little thing I would ask for: Better parts descriptions on the BOM. That would be a big help for the inexperienced.
Sorry, I don't believe that.
A "normal" type 300B does not have max 200VRMS anode swing with normal operating points, let alone distortion...
Please prove your claim with well documented measurement.
That is the "experimental" amp and it does 6-7W at 1% THD. It is obvious that the cascode and the selection of tubes offers a great deal of tunability. So no reason why one should not be able to get the 1% figure up to 8W. I managed to get 1.3% at 10W!
Many boutique 300B amps that cost zillions have much worse distortion figures....those you should criticize.
The datasheets claim 16 watts is possible but you’d probably need to use one of the expensive boutique 300Bs, say from Elrog.