I came up with this idea after researching tubes I could use to drive the 300B, 2 stage, non-inverting, tubes that are manufactured today. I remember building the Salas preamp years ago and it sounded pretty sweet. I've heard the driver stage makes or breaks the 300B magic. What are your thoughts, should I give it a try?
I also have a pair of the Hashimoto HC203U iron... note that the standard phase configuration of the Hashimoto SE OPTs result in a non-inverting output referenced to the grid of the output tube. As such, your circuit will have inverted output unless you decide to either reverse the secondary output winding or add an additional driver stage. In short, not my first choice for the 300b, but certainly a good match for 2A3 or 6B4G tubes.
I've also done some initial prototyping with a matched pair of Western Electric 300b tubes (their initial reproduction tubes from 2007/2008) using Hashimoto H-20-3.5U OPTs. I was using a 2-stage DC coupled input/driver based on a 6FQ7. Overall, I feel that the 300b will be best driven via transformer coupling for optimal results. I also found that AC filaments resulted in poor signal/noise and required a DC filament supply to keep them quiet. I finally opted for a pair of Hashimoto H-30-5S OPTs. I'm still deciding on which interstage driver transformer to use, but will be sticking with Hashimoto for all iron in the amps.
I think you've got sufficient drive current in the 6V6 to overcome the input capacitance of the 300b, but the (300b) grid resistor is bit higher than what WE datasheet recommends, being 250K as a maximum for self bias operation. In any case, it's worth doing some prototyping to see how your performance ends up. I think you'll likely need higher gain to get sufficient drive from a typical line level feeding the amplifier input.
I've also done some initial prototyping with a matched pair of Western Electric 300b tubes (their initial reproduction tubes from 2007/2008) using Hashimoto H-20-3.5U OPTs. I was using a 2-stage DC coupled input/driver based on a 6FQ7. Overall, I feel that the 300b will be best driven via transformer coupling for optimal results. I also found that AC filaments resulted in poor signal/noise and required a DC filament supply to keep them quiet. I finally opted for a pair of Hashimoto H-30-5S OPTs. I'm still deciding on which interstage driver transformer to use, but will be sticking with Hashimoto for all iron in the amps.
I think you've got sufficient drive current in the 6V6 to overcome the input capacitance of the 300b, but the (300b) grid resistor is bit higher than what WE datasheet recommends, being 250K as a maximum for self bias operation. In any case, it's worth doing some prototyping to see how your performance ends up. I think you'll likely need higher gain to get sufficient drive from a typical line level feeding the amplifier input.
Thanks for that information. I didn't realize the HC203U Transformers would do that in the standard configuration. That Opens the door to many 3 stage configurations using 6SN7's and so on. I'l reduce the grid resister down to 220K I was using the Electro-Harmonix data sheet that had the max Grid resister at .5Meg.
Well, the entire line of Hashimoto SE OPTs are configured this way. However, they recently released two variations of their 7W SE OPTs with inverted output windings. These are listed as H-507D and H-203D and state for use as a 2-stage amplifier.
Attached is their current catalogue in English.
Attached is their current catalogue in English.
6V6-triode as a driver tube is a good idea, I think.
I heard this ca. 1990 Komuro design SE 300B at the Fi store in NYC, playing "Waltz For Debby" by Bill Evans (LP). The trick is that it's all DC-coupled, no interstage capacitors or transformers. I remember it sounded pretty darn spectacular.
It might be possible to improve on the basic idea. Maybe start by finding a plate choke with higher primary inductance for the 6V6-triode...
I heard this ca. 1990 Komuro design SE 300B at the Fi store in NYC, playing "Waltz For Debby" by Bill Evans (LP). The trick is that it's all DC-coupled, no interstage capacitors or transformers. I remember it sounded pretty darn spectacular.
It might be possible to improve on the basic idea. Maybe start by finding a plate choke with higher primary inductance for the 6V6-triode...
I do not know if this is helpful or not.
I have an old Chinese SE 300B amp which runs a 12ax7 input valve, to the volume pot which then feeds the input of a 6VS/6p6 which then drives the 300B,
I have never really traced out the circuit as it works just fine, and have never measured its response, but it certainly sounds good with no major shortcoming i can hear. I also have a Glasshouse 300B amp which has a few more expensive caps and resistors that uses triode connected EF86 followed by 5687's with both plates in parallel to drive the 300B and there is not much of difference.
Consequently in my ears there is nothing wrong with using 6V6, maybe your use of the Hashimoto transformer is the clever bit.
I have an old Chinese SE 300B amp which runs a 12ax7 input valve, to the volume pot which then feeds the input of a 6VS/6p6 which then drives the 300B,
I have never really traced out the circuit as it works just fine, and have never measured its response, but it certainly sounds good with no major shortcoming i can hear. I also have a Glasshouse 300B amp which has a few more expensive caps and resistors that uses triode connected EF86 followed by 5687's with both plates in parallel to drive the 300B and there is not much of difference.
Consequently in my ears there is nothing wrong with using 6V6, maybe your use of the Hashimoto transformer is the clever bit.
Funny I was originally thinking a 5K interstage transformer on the plate of the 6V6. Thanks I'll start looking further into improving the design. Thanks
A triode-wired 6V6 is a lovely driver for the 300B but you need a voltage amplification stage or the amp will just sort of sit there, with no real gain or dynamics. Here's an old three-stage design I built years ago. Very nice:
https://www.bonavolta.ch/hobby/en/audio/300b_7.htm
https://www.bonavolta.ch/hobby/en/audio/300b_7.htm
Also, perhaps consider using an EL84/6BQ5 in triode mode as a driver. The plate curves for these in triode mode look quite good, perhaps better than the 6V6. You'll still need an extra gain stage of course....
Another alternative could be a 6AH4GT, which is an octal based triode.... options...
Slightly off topic, I'll be using a 5687 twin triode for my WE300b SE amps. Two-stage direct-coupled with the second stage driving an interstage transformer, most likely the Hashimoto A-115.
Good luck on this project and keep the thread updated.
Another alternative could be a 6AH4GT, which is an octal based triode.... options...
Slightly off topic, I'll be using a 5687 twin triode for my WE300b SE amps. Two-stage direct-coupled with the second stage driving an interstage transformer, most likely the Hashimoto A-115.
Good luck on this project and keep the thread updated.
Having looked at Thorsten's legacy amp, then compared with EL84 pentode curves, it is clear getting the operating point correct for the screen grid is critical. Thorsten mentioned on this site that the quality and value of the decoupling cap for the screen grid is important, but the voltage from the resistor divider must be set just right for the amount of drive.
I stumbled across a few EF55's. It is a sharp cut off RF pentode with 12W dissipation, and looks like a more sensitive EL84 on paper.
What is the design thinking when it comes to the screen for using a pentode as a driver? I am assuming that it is kept simple with a fixed G2 and no clever stuff with feedback and suppressor grids, like some of the designs from the golden age.
I stumbled across a few EF55's. It is a sharp cut off RF pentode with 12W dissipation, and looks like a more sensitive EL84 on paper.
What is the design thinking when it comes to the screen for using a pentode as a driver? I am assuming that it is kept simple with a fixed G2 and no clever stuff with feedback and suppressor grids, like some of the designs from the golden age.