How much gain is required to drive any output stage is dependent on *a lot* of things.
You *must* begin by measuring the output of your sources / preamp at full volume and doing the math required from there to know how much gain will drive the 300B into clipping. In other words *consider the gain structure of your entire system.* If you've got a preamp with loads of gain, you might be able to build a 2-stage amp using only a trioded power tetrode as driver and the 300B - no gain stage! Or rather, the preamp is the gain stage.
Then *I* would take that calculated gain number and double it. If you are struggling with power limits (as most are when using normal box speakers with SET amps) the extra gain will come in handy on those recordings that are recorded at too low of a level, or with a large difference between the average and peak levels. In the latter case, it may be acceptable to set the volume so the 300B just enters clipping on the loud parts so that you can hear the quiet parts at a satisfying level.
You *must* begin by measuring the output of your sources / preamp at full volume and doing the math required from there to know how much gain will drive the 300B into clipping. In other words *consider the gain structure of your entire system.* If you've got a preamp with loads of gain, you might be able to build a 2-stage amp using only a trioded power tetrode as driver and the 300B - no gain stage! Or rather, the preamp is the gain stage.
Then *I* would take that calculated gain number and double it. If you are struggling with power limits (as most are when using normal box speakers with SET amps) the extra gain will come in handy on those recordings that are recorded at too low of a level, or with a large difference between the average and peak levels. In the latter case, it may be acceptable to set the volume so the 300B just enters clipping on the loud parts so that you can hear the quiet parts at a satisfying level.
I've got my own ideas for a SE 300B amp. It will (gasp) use NFB of the LOCAL variety.
Dennis Boyle's combination bias will be used. An unbypassed 330 Ohm cathode resistor will be combined with a C- supply.
5 KOhm primary O/P "iron" will be employed.
BTW, take a look at the 12GN7 as a single stage voltage amp/driver for the 300B. Regulate the screen grid B+ at about 1/3 the plate rail voltage and it will be quite linear.
Dennis Boyle's combination bias will be used. An unbypassed 330 Ohm cathode resistor will be combined with a C- supply.
5 KOhm primary O/P "iron" will be employed.
BTW, take a look at the 12GN7 as a single stage voltage amp/driver for the 300B. Regulate the screen grid B+ at about 1/3 the plate rail voltage and it will be quite linear.
No, this is just an expensive recipe for sonic mush. You would require a transformer with a step up ratio of at least 20X which means that even with very low source impedance drive you are going to have a very high driving impedance at the grid.. Miller capacitance in the 300B isn't terribly high, but assuming a required ratio of about 20X and assuming a 600 ohm source Z the effective impedance at the grid is going to be 240K - far too high for good HF performance.
1:1 IT coupled D3A or 1:1.5 with the 417A/5842 would give you sufficient gain and a low driving impedance. (Well under 5K ohm in both cases.)
Generally for the 300B 160Vpp is a typical drive requirement for full power and many would argue for a 6dB margin over this, so 320Vpp which is about 110Vrms. I generally aim for at least 3dB margin in my designs. Unless you have a very robust driver design that can handle some grid current it is over at the point where the drive approaches the 0 bias point in the curves as the grid current starts to increase rapidly.
I've been fooling with the 300B for quite a while now. I've tried various high-mu driver tubes (like the previously mentioned 5842, d3A, 6SL7, etc). The challenge really is to find a tube that will drive the needed 160~200 Vpp at low distortion and still have a little margin left. Many of the high-mu tubes can't handle that much voltage and those that can handle the voltage tend to want to operate at quiescent points where the Vgk is rather low. So low that you can't drive the input tube to the point where it'll produce the needed swing on its output.
So I'm actually back looking at medium-mu triodes again. JJ has a nice little one - ECC99 - in current production. I'm also going to try 6SN7 (how original, I know...
)
Of course, I've painted myself into a bit of a corner by insisting on a single tube driver. With two tubes (or one 6SN7 coupled as a 2-stage amp), life becomes easier.
And if you don't mind a little sand in your glass, you can always add a MOSFET source follower on the driver tube for low drive impedance.
~Tom
So I'm actually back looking at medium-mu triodes again. JJ has a nice little one - ECC99 - in current production. I'm also going to try 6SN7 (how original, I know...
)Of course, I've painted myself into a bit of a corner by insisting on a single tube driver. With two tubes (or one 6SN7 coupled as a 2-stage amp), life becomes easier.
And if you don't mind a little sand in your glass, you can always add a MOSFET source follower on the driver tube for low drive impedance.
~Tom
Thanks, Kevin and Tom for your responses.
I have a 300B SE amp with 6sn7 at driving stage but am thinking that a different tube at driving stage would fade/color the sound of 300B and that's why I am thinking of input transformers.
240K is far too high for good HF performance but would 150:100Kpp input transformers be ok to drive 300B at push pull mode?
Thanks
Andersen
I have a 300B SE amp with 6sn7 at driving stage but am thinking that a different tube at driving stage would fade/color the sound of 300B and that's why I am thinking of input transformers.
240K is far too high for good HF performance but would 150:100Kpp input transformers be ok to drive 300B at push pull mode?
Thanks
Andersen
I'm curious here. Which issue or problem are you trying to solve? Does the 6SN7 not offer enough sensitivity?
You also mention push-pull. Does your current amp not offer enough power for you?
Or are you just trying to turn something into a project?
All answers are valid. I'm just trying to get a sense of where you're trying to go with this.
~Tom
You also mention push-pull. Does your current amp not offer enough power for you?
Or are you just trying to turn something into a project?
All answers are valid. I'm just trying to get a sense of where you're trying to go with this.
~Tom
Hi!
As Kevin already mentioned, it is not really feasable. Transformers with such a winding ratio will normally not be able to handle such large voltage swings as needed to drive a 300B.
You would need to use something like an output transformer in reverse. Still the miller cap of the 300B would probably cause high frequerncy roll off. Also the preamp would need to have extremely low output impedance to drive this.
You would certainly create many more colorations and problems with such a solution compared to a good driver stage.
Best regards
Thomas
As Kevin already mentioned, it is not really feasable. Transformers with such a winding ratio will normally not be able to handle such large voltage swings as needed to drive a 300B.
You would need to use something like an output transformer in reverse. Still the miller cap of the 300B would probably cause high frequerncy roll off. Also the preamp would need to have extremely low output impedance to drive this.
You would certainly create many more colorations and problems with such a solution compared to a good driver stage.
Best regards
Thomas
Hi!
do a google search for Sakuma amps, he built 300B/300B amps and publsihed schematics. These are either 3 stage or still use input transformers with large step up ratio, which requires a suitable preamp to drive them. amps with DHT drivers can become very big and heavy. The filament supply in an all DHT amp is crucial.
The best 300B amp I built used a 10Y driver.
Best regards
Thomas
do a google search for Sakuma amps, he built 300B/300B amps and publsihed schematics. These are either 3 stage or still use input transformers with large step up ratio, which requires a suitable preamp to drive them. amps with DHT drivers can become very big and heavy. The filament supply in an all DHT amp is crucial.
The best 300B amp I built used a 10Y driver.
Best regards
Thomas
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