300b lovers

@donsachs 

What I meant above is the design of the driver section of the circuit.  Your amp is using a 6L6 to drive a 300b, which is a good choice.  I was referring to many 300b amps that are pushing tubes like a 6SN7 to the max and barely driving a 300b.  The first rule of amp design is your driver section should never strain or clip before your output section does

It seems perfectly rational that the driver stage of a tube amplifier is of paramount importance. If the output tube isn’t adequately driven then how could optimal sonic performance be achieved?  The builder of my 300b SET chose the 6EM7 as a very suitable driver for the 300b. His reasoning was the same as you mentioned.

I am not an electrical engineer but I wonder how much is it the particular driver tube versus the implementation of said tube. Ralph has written in the past that the 6SN7 is a sufficient driver tube “dependent “ on its implementation in a given circuit. I have been told that the 6EM7 provides much more current, power and drive capability compared with the 6SN7.

I will say that I’ve heard an excellent sounding PSET 845 amplifier that utilized the 6SN7. I could certainly be wrong, but isn’t the 845 a more difficult tube to drive than the 300b? Anyway, no question that driver stage rivals power supply and output transformers in getting it right for tube amplifiers. So it appears that there are alternative executions that can get one there successfully.

I very much appreciate the comments this thread has generated. The education never stops.

Charles

Hi 

The 6SN7 will barely drive a 300b, which is a very difficult load.   I have not worked with 845 tubes nor will I given the high voltages and heat involved, but I did look at the data sheet.  Yes, it takes more negative grid voltage to bias one than a 300b, but I have no idea of how capacitive a load an 845 is, and I really don't have time to research it.  Ralph is right in that a 6SN7 is adequate to drive many tubes.  It makes a wonderful driver for any of the usual octal pentodes like kt88 or el34, or a 6V6 or 6L6.  Works great.  But a 300b taxes a 6SN7.  It works as in the original Reichert amp, but if you want to unlock the unlimited dynamics of the 300b you need a much better driver setup.  The reason the original Dynaco ST70 was a marignal amp was it had a wimpy driver section that would clip before the output tubes, or about the same time, plus it had a very mediocre power supply.  It was a pleasant amp and a zillion of them sold.  But if the driver and power supply were better it would have been a much better amp.  The output transformers are not world class, but they are quite good and capable of more than the rest of the amp could provide.   

You can also improve the performance of any driver tube by changing the way it is loaded.   For example, a CCS will give far better drive capability than just typical RC coupling.   But CCS circuits do have some coloration.  Not so bad in a pentode amp, but very noticeable in a DHT amp.   The other ways are to add inductance to the plate of the driver with either LC (choke cap) coupling or full on interstage transformer coupling.  The inductance the driver tube plate sees "helps" it out to drive the 300b.  There are differences in the way all of these approaches sound, but all are better than just a plate load resistor and coupling cap.  I have explored all of them thoroughly for a year with the 300b project and found a solution that Lynn and I like and sounds the most musical.

I have had a DIY 300B integrated stereo amplifier since 2006 built by one guy.

But in all this long period of time I did dozents upgrade myself.

The amplifier has 6sn7 input, 6f6 (in triode mode) driver and 300B. Cathode bias and C coupling between stages.

I upgraded: output transformers, power supply choke, interstage capacitors, power supply capacitors, resistors, driver and input tube idle current...

My experience shows that big value power supply capacitors (after the choke), big value cathode capacitors, correct driver idle current gave me much better improvement versus change of output transformer from James Audio to Hashimoto H30 3.5 (3 times more expensive!).

Coupling capacitor quality and sound signature are very important too and cathode resistors as well. Also the quality of power supply capacitors is very important. To get big values and quality without spending too much I use a mix of electrolytic, polypropylene film and vintage German oil capacitors.

I don’t understand why producers put such small capacitors after a choke in 300B amplifiers (typically 100-200uF per channel). My experience shows x10 value gives huge improvement in bass control, transparency, focus, soundstage.

I don't have as much experience as @atmasphere @donsachs have. I also don't understand how audio designers make decisions. Probably a much cheaper change of schematics can lead to great sound improvement without spending too much on parts and size of amplifier.

Or leaving conventional power supply topology behind, and using multiple independent supplies in the proper way😀

@alexberger 

You make salient and pertinent points. This is why there’s such a price discrepancy/spectrum for 300b amplifiers. They range from 500.00 to 50,000.00 USD. Implementation and quality of the power supply/driver stage and output transformer has so much do with what you get and hear.

@donsachs 

I wish you much success with your new 300b PP amplifier at the Seattle audio show. It sure seems it is a winner.

Charles