This is the current state of my sunvalley 300b. I am back to using the red HV supply wires. Ultimately I found that I could not quite get the output tube bias high enough with the purple wires. So now with a 5u4gb I am at ~420V and 75-80ma cathode current. With a 5u4g I get a bit less B+, ~400V and 68-70ma cathode current.
Notice that I have changed the cathode resistors for 300b’s to ~935 ohms.
Also to help with turn on inrush I am using a current inrush limiter CL-40.
Notice that I have changed the cathode resistors for 300b’s to ~935 ohms.
Also to help with turn on inrush I am using a current inrush limiter CL-40.
If you are referring to R107, that resistor is dropping the high voltage down for the preamp(12at7) and driver stage(12au7’s). It is fairly critical that this voltage remain at about what the schematic shows, which is 432V. If you have lowered the power supply B+ voltage to near 425V then you will need to lower R107 to keep the preamp section voltage up as much as possible. I am using 820 ohms at 3watts. There is only about 25ma current draw in the preamp circuit so the resistor doesn’t need to be huge. The driver stage will not work as well if the voltage drops too much below 400V. The 12au7’s will not be able to swing enough voltage to drive the 300b to full power.
Look at B2 and B3 test points. B2 is 388v. Personally I would not go any lower than that. I would decrease the resistance to get that voltage above 400v. You will begin to lose drive and power if you go lower than 388v. It will sound wimpy.
My biggest problem with this amp in stock configuration is that it sounded wimpy and hard-with glare in the midrange and lacked details in the high frequencies. It wasn’t open and full and rich and relaxed which is what I’m used to when using 300b. It was too tight and lean and lacked power. I found most of the problems were solved by rebiasing the input tube. I would encourage you to try a 5751 and 12ax7 as input tube. I never tried them because I wanted to have a little less gain not more and I wanted a tube that wasn’t so expensive to buy. 12AV7 is almost the same as 12AT7, but to me sounds a bit better.
All you need to do to rebias the 12at7 is to lower the 2 plate resistors. It’s easy if you don’t like it just put the 220k resistors back.
My biggest problem with this amp in stock configuration is that it sounded wimpy and hard-with glare in the midrange and lacked details in the high frequencies. It wasn’t open and full and rich and relaxed which is what I’m used to when using 300b. It was too tight and lean and lacked power. I found most of the problems were solved by rebiasing the input tube. I would encourage you to try a 5751 and 12ax7 as input tube. I never tried them because I wanted to have a little less gain not more and I wanted a tube that wasn’t so expensive to buy. 12AV7 is almost the same as 12AT7, but to me sounds a bit better.
All you need to do to rebias the 12at7 is to lower the 2 plate resistors. It’s easy if you don’t like it just put the 220k resistors back.
After rebiasing the input tube the next thing I wanted to address was the output tube bias. I see that your 300b’s are running at about 67-68ma. In my amp I changed the cathode resistors to make them draw 75-80ma. When you do this it will cause the B+ to sag down to about 420v. So now plate voltage 420 minus cathode voltage 72 is right at 350v. If your target voltage is 360, then maybe try 1k cathode resistors. I am at 935 ohms.
As you can see I have a lot to say about this amp. I want to point out that you should check the filament voltage on the preamp tubes. It supposed to be 12.6v mine was at 14v. This will prematurely burn out your tubes. Also it will sound better if you get it to 12.0-12.6v. I set mine at 12.3v.
I just had a closer look at the input tube resistors in my amp. I didn’t remember that I did change the plate resistors and the cathode resistors. So there are 4 resistors that need to be adjusted. If you’re interested to know exactly what I did let me know and I will give all the details. Cheers Thomas