I did two upgrades on my 300B amplifier yesterday evening,
1. Added two ferrite beads on the grid stopper resistor lead. Did not notice any degrade or upgrade on the SQ with this. There was anyway no RF hiss or howling in the amplifier before this too. Its more of a placebo for me.
2. Restored my partial negative feedback loop from the 300B plate to the grid stopper resistor of the 300B tube. Actually this part of the ckt was initially tested couple of months ago but i removed them due to improper value of resistor and capacitor. I yesterday added the exact same value of resistor and capacitor as used in the T-Rex 300B amplifier designed by John Broskie some decade and half ago. This is a huge change in way the amplifier sounds w.r.t. gain, distortion and bandwidth. Anybody who have listened to a SET based 300B would have experienced a bit higher 2nd harmonic distortion as high as 2% which gives the vocals a bit edgy tone which could extend in some recordings which could be sometime sweet but some high pitch sound could lead to listen fatigue. This partial negative feedback removes all the edginess and reduces the distortion to very low minuscule level not only on paper but also in listening tests.
I was amazed with the clean sound of the 300B which i had never experienced before in my setup and was getting used to it all these years as nothing much could be done as its the signature of the tube due to its Trans-conductance and gain. The partial negative feedback uses the input voltage swing to adjust the tube trans-conductance to prevent adjusting of the plate current / voltage w.r.t the swing and there by reducing the distortion.
I was intrigued to measure the distortion which as per the original manual was 2% THD. I had downloaded the RMAA 5.5 couple of years ago and the application was lying unused in my laptop which i put to use today. I connected the input/ output and measured the -dB range of 1KHz fundamental and its associated harmonic distortion. I noticed that the 2nd harmonics was -84dBu and 3rd harmonic was -96dBu. converting this to % scale means its 0.0062% for 2nd harmonic and 0.0015% for 3rd harmonic. This was the first version of my test and will like to also cross-verify by removing the feedback loop on the same amplifier. But listening test can for sure confirm the loss of the edginess in the vocals.
1. Added two ferrite beads on the grid stopper resistor lead. Did not notice any degrade or upgrade on the SQ with this. There was anyway no RF hiss or howling in the amplifier before this too. Its more of a placebo for me.
2. Restored my partial negative feedback loop from the 300B plate to the grid stopper resistor of the 300B tube. Actually this part of the ckt was initially tested couple of months ago but i removed them due to improper value of resistor and capacitor. I yesterday added the exact same value of resistor and capacitor as used in the T-Rex 300B amplifier designed by John Broskie some decade and half ago. This is a huge change in way the amplifier sounds w.r.t. gain, distortion and bandwidth. Anybody who have listened to a SET based 300B would have experienced a bit higher 2nd harmonic distortion as high as 2% which gives the vocals a bit edgy tone which could extend in some recordings which could be sometime sweet but some high pitch sound could lead to listen fatigue. This partial negative feedback removes all the edginess and reduces the distortion to very low minuscule level not only on paper but also in listening tests.
I was amazed with the clean sound of the 300B which i had never experienced before in my setup and was getting used to it all these years as nothing much could be done as its the signature of the tube due to its Trans-conductance and gain. The partial negative feedback uses the input voltage swing to adjust the tube trans-conductance to prevent adjusting of the plate current / voltage w.r.t the swing and there by reducing the distortion.
I was intrigued to measure the distortion which as per the original manual was 2% THD. I had downloaded the RMAA 5.5 couple of years ago and the application was lying unused in my laptop which i put to use today. I connected the input/ output and measured the -dB range of 1KHz fundamental and its associated harmonic distortion. I noticed that the 2nd harmonics was -84dBu and 3rd harmonic was -96dBu. converting this to % scale means its 0.0062% for 2nd harmonic and 0.0015% for 3rd harmonic. This was the first version of my test and will like to also cross-verify by removing the feedback loop on the same amplifier. But listening test can for sure confirm the loss of the edginess in the vocals.
