![[-]](https://theultimatetone.com/images/collapse.png "[-]")
05-25-2023, 11:37 PM
Hi Guys
A member of the forum brought up a question about adjusting idle in a cathode-biased amp and bought the BMK2 Bias Mod Kit for 2-tubes from us. The BMKs are intended for use with fixed-biased amps and the notes reflect that application. However, you can use BMK2 to hum-balance a cathode-biased amp. Each pot supplied will control one side of the push-pull circuit.
Where is the bias voltage coming from? It is the voltage across Rk. Each tube sees this voltage and adjusts its own current in a bit of a tug and war with the opposite tube(s). The two sides are unlikely to have equal currents, let alone end up at the more important hum-balance point for the OT. Being able to tap this voltage with the pots allows the effective bias voltage to one side to be REDUCED. This will make that side run hotter.
Since cathode-biased amps are notoriously poorly designed, lacking grid-stops and screen-stops, sharing one bias resistor for the entire output stage, and aiming to run each tube at its full dissipation, having a bias adjustment that makes things even hotter is not really a good idea, but it is what we can do. The fix is to increase the value of Rk to a safer operating point that gives leeway for making one side (or both) a little warmer. To do this, we can either add another resistor in series with the stock Rk, or change Rk outright with a higher value. The new net value should be about 20% higher or more. So, a 250R would change to 300R, maybe go to 330R to use a standard value. The 100R in an AC30 would change to 120R minimum. Higher is better as we can make things warmer at will using BMK2.
To limit the adjustment range of the pots, we use the 6k81s in series with the X end of the pot, just as in fixed-bias.
The drawing shows how BMK2 is incorporated into a 2-tube cathode-biased amp. The RED-1, RED-2 and BLK are the meter jacks, along with the 1R current-sense resistors. We can measure 1mV per 1mA through the tube and know how much heat it is dissipating.
If the amp is a type with independent cathode-bias resistors and bypass caps, the BMK2 can still be used with half the kit applied to each tube. You would need a second BLK meter jack at the top of the second Rk,Ck.
A member of the forum brought up a question about adjusting idle in a cathode-biased amp and bought the BMK2 Bias Mod Kit for 2-tubes from us. The BMKs are intended for use with fixed-biased amps and the notes reflect that application. However, you can use BMK2 to hum-balance a cathode-biased amp. Each pot supplied will control one side of the push-pull circuit.
Where is the bias voltage coming from? It is the voltage across Rk. Each tube sees this voltage and adjusts its own current in a bit of a tug and war with the opposite tube(s). The two sides are unlikely to have equal currents, let alone end up at the more important hum-balance point for the OT. Being able to tap this voltage with the pots allows the effective bias voltage to one side to be REDUCED. This will make that side run hotter.
Since cathode-biased amps are notoriously poorly designed, lacking grid-stops and screen-stops, sharing one bias resistor for the entire output stage, and aiming to run each tube at its full dissipation, having a bias adjustment that makes things even hotter is not really a good idea, but it is what we can do. The fix is to increase the value of Rk to a safer operating point that gives leeway for making one side (or both) a little warmer. To do this, we can either add another resistor in series with the stock Rk, or change Rk outright with a higher value. The new net value should be about 20% higher or more. So, a 250R would change to 300R, maybe go to 330R to use a standard value. The 100R in an AC30 would change to 120R minimum. Higher is better as we can make things warmer at will using BMK2.
To limit the adjustment range of the pots, we use the 6k81s in series with the X end of the pot, just as in fixed-bias.
The drawing shows how BMK2 is incorporated into a 2-tube cathode-biased amp. The RED-1, RED-2 and BLK are the meter jacks, along with the 1R current-sense resistors. We can measure 1mV per 1mA through the tube and know how much heat it is dissipating.
If the amp is a type with independent cathode-bias resistors and bypass caps, the BMK2 can still be used with half the kit applied to each tube. You would need a second BLK meter jack at the top of the second Rk,Ck.
