06-02-2023, 01:38 AM

Hi Guys

Post-20 says how to size the bias range resistors based on doubling Rk as Champ81 did.

In general, the range resistor should be scaled with the bias pot value based on the increase of Rk that you incorporate. For example, if you increase Rk from 150R to 220R, this is an increase of 70R, which 0.47 of the stock Rk. We want the voltage across the bias pot to equal the portion of Rk added to limit the maximum current through the tube to the stock value. The range resistor then has the remaining voltage across it equal to stock Vk.

It is probably simpler to view the two parallel resistor pairs: the first is stock Rk at top with the added amount below, so 150R over 70R. Say you have 25k pots. The pot portion of the bias divider is the lower part and the range resistor is the upper part. We could simply divide 25k by 70 to get a multiplication factor to calculate R-range. 25k / 70 = 357. Now, 357 x 150 = 53k6.

If the bias pot was 1k, then the range resistor would be 2k14.

If we actually built the new Rk by stacking two resistors as post-6 suggests, where the upper portion is the stock Rk, then we can eliminate the need for this calculation and tie the bias pot across the added resistance to ground.

Since the example 2xEL-84 amp begins with Vk=15V, we can see that the 25k pot voltage divider dissipation is so low as to be unimportant even with 22V maximum across the new "220R" Rk. The divider total resistance is 78k. The 1k pot divider warrants investigation as its net resistance is 3k2, roughly, but total heat is 150mW, shared as about 100mW in the resistor and 50mW in the pot.

In Champ81's amp, Rk was doubled, so the total Vk could be 30V. You can see that the dissipations do not increase greatly compared to the example and 250mW resistors are fine even using 1k pots.

Post-20 says how to size the bias range resistors based on doubling Rk as Champ81 did.

In general, the range resistor should be scaled with the bias pot value based on the increase of Rk that you incorporate. For example, if you increase Rk from 150R to 220R, this is an increase of 70R, which 0.47 of the stock Rk. We want the voltage across the bias pot to equal the portion of Rk added to limit the maximum current through the tube to the stock value. The range resistor then has the remaining voltage across it equal to stock Vk.

It is probably simpler to view the two parallel resistor pairs: the first is stock Rk at top with the added amount below, so 150R over 70R. Say you have 25k pots. The pot portion of the bias divider is the lower part and the range resistor is the upper part. We could simply divide 25k by 70 to get a multiplication factor to calculate R-range. 25k / 70 = 357. Now, 357 x 150 = 53k6.

If the bias pot was 1k, then the range resistor would be 2k14.

If we actually built the new Rk by stacking two resistors as post-6 suggests, where the upper portion is the stock Rk, then we can eliminate the need for this calculation and tie the bias pot across the added resistance to ground.

Since the example 2xEL-84 amp begins with Vk=15V, we can see that the 25k pot voltage divider dissipation is so low as to be unimportant even with 22V maximum across the new "220R" Rk. The divider total resistance is 78k. The 1k pot divider warrants investigation as its net resistance is 3k2, roughly, but total heat is 150mW, shared as about 100mW in the resistor and 50mW in the pot.

In Champ81's amp, Rk was doubled, so the total Vk could be 30V. You can see that the dissipations do not increase greatly compared to the example and 250mW resistors are fine even using 1k pots.