calculate resistor to drop voltage..

[diddymix]

Hi People,

I was hoping I could ask for some advice on how to achieve a voltage drop on the B+ of my amp to feed my V1 plate.. At the moment it is around 216V and I would like to move it down to 200V and not too sure on how to work out the resistor I need to achieve that 200V value. I changed the V1 plate resistor to 220K, and to compensate for the large drop in voltage on the plate.. I moved the B+ source from R76 (taking the resistor out of pcb) to tapping it from R78 (the Cathode follower side HT4). So now I have a higher voltage of 216V I want to drop down to 200V. Thats the reading on the plate though.. and I dont want to change the 220K plate resistor... The voltage at R78 HT4 is 366V... but then that obviously hits the 220K resistor to land at 216V... so I need to figure out the resistor to go in between the two.
I'm so sorry about going into this much detail but Ive never done this before and any pointers on working this out would be most appreciated! Is there a formula I can use to calculate the required resistance?

[diddymix]

Hi People,

I was hoping I could ask for some advice on how to achieve a voltage drop on the B+ of my amp to feed my V1 plate.. At the moment it is around 216V and I would like to move it down to 200V and not too sure on how to work out the resistor I need to achieve that 200V value. I changed the V1 plate resistor to 220K, and to compensate for the large drop in voltage on the plate.. I moved the B+ source from R76 (taking the resistor out of pcb) to tapping it from R78 (the Cathode follower side HT4). So now I have a higher voltage of 216V I want to drop down to 200V. Thats the reading on the plate though.. and I dont want to change the 220K plate resistor... The voltage at R78 HT4 is 366V... but then that obviously hits the 220K resistor to land at 216V... so I need to figure out the resistor to go in between the two.
I'm so sorry about going into this much detail but Ive never done this before and any pointers on working this out would be most appreciated! Is there a formula I can use to calculate the required resistance?

[tubeswell]

V1a is supplied by R76

V1b is supplied by R81

Which ‘V1’ do you want to change?

Whichever one it is, going from 216V to 200V won’t make any difference in the way the amp sounds. 16v is not a significant enough change to make a noticeable difference. However, if you wish to find this out for yourself, then assume that each AX7 triode pulls about 1mA at idle. 16v/0.001A = 16k. If it’s V1a you’re concerned about, R76 is 1k, so you want to make it 17k (or use the nearest available standard size 18k, which will drop a teeny weeny bit lower than 200V)

(Note: increasing the size of the plate resistor from 100k to 220k will increase the gain of the stage, which may or may not be what you intended.)

[diddymix]

V1a is supplied by R76

V1b is supplied by R81

Which ‘V1’ do you want to change?

Whichever one it is, going from 216V to 200V won’t make any difference in the way the amp sounds. 16v is not a significant enough change to make a noticeable difference. However, if you wish to find this out for yourself, then assume that each AX7 triode pulls about 1mA at idle. 16v/0.001A = 16k. If it’s V1a you’re concerned about, R76 is 1k, so you want to make it 17k (or use the nearest available standard size 18k, which will drop a teeny weeny bit lower than 200V)

(Note: increasing the size of the plate resistor from 100k to 220k will increase the gain of the stage, which may or may not be what you intended.)

Thanks!!! You were dead on and 18K lands it at 200V!, actually it does sound a little different.. less clean and a little richer in the tops.. however it is also a little more spongy so I may try the LNFB like the dumble 102, maybe bring some dynamics back..

[diddymix]

V1a is supplied by R76

V1b is supplied by R81

Which ‘V1’ do you want to change?

Whichever one it is, going from 216V to 200V won’t make any difference in the way the amp sounds. 16v is not a significant enough change to make a noticeable difference. However, if you wish to find this out for yourself, then assume that each AX7 triode pulls about 1mA at idle. 16v/0.001A = 16k. If it’s V1a you’re concerned about, R76 is 1k, so you want to make it 17k (or use the nearest available standard size 18k, which will drop a teeny weeny bit lower than 200V)

(Note: increasing the size of the plate resistor from 100k to 220k will increase the gain of the stage, which may or may not be what you intended.)

Thanks!!! You were dead on and 18K lands it at 200V!, actually it does sound a little different.. less clean and a little richer in the tops.. however it is also a little more spongy so I may try the LNFB like the dumble 102, maybe bring some dynamics back but hopefully keep that base tone..