Simple SE Cathode Resistor Values

[skidave]

Hello,

I'm slowly working to get my Simple SE assembled. I'm looking for some suggestions for the Cathode resistors for various tubes.

I'm using a Hammond 274 transformer, 5AR4, and a Triad C-14X choke. My output transformers are 5K Edcors.

I'd like to install a rotary switch with a few resistors for various tubes. I looked at George's site and the information provided for the B+, tubes and resistor combos. I'm looking for suggestions...

I'd like to run KT88s, 6L6s, maybe 6V6s for now. I have a dual gang 6 position switch I ordered from Mouser so I can select various resistors.

I did see some posts with switches, but it looks like people were switching a resistor in parallel for one different tube. I thought the rotary would give me a few more options.

Thanks...Dave

[rknize]

This is mine:



There is a 680 ohm on the board and the following are switched in parallel by the 6-position rotary:

3.3k (for ~560ohms)
1.5k (for ~470ohms)
1.2k (for ~430ohms)
910 (for ~390ohms)
750 (for ~360ohms)
680 (for 340ohms)

I made a spreadsheet to help come up with target values and calculate the necessary power ratings.

[rknize]

...oh and you won't be able to run 6V6 with that amount of B+. It's possible to do it by modifying the power supply filter to switch from CLC to LC, as was brought up recently in another thread.

[skidave]

Russ,

Thanks for sharing and I ejoyed your website. I did not see your spreadsheet posted anywhere on your site. Do you still have your data available.

When you did your test points, did you just set it up so the points measured across the cathode resistor. I plan to do the same.

Dave

[rknize]

Yes, the test points are across the resistor so I have to do the math when measuring. I have the original spreadsheet kicking around. I'll post it tonight if I remember.

[skidave]

Thanks. Your website is fantastic.

[squiffiness]

Originally Posted by rknize

This is mine:
(photo removed for space)

There is a 680 ohm on the board and the following are switched in parallel by the 6-position rotary:

3.3k (for ~560ohms)
1.5k (for ~470ohms)
1.2k (for ~430ohms)
910 (for ~390ohms)
750 (for ~360ohms)
680 (for 340ohms)

I made a spreadsheet to help come up with target values and calculate the necessary power ratings.

Russ,

The resistors in the photo are 3W rating, unless mine eyes deceive me. Is your 680R on the board in R17/R27 the same? Or is it 5W as a hedge against your switch failing or the resistor you are placing in parallel failing open?

Just wondering, or are you playing a little V^2/R action for each leg to determine a power handling requirement?

Since I am planning a similar scheme (in another recent thread), I am interested in this too.

~Brad

[rknize]

The 680s are the 5W type. The rest are 2 or 3W, depending. See the attached spreadsheet. The columns may warrant some explanation:

B) The expected cathode current from George's table.
C) The total cathode resistance I am shooting for. Tweak these to get the Vk that you like.
D) The actual cathode resistance calculated from "R1" and "R2 avail".
E) The target cathode-to-grid voltage from Ik and Rk.
F) The total power dissipated by the cathode resistor(s)
G) The on-board resistor "R1".
H) The power dissipated by "R1".
I) The exact "R2" value needed in parallel with "R1" to get the target "Rk" value.
J) The power dissipated by the ideal "R2".
K) The actual "R2" value, based on what is available.
L) The power dissipated by the actual "R2" resistor. The resistor should be rated at least twice this amount.

[skidave]

Thanks Russ. I had a whole 2 minutes to look at your information today because work was so busy. I see you are giving yourself a few different current settings for each tube with your switch resistor configuration.

I'm in the middle of my build and I need to order a few more items from Mouser. I think I am going to determine safe values for the tubes (KT88, EL34 and 6L6GC) and order these values.

I appreciate you sharing your data. I plan to look more tomorrow. I am also going to provide a set of jacks for current measurement for cathode resistor (like you have).

Dave