So, the cathode follower has lower output impedance compared to a traditional gain stage to allow it to drive current demanding circuits like tone stacks. It's not actually amplifying current but allowing more to flow.
You are spot on on the cathode follower operation. When you get back to it try this: put a 1/4" plug or cable end into each of the four inputs and measure the resistance from that input to ground (power off). You should see 1M on the #1's and 136k on the #2's.
Last edited by martin manning on Thu May 02, 2013 2:27 pm, edited 1 time in total.
martin manning wrote:You are spot on on the cathode follower operation. When you get back to it try this: put a jack or cable end into each of the four inputs and measure the resistance to ground, with the power off. You should see 1M on the #1's and 136k on the #2's.
Also measure from the grid pins (2 and 7) to ground at the same time. The triode not in use should always read 34k, the other one should read 1034k when the plug is in the #1 input and 68k when it is in the #2 input.
Edit: corrected resistance to ground from grid of triode not in use.
Last edited by martin manning on Sat May 11, 2013 10:44 am, edited 1 time in total.
martin manning wrote:Also measure from the grid pins (2 and 7) to ground at the same time. The triode not in use should always read 0 ohms, the other one should read 1034k when the plug is in the #1 input and 68k when it is in the #2 input.
Thanks Martin! Will do.
I was in live audio for 15 years doing one-off shows most of the time so I can appreciate the value of being able to troubleshoot. You guys are amazing for taking interest in a tube-noob like myself. I'm learning so much about this amp.
And, I know I'll never build another chassis with Spraque Orange Drops...they're a real PITA when trying to meter things.
Peace,
Mark
Last edited by Guitarnut on Thu May 02, 2013 11:25 pm, edited 1 time in total.
Mark, check the ground from the presence pot's CCW lug, and measure resistance to ground from the CCW lug, CW lug, and at the ground side of the 10k resistor on the board. I'm thinking that the PI tail is not grounded.
martin manning wrote:Also measure from the grid pins (2 and 7) to ground at the same time. The triode not in use should always read 0 ohms, the other one should read 1034k when the plug is in the #1 input and 68k when it is in the #2 input.
I just checked the inputs as you suggested. My 1M are on #2 jacks. Looks like I have work to do on the normal jacks.
Hot to Ground:
Norm - Unmeasurable - both started around 1M and rose off the scale.
Bright - Jack1 135K Jack2 1030K
martin manning wrote:Mark, check the ground from the presence pot's CCW lug, and measure resistance to ground from the CCW lug, CW lug, and at the ground side of the 10k resistor on the board. I'm thinking that the PI tail is not grounded.
The ground looks good. It's going straight down to the bus and is a solid joint.
Presence CCW lug to ground is 0V, CW lug and the 10K to ground are 4.25K.
I'm going to start over on my inputs and see where that gets me.
