Here's a little clearer and more complete pic with some resistance checks. I don't know if the values in the schematic match yours, but they should be close.
Edit: see later diagram below.
Last edited by martin manning on Mon Jan 11, 2016 10:11 am, edited 1 time in total.
These don't look out of line. The schematic shows a 7025 in the PI spot, but the resistor values are more like for a 12AT7 as shown in Tony's layout. That shouldn't kill the sound, though; there must be some wiring error or a bad component.
When you measure the PI voltages, get the pin 2 and 7 voltage from the bottom of the bias resistor where the two 1M's join (same value for both). If you measure at the grid your meter upsets the bias. Same thing for the loop driver stage, but the 30.75 on the cathode looks good there.
martin manning wrote:Here's a little clearer and more complete pic with some resistance checks. I don't know if the values in the schematic match yours, but they should be close.
Thanks Martin,
1) At Loop input to ground (pin3?), nothing in OD or Clean. Am I measuring in the right place?
2) At recovery output to ground (off the anode .1uf cap), clean = 1.07meg
For the loop input, measure point A to ground, which you can get at the relay board or pin 2 on the loop tube. When you say "nothing" do you mean infinite or zero ohms? Infinite means the ground is missing, zero means it's shorted to ground.
martin manning wrote:For the loop input, measure point A to ground, which you can get at the relay board or pin 2 on the loop tube. When you say "nothing" do you mean infinite or zero ohms? Infinite means the ground is missing, zero means it's shorted to ground.
Loop tube Pin2 to ground is infinite (meter reads the same wether connected or not).
Where do I take the gound from? Stupid question I know. Humour me!
I'm confused now. I don't know how you could get sound out of the send jack and the voltages above if pin 2 of the loop tube has no ground reference. Here's an update to the above diagram.
Edit: added cap at entrance to loop send stage.
You do not have the required permissions to view the files attached to this post.
Last edited by martin manning on Mon Jan 11, 2016 9:43 am, edited 1 time in total.
Martin
Both the cathode ends of the 27k and the 1k5&5uf ARE grounded to the mid pot casing, which is grounded to the tone stack buss bar. Should this be moved to the same point as the B+ ground for this stage (which is on the Return pot casing at the rear of amp)?
I would say yes make all of the loop grounds where the B3a and B3b filter caps are grounded, preferably at the rear of the amp. I'd like to see a ground lug for that rather than the pot casing.
I'm still not understanding the send stage input into the loop tube's pin 2 grid... Is it wired like the diagram above? Shown there are a 27k, a 1k8 and a 1M, but only the 27k goes to ground.
martin manning wrote:I would say yes make all of the loop grounds where the B3a and B3b filter caps are grounded, preferably at the rear of the amp. I'd like to see a ground lug for that rather than the pot casing.
I'm still not understanding the send stage input into the loop tube's pin 2 grid... Is it wired like the diagram above? Shown there are a 27k, a 1k8 and a 1M, but only the 27k goes to ground.
Will do a ground lug like the rest of the B+ ground. I didn't use a lug here because of space.
Yes the send stage is wired as per your diagram. Have checked it many times.
Yes the 27k is the only one grounded.
The end of the cathode for for the return (1k5&5uf) are also grounded.
Here is a pic of B+ ground (thick green/yellow cable)
You do not have the required permissions to view the files attached to this post.
Ok, so you are still getting sound out of the send jack in clean, but not in OD? That's the bit I was trying to solve by checking resistances to ground.
martin manning wrote:I'm confused now. I don't know how you could get sound out of the send jack and the voltages above if pin 2 of the loop tube has no ground reference. Here's an update to the above diagram.
Thanks Martin will go over in the morning with a fresh mind.
Dont stress about being confused. You must see it through my eyes!!!! Been at his all day!!
Thanks for hanging in with me. Unfortunately, my leave is over and I am back to work in the morning. So close yet so far.
martin manning wrote:Ok, so you are still getting sound out of the send jack in clean, but not in OD? That's the bit I was trying to solve by checking resistances to ground.
OUT OF SEND JACK
Noticed before I turned in, the OD does now make a distorted time sound, a almost like it is trying to "break through". Scratchy and low in volume. Checked voltages of V2 and get
147v on pin 1 (low?)
201v on pin 6
2.222v on pin 3
1.5i8v on pin 8
I get 16.8v on pin 2 to ground on send tube (in clean mode). In OD mode, this jumps to 3.76v!
So, how can there be no "reference to ground" here? Confused. I checked again, and sure enough, infinity to ground!. I am reading voltage to ground, but no resistance.
