Hello,
    I am going over this whole thread again to try to come to grips with it. I went back to the very beginning to try to get it all in context. When I gave you the reading of 68 it was off of pin five of the 6AU6 as that is the next in the circuit from the plate of the 12AX7 to the grid of the 6AU6.
    I will take a bit of time to reread all of the thread and ponder it in depth. I will get back to this thread after I go through a download of (A Glossary of Common Amplifier Terms), Volume 2 of Basic Electricity by Neville, Nooger & Van Valkenburgh. It covers Ohm's & Kirchhoff's Laws. It is illustrated so it is not just blah, blah, blah, reading. That should help a bit
    I guess I let all of this get to me by being impatient, sorry about that. I will try to understand things in
relation to Ohm's law.

Hello,
      Given a circuit with ballpark values to build, is a far cry from suggesting a circuit to duplicate. Previously I had explored other options. The Fender Princeton, or Tweed Deluxe tone stacks. There is a circuit with ballpark values that already works and may need some modification.
      The value substitution and calculator number crunching is the mathematics I am talking about. It seems to be endless. I don't know where to start what to crunch and to what end or for what purpose. I told you I am not as well versed as you are in electronics.
       So as opposed to endless riddles, questions, and dead ends , I have decided to go in another direction. If the solution was as simple as modifying the preamp triode to suit the guitar why was that not the target from the start as opposed to starting from the opposite end of the amp increasing all of the voltage causing
the need to change all of the resistance values and make all sorts of calculations. It all just seems pointless
as the amp was working after a fashion before all of this started.
       I have put it back to stock and will start from there adjusting values to get the input similar to one of the Fender schematics.
       Once again Thank you for all your time and effort, but I am simply not capable of the disassembling and completely redesigning one component into another

Quote from: Roly on January 01, 2014, 10:19:34 AM
The attached circuit is unfinished, potentially full of value errors because the original was so lo-def; hasn't run yet in LTSpice because the OPT is still unknown
http://www.r-type.org/pdfs/6au6-1.pdf

Hello,
      Here is where this began some time ago, I have made no progress, I am just as confused now as when I first started. The constant mathematical equations are overwhelming. To completely redesign the circuit and change all of the values and voltages seems a bit of overkill when the amp was functional at one point. All I wanted to do was install a tone stack between the two existing tubes, and it has spiraled out of control to the point that I cannot plug it directly into the wall to see if any progress is being made or not.
      I thank you deeply for all your time and effort, but I have to bow out of this project. I am going to restore the circuit to its original specifications and make small changes one at a time, the first being the tone stack in the LTSpice drawing drafted into the original circuit. I am going to replace the 180K resistor with a potentiometer. Change the value of the 560K resistor from power supply to #1 pin on preamp side of the 12AX7.
       I think those changes will help. Once again thank you very much for your time and effort but I feel as if
I am in over my head.

Hello,
      I am not sure I did this right, but here goes.

6AU6 Supply side R7 68 - Anode side 67
68-67= 1
1/68=0.014
Actual meter reading - 0.010
Preamp voltage - Anode 68 - Cathode .526

Hello,
      Thanks, it certainly feels good to be making progress. I guess that grounding strip was causing most of my
problems all along. No matter what I tried I was fighting an open circuit. Without knowing where or what to check I was just beating my head against a wall. Oh well onward and upward.
       I will check the readings once again. There is a location on the meter for lesser readings. I will utilize it for the cathode reading, and post it back.

        I set the meter on the more sensitive setting and here is the result, (12AX7) Cathode (0.532). It may not be as accurate as it should be as the meter is not high dollar,







90.532

Hello Roly,
       Well I found the grounding problem.  A is strip riveted and soldered to the chassis. It has 3connections.
I thought they were all grounds, I was wrong. Only the middle is ground. so I soldered the grounds to that. I now have some new numbers and they are radically different. So here goes.
C1 = 296
C2 = 289
C3 = 226
R15 = (55)
12AX7 = ANODE (67) CATHODE (0)
6AU6 = ANODE (67) CATHODE (0) PIN 1 (.718)
6V6 = U1-ANODE (289) CATHODE (16)
6V6 = U2-"               " "                    "
Master Volume (0)
       I also had to change a couple of wires on the tone controls. As I was working on it I was looking at it backwards. As I said before to most of you guys on this forum, this is all second nature to you. I am not as well versed in electronics. I am sorry if you get impatient with me.

Hello,
      OK, here are the resistor readings using alligator clip. Some are slightly different but should be in spec.
Master wiper = (234)                           12AX7 = PIN 1 = (294)        6AU6 = PIN 5 = ANODE (291)           
C3 = (295)                                                     PIN 2 = (20)                    PIN 7 = CATHODE (0)
R15 = (295)                                                    PIN 3 = (270)
6AU6 = PIN 5 = (291)                                       PIN 4 = (0)           U1 6V6 = PIN 3 ANODE (261-269)*
6AU6 = PIN 1 = (-2)                                         PIN 5 = (0)           U1 6V6 = PIN 8 CATHODE (16)
12AX7 = PIN 6 = (295)                                      PIN 6 = (295)        U2 6V6 = PIN 3 ANODE (271-275)*
12AX7 = PIN 7 = (0)                                         PIN 7 = (0)           U2 6V6 = PIN 8 CATHODE (16)
6V6 = PIN 5 (0) Both tubes                                PIN 8 = (271)
6V6 = PIN 4 (285) Both tubes                             PIN 9 = (0)           6V6 CATHODE RESISTOR 16V ACROSS
                                                                                               VALUE = 300-OHMS
                                                                                            COLOR -ORANGE-BLACK-BROWN-GOLD
                                                                                             
                                                                                       *Indicates fluctuation between two values
   Hope this helps. Alligator clip was a tight squeeze in some places. One more question. I am working on this
out of the cabinet. Should the speaker be hooked up ? It is of great concern to me as it is said not to power it up with the speaker disconnected. However I am not putting under a load, but I am thinking it should be there to complete the circuit.



       

Hello,
      Glad to know things are somewhat normal. I will do all of the readings over again using alligator clips. The
way I took the readings I posted was using an alligator clip for ground and a probe to get the readings, that may have led to the one volt difference. I will check them ALL again just to be sure.
       I will also be sure to do the 6V6 cathode resistor. 

Hello,
   With limiter still on the voltages are as follows. I only have voltages as I am not sure what Va,Vs,or Vk,are.
1.6AU6 pin 1= (-2)                                    1. 12AX7 = Anode pin 1 (286) / Cathode pin 3 (259)
2.6AU6 pin 5=(293)                                   2. 6AU6 = Anode pin 5 (282) / Cathode pin 7 (0)
3.C3 = (293)                                            3. U1-6V6 = Anode pin 3 (265) / Cathode pin 8 (15)
4.R15 = (293)                                          4. U2-6V6 = Anode pin 3 (265) / Cathode pin 8 (16)
5.12AX7 pin 6 = (61)                                 5. 6X4 = Anode pin 6-1 (fluctuates 1-4) / Cathode (295)
6.12AX7 pin 7 = (0)
7.6V6 pin 5 = (0) both tubes
8.6V6 pin 4 = (284) both tubes

         Removed resistor 560K right of pin 5 6AU6 going to pin 6 of second half of 12AX7 on original schematic
as it was missing from appended cct.
          Is it now ready to plug directly into the wall ?
         

Hello,
      I used a light bulb limiter with 60 watt bulb. Light came on bright then went to nothing. Second try light came on dim also went to nothing. I unhooked the speaker changed to 25 watt bulb now I have maybe half the
output of the bulb constant glow.
      I checked the voltage on the 6V6's it was 95v with light bulb. Is it ready to plug directly into the wall ? Or should I take more readings first ?

Hello,
      I spent most of the day scratching my head and soldering. I am almost done. I have to make another hole in the front panel for the gain control. That is going to regulate the value of the 25uf bypass cap correct ? I
removed the pot for the treble control (original schematic) was stacked on top of an on and off switch. I just
removed it since that treble circuit was going to go anyway, and replaced the rotary on and off switch with a toggle switch
      Now I have the on and off toggle, bass, treble, and master volume. That makes it cleaner and the pots are the proper value. When I get the other hole for the gain everything will be arranged in better order.
      OK as soon as I complete all of this tomorrow I will do the readings. I thought I was going to get it all done today (Ha Ha Ha) !!!!
       Thank you for all your help so far. I could have never imagined doing all of this on my own. I didn't know
where to start or what to do.
                                                             

Hello,
      OK, I'm on it will post results later today.

Hello,
      Yes Roly, I will post those readings as soon as I finish cobbling it together. I should have it done sometime
tomorrow. I will also give you the readings of the 6AU6.
       Should the guitar be plugged in, or will that matter ?
                                                                                                                           

Hello Roly,
      I have spent a lot of time pondering over the diagram you posted. I do not know how to check all of the voltage measurements of supplies, warm idle, or normal program drive. I intend to incorporate all of the changes to the left of R6.
      I have one question at this point.  Coming off the plate of the 6AU6, (on the original schematic) there is a resistor immediately to the right, a 560K that ties the 6AU6 plate to the plate of the second half of the 12AX7. Is that connection to be eliminated ? As on the drawing you posted it is not there. I want to be sure before I start tearing things out. I will have both schematics and make notations as I go along. So in the
worst case scenario I will know how things were before I started
      Thank you for the effort you expended in creating the drawing. Also in case no one else told you yet,
                                                            Happy New Year !!!!!

       

Hello,
      I hope this works. (GREAT), are we on the same page ? I think I put it in the right position. I had never thought of putting the tone stack in front of the 12AX7, Duh !!!! Maybe that's why they call it a preamp.
      I made one mistake, on the cathode resistor of the 12AX7 I marked it 1.5K, but I know it is 1500 as per the schematic. Thank you for the diagram. I am going to start on it right away.