It's been a while since I started working on it because the part's took a long time to get here but I have it working now. It plays good but the volume is really low with the guitar plugged in. But it is loud unplugged with the volt meter probe when you touch it on the input positives. And you can hear the volume and tone change when moving the pots. I had to change the plate resistors from 100k to 27k to get the Voltages right on the first 12AX7 tube. Here's what I have for voltages On the two 6l6 GCs 431/435 Plates and 334/335 screen's, 59/60 ma's, -25/-25 G1 Bias. Tube #4 is A 12AT7 Phase Inverter 141/144 plates, 9 volts on Cathodes, two 40K res. Tube #3 is 1/2 of A 12AX7 334 plate,155 grid and 155 Cathode, Tube #2 is 1/2 of A 12AX7 154 plate. Tube #1 is A 12AX7 154/154 Plates. The grid's Go to double 68k resistors with four input jacks, The Fender Bassman design with two separate volumes. I have been trying to figure out why the volume is loud when I touch the input jacks with my volt meter and very quiet when I plug in my guitar. I tried to remove the two 1 Meg resistors on the input jacks but it was the same.
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If you had to change the plate resistor to 1/3 to get the voltages right in an otherwise proven circuit, something more fundamental must be wrong. Wiring error, solder joint, cathode resistor, grid leak resistor ... etc ... etc ...
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Analog Gary,
Welcome to Tubes / Valves, the threads that deal with HI Fi Stereo, other vacuum tube products, and . . . Guitar Amps.
There are also the 'Instruments and Amps' Threads, where some may be watching who have knowledge of the Fender Bassman circuit.
I am sorry if you saw the unkind word that I first wrote for the word 'watching' above.
I agree with others, A Complete and Accurate Schematic is needed.
It is worth 1000 words, can also cut the number of posts required to solve the problem(s).
Welcome to Tubes / Valves, the threads that deal with HI Fi Stereo, other vacuum tube products, and . . . Guitar Amps.
There are also the 'Instruments and Amps' Threads, where some may be watching who have knowledge of the Fender Bassman circuit.
I am sorry if you saw the unkind word that I first wrote for the word 'watching' above.
I agree with others, A Complete and Accurate Schematic is needed.
It is worth 1000 words, can also cut the number of posts required to solve the problem(s).
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Make sure you have correct grounding. The grounds on the input and outputs must be connected.
Double check the Cathode resistors on the preamp tubes. Too high, no signal gets thru.
Ya, let us see the schematic, and verify all parts match the schematic.
w
Double check the Cathode resistors on the preamp tubes. Too high, no signal gets thru.
Ya, let us see the schematic, and verify all parts match the schematic.
w
Let me go back over everything again and make sure I didn't make a mistake with something. I found a Dead Brand New 1 Meg volume pot on A half of the first 12AX7 tube. I need to check everything again for the New part's and old part's integrity. It seems like it definitely could be a grounding issue or tone stack issue. When I touch it on the inputs with my fluke meter on voltage I can hear it loud through the speaker but it goes almost quiet when I plug in the guitar. I also noticed that Bass is louder than the mids and highs and I can hear a slight intenuence almost like a tremolo.
It's chassis grounds for the most part. I tried putting 1 Meg grid resistors on the inputs and on the grid's before the 68k resistors and it doesn't seem to make any difference. But if I touch the grid on the second tube coming out to the first volume pot before the resistor it is very loud. It seems like something is blocking the first tube from being loud. I just checked the resistor and I Read 394 K and I don't know if I got resistors that are marked wrong or not. That seems pretty high going to the middle of the volume pot from the grid on a 12AX7 tube. Both volume pot's come off of the same place on the grid of 1/2 of the second 12AX7. I would call it stage 3 grid.
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I'm going to try to make a wiring diagram for it because I have mixed up a few designs. Some is Fender Bassman, and Some is Marshall 50 and Sunn Model T 100 and Sovtek 50. I tried to use what I thought would sound good together and play good together. And The basic output section and negative bias is Whirlizer Organ. I need to draw it out on paper to make things easier to understand.
I think that I used The Blue's DeVille design for the pre Amp section. But I didn't use the 25k mid pot and subbed in A 15k resistor to ground instead.
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I changed it to the blue's DeVille design last night for the Preamp section. And the spitter is A Sovtek 50. I eliminated the mid pot and used a 15k resistor to make it fixed and it goes to chassis ground. The third Preamp tube is not really doing anything but glowing and A hookup point. And only 1/2 of the second tube is hooked up. 3 - 12AX7 Half sections going into a 12AT7 splitter. Volume is working but I only get humming and when I touch the input jacks with the guitar chord plugged into the guitar but not grounded it is very loud. I get nothing with the guitar plugged in grounded.
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The plate voltages Are as follows #1 The first half of a 12AX7 =131 , second half is 197 , Second tube first half is 141 vdc. The first 3 stages have 100k plate resistors on the power side.
The splitter is a 12AT7 and the plate voltages Are 132 and 125 and The resistors Are 47 k each. The splitter wiring is A Sovtek 50 design. The third Preamp tube is not doing anything.
The splitter is a 12AT7 and the plate voltages Are 132 and 125 and The resistors Are 47 k each. The splitter wiring is A Sovtek 50 design. The third Preamp tube is not doing anything.
I got it working now. I added A Grid #2 stopper 1 Meg and A ground wire from the test speaker to the chassis because I have not wired it into the chassis grounded speaker Jack yet. I'm using A separate test speaker. It's Playing very loud now At very low volume. It looks like I need different input jacks with the extra ground on them to keep it from squeeling when I unplug the guitar. It's loud now but pretty dirty, the tone is good and I just need to tame it down some. It seems to play pretty well with A good feel to it. It's definitely powerful. It's only clean enough to play with the volume down low. Turn it up and it distorts a lot. I Used The Sovtek 50 phase inverter wiring and I have a negative feedback connection that I have that is not hooked up, I was thinking maybe that would help to calm it down some ? . It's A 12AT7 phase inverter. The 12AX7 pre Amp tubes seem to be within spec's on the tube data sheets but I really wonder if they are at the best place or not ? . I have 473 Caps on the output tubes and A 223 coming out of the phase inverter and 223's for the rest of the Preamp section other than one 473 on the Bass pot. I've built a lot of desktop stuff from scratch in the past but this is my first guitar 6l6 built from scratch. I'm used to using lower gain tubes in the Preamp section. Most of it was built from some really Old stuff. Even This one is with 1960 part's. I guess that I will start studying the different Amplifier designs using the same tubes and see what they have done differently than The Blue's DeVille design. And read up on Negative Feedback and see what people say about that.
Your second triode, pin 7 grid only has a capacitor connected to it. Post # 16
No DC return resistor from Pin 7 to ground?
Without a DC return resistor, that is an Un-Defined state for a triode tube!
No DC return resistor from Pin 7 to ground?
Without a DC return resistor, that is an Un-Defined state for a triode tube!
Is that the one on The grid going to the tone pot ? I thought that the pot is acting like a stopper resistor for the grid, even if it is only 250k plus the 30k of resistors. Do I need to add a 1 Meg on the grid there also ? . I was getting ready to pick a Negative Feedback Resistor to it to tame it down some. It has nothing Now but I did set it up for one during the build. I'm just not sure what to Use. I have a Sovtek 50 phase inverter with a 12AT7 tube and a Blue's DeVille designed Preamp. It's very loud now on 1 but it's dirty distorted sounding. You can play it there but not louder than that. I was thinking of changing the cathode resistance to lower the gain down some on the 12ax7 but I thought that I should try hooking up the negative feedback first before I do that.