Trying to build from scratch normal channel AB763 (twin style)

[tedsorvino1]

Hi there all you kind people.
I have a twin replica which only had a vibrato channel and a useless parametric eq. After throwing away the parametric eq. and changing the whole vibrato channel and the rest of the amp to blackface AB763 specs with really good results (partially using the help of the forum), I decided to build from scratch a normal channel.
There are the appropriate sockets on the chassis, for tube, pots, jacks, switch and enough space for the few components that make the normal channel.
I'm using:
2 x 022uF and 1 x 250pf for bass, mid, treb caps.
1x 56k for slope resistor.
2 x 100k for plate caps
1x 25uf + 1x 1.5k for preamp cathode resistor & cap.
1x  022uF coupling capacitor. 
All pretty much similar to the ones on the vibrato channel part.
I'm just trying to build it with a slight tonal twist, for possible further "marshallesque" moding.

I'm taking about 450V DC from point D (around the reverb recovery plate resistors area) with a jumper lead to the preamp plate resistors, and it gives me about 280 V on either tube plates (pin 1 & 6).
 I also send a jumper lead from the 022uf coupling capacitor to the 220k mixing resistor (next to the other mixing resistor where the vibrato channel coupling cap is soldered, around the P.I. area).
The pots I'm using are the appropriate values and in good condition.
The result is a very faint sound. I've measured all the voltages on every tube 1 pin and everything seems correct and similar to the ones of tube 2. The 12AX7 tube I'm using is in excellent condition and tested.

Any ideas where things could have gone wrong?

[SILVERGUN]

Try to mock up a schematic snip so we can see exactly whats going on.
I tried to follow along just by your description, but there's just not enough time in life and I had to move on.
We really do want to help, we just need you to help us help you.

[tedsorvino1]

Here's a little layout of the channel, including my voltages and component values

https://imgur.com/a/nO0Rj

[SILVERGUN]

Pulled it over for anyone else who might play along...doesn't look bad

[SILVERGUN]

You'll have to have the tone controls turned up to get a decent signal

[tedsorvino1]

Thanks Silvergun. But I've already done it.
There is some proper reaction on the tone controls (tonewise) but the signal remains faint.
And I have to admit that the volume pot doesn't react at all. It remains really quiet (and in a way it reacts in strange way, when I turn towards 10 it becomes treble and then dies). I 've already replaced the pot.

[SILVERGUN]

I'd still love to be looking at the whole picture from a schematic standpoint, but I'd have to try taking "X" and jumping it right into the cap feeding the PI

[sluckey]

Your schematic shows 180v on pin 1 but you said you have 280v on both plates. Which is right?

Where does pin 8 connect?

Measure filament voltage like this... one probe on pin 9, the other probe on pin 4/5. Be sure there is a jumper between pins 4 and 5. What have you?

The most likely problem is a wiring error or incorrect component value. Input jacks is a very common point for a wiring error to hide. Don't think about a Marshall sound until the Fender sound comes roaring through. You do have it wired like the schematic you posted, right?

[tedsorvino1]

Hello Sluckey. You're right.
I've made a measurement mistake. The voltage is 180V on pin1. Is it ok (combined with a 56 k slope resistor...)?
Pin 8 is connected to pin 8 of tube 2
Filament measurement this way is 6.7VAC. My pin 4/5 is jumpered properly.
I suspected the input jacks because of the resistors but...i did the connections very carefully.
I did the wiring according to this schematic ( I haven't soldered the bright switch yet)

[sluckey]

The voltage on pin 1 should be about the same as the voltage on pin 6. I would find out why they are so different. The 56K slope resistor has nothing to do with the idle dc voltages on the tube.

I would disconnect the 56K and the 250pF (This leaves only a 100K connected to pin 1). Is the pin 1 voltage now about the same as the pin 6 voltage?

[tedsorvino1]

I disconnected the slope resistor and the treble cap and the values raised to 187V (not that different. I disconnected the wire from pin 8 and the voltage was 410V on the wire end. Pin 6 remained 270V throughout the test.

[SILVERGUN]

I disconnected the wire from pin 8 and the voltage was 410V on the wire end.

Can you clarify?

[sluckey]

... I disconnected the wire from pin 8 and the voltage was 410V on the wire end. Pin 6 remained 270V throughout the test.

Really! That's impossible if the other end of that wire is connected to V2 pin 8 as you said.

Connect the cap, resistor, and wire back like they were. Keep looking for a wiring error or wrong component value.

[tedsorvino1]

Sorry about that....My mistake. I mean pin1.

[tedsorvino1]

After "playing" with the 1M input jack resistor (always with a stick), the voltage on pin1 raised to 250V. But still the sound is faint. I also change the slope resistor back to 100k.

[sluckey]

Time to see your working schematic and some hi-rez pics.

[tedsorvino1]

Thanks Sluckey but I don't know if my images would be any helpful since it's an old printed board and I've managed to avoid any bridging. But it's really hard to follow from an image.

Here's an image of the tone caps and the 100k slope resistor

https://imgur.com/a/FR8zW

Another image of the tube pins

https://imgur.com/a/UZwjO

and an image of the 100k plate resistors and 047 coupling capacitor (the black jumper wire goes to the 220k resistor and the red gives 400VDC)

https://imgur.com/a/N4MjN

Is it ok if I resend the layout I've sent before (with corrected voltages), since it's the only one I follow?

What measurements should I have on pin 2 and pin 7?

[tedsorvino1]

 I think the main problem is the circuit board. I made some point to point soldering and the sound came through (ok....there's no real control yet...but I guess I 've found where the main problem starts).
Is there a way to cover tha circuit board with something and solder there?

[tedsorvino1]

Sorted. Thanks for the help Sluckey. I guess I will have to buy a short breadboard or something to cover the circuit board.

[sluckey]

You're right, those pics don't help, except to make me realize how easy it would be to make an error. I see no similarity between your partial layout and what you actually have. My biggest advice would be to clean that board with denatured alcohol.

It's a simple circuit. I gotta fall back on Hoffman's law... "If it were wired correctly it would be working." Just keep looking. Something ain't right yet.

Sorted. Thanks for the help Sluckey. I guess I will have to buy a short breadboard or something to cover the circuit board.

Does that mean you found the gremlin and the amp now works properly?

[tedsorvino1]

Yes Sluckey. Thanks for the help once again.  It was some kind of not so obvious bridging on my point X (where I was soldering the jumper from the mixing resistor). Now everything is working as it should (I guess...). At least the levels are similar to the other channel (I think it's slightly lower. I mean I'm trying it while I'm using an attenuator and I attenuate the hell out of it.If I go beyond 4 without it.I need to go out of the building.It's 100Watts...)
Is it ok that my tube 1 pin plates have symmetrically lower voltages to the ones on tube 2? Tube 1: Pin 1 250VDC, Pin 6 270VDC                     Tube 2:   Pin 1 265VDC, Pin 6 285VDC
Is there any nominal voltage or current values for Pin 2 and Pin7?

[sluckey]

Those voltages look fine now. I was concerned back when you only had 180v on one pin.

Pins 2 and 7 on V1 and V2 should be zero volts.

[tedsorvino1]

Ok with the 0V. But just out of interest, how can we figure out if there is a problem on these two pins (pin 2 and 7). Is there something in particular we 're looking for? Anything to measure on the volume pot and input jacks?

[sluckey]

Zero is a valid number. You will have a problem if you have something other than zero volts on pin 2 or 7. This applies to your V1 and V2. Other circuits such as a long tail phase inverter may need a different voltage.

The actual voltage on pin 2 (grid) is not so important. The important voltage is the voltage between pin 2 and pin 3 (cathode). This is the bias voltage for the tube. For the particular circuit you have for V1 this bias voltage should be approx. 1 to 2 volts and the grid must be negative with respect to the cathode.

[tedsorvino1]

Thanks for the info Sluckey.
How can we measure  the voltage between pin 2 and pin 3 (cathode) in order to find the results you mention?
I can't really understand how can I get these results.
Where do I place the probe?
I do all my measurements with the black probe grounted on the chassis.

[tedsorvino1]

Another quick question since we are here.
 Is there a way to "swap" plate voltage values between Tube 1 and Tube 2? I think Tube 2 is louder than Tube 1. And the voltages on its plates are by 15V louder.

[sluckey]

How can we measure  the voltage between pin 2 and pin 3 (cathode) in order to find the results you mention?

I bet if you think about it a few seconds the answer will come to you. Hint... the answer is in the question. 

Is there a way to "swap" plate voltage values between Tube 1 and Tube 2? I think Tube 2 is louder than Tube 1. And the voltages on its plates are by 15V louder.

Move the tubes around. V1 and V2 circuits are identical and the voltages should be identical.

But, the difference you think you hear in volume is very likely due to the fact that the reverb channel has more gain stages than the normal channel, so the comparison is really not fair.

[tedsorvino1]

Thanks man. I know what you mean. Everything seems clear. I think you 're right about the reverb channel gain stages. More 12AX7 tubes involved in this channel.
It's not a matter of moving tubes since they re more or less the same. I would just like to have 15 more Volts on tube 1 plates and 15 less on tube 2.plates. And I guess the whole thing happens on pin 1 and 6 of each tube. Now I have them the other way around and I wouldn't like to resolder all pins. Is there any other way.

[shooter]

I would just like to have 15 more Volts

run the amp for 60 -100hrs then re-check