Vox Ac 15 vibrato problem

[rafaelctt]

Hello!! I hope I don't bother you with my questions. Thanks in advance. I have built a Vox AC 15 from the year 1960 and I am facing the following problem. The clean channel works perfectly but the vibrato channel has a very low signal and also does not modulate. I have made different measurements and the result is that of the attached image. Could you help me understand it? Thank you

[sluckey]

I drew that schematic. Did you also use my board layout?

The footswitch contacts must be closed to enable the oscillator. IE, the bottom of R68 and R65 must be connected to ground.

The normal channel is noticeably louder than the vibrato channel. That's just the way it is.

[rafaelctt]

Thanks for answering. I am sure that the clean channel must sound much louder, but the vibrato with maximum volume, is heard very low and does not have a vibrato effect.

[trobbins]

Did the amp work properly before, and this problem just happened?

[rafaelctt]

No, I am finishing building this amplifier, and I am in the testing stage.

[rafaelctt]

The vibrato sounds already!! The problem is that I have to turn the volume to maximum and it sounds very low and with a lot of distortion. What has me concerned is the 32V voltage at pin 1 of V4 (Ecc82). According to the colleague who made this version, there should be the same voltage on pin 1 and pin 6 of V4. I attach an image. In blue my measurements and in red those of the previous one.
Thank you

[pdf64]

Something seems fishy, I suggest to verify the values of R35,36,37.

[rafaelctt]

I just measured the values ​​of the three resistors R35,R36 and R37 and they are correct.

With SW4 open, the voltage on pin 1 of V4 goes up about 8 V and it sounds louder.

[trobbins]

Have you swapped tubes?  Apart from the obvious anode voltage difference on V4, have you confirmed appropriate LFO AC voltage (measured from a scope, as a meter is unlikely to give an accurate rms voltage) on top of R45, and also top of R32 and R33?

[sluckey]

Let's see a hi-rez pic that shows V1 and V4 and the section of the board that contains the modulator components. Pic needs to clearly show components so we can read values. We also need to see all the components between V1 and V4.

[rafaelctt]

Have you swapped tubes?  Apart from the obvious anode voltage difference on V4, have you confirmed appropriate LFO AC voltage (measured from a scope, as a meter is unlikely to give an accurate rms voltage) on top of R45, and also top of R32 and R33?

At this moment I don't have another Ecc82, only Ecc83 from other parts of the circuit.

The oscilloscope voltages that I obtain are: 2.90 rms in R32, 3.02 rms in R33 and 10.3 rms in R45. These measurements are with SPEED at halfway and DEPTH at maximum

[rafaelctt]

Let's see a hi-rez pic that shows V1 and V4 and the section of the board that contains the modulator components. Pic needs to clearly show components so we can read values. We also need to see all the components between V1 and V4.

<a href='https://postimg.cc/tYHtPK91' target='_blank'><img src='https://i.postimg.cc/VLJGywTF/medidas-tensiones-vibrato-para-foro-2.jpg' border='0' alt='medidas-tensiones-vibrato-para-foro-2'/>[/url]

[sluckey]

Let's see a hi-rez pic that shows V1 and V4 and the section of the board that contains the modulator components. Pic needs to clearly show components so we can read values. We also need to see all the components between V1 and V4.

This means post a PHOTOGRAPH of the inside of your chassis, not the schematic.

[rafaelctt]

[PRR]

If it is not the plates, and common cathode, then look at the grids.

Resistance from pins 2 and 7 to ground? Examine all solder joints closely.

[rafaelctt]

Are you referring to V4?


I measure 1.3M resistance

[sluckey]

What dcv do you measure on pin 2 and pin 7 of V4?

[rafaelctt]

3,45 V  1,78V

[sluckey]

Pull V6 to kill the oscillator. Now what dcv do you measure on pin 2 and pin 7 of V4?

[rafaelctt]

Do you mean extract  v6 from the socket or just disable sw4(open sw)?

[sluckey]

extract

[Latole]

It is the picture or ?

[rafaelctt]

It is the picture or ?

connected to power point D via jumper

[rafaelctt]

extract

Extracted the V6 and the problem of the voltage difference continues

[Latole]

It is the picture or ?

connected to power point D via jumper

Why I did not see a wire ?

[sluckey]

The jumper is under the board. Here's the complete docs, including a board layout...

     http://sluckeyamps.com/VAC15/Vox_AC15.pdf

[Latole]

The jumper is under the board. Here's the complete docs, including a board layout...

     http://sluckeyamps.com/VAC15/Vox_AC15.pdf

Thank you sluckey, now I see.

[sluckey]

Extracted the V6 and the problem of the voltage difference continues

OK, so just to make sure you know where I'm headed... That 3.45V on pin 2 is causing the voltage on pin 1 to be low. I'm trying to find the source of that 3.45V. I'm gonna do this one step at a time so we can stay on the same page, so don't get ahead of me. Leave V6 tube out of the socket. Now unsolder and lift one end of C24. What is the voltage on pin 2 now?

[rafaelctt]

Sorry, I don't see that 3.45V on the schematic. Right now I'm not near the circuit, I'm working. I will do it as soon as possible and we keep in touch. Thank you

[sluckey]

Sorry, I don't see that 3.45V on the schematic. Right now I'm not near the circuit, I'm working. I will do it as soon as possible and we keep in touch. Thank you

The 3.45V ain't on the schematic! YOU measured that 3.45V on pin 2 yesterday. THAT'S A PROBLEM. Do as I say and we will soon find out why that 3.45V is on pin 2.

[rafaelctt]

ok, i will do it as soon as possible. Thank you

[rafaelctt]

pin2     4V

[sluckey]

pin2     4V

So, with V6 extracted and C24 disconnected you still have 4V on pin 2? Extract V4. If pin 2 now reads 0V replace V4 with a good 12AU7. If you don't have a 12AU7 then use a 12AX7 while you are waiting for a 12AU7 to arrive.

[rafaelctt]

Ok, so I will do it, in fact I have prepared an Ecc83 to be able to put it provisionally. What measurements should I do when I insert it?

[sluckey]

Answer this question. Does V4 pin 2 read zero volts when V4 is extracted?

[rafaelctt]

I'll measure it and tell you

[rafaelctt]

Answer this question. Does V4 pin 2 read zero volts when V4 is extracted?

yes, 0 volts

[sluckey]

If pin 2 now reads 0V replace V4 with a good 12AU7. If you don't have a 12AU7 then use a 12AX7 while you are waiting for a 12AU7 to arrive.

[rafaelctt]

In agreement.
What measures do you recommend me to measure once replaced?

[sluckey]

Measure V4 pins 1, 2, 3 and 6, 7, 8. Is the sound better?

[rafaelctt]

I'll do it when I get home and write down the result here.
Thank you very much

[rafaelctt]

1   :60v
2   :2v
3   :3v
6   :60v
7   :2v
8   :3v

[sluckey]

Looks like the voltage readings are good now. How does it sound?

Are you using a 12AU7 for V4?

[rafaelctt]

I am using 12AX7 and I have measured again and on pin1 there is 95v and on pin6 110v. It is NOT stable and varies constantly. The vibrato channel sound is still weak and distorted.

[sluckey]

I am using 12AX7 and I have measured again and on pin1 there is 95v and on pin6 110v. It is NOT stable and varies constantly. The vibrato channel sound is still weak and distorted.

That's quite different from what you posted yesterday.

[rafaelctt]

Yes, it is very different. The supply voltage was very low (about 280 V) so I secured the valves and now the voltage is 315 V (normal) and with that value the measurements are the ones I have set. In addition, they do not remain at a fixed value but vary a few volts up and down.

[rafaelctt]

The clean channel sounds great, although there are clicks

[pdf64]

Looks like the voltage readings are good now. How does it sound?

Are you using a 12AU7 for V4?

I’m stuck getting my head around how 2V on the grids is ok? They look to be referenced to 0V common to me, with no other obvious DC source there to pull them above that.

[rafaelctt]

Looks like the voltage readings are good now. How does it sound?

Are you using a 12AU7 for V4?

I’m stuck getting my head around how 2V on the grids is ok? They look to be referenced to 0V common to me, with no other obvious DC source there to pull them above that.

I have simulated the entire amplifier and if I get voltages that oscillate around 2V

[pdf64]

Are you sure that’s a VDC (average) measurement, eg rather than instantaneous?
One grid looks to be -2V, the other +2V.