My Vox Ac15 playing

[rafaelctt]

Os muestro mi ampli sonando. Noto demasiada distorsión y ruido de fondo en el canal clean. Es asi?

Thank you

[sluckey]

Very good looking amp! Vibrato sounds good. Normal channel does not sound right. How does it sound with Brilliance switch off?
 

[rafaelctt]

Thank you
That sound is with the brightness off. (I have to mechanically invert the switch by turning it)
With the brightness ON it sounds too high pitched and lacking in body.

[rafaelctt]

Very good looking amp! Vibrato sounds good. Normal channel does not sound right. How does it sound with Brilliance switch off?

Would there be any solution to make that channel (clean) sound cleaner (less distortion)?

[sluckey]

You just barely turned the normal volume pot up. Does your guitar have active pickups, ie, need batteries?

[rafaelctt]

You just barely turned the normal volume pot up. Does your guitar have active pickups, ie, need batteries?

No
They are passive pickups and I have also tried with other guitars (also passive pickups) and exactly the same thing happens to me.

[sluckey]

How does it sound with the volume turned halfway up?

[pdf64]

Would there be any solution to make that channel (clean) sound cleaner (less distortion)?

Perhaps your amp isn’t working correctly?

[rafaelctt]

How does it sound with the volume turned halfway up?

The camera's microphone compresses. When I turn up the volume it gets louder even though it's not noticeable. It distorts a lot at any volume level.

[sluckey]

Gotta be something wrong. Since the VIBRATO channel sounds OK the problem should be around V2. Change EF86 tube. Verify wiring. Verify component values. Measure V2 voltages. Should not be to difficult to find.

[rafaelctt]

Thank you very much slukey for your interest.
I have checked the wiring over and over again.
I have replaced the EF86 with a new one.
The values ​​are:
pin 6: 95V
pin 1: 119V
pin 3: 2.7V
I don't understand what can go wrong.

[rafaelctt]

I have also made measurements with a signal generator at 1 Khz and 124mV at the input, obtaining these values ​​without appreciable distortion on the oscilloscope.
The two values ​​in brill is with brill acting and without acting.

[sluckey]

V2 circuit is very sick. Your gain is only 4.3. That's very, very weak. I would expect the gain to be more than 50. IOW, if your input is 124mV at pin 9 your output on pin 9 (plate) should be more than 6V.

Verify your input jack is wired correctly. Measure resistance of R13, R14, R15, and R20 and post four readings. Then measure resistance between ground and EVERY tube pin on V2 and post all nine readings.

You do understand that BRILLIANCE is active when the switch is open, ie, not shorting C17?

[rafaelctt]

ok, i will need time but i will do all those measurements and checks. i will write the results. thanks

[rafaelctt]

R15: 213K
R14: 21K5
R20: 972K
R13: 2K18

V2

PIN1:1M1
PIN2:2,6 OHM
PIN3:2K1
PIN4:3 OHM
PIN5:2,8 OHM
PIN6:343 K
PIN7:2,5 OHM
PIN8:2K1
PIN9:32K8

About the BRIGHTNESS, I understand that when passing a signal through a low capacity capacitor, it will preferentially let the highest frequencies pass. Therefore it will work as BRIGHT when sw3 is NOT short-circuited. I already told you that I have to turn the switch to match the serigraphy.

Thank you

[sluckey]

What voltage do you measure from Node C (C3) to ground? What resistance do you measure from Node C (C3) to ground? Reverse your meter leads and measure resistance again.

Show us some pics of your board in the area of the normal channel circuitry. You had some issues with the board earlier. Did you correct that?
 

[rafaelctt]

Tensión en nodo C :317 V
Resistencia en nodo C: 130K



I did the circuit again divided into two zones

[rafaelctt]

I am going to measure and view the entire signal path with an oscilloscope to determine where the distortion is occurring.
Could I do it with an 8 Ohm load instead of speakers without damaging the OT?

[shooter]

I use a dummyload most of the time, works well.  the wattage is important, a 25W R in a 100W amp might not last long

[sluckey]

Yes. However, you have already demonstrated that the problem lies between the input jack and V2 pin 6. Not much there. The tube is the main suspect.

[rafaelctt]

I use a dummyload most of the time, works well.  the wattage is important, a 25W R in a 100W amp might not last long

The amp is 15w and the load is 8 ohms 25w

[PRR]

> the load is 8 ohms 25w

That type resistor needs additional heatsinking to dissipate 25W, and maybe for 15W.

For short-term, let it set naked on a fire-resistant bench and smear a little meat on top. When it smells like dinner, take a break to cool down.

For long-term runs a paint-can full of oil will dissipate a lot more power all night.

[rafaelctt]

V2 circuit is very sick. Your gain is only 4.3. That's very, very weak. I would expect the gain to be more than 50. IOW, if your input is 124mV at pin 9 your output on pin 9 (plate) should be more than 6V.

Does the measurement to obtain 6 volts have to be done with the oscilloscope in AC or AC+DC?

[shooter]

when measuring an audio signal, use AC coupling

[rafaelctt]

I have bought a new EF86 and all the components that surround it.
I have built a simulator and made measurements.


The amplification is correct when the power tubes (2xEl84) are not inserted.
I give you measurements:
POINT C 345 V WITH POWER LAMPS INSERTED
AMPLIFICATION EF86 1.3 V with INPUT 90mV

POINT C 427 V WITH POWER LAMPS NOT INSERTED
AMPLIFICATION EF86 3.5 V with 90mV INPUT

POINT C 430 V WITH POWER LAMPS NOT INSERTED (and disconnected C12 to Brightness)
AMPLIFICATION EF86 4.7 V with 90mV INPUT

[sluckey]

There should be an 8µF filter cap connected to the junction of that 22K, 1M, and 220K resistors.

[rafaelctt]

that resistance is set to about 22k with 8 mF from point C. The idea is to lower the voltage a bit.

[sluckey]

There should be an 8µF filter cap connected to the junction of that 22K, 1M, and 220K resistors.

[rafaelctt]

Playing ok

[sluckey]

So, what was the problem with the EF86 channel?

[rafaelctt]

I bought a new Ef86 and it still sounded the same. I made an external simulator and the measurements were the same. So I decided to modify the output impedance just after the brightness SW. I connected a 47k resistor to ground and I got my goal: minus background noise and be able to vary the signature volume more proportionately.

[sluckey]

very strange!

[rafaelctt]

With these two modifications I have achieved exactly what I was looking for. I know it seems strange but there are many tests, measurements and changes that I have made to reach this conclusion. The sound is that of the video. I think it is fine.


There is an error in the image: the resistor is 47k