Vox Ac 15 vibrato problem

[sluckey]

I've also been bothered by the positive voltage on pins 2 and 7. So, I put my amp on the bench, set the variac to exactly 120VAC, and warmed the amp for a few minutes. Here are the voltage readings for V4 on a properly working amp...

pin 1 = 73vdc
pin 2 = 0vdc
pin 3 = 3.3vdc

pin 6 = 74vdc
pin 7 = 0vdc
pin 8 = 3.3vdc

I'm beginning to think your board 'may' be conductive. What material is the turret board? Doesn't look like fiberglass. The board looks pretty dirty in that poor quality pic you posted. The turret column spacing is 1/4". That's pretty tight. A clean, high quality board is important.

What brand and model number is your meter?

[rafaelctt]

I have measured it with a multimeter in VDC. I will measure it with an oscilloscope

[rafaelctt]

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

I have measured it with a multimeter in VDC. I will measure it with an oscilloscope

[PRR]

I have measured it with a multimeter in DC.

No, the question was:

....What brand and model number is your meter?

The meter details matter. Since you may not know the meter's specs, Sluckey wants to help by looking-up the specs of YOUR meter and giving advice accordingly.

If you switch to an oscilloscope, he'll again want to know brand/model. For a short time in the late 20th century, "all 'scopes were the same", but newer ones can be very different. (And some un-suitable for tube amplifier work.)

[rafaelctt]

Fluke model 123

[rafaelctt]

[sluckey]

When you measure 2vdc on pin 2 or pin 7 is it possible that your scopemeter is really displaying 2mV and you are mistakenly interpreting that as 2V?

We've eliminated all the logical paths for 2vdc to get on the grids. Time to look at some illogical paths such as conductive board. Try this. Connect one meter probe to chassis ground. Now use the other probe to poke the board near the turrets that the V4 components are connected to. You should not measure any voltage on the board at any point.

[trobbins]

Or reading AC+DC, rather than just the DC mode of the Fluke 123.

[rafaelctt]

When you measure 2vdc on pin 2 or pin 7 is it possible that your scopemeter is really displaying 2mV and you are mistakenly interpreting that as 2V?

We've eliminated all the logical paths for 2vdc to get on the grids. Time to look at some illogical paths such as conductive board. Try this. Connect one meter probe to chassis ground. Now use the other probe to poke the board near the turrets that the V4 components are connected to. You should not measure any voltage on the board at any point.

Tomorrow I will do the measurements again with an oscilloscope. Should I do it in VDC or VDC+AC? For these measurements, do I disconnect C24 and C23 or is it not necessary?
I understand that I should get 0V on pin2 and pin7.
I have bought all the components that surround V4 and if I don't get those measurements I will simulate it externally. I'll do it with a 12AX7. I imagine I must also get 0V on those pins.

[sluckey]

VDC only. No need to disconnect anything.

[rafaelctt]

VDC only. No need to disconnect anything.

Do I leave the V6 connected? Should I take any other precautions? Does the 12AX7 serve me for these measurements?
Thank you

[sluckey]

Put V6 in the socket. No precautions. 12AX7 is OK but a good 12AU7 would be better.

What material is your board?

[pdf64]

It seems to me that if the grid voltage was due to a conductive board (or leaky coupling caps), it would still be there, perhaps even worse, with the valve in V4 pulled?
As it isn’t (see reply #37), I wonder if there’s a error in implementing the circuit, eg missing link, incorrect value resistor?

[rafaelctt]

Put V6 in the socket. No precautions. 12AX7 is OK but a good 12AU7 would be better.

What material is your board?

This is the only one I found in Spain that would fit the size. The truth is that I did not like it from the beginning.
https://www.retroamplis.com/epages/62070367.sf/sec0beea035dd/?ObjectPath=/Shops/62070367/Products/BO-PHP2-200X400

[rafaelctt]

I have managed to get it to work at 90%, the vibrato channel already works well, enough volume and without distortion but it has background noise. I have to keep reviewing. It is a problem of lack of insulation at different points on the board. Little by little I am solving them. I have too much input gain on the channels, which gives me distortion in the first stage.

[rafaelctt]

The problem I have now is the voltage of pin1 of v6. My blood pressure is too low. I have removed the V6 and the tension is correct, but when I put the valve on, the tension drops too much. do you know what it could be?

[sluckey]

Read note 1.

[rafaelctt]

Yes Sluckey, I have R65 and R68 grounded. I just checked again.

[sluckey]

Yes Sluckey, I have R65 and R68 grounded. I just checked again.

NOTE 1: Oscillator voltages taken with oscillator disabled.

By grounding those resistors you have ENABLED the oscillator!

[rafaelctt]

With the oscillator disabled (r65 and r68 not grounded) the voltage is even lower at pin1. I just measured 71V

[rafaelctt]

The oscillator does not oscillate. I measure it with the oscilloscope and find no oscillation. n the simulator (Proteus) if it oscillates and I have the same component values

[sluckey]

You told me in a PM that you measured 30V on the board, meaning the board is conductive. You are wasting your time doing any further checks. Replace the board with a high quality G-10 fiberglass board and start over.

I understand and sympathize with you since Hoffman won't sell you the board due to the crazy VAT laws. You probably would not pay the price even if Hoffman would sell to you. So, you will have to make your own board again with a better quality board. I know you said you could not find a blank board that's long enough, but you still have options. The board is laid out such that there are several logical places where you could split the board and use two shorter boards. Or, you could cut your losses and use the parts to build another amp that uses a shorter board, such as a Marshall 18W or my Dual Lite.
 

[rafaelctt]

Tomorrow I'll start building the circuit around the V6 on a separate board as a prototype. I'll go step by step. The clean circuit works very well and so does the vibrato preamp stage. So I will focus on the V6. Thank you very much. I will inform you.

[pdf64]

Tubetown sell nice boards, 0.5m lengths!

[rafaelctt]

Hello!! I have already built a simulation of the circuit around V6(oscillator). How can I check with the oscilloscope that it is oscillating? At what frequency and with what voltage does it oscillate and on which pin can I measure it?
Thank you

[sluckey]

The oscillator signal is a big sinewave. Frequency is between 3 and 10Hz, depending on the speed pot setting. I suggest setting the speed pot about midrange and set the intensity pot to zero.

Connect your scope to the ungrounded lug of the intensity pot to view the waveform.

Or you may be interested in the LED indicator mod I did to my AC-15. The LED replaces R55 and the anode connects to pin 8 of V6. The LED blinks at the speed of the oscillator and will be steady if the oscillator is disabled or the intensity pot is set to zero. My LED is mounted on the front panel. Look here if interested...
 
        http://sluckeyamps.com/VAC15/Vox_AC-15.1.pdf

[sluckey]

Another way to tell if the oscillator is working is to measure the DCV on V6 pin 1. If the voltage is steady, it ain't working. If the voltage is constantly changing or erratic it is working. The bar graph on your meter should be moving at the speed of the tremolo.

[rafaelctt]

The oscillator signal is a big sinewave. Frequency is between 3 and 10Hz, depending on the speed pot setting. I suggest setting the speed pot about midrange and set the intensity pot to zero.

Connect your scope to the ungrounded lug of the intensity pot to view the waveform.

Or you may be interested in the LED indicator mod I did to my AC-15. The LED replaces R55 and the anode connects to pin 8 of V6. The LED blinks at the speed of the oscillator and will be steady if the oscillator is disabled or the intensity pot is set to zero. My LED is mounted on the front panel. Look here if interested...
 
        http://sluckeyamps.com/VAC15/Vox_AC-15.1.pdf

I find it fantastic!!
I will also make the led circuit!!
Thank you!

[rafaelctt]

Well, it seems that everything has gone well.
I have built it off the board and it has worked perfectly.
With which I only have to implement it on the circuit.

[sluckey]

Well, it seems that everything has gone well.
I have built it off the board and it has worked perfectly.
With which I only have to implement it on the circuit.

Does this mean you have built the ENTIRE AMP off board? Could we see pictures?

[rafaelctt]

No, I have built the vibrato circuit (V6) off the board. the rest the same

[rafaelctt]

Good morning, a question. I could have a direct output or use the power zone (EL84) with an external jack if I interrupt the central volume contact (R29)?

[pdf64]

That will work as a power amp input.
As drawn, the signal from that preamp will be disabled when a plug is inserted.
Is that how you want it to work?

[sluckey]

You need two jacks if you want a preamp output ***AND*** power amp input available. Notice this does not apply to the VIBRATO channel!

[rafaelctt]

Yes, I had thought of that or a jack and switch to use it as a direct out or power amplifier. It could even be used to daisy chain an external effect. Of course it would only work on the clean channel.

[Eythor]

In case you want to rebuild the board, modulus amps in the UK has boards long enough for this and very affordable shipping. I got this one from him.