VHT Special 6 out of volume

[mhuss]

Suggest you download and read (many times ) Jack Darr's classic book "Repair your guitar amplifier" where the introduction is available as a .pdf at some sites.
Of course, if you like it try to get the full printed book (often available at Amazon and other online bookstores) .
VERY clear and to the point explanations.

+1

FWIW I started with it 45 years ago

well, maybe 40 in my case.

[wole]

Mick suggested tracing back >in #24 here<

This is perfectly sound approach, especially as you can compare can test results from one amp to the other.

Put in sine wave at the input, connect the black lead to ground as before but this time you put the meter on ACV probe the signal path. There's a gotcha tho'. Some meters don't accurately report AC if there is DC present so avoid probing the anodes as they carry a high DC voltage.

Edit: The red dots indicate places to probe. Also you never told us if the controls work normally. I'm particularly interested in the tone control in case it has an open track at the top which would cause volume and HF loss.

[ATTACH=CONFIG]37811[/ATTACH]

Ok I finally received the new tester and did the AC tension measures!
I used the sine wave at 1000Hz generated by this site Online Tone Generator - Free, Simple and Easy to Use.
I also used a tester with a +-2mV accuracy
The amp was in High power mode and the signal was in the high input
V1A pin2 : 23mV
V1B pin7 : 23mV
V2 pin5 : 25.5mV
Point you indicated near R16: 23.5mV
Point you indicated near R10 and R12: 22.6mV
Point you indicated near R18 and C14: 24mV (the tension was varying moving the volume knob, this value was measured with volume at 1/4 power)
Point you indicated "ORG_16ohm": 25mV (the tension was varying moving the volume knob, this value was measured with volume at 1/4 power)

By the way I also tried to vary the tone knob while playing and it was working fine in my opinion: no dead zones

[nickb]

Ok I finally received the new tester and did the AC tension measures!
I used the sine wave at 1000Hz generated by this site Online Tone Generator - Free, Simple and Easy to Use.
I also used a tester with a +-2mV accuracy
The amp was in High power mode and the signal was in the high input
V1A pin2 : 23mV
V1B pin7 : 23mV
V2 pin5 : 25.5mV
Point you indicated near R16: 23.5mV
Point you indicated near R10 and R12: 22.6mV
Point you indicated near R18 and C14: 24mV (the tension was varying moving the volume knob, this value was measured with volume at 1/4 power)
Point you indicated "ORG_16ohm": 25mV (the tension was varying moving the volume knob, this value was measured with volume at 1/4 power)

By the way I also tried to vary the tone knob while playing and it was working fine in my opinion: no dead zones

I think maybe you need to switch your meter to ACV from DCV and try again

[wole]

I think maybe you need to switch your meter to ACV from DCV and try again

wow I can't believe I got it wrong again Thank you again for your patience. I'm quite sure I turned the meter on ACV with scale in millivolts so maybe I connected something wrong.. Tomorrow I will redo the measures, but just to be sure: which values should I expect?

[nickb]

wow I can't believe I got it wrong again Thank you again for your patience. I'm quite sure I turned the meter on ACV with scale in millivolts so maybe I connected something wrong.. Tomorrow I will redo the measures, but just to be sure: which values should I expect?

Set the input (V1-2) to read 100mV. I would expect about 500mV at the top of the volume control. The wiper of that control should vary from zero to the top level, depending on where you have the control set. V1-5 will be about 50x whatever is on the wiper. The 16 ohm output should be getting on for 9-10V before you hear too much distortion.

Just to test if your meter does or doesn't reject DC when used on an ACV range also measure V1 pin 1.

[wole]

Set the input (V1-2) to read 100mV. I would expect about 500mV at the top of the volume control. The wiper of that control should vary from zero to the top level, depending on where you have the control set. V1-5 will be about 50x whatever is on the wiper. The 16 ohm output should be getting on for 9-10V before you hear too much distortion.

Just to test if your meter does or doesn't reject DC when used on an ACV range also measure V1 pin 1.

You were right, I was connecting the probe in the wrong terminal..
I hope this time the measures make more sense:

V1A pin1 : 2.2V
V1A pin2 : 100mV
V1B pin7 : 7mV
V2 pin5 : 336mV
Point you indicated near R16: 7mV
Point you indicated near R10 and R12: 393mV
Point you indicated near R18 and C14: 1090mV
Point you indicated "ORG_16ohm": 309mV

The tension at V1A pin2 was not varying turning the volume knob, it was varying with the intensity of the input signal.
The tensions varying with the volume and tone knob where the one near R18 and C14 and the one on the 16ohm output. The measurements were made with the tone knob at 12o'clock and volume at 9o'clock in low power mode

[nickb]

You were right, I was connecting the probe in the wrong terminal..
I hope this time the measures make more sense:

V1A pin1 : 2.2V
V1A pin2 : 100mV
V1B pin7 : 7mV
V2 pin5 : 336mV
Point you indicated near R16: 7mV
Point you indicated near R10 and R12: 393mV
Point you indicated near R18 and C14: 1090mV
Point you indicated "ORG_16ohm": 309mV

The tension at V1A pin2 was not varying turning the volume knob, it was varying with the intensity of the input signal.
The tensions varying with the volume and tone knob where the one near R18 and C14 and the one on the 16ohm output. The measurements were made with the tone knob at 12o'clock and volume at 9o'clock in low power mode

Those look fine. Now, turn up the volume until it just starts to distort and repeat - then we can see how much power you can actually get out. Usually when doing this, people use a dummy load resistor instead of the speaker for hopefully obvious reasons - it's loud!

BTW your meter seems to work in the presence of big DC voltages as 2.2V at V1-1 is reasonable - you can add V2 pin 3 into the list of things to measure.

PS: Don't forget to take extreme care around a live amp.

[wole]

Those look fine. Now, turn up the volume until it just starts to distort and repeat - then we can see how much power you can actually get out. Usually when doing this, people use a dummy load resistor instead of the speaker for hopefully obvious reasons - it's loud!

BTW your meter seems to work in the presence of big DC voltages as 2.2V at V1-1 is reasonable - you can add V2 pin 3 into the list of things to measure.

PS: Don't forget to take extreme care around a live amp.

Good!! Finally I got it right!!
But I don't have a dummy load... Will 4 4ohm resistors in series work? 25watt each is reasonable?
By the way, if I'm using a dummy load, how can I know when it's starting to distort?
Thank you so much nickb!

[nickb]

Good!! Finally I got it right!!
But I don't have a dummy load... Will 4 4ohm resistors in series work? 25watt each is reasonable?
By the way, if I'm using a dummy load, how can I know when it's starting to distort?
Thank you so much nickb!

Yes- well done

Doing this by ear is very rough. Connect the speaker and have it low enough so it's not distorted - get used to that nice pure sound. Now increase the volume and you'll hear the harmonics (octaves) become audible. At some point they will increase very rapidly - turn back down just a little and leave the volume there. Switch off and substitute your two 4 ohm resistors in series to the 8 ohms jack and do the measurements. The proper way is to use a scope but we don't have one.

[wole]

Yes- well done

Doing this by ear is very rough. Connect the speaker and have it low enough so it's not distorted - get used to that nice pure sound. Now increase the volume and you'll hear the harmonics (octaves) become audible. At some point they will increase very rapidly - turn back down just a little and leave the volume there. Switch off and substitute your two 4 ohm resistors in series to the 8 ohms jack and do the measurements. The proper way is to use a scope but we don't have one.

thank you so much. I'll have to buy the resistors, but as soon as I'll get them I'll redo the measurements

[nickb]

thank you so much. I'll have to buy the resistors, but as soon as I'll get them I'll redo the measurements

..or you could just put up with the noise for a couple of minutes and use the speaker. Depends on the neighbors...

[wole]

..or you could just put up with the noise for a couple of minutes and use the speaker. Depends on the neighbors...

yes.. maybe.. I don't know if they'll allow that.. I live in a condo with thin walls

[nickb]

yes.. maybe.. I don't know if they'll allow that.. I live in a condo with thin walls

Ah, in that case you need neighbors with think skins...

(Hoping the idiom doesn't get lost in translation here)

[wole]

Ah, in that case you need neighbors with think skins...

(Hoping the idiom doesn't get lost in translation here)

yeah I know what you mean

[g1]

What are "think skins" ?