VHT Special 6 out of volume

[nickb]

What are "think skins" ?

Where's the stupid EDIT button when you really need it...?

[g1]

Well, if that's not what you meant, then even more surprising the idiom didn't get lost in translation.

[nickb]

Well, if that's not what you meant, then even more surprising the idiom didn't get lost in translation.

I know! The OP must be pretty switched on. Of course, I meant "thick skins" but it's just not funny any more..

[g1]

I was hoping it was something like putting on a thinking cap.

[wole]

ahah yeah I imagined you meant thick

[nickb]

"Thinking cap"? Now I have images of Wole's apartment filled with an army of inquisitive and helpful neighbors all trying to fix his VHT. It's not going to end well.

[wole]

Hi guys! Finally I'm back.. I disappeared and I'm very sorry for that.. I've had some issues which kept me away from the amp.. But finally I did all the measures you asked:

AC Measures: (done in low power mode, because the loss of volume was present in both low and high power mode)

Low Power mode, Tone 12 o'clock, Volume 9 o'clock (1/4 of total volume)
V1A Pin 1: 2.2V
V1A Pin 2: 100mV
V1B Pin 7: 21mV
V2 Pin 5: 1.18V
Pt indicated near R16: 21mV
Pt indicated near R10 and R12: 405mV
Pt indicated near R18 and C14: 1.184V
Pt indicated Org 16ohm: 335mV

Low Power mode, Tone 12 o'clock, Volume 3 o'clock (3/4 of total volume)
V1A Pin 1: 2.2V
V1A Pin 2: 100mV
V1B Pin 7: 150mV
V2 Pin 5: 8.84V
Pt indicated near R16: 150mV
Pt indicated near R10 and R12: 400mV
Pt indicated near R18 and C14: 8.85V
Pt indicated Org 16ohm: 2.5V

[nickb]

The whole point of the test was to estimate the max output power and then see all the gains of each stage. That isn't going to happen in low power mode. Right now you are only getting about 0.4W (2.5 * 2.5 /16). For 6 watts out the rms voltage at the 16 ohm output would be 9.8 to give you and idea of what we are looking for.

So, you have to be in high power mode AND you have to turn it up to just short of bad distortion see VHT Special 6 out of volume - Page 2

Also, since you meter does measure AC in the presence of DC add V2 Pin 3 to the list of probe points.

[wole]

The whole point of the test was to estimate the max output power and then see all the gains of each stage. That isn't going to happen in low power mode. Right now you are only getting about 0.4W (2.5 * 2.5 /16). For 6 watts out the rms voltage at the 16 ohm output would be 9.8 to give you and idea of what we are looking for.

So, you have to be in high power mode AND you have to turn it up to just short of bad distortion see VHT Special 6 out of volume - Page 2

Also, since you meter does measure AC in the presence of DC add V2 Pin 3 to the list of probe points.

High Power mode, Tone 12 o'clock, Volume 3 o'clock (3/4 of total volume) (I'm not 100% sure about this, but I think this is the point the amp started to distort)
V1A Pin 1: 2.2V
V1A Pin 2: 100mV
V1B Pin 7: 146mV
V2 Pin 5: 8.13V
V2 Pin3: 233V
Pt indicated near R16: 146mV
Pt indicated near R10 and R12: 393mV
Pt indicated near R18 and C14: 8.13V
Pt indicated Org 16ohm: 9.46V

So, I think my tested doesn't separate ac from dc current..

[nickb]

High Power mode, Tone 12 o'clock, Volume 3 o'clock (3/4 of total volume) (I'm not 100% sure about this, but I think this is the point the amp started to distort)
V1A Pin 1: 2.2V
V1A Pin 2: 100mV
V1B Pin 7: 146mV
V2 Pin 5: 8.13V
V2 Pin3: 233V
Pt indicated near R16: 146mV
Pt indicated near R10 and R12: 393mV
Pt indicated near R18 and C14: 8.13V
Pt indicated Org 16ohm: 9.46V

So, I think my tested doesn't separate ac from dc current..

Super thanks.

Going from output back to input:

There is nothing wrong with the output power - 9.46v into 16 ohms is 5.5W. Good.

233V on the V3 pin 3 (plate)- seems about right. It would make the output transformer have a turns ratio of 233/9.46 ~= 25:1 i.e plate load impedance of 25^2*16 ~= 10k ohms, reasonable.

The reading for V2 pin 5 (grid 1) is not consistent with the previous set so I'm ignoring the old one. 8.13v rms is reasonable.

V1 pin 7 (2nd stage grid 1) 146mV means the gain of the 2nd stage is something like 8.13/0.146=55 about right

The numbers around the tone stack seems correct provided you are NOT in boost mode. If you are in boost mode then there's a problem. Measure the DC resistance across the 68K in both both and non-boost modes.

The first stage gain is 2.2/0.1 = 22. This about half of what I would expect. Maybe there is something else wrong with the tone stack that is loading it down. Power off, try doing a series of DC resistance measurements. Clip one lead to ground and then report the
ohms at the top of each of R22 R15 R13 R12 R10. Maybe the 12AX7 is weak. Try another.

[wole]

Super thanks.

Going from output back to input:

There is nothing wrong with the output power - 9.46v into 16 ohms is 5.5W. Good.

233V on the V3 pin 3 (plate)- seems about right. It would make the output transformer have a turns ratio of 233/9.46 ~= 25:1 i.e plate load impedance of 25^2*16 ~= 10k ohms, reasonable.

The reading for V2 pin 5 (grid 1) is not consistent with the previous set so I'm ignoring the old one. 8.13v rms is reasonable.

V1 pin 7 (2nd stage grid 1) 146mV means the gain of the 2nd stage is something like 8.13/0.146=55 about right

The numbers around the tone stack seems correct provided you are NOT in boost mode. If you are in boost mode then there's a problem. Measure the DC resistance across the 68K in both both and non-boost modes.

The first stage gain is 2.2/0.1 = 22. This about half of what I would expect. Maybe there is something else wrong with the tone stack that is loading it down. Power off, try doing a series of DC resistance measurements. Clip one lead to ground and then report the
ohms at the top of each of R22 R15 R13 R12 R10. Maybe the 12AX7 is weak. Try another.

First of all let me say thank you so much nickb!
I've already tried other 12AX7 but the problem persisted. I've also tried switching the 12AX7 between the good working amp and this one. So I think we can exclude a tube problem.
I was in NOT boost mode.
I will do the DC resistance measurements of R22 R15 R13 R12 R10. By 68K you mean R2, R6 or R22?

[nickb]

First of all let me say thank you so much nickb!
I've already tried other 12AX7 but the problem persisted. I've also tried switching the 12AX7 between the good working amp and this one. So I think we can exclude a tube problem.
I was in NOT boost mode.
I will do the DC resistance measurements of R22 R15 R13 R12 R10. By 68K you mean R2, R6 or R22?

I was pretty sure you had tried another 12AX7, but best to ask.

R22 - the one that does the boost thingy. Try using the front panel switch and no foot pedal. Also repeat with the the foot pedal.I note that if the front panel switch is in non-boost then the footswitch doesn't do anything and vice-versa.

Also, just checking - when you compared the two amps the boost was in the same setting, right?

[wole]

I was pretty sure you had tried another 12AX7, but best to ask.

R22 - the one that does the boost thingy. Try using the front panel switch and no foot pedal. Also repeat with the the foot pedal.I note that if the front panel switch is in non-boost then the footswitch doesn't do anything and vice-versa.

Also, just checking - when you compared the two amps the boost was in the same setting, right?

Yes of course, when I compared the two amps the boost was off in both

[wole]

I was pretty sure you had tried another 12AX7, but best to ask.

R22 - the one that does the boost thingy. Try using the front panel switch and no foot pedal. Also repeat with the the foot pedal.I note that if the front panel switch is in non-boost then the footswitch doesn't do anything and vice-versa.

Also, just checking - when you compared the two amps the boost was in the same setting, right?

Hi, I measured the resistances, they were all ok except for R22... When I tried to measure it the tester measured some random values for a second and then 0.
By the way, to get the right values of DC resistance I had to put one probe on one end of a each resistance and the other probe at the other end.. If I put one probe fixed at mass the values were strange.. Did I mess up again?

[nickb]

Hi, I measured the resistances, they were all OK except for R22... When I tried to measure it the tester measured some random values for a second and then 0.
By the way, to get the right values of DC resistance I had to put one probe on one end of a each resistance and the other probe at the other end.. If I put one probe fixed at mass the values were strange.. Did I mess up again?

Sorry for delay - I've been away.


The idea of keeping the probe in one place was to check the continuity across the whole chain as well as the resistances. The total resistance you measure should be the sum of the resistance in the chain between the two probes.

No matter since you measured R22 as shorted. The boost switch shorts R22 in normal so you are in low gain mode. If the foot switch is connected, remove it and check the operation of the boost switch. Does it work the same on both amps?