VHT Special 6 out of volume

[Mick Bailey]

I really don't think we know that yet - the listening test is too inaccurate for my taste. I'd wan't see a real measurement to back that up, so, as far as I'm concerned the preamp is still on the table. Especially as the OP said that it was 'like it had a blanket on it' and ' lost all it's sparkle'. This shouts HF loss, surely?

Agreed, now the OP has determined the amp suffers the same loss in both modes. I'd based my comment on the later suggestion that it was fine in triode mode, though.


So wole, if you generate an AC voltage and set it to a convenient value (I use 100mV), take a voltage reading at V1a pin 2, V1b pin 7, and V2 pin 5 this will show the signal levels on the grids and give some idea of the signal path. I would suggest 1khz to begin with.

What make/model DMM do yuo have?

[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]

Thank you! I did the test comparing the two amp with the tone knob at 12o'clock and didn't try accurately if the tone control was working.. I will sure do as soon as possibile

[wole]

Agreed, now the OP has determined the amp suffers the same loss in both modes. I'd based my comment on the later suggestion that it was fine in triode mode, though.


So wole, if you generate an AC voltage and set it to a convenient value (I use 100mV), take a voltage reading at V1a pin 2, V1b pin 7, and V2 pin 5 this will show the signal levels on the grids and give some idea of the signal path. I would suggest 1khz to begin with.

What make/model DMM do yuo have?

Again I'm sorry it was my fault, I first said the problem was only in high power mode, then I realized it was in both high and low power mode..

I was thinking about using the program "tone generator" to generate the sine wave (I have a mac).
I don't know the model of my tester but I will check it as I come back home.

Thank you very much again!

[wole]

Again I'm sorry it was my fault, I first said the problem was only in high power mode, then I realized it was in both high and low power mode..

I was thinking about using the program "tone generator" to generate the sine wave (I have a mac).
I don't know the model of my tester but I will check it as I come back home.

Thank you very much again!

It's a maurer 86477

[J M Fahey]

It's a maurer 86477

Ouch!!!
It's a "$10 multimeter"


Sorry, the problem is not the price per se, we are not snobs here, but it has only 2 AC scales, 200VAC and 500 or 750VAC and much worse, they go crazy if there is DC and AC present at the same time (as in a plate or cathode or power supply) so you basically can't measure signal at plates with them.

Anyway, let me look at the circuit, there are a few points where Audio (which is AC) is present, but after some coupling capacitor so DC was left behind so although that limits us somewhat, we still can measure something.

[wole]

Ouch!!!
It's a "$10 multimeter"


Sorry, the problem is not the price per se, we are not snobs here, but it has only 2 AC scales, 200VAC and 500 or 750VAC and much worse, they go crazy if there is DC and AC present at the same time (as in a plate or cathode or power supply) so you basically can't measure signal at plates with them.

Anyway, let me look at the circuit, there are a few points where Audio (which is AC) is present, but after some coupling capacitor so DC was left behind so although that limits us somewhat, we still can measure something.

Yes it's not a good tester.. Are you saying that my previous measures are useless because of the poor quality of the tester I used?
I can buy a new one if it's needed or could help me out, I was planning of buying a new one anyway. If you can list me the specs it should have it will be very appreciated

[J M Fahey]

Half and half.

The DC voltages you measured are accurate and useful.

If you measure "pure" AC as in a wall socket you will get proper 230V , same if you measure , say, a 12V dicroic lamp transformer, BUT if you set it to AC and measure a 9V battery, it will show you some "phantom" 20VAC which of course do not exist.

By the way, I *use* those cheap multimeters all day long, no problem if I know their limitations.

We can still continue with it today, but when you buy, basically get one which has more sensitive AC scales, typically 200mVAC , 2VAC, 20VAC, 200VAC, "high voltage"VAC (500/750)

Those are more sophisticated , have better electronics inside , are more sensitive and, most important, include an internal capacitor which blocks DC .

You should get them starting around 50 Euro .

IF for extra 20 Euro or so you can get one with "True RMS" AC measurement, even better; if price difference is too high, it's not *that* important, at least for troubleshooting.

Get into some shop page where you buy and link it here, maybe we can help you pick one or another.

No language barrier , of course

Porteño still is 95% Castellano

[wole]

Half and half.

The DC voltages you measured are accurate and useful.

If you measure "pure" AC as in a wall socket you will get proper 230V , same if you measure , say, a 12V dicroic lamp transformer, BUT if you set it to AC and measure a 9V battery, it will show you some "phantom" 20VAC which of course do not exist.

By the way, I *use* those cheap multimeters all day long, no problem if I know their limitations.

We can still continue with it today, but when you buy, basically get one which has more sensitive AC scales, typically 200mVAC , 2VAC, 20VAC, 200VAC, "high voltage"VAC (500/750)

Those are more sophisticated , have better electronics inside , are more sensitive and, most important, include an internal capacitor which blocks DC .

You should get them starting around 50 Euro .

IF for extra 20 Euro or so you can get one with "True RMS" AC measurement, even better; if price difference is too high, it's not *that* important, at least for troubleshooting.

Get into some shop page where you buy and link it here, maybe we can help you pick one or another.

No language barrier , of course

Porteño still is 95% Castellano

Hi, I'm sorry, I didn't do the checks yet.. I'm working very hard these days and I have time to check the amp only during the week end.. I hope to find a better tester tomorrow and do the checks!

[wole]

Half and half.

The DC voltages you measured are accurate and useful.

If you measure "pure" AC as in a wall socket you will get proper 230V , same if you measure , say, a 12V dicroic lamp transformer, BUT if you set it to AC and measure a 9V battery, it will show you some "phantom" 20VAC which of course do not exist.

By the way, I *use* those cheap multimeters all day long, no problem if I know their limitations.

We can still continue with it today, but when you buy, basically get one which has more sensitive AC scales, typically 200mVAC , 2VAC, 20VAC, 200VAC, "high voltage"VAC (500/750)

Those are more sophisticated , have better electronics inside , are more sensitive and, most important, include an internal capacitor which blocks DC .

You should get them starting around 50 Euro .

IF for extra 20 Euro or so you can get one with "True RMS" AC measurement, even better; if price difference is too high, it's not *that* important, at least for troubleshooting.

Get into some shop page where you buy and link it here, maybe we can help you pick one or another.

No language barrier , of course

Porteño still is 95% Castellano

I've found this tester at my local shop for around 40€
http://www.soselectronic.hu/a_info/r...T132C-132D.pdf
is this good enough to check the ac voltages?

[Dave H]

I've found this tester at my local shop for around 40€
http://www.soselectronic.hu/a_info/r...T132C-132D.pdf
is this good enough to check the ac voltages?

I don't think so. It doesn't have any low voltage AC ranges.

[wole]

I don't think so. It doesn't have any low voltage AC ranges.

Ok and this one would be ok ? http://www.amazon.it/HYELEC-multimet...eywords=tester

[g1]

Yes, that looks good.
(english specs here: http://www.amazon.com/Digital-Multim.../dp/B00ME4F6EK )

[wole]

Yes, that looks good.
(english specs here: http://www.amazon.com/Digital-Multim.../dp/B00ME4F6EK )

good. then I will order it and once I'll have it I'll measure the ac voltages

[wole]

Ok I've ordered the tester, as soon as it arrives I'll measure the voltages.

One quick question: could the loss of volume be caused by a damaged transformer? I'm thinking that maybe the friend to whom I lent the amp could have connected it to an external cab mismatching the impedence

[Mick Bailey]

It's possible, but transformers are way down on the list when it comes to failure. You can test the output transformer just to be sure;

The Super-Secret Transformer Tester