VHT Special 6 out of volume

[g1]

That works fine. It's the pdf I tried to upload. How come I only got a single pixel?

I re-saved the file via adobe reader. That alters the filesize by a few bytes which makes it acceptable to the forum software.

[Dave H]

I notice that the -ve side of the speaker has to run through three jack switches before it connects to the OT. Any one of the switches could be high resistance. It looks like the speaker connects to a jack socket on the underside of the chassis. Try connecting it to the 16 ohm speaker output to bypass the jack switches.

[wole]

I notice that the -ve side of the speaker has to run through three jack switches before it connects to the OT. Any one of the switches could be high resistance. It looks like the speaker connects to a jack socket on the underside of the chassis. Try connecting it to the 16 ohm speaker output to bypass the jack switches.

[ATTACH=CONFIG]37719[/ATTACH]

ok, but I have a 8 ohm speaker which is connected to the 8ohm output in the back of the chassis. Wouldn't it be dangerous for the speaker to connect it to the 16ohm output?

[Dave H]

ok, but I have a 8 ohm speaker which is connected to the 8ohm output in the back of the chassis. Wouldn't it be dangerous for the speaker to connect it to the 16ohm output?

I see, it's only going through one jack switch then not three. It will be OK to connect it to the 16ohm output for a quick test to eliminate the jack switch. The switching jacks in effects loops are notorious for bad contacts. It's unlikely to be a fault in a new amp but it's worth trying.

[wole]

Hi everyone! Finally I found time to test the amp.
Just one note: there is a loss in volume only in high power mode. If there is a loss in volume also in low power mode it is very slight.
First thing I tried the amp with the stock speaker on the external 16ohm output, so I think what Dave H said can be excluded right?
Here are the voltages you asked:
Low Power:
V101A
Pin 1 : 140V
Pin 2 : 0V
Pin 3 : 1,2V
V101B
Pin 6 : 138,2V
Pin 7 : 0V
Pin 8 : 1,2V
V102
Pin 3 : 336V
Pin 4 : 330V
Pin 5 : 0V
Pin 8 : 21V

High Power:
V101A
Pin 1 : 137V
Pin 2 : 0V
Pin 3 : 1,1V
V101B
Pin 6 : 135,2V
Pin 7 : 0V
Pin 8 : 1,2V
V102
Pin 3 : 331V
Pin 4 : 332V
Pin 5 : 0V
Pin 8 : 21V

I couldn't find the points marked A,B,C on my circuit, because my circuit is like this http://music-electronics-forum.com/a..._5-17-10-1.png
If the values of the voltages at these points are essential to understand the problem can you point me out where the points are on the schematic I linked?

Any comment or idea on what the problem could be is welcomed
Thank you

[nickb]

You are right that the 16 ohm output bypasses any potential jack switching problems.

Thanks for the neat table - that was very easy to read. Exemplary

Those voltages are fine. Based on the testing you have done and that data I would I doubt there is really a power loss. The only way to know for sure it to measure it. What test equipment do you have access to?

So, I thinking that what you really have is a high frequency and/or gain loss let's try some other things.

Do the tone and volume controls work identically in the two amps? i.e. are the controls at about the same settings for a similar volume and tone.

Using the sch you linked to in #20 C6 and C11 are critical to the HF content. You could try one at a time lifting one end of each to see if they make a difference. You could also try, with one end floating on the originals, soldering a new one in place.

[Mick Bailey]

The difference between high and low power is just the switching of the 470R screen resistor R21 - high power connects it to B+2, and in low power it gets connected to the 6V6 plate in pseudo-triode configuration.

So, the amp works OK in low power mode. Now, with a power tube the screen voltage has a bearing on the current flowing through the tube. If there's not enough screen voltage you won't develop the full power.

The voltages are fine, but those are static voltages with no signal running through the amp. My thought is that maybe the screen voltage is collapsing under signal load. Either there's a high-resistance contact in your High/Low power switch, or R26 or R21 are high resistance under load, or there's a bad solder joint between B+2 and pin 4 of the 6V6.

An initial check would be to measure the voltage on 6V6 pin 4 in high power mode while someone hits (say) an open E chord with the amp turned up. This will establish whether the screen voltage is holding up. It will drop some, but we need to know how much.

[wole]

Thank you very much!
Ok, I'm going to do the checks you suggest.

First of all I'm going to record the two amps (the well working and the broken one) with the same settings and the same speaker in low power mode. I want to be 100% sure if the broken one is working fine in low power mode.

@nickb: yes, when I tested the two amps in high power mode the volume and tone knobs where at the same level in both.
Is there a way to do the check you suggest without unsoldering the caps and lifting them with the amp on? this seems dangerous

@ Mick Balley: ok I'll check the voltage value you suggest

[Mick Bailey]

If the amp is OK in low power mode, than this pretty much eliminates the preamp and its controls, because that is all shared between both modes.

[wole]

I'm sorry, but I just did a test and the broken amp has less volume BOTH in low power and high power mode.
I'm sure because I recorded both amps and then checked the amplitude of the waves: there's no doubt about the loss of volume both in high and low power mode.
Again I'm very sorry for the wrong information.

What should I check now, given this information?
Should I still check the things you said before?

[Mick Bailey]

That's OK.

The way I troubleshoot low volume (after eliminating connections, tubes and the speaker) is to set up a low level sine-wave test signal at the input and use an oscilloscope to trace back from the output stage. If you don't have a scope, a meter set to AC volts is fine.

Are you able to generate an audio signal from a soundcard? You don't need anything more than 100mV. I use 1KHz, or if that gets annoying, around 400Hz is fine. The problem you'll run into is sound output. Most people use a dummy load and use that instead of a speaker. You could use a longer speaker lead and muffle the speaker with a blanket over the cabinet for testing.

[nickb]

Thank you very much!
Ok, I'm going to do the checks you suggest.

First of all I'm going to record the two amps (the well working and the broken one) with the same settings and the same speaker in low power mode. I want to be 100% sure if the broken one is working fine in low power mode.

@nickb: yes, when I tested the two amps in high power mode the volume and tone knobs where at the same level in both.
Is there a way to do the check you suggest without unsoldering the caps and lifting them with the amp on? this seems dangerous

@ Mick Balley: ok I'll check the voltage value you suggest

With the amps on??? No!! Whatever gave you that idea? You turn off, make the change and turn back on again.

[nickb]

If the amp is OK in low power mode, than this pretty much eliminates the preamp and its controls, because that is all shared between both modes.

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?

[wole]

With the amps on??? No!! Whatever gave you that idea? You turn off, make the change and turn back on again.

yeah that seemed strange, my bad. Ok I get it now, so I will try to unsolder one end of C6 and C11 and see if that makes a difference to determine if one of them is the broken component.

Is there anything else I can do?
I've seen Mick Balley was talking about troubleshooting (but the comment seems deleted now): I know how to generate a sinewave, but I don't know what to check with the AC tester.

[nickb]

yeah that seemed strange, my bad. Ok I get it now, so I will try to unsolder one end of C6 and C11 and see if that makes a difference to determine if one of them is the broken component.

Is there anything else I can do?
I've seen Mick Balley was talking about troubleshooting (but the comment seems deleted now): I know how to generate a sinewave, but I don't know what to check with the AC tester.

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.