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Old 16th May 2012, 10:04 AM   #1
azvrt is offline azvrt  Netherlands
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Default 12AX7 questions

Hi,

I recently bought two car tube hybrid amps.
Since my question is mostly regarding the tubes, I chose to start a thread in this section, hope that's okay.

The amplifiers are Planet Audio HVT7100 and 7150.
They were made by ZED Audio around 1997 / 2000 I believe.
Both are two channels amplifiers, the 7150 a larger and a bit more powerful and fan cooled and slightly different in design in some sections.

Anyway, I have tested and compared both.
The 7150 is perfect for me, I love its sound. It has slight laidback highs and upper mid which is great for my personal taste as my ears are very easily fatigued by higher frequencies. I could listen for hours and hours to this amp.

The 7100 however, is more pronounced in the highs and upper mid and I find it more fatiguing there. Since I am running active in my car that does not necessarily have to be a problem as I could use it for midbass/lower mid.

But I also noticed that for instance with Fleetwood Mac - Dreams, which has pretty low bass, it seems to distort or something ? Also, I thought that with some high quality piano recording some notes were distorted. Same as with the bass, there appeared to be an extra sound which shouldn't be there.

My father thinks it's impossible for an amp to distort only at certain frequencies, but since these amplifiers not only use transistors but also tubes and we basically know nothing about tubes, I was wondering whether the tubes might need to be replaced on the 7100 ?

These amps use Sovtek 12AX7 tubes, any recommendations as to trying a different model tube ?
I am going to ask these questions to ZED Audio as well, but I am always interested in different opinions from different people.

I like my sound pretty warm with slightly laidback upper mid / highs.

Thanks for your advice !

Here are some pics:

ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
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Old 16th May 2012, 10:22 AM   #2
DF96 is offline DF96  England
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If a valve is failing then it is likely to distort at all frequencies, but as bass notes tend to be stronger signals and stronger signals will almost always distort more then your perception could be that bass only is distorting.

I assume the valves in your system are there either as fashion accessories or as part of an FX circuit so I can't help you on valve choice as I never use such systems myself.
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Old 16th May 2012, 10:27 AM   #3
azvrt is offline azvrt  Netherlands
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Hmm yes this could very well be the case, and might explain why I experienced the amplifier as much more fatiguing. it might be wise to check for distortion with an oscillosope and a testtone cd. What would the distortion look like on a sine wave ?

I do not think the tubes are in there just for the looks. I don't think ZED could afford to risk his reputation. They must be in the signal path.
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Old 16th May 2012, 10:36 AM   #4
DF96 is offline DF96  England
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'being in the signal path' and 'being there for fashion' are not mutually exclusive. Some designers plop in a valve where an op-amp would do an equally good job, because they know it will help sales by increasing the price.

Distortion on a sine wave could take several forms. You might see clipping on peaks, or a wiggle near the zero crossing, or the top and bottom of the wave being a different shape (e.g. one looks sharper, the other more rounded off). A very rough rule of thumb is that if you can just about see it on a 'scope then you will probably be able to hear it without too much difficulty.
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Old 16th May 2012, 10:41 AM   #5
azvrt is offline azvrt  Netherlands
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Ok thanks !
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Old 16th May 2012, 04:59 PM   #6
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Swap Tubes between amps and check for any changes?
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Old 16th May 2012, 07:49 PM   #7
azvrt is offline azvrt  Netherlands
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I've sent a big email to ZED see what he says.

I did a little test with a testtone cd with sine waves and the oscilloscope.
Got some pretty weird results on both amps !
I thought maybe I was doing something wrong, but then I took one of my normal (transistor) car amps and the signal was perfectly clean.

http://imageshack.us/photo/my-images...201293357.jpg/
Here's a very ugly drawing of mine which represents what I saw on the scope.
The top half shows the smaller 7100, the bottom half the more powerful 7150.
The signal was far from clean on both amps. Strangely they sound very clean when playing music. Anyway, the first 10 seconds (after the tubes have warmed up which takes approx. half a minute) the output on the 7150 seemed clean, therefore I drew a clean signal on the bottom left, but after that it got pretty messy same as on the other amp.

There were multiple lines at the peaks, and in between the peaks they merged into a blurry thick line instead of a thin thight line. At the peaks, in between the multiple lines, the area was filled as well. With the cd on pause, the straight clean line one would normally see was very noisy.

The fact that the amps sounded different from each other was probably caused due to the fact they were not level matched (at all) when playing music. Strangely, the supposedly less powerful 7100 is way louder at the same gain setting (gains turned all the way down to 7 volt gains) than its larger brother. So it was playing much louder which explains the fact that it sounded fatiguing to me, and more present in the highs even though basically everything was louder, the highs simply annoy me rather quickly as compared to other frequencies, which might explain why I found it to be more pronounced in the higher regions.

I did test Fleetwood Mac - Dreams on both amps, and on the smaller one there is something strange going on in the bass line. I had the larger amp play louder to approximately match the output of the smaller amp, and did not notice the same peculiar thing on the Dreams' bassline.

I put the smaller one in my car, it's driving a pair of speakers which are on a 80 Hz high pass and 180 Hz lowpass and it sounds pretty damn good. I'll await ZED's mail, see what he has to say on the matter.

What's strange is that the amps sound very clean to me. I mean they could sound good to me while being noisy / distorted (or whatever is causing the unclean sine waves) if that's what I liked and I'd be fine with that if that would be the case, but besides sounding pleasing to me they sound clean, not noisy or distorted (except for that bassline in Dreams and maybe some notes in that piano track on the 7100, the 7150 does not do that).

Last edited by azvrt; 16th May 2012 at 07:53 PM.
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Old 16th May 2012, 08:34 PM   #8
DF96 is offline DF96  England
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I could not look for long because almost as soon as I arrived on your pics page I got a (spurious? hopefully) warning about viruses.

'Scope traces can show all sorts of confusing things, including hum and RF pickup. A clean trace is not necessarily good; a dirty trace is not necessarily bad. It all depends on how well you know your scope and its probes.
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Old 16th May 2012, 08:49 PM   #9
azvrt is offline azvrt  Netherlands
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I'm sorry to hear that. Though I must look up 'spurious' in the dictionary

But the signal was clean with an other amp with the same scope.
I must admit I am not very experienced at operating and reading scopes, though.

Let's see if I can get the pic to show up in the thread. Usually, I fail on this site.
I'll try 3 different methods.

Click the image to open in full size.


[img=http://img823.imageshack.us/img823/2369/165201293357.th.jpg]



<a href="http://imageshack.us/photo/my-images/823/165201293357.jpg/" target="_blank"><img src="http://img823.imageshack.us/img823/2369/165201293357.th.jpg" border="0"/></a>
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Old 16th May 2012, 08:50 PM   #10
azvrt is offline azvrt  Netherlands
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Nope, failed again. Can't zoom.
I can forward you the pic by email if you'd like.
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