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Old 28th October 2006, 03:11 PM   #1
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Question 6sl7 or 6sl7GT for Broken Tubalizer

Hi everyone,
I have just received an Audiodigit tubalizer that unfortunately isn't very healthy.

http://www.audiodigit.com/?section=81
This is my first experience of valves and hopefully will lead to more adventurous things.

THe Tubalizer is placed between my CD Player and Integrated (14KOhm input impedance) solid state amplifier.

It distorts very badly with any input. It is slightly better with very small signals (-12dB) but not at all satisfactory. I have tried many sources and different power supplies and 'tuning' settings.

I am using a 12v 1.2Amp regulated linear supply with the correct polarity. I have also tried other supplies with no change. My power supply gives 12.06 Volts (unloaded) and the regulator is passing approximately 6 Volts and the tube glows slightly near the top as it should.
I have redone all the soldering to be sure and measured 5.7Volts at the ground of the RCA outputs (This is as it should be.)
This device is only made from a handful of caps and resistors with a 6 Volt regulator and a Sovtek 6SL7.

Surely it must be a faulty Tube? What else could be wrong here? Any help would be very much appreciated. I would eventually like it to be my CD players output device when I get it working.

I feel I should buy a spare valve for test purposes but what is the difference between a 6sl7 and a 6sl7GT?
Please help
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Old 28th October 2006, 09:11 PM   #2
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Martin,

Was this a kit you put together? Do you have a schematic for it or could you draw one? Changing the 6SL7 for a 6SL7 GT isn't going to make it work any better. We need to see the schematic and take a closer look.
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Old 6th February 2007, 12:49 AM   #3
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Sorry for the long delay.........

I bought this already assembled (only option) from Audiodigit in Italy.
http://www.audiodigit.com/?section=81

I have bought six more (unbranded) 6SL7 and no change.
I tried it between a CD player and soundcard and could quite clearly see that the negative cycles were being clipped. The distortion lessens when I reduce the volume on the CD player.

I have emailed the manufacturer and they think that it is probably a combination of bad tube and my amplifiers low input impedance.
I have tried many tubes and amps and have even built an opamp buffer to buffer the buffer!! So, this cannot be the problem.

I am feeding it with a 1250mA 12V regulated linear supply.
I should send it back under warranty but I would like to learn more about tubes and save the hassle of posting and waiting....

The thing that bugs me most is Why does it only clip on the negative cycles?
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Old 6th February 2007, 01:01 AM   #4
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I don't have a schematic but may be able to make my own if it would be helpful. It would be good practise with this double sided board.
The pot at the front is supposed to be "Tube operating point"
Out of view at the back is a 6 Volt reg and heatsink.
The green croc-clips were there to try parallel resistor across the originals. This was advised by the seller. Lowering this resistance does help but lowering too much makes the sound very anaemic.

Burnedfingers: (Off Topic) I have designed an avatar that you may like and posted it at the avatar thread: What are your avatars?

Many thanks,
Martin.
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Old 6th February 2007, 02:03 AM   #5
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It looks as though the thing is behaving like an "infinite impedance detector", i.e. half-wave rectifying. I'm amazed that a 6SL7 can operate at all with only 12v B+. I'd love to know what the "operating point adjustment" is supposed to do!
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Old 6th February 2007, 02:37 AM   #6
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Hi Ray,
All I can say is that the pot seems to control the treble content.

Fully clockwise seems to give least distortion and a more open sound.
Anti clockwise 'sounds' like more distortion with rolled off treble / more bass.

I don't know much about tube circuits and would like to learn,

Am I right in thinking that......

The circuit is running from a single 12V supply which is divided to give +/- 6V and the resulting positive and negative supplies are unequal for some reason?. i.e. +9V and -3V.
Am I on the right tracks here or are there more possibilities?

Yes, I would like eventually to increase B+ when I get the darn thing working!!

Cheers,
Martin.
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Old 6th February 2007, 07:19 AM   #7
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I may be completely wrong here, but I doubt if you'll get it working without increasing B+ to about 40v (and preferably at least 70v). This use of a 6SL7 is beyond my experience. There may be some trick the supplier is using. Seeing the schematic would answer important questions.
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Old 6th February 2007, 08:29 AM   #8
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Quote:
Originally posted by Sonusthree
I have emailed the manufacturer and they think that it is probably a combination of bad tube and my amplifiers low input impedance.
As I'm a moderator I'm not allowed to type what I said when I read that. The manufacturer is either lying through their teeth or they haven't a clue what they're talking about. Probably both. As others have pointed out, there is no way on earth that you are going to get a 6SL7 cathode follower to operate properly from 12V. Or even 40V, not even if you drove the valve with grid current.

I hate to give in, but you'd do better to give this horrible circuit up as a bad job and use your 6SL7 for something else. By the way, the suffix GT simply means "glass, tubular" whereas "G" means "glass" and is usually Coke bottle shape. No suffix ought to mean metal tube. The innards in all cases are the same.
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Old 6th February 2007, 11:25 AM   #9
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Martin,

Take a look at the -3 volt supply. See if you can't sketch something out. Is there anyway you can bypass the regulators and send voltage to it from another supply?
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Old 6th February 2007, 02:25 PM   #10
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Quote:
Originally posted by burnedfingers
Take a look at the -3 volt supply. See if you can't sketch something out. Is there anyway you can bypass the regulators and send voltage to it from another supply?
Hi, Burnedfingers,
I only mentioned a -3Volt supply to illustrate the point that I was trying to make.

I'll have a look later but I don't want to lose my warranty! I'm going to attempt to draw a schematic but 6SL7 is not available in my software. I'm hoping that I may substitute somehow for two triodes.
I cannot find an available equivalent for simulation purposes.

The only regulator is for the 6v heater supply. The whole circuit is driven by a 12V regulated external power supply. I guess that this is split into two somewhere or maybe there is a voltage doubler?
I'm not very experienced with either but learning will be fun!! Hopefully my schematic will shed some light.

Here's something from their website:
"IMPORTANT NOTE:
Due to the biasing scheme for the tube the Tubalizer needs its own power supply, in other words you cannot share the supply with other modules, otherwise you will short the signal to ground. No damage, but the sound will appear "clipped" at all levels. This is not a problem, of course, as any cheap (but good) 12V regulated supply is sufficient. You can buy one in any electronic shop, or buy ours."

This seems to be what is happening to me but I have tried many different supplies. Maybe there is a clue here somewhere.

It is supposed to have a gain of 0.98 so does this make it more believable that a 12V supply would work?

Hopefully, when this is fixed, it would a simple job to isolate the 6V reg for the heater and increase the voltage to the rest of the circuit.

Cheers,
Martin.

P.S. Here's the Burnedfingers avatar I mentioned earlier. I think it would look great under your name!
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