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Old 17th February 2007, 11:28 PM   #21
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For the tube to work, it must conduct current....

For it to work with little or negligable distortion, it must conduct All the time the signal swings, ie, Class A....

Whats happening I believe is that its only conducting during one-half cycle, the other being cut-off, as there isnt sufficient 'headroom' or current flowing through the valve in the first-place....

Why one-half-cycle?....

Its to do with the way the tube is biassed I believe... During positive half-cycles, this is adding to the grid, a positive potential greater than its standing no signal set value, which causes the valve to conduct more current...No problem so far....

But when the neg half cycle comes along, and as the valve isnt conducting much in its 'standing' no-signal state, it drives the grid sufficiently negative to completely cut off current flow all together...

This is why it appears to 'work' with Very low signal levels on its input, There is just sufficient current flowing to allow very small signal operation

The result is the loss of the half cycle you can see when more 'normal' signal levels are used....


The cause?--

Not enough supply volts to the tube, or its operating point is chosen badly with insufficient current being passed in its 'standing' or no signal state, This second point probably caused by the first point!...

What value is the resistor network on the grid, and what potential do you read at the grid in relation to the cathode?

At a wild guess, to work at all it must have a positive potential at the grid in relation to the cathode or I would doubt a 6SL7 would conduct any current at all at 12v!

We really need the 'schematic' and some voltage readings would be interesting to see what the supplier is trying to do....,
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Old 20th February 2007, 02:16 AM   #22
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Thanks to everyone for your help so far.

I have draughted a schematic. It needs a lot of tidying up and a proper tube diagram in the middle instead of the pins but I thought that I'd throw this out there for an initial response.

The values are a little hard to read at the moment. Sorry.
I will add a final and more legible version soon...........

Any hints as to how this thing is working and also what the fault could be?

Regards,
Martin.

Edit: Please ignore the polarity of the electrolytic caps if I have got them reversed by mistake!! (It's very late here.)
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Old 20th February 2007, 03:08 AM   #23
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Darn it!! I need to check where the B+ is coming from but I would assume it's simply the full 12 Volts.
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Old 20th February 2007, 04:25 AM   #24
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Hi Sonusthree ,

Anything wrong with your schematics !!!

The 6 Volts regulator’s ground , NEEDS to be
connected to the ground , if not , the regulator
will “float” and the output will not be 6 Volts .

Pin 2 and pin 5 are tied to ..... ???

Please , verify all stuff again . Thanks .

Carlos
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Old 20th February 2007, 11:59 AM   #25
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Quote:
Originally posted by Sonusthree
Darn it!! I need to check where the B+ is coming from but I would assume it's simply the full 12 Volts.
Quote:
Originally posted by refference
The 6 Volts regulator’s ground , NEEDS to be
connected to the ground ...............

Pin 2 and pin 5 are tied to ..... ???.
Sorry for the silly mistakes but it was very late when I finished it and I am still learning all the time.

Here is the revised schematic. There may still be some errors but I think it is correct now.

Cheers,
Martin.
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Old 20th February 2007, 02:23 PM   #26
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Hmm, Looks like a pair of cathode-followers, with a variable cathode load....

Varying the cathode-load will change the op. point and also increase/decrease its distortion and out-put impedance I believe....

Out of interest and because of this thread, Ive delved into low volts tube stuff and come up with a scheme Ill post later..At work currently, nothing interesting here!

Ive chosen a different valve to your 6SL7g/gt, Ive used the cheap and chearful computer triode 5687, which is a very popular pre-amp driver and has developed quite a following in the DIY 'scene'...

I chose this as it has a 6 and 12V heater option and very high 'pervayence' My little experiment has a gain-stage followed by cathode-follower O/P.-- Works well providing you keep its input below 800mV P-P or it will clip, otherwise its sound is fairly neutral.--It has a gain of around 10 ish, currently...

The Tubaliser scheme will have a negative gain of around .8 ish....

If I get the time, Ill play with the 6SL7 in the same scheme and tweak values to suit...

For the purposes of your 'Simulation' work, choose the ECC83 or 12AX7 in Proteus, while not the same as the SL7 its similar enough to work out whats happening, seeing you have such a 'drastic' fault...

I used the ECC88 for my 'sim' work in Proteus before building the breadboard with the 5687. It was similar enough at low volts to work out OK...

(To be quite honest, Its the first time Ive used the 'Simulator' in Proteus....!)
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Old 20th February 2007, 05:20 PM   #27
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Alastair, I cannot thank you enough.

I chucked a 12AX7 into the sim and was able to see what is happening. I will, tonight, change all of the resistors in the software for realtime potentiometers. Why not?

I am a complete tube noob and this experience has been most enlightening. I know understand the biasing mechanism involved here. From the simulator (and your advice) I can see that it is a little 'off centre'. I believe that increasing the resistors around the 50K pot is the solution.
Please correct me if I'm wrong. I hope I'm not.

This theory also matches my experiences. When turning the pot clockwise (increasing resistance) the treble opens up and the distortion lessens. Turn it anti-clockwise and the treble goes and distortion increases.

Theoretically: 52K of resistance still distorts so I should probably try swapping the resistors (2.26K) for ~ 27K. If this works then I will perfect this value to give the most usable range with the pot.

The simulation seems rather inaccurate with regards to input amplitude and resulting gain but I'll just have to try it in the real world.

I will report back later.
Thanks again, and wish me luck.......
Martin.
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Old 20th February 2007, 08:32 PM   #28
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Looked at your scheme again having delved into my own experiments....I was going to try a 6SL7 and 6SN7 in my scheme to see what would happen, but my soldering-iron has quit this life...Typical!

The 'Grid-Bias' resistors in your machine are 220K....

Ive found that at low volts, that the grid-bias resistors need to be below 100K for a good op point. I settled for 47K in my experiments.

Usual values for 'Normal' high-volts operation are around 1 to 2 Meg-Ohms...

The Proteus simulator will still 'say' it works without the grid-resistors in place, but it won't in the real-world, so treat everything in the sim with a little contempt........

If you increase the cathode-load resistance as you plan, this will reduce the amount of available current through the valve, ....At 12V, this is really a very tiny current already....
In my set-up, its just a couple of hundred Micro-amps, and that is with a valve that has comparatively Massive cathodes in comparison to the SL7, so huge area for electron-emission!....
With a 6SL7, and above 50 odd K cathode-load, I would be surprised if you got a few tens of micro-amps Your O/P impedance will also increase if you add resistance here-Not so good, H/F response could well suffer ...

Instead of adding resistance there, try putting a couple of 100K resistors accross the original 220K R6 and R5 grid-resistors rather than increasing the cathode-load. These resistors are'nt 'bootstrapped', as in my layout so will affect the input impedance a little but shouldnt be noticable unless you have a very high O/P impedance of your source-device (Very unlikely!)

If this helps, try going lower, say down to 60 odd K....At least this will increase the current through the valve without increasing its O/P impedance. The cathode voltage can be measured which will give you an idea of the current flow.......

Even if you were to place a short here, at 12V you wont damage the valve, as its normal HV operation it passes around 2-3mA...You will not get anywhere near that amount of current at 12V, so feel free to mess around there all you like!

Right, Now Im gonna try and copy out the scheme of my 12V amp and post it here... Ill add real-world voltage values at various places for interest--Im currently playing the TV sound through this bread-board and my OTL.....
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Old 20th February 2007, 09:16 PM   #29
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Default well, here goes

Hopefully the schematic attached...

(Attaching that, took longer than designing the daft thing!)

N.B. Voltage measurement on Pin 7 (Grid) should read Minus 0.34V, not 0.34V
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Old 20th February 2007, 09:18 PM   #30
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Update: My 4 week old soldering iron is starting to melt (plastic handle) and hiss. Not good!

I have just changed the 2.26K cathode load resistors to 30K (before I read this). Less distortion than before but still not completely gone. The pot now has less effect on the sound. I need to analyse the output on my soundcard to see what effect this has had.

The manufacturer once advised me to use 33K - 47K resistors in parallel with the 'Grid-Bias'. This helped but did not cure the problem. In the end I added a stereo pot across these resistors. The distortion lessened with resistance but never went entirely. At very low R, the low frequencies would begin to fade rapidly just as the clipping was!!!
I guess that I should probably adjust the 'Grid-Bias' resistors to the lowest possible before the L.F. response fades. and then try raising the 'cathode load' slightly.

Darn!!! My simulation seemed to indicate that increasing the cathode load was a good idea. How naive am I?

Could it be a combination of two or more things?

I'll be back with more results!!
Martin.
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