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Old 23rd May 2007, 10:43 AM   #1
arnoldc is offline arnoldc  Philippines
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Default LTP w/ positive only supply

Folks,

I've been toying around with a LTP with positive only supply (shared cathode resistor for both) and I have a concern about imbalance and distortion.

With a CCS on the tail, I don't think this is much of an issue.

Any thoughts?
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Old 23rd May 2007, 03:45 PM   #2
kevinkr is offline kevinkr  United States
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I have done this, the only caveat is making sure you have enough compliance voltage for the CCS.. Usually this means ac coupling to the inputs of the LTP and sufficient positive grid bias voltage to provide the compliance voltage the LTP requires. I have not had good results even with 6SN7 without making provisions for this, and with say 12AX7 or 6SL7 the provision for additional compliance voltage (or a negative supply) has been necessary for good (read acceptable) linearity.
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Old 23rd May 2007, 05:28 PM   #3
SY is offline SY  United States
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The issue with 6SN7, despite the rather high bias voltage, is probably the low gain, which bites hard into the compliance. In an LTP splitter, you've got half the input at the cathodes.

Your advice on using an RC coupling to float things a little higher is spot-on.
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Old 23rd May 2007, 06:13 PM   #4
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Default Re: LTP w/ positive only supply

Quote:
Originally posted by arnoldc
With a CCS on the tail, I don't think this is much of an issue.
It isn't. Cascoded BJTs work very well for the CCS.

How much gain do you need? You won't be needing a negative tail rail if you can DC couple to a previous stage. Just connect a 1.0M resistor between the LTP control grids, connect one half of the LTP to the preceeding voltage amp, AC ground the opposite half with a 0.1uF capacitor. Be sure to keep an eye on those Vhk ratings, as they are usually rather limited for small signal VTs.

Problem solved.

If there's no stage in front of the LTP, simply DC bias the AC grounded end of the grid resistor with a bypassed voltage divider, and AC couple to the LTP's input side.

Problem still solved.
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Old 23rd May 2007, 06:20 PM   #5
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Arnold,

The Schmitt circuit (see attachment) seems to be called for. Just replace RK2 with the current sink.

The previous posters are correct in warning about sufficient voltage for CCS compliance. If you use the "classic" CT full wave B+ topology, Pete Millett's 2 rails from 1 trafo "trick" will give you all the Volts you could possibly need. Look here.
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Old 23rd May 2007, 07:47 PM   #6
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Would this work as well.

Shoog
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Old 23rd May 2007, 11:56 PM   #7
arnoldc is offline arnoldc  Philippines
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Folks, thanks for the inputs, however, I should knock my head with a stone for not being clear, and being terse (no apologies, but it was close to midnight when I posted)...

1. I actually wanted to avoid the CCS on the positive supply-only LTP (I'm not doing this because I'm lazy ok? I just know that LTP with (-) supply always works, and most use it but a few (+) only)

2. But if I just tie the two cathodes to ground via a resistor, have a 1M or so tied to both the grids, would I get imbalance and or distortion?

3. Now that the CCS is somewhat endorsed and seems a bit obvious to be a logical choice, I can use the diyAudio CCS board for this? I should right? I plan to use a WE396A with 22K on the plates, 2K2 on the cathode, 8mA per triode. It should give me something like 23dB gain and +-85V on the output (not enough to drive a 300B ) I would replace the 2K2 resistor with the diyAudio CCS set for 16mA as a current sink... Will this be enough? or LM317 will do the job?

4. Now with the CCS in the "tail" I have to bump up my B+ to 400V?

5. I don't have an input stage as I have a preamp, so I have to bias the grid of the input triode to around -12V (because my cathode is at 6V), is this right?

Thanks again in advance.

edit:

I attached a schematic
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Old 24th May 2007, 12:15 AM   #8
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Bias the grids positive, the cathode will be x volts more positive as a function of your chosen operating point. The difference in voltage between your grids and cathodes is the operating bias. (It will be negative if you have done your calculations correctly.) Something like 30V should provide more than enough compliance, and yes you will need to compensate for the voltage eaten up by the compliance requirements of the ccs.

Note that at these currents the tail resistance for the required current would not be large enough to result in good ltp balance, you are far better off with a ccs. (Or you would need obscenely high plate voltage or a -200V or better negative rail.)

I would think the diyaudio ccs would be perfect for this application.
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Old 24th May 2007, 12:23 AM   #9
arnoldc is offline arnoldc  Philippines
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Here's the one with CCS:

edit:

Just saw the post of Kevin...

So I'll need +30V something volts on the input grid... and really ditch the Rk in favor of the CCS...

Is this circuit good enough then?
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Old 24th May 2007, 12:48 AM   #10
kevinkr is offline kevinkr  United States
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Looks good except the 100K should be more like 1K to audio signal source,(see my earlier posts for more detail) grid bias voltage can probably be less than what I indicated in my last post without problems, but it is not too big a % of the available supply and will keep you far away from non-linear behavior in the ccs. You can derive it from your existing plate supply, just make sure it is well filtered.
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