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Old 11th November 2011, 09:29 PM   #11
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Thanks for your patience SY. Putting the 56k resistors back in I have adjusted the bias so that I have 167 volt drop on one side along with 163 volt plate to grid difference.

The other side is 162 volt drop and 186 volt plate to grid difference. It falls into what you have said. Isn't the almost 20 volt difference on the grids important or is that not an issue? It gives me around 3 ma per side. Is everything kosher?
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Old 11th November 2011, 09:31 PM   #12
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Sorry I had 119v on one grid and 100v on the other. The voltage drop on the plate resistors seems good.
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Old 11th November 2011, 09:46 PM   #13
tricomp is offline tricomp  Denmark
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You can't measure the grid-voltage on the second tube in the LTP 100% correctly.
The grid-circuit impedance is far too high and your voltmeter will be loading the actual value down. Even with a 10Mohm/V hi-impedance voltmeter you will still be adding a resistor in parallel with the present 1Mohm grid-resistor and read voltage out of what's expected. And that's with a 10Mohm/V meter, most digital-meters have a 1Mohm input-impedance making things even worse.
Never take and trust grid-1 readings unless the grid is DC-driven from a low-impedance circuit like the input triode in the LTP or like the g1 of the 2C34 output triode in the 2C34-PP amp above. Here the g1 circuit is a low-impedance cathode-follower and readings will be valid.

3mA per side totals 6mA in the LTP which is very close to the estimated original 6,8mA. I wouldn't worry about the difference.
I WOULD worry about the output-impedance of the CCS (see MJ on impedance and how to calculate)
rgds,

/tri-comp

Last edited by tricomp; 11th November 2011 at 09:49 PM.
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Old 11th November 2011, 09:50 PM   #14
SY is offline SY  United States
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What tricomp said. The drops you're reporting look excellent. I think you're doing great.

One other thing- the impedance of your CCS is likely to be pretty high (that's good). What makes life even sweeter is that an LTP CCS doesn't have to be as good as a plate one because the impedance at the cathodes is pretty low in comparison.
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Old 11th November 2011, 10:30 PM   #15
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Thank you guys very much. I've been up all night here with this thing but I'm really happy I'm learning something.

Tricomp your point about the BD139 is taken. I will look to source the MJE340. I will need to do a lot more reading to take in MJ out-put impedance math. I do note that he said the output impedance is greatly increased using the d design. "the one you recommended to me in the first place. I will finish the other channel and give them a listen tomorrow "uh" later today. Haven't slept yet.
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Old 12th November 2011, 07:00 AM   #16
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Spoken like a real tube-geek

Don't always trust what you see on schematics, people make errors.
Even MJ on his proposed CCS-designs.
Exactly in the 'd'-circuit the wrote the wrong voltage-reading.
The MJE340-Emitter/BC549-Collector junction should read 134V, not 124.
Just goes to show errors creep in where you least expect them to.

I'll help you calculate the CCS if you want but doing it yourself and posting the result here will be more educational

rgds,

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Old 12th November 2011, 10:14 AM   #17
tricomp is offline tricomp  Denmark
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Quote:
Originally Posted by tricomp View Post
Exactly in the 'd'-circuit the wrote the wrong voltage-reading.
You see what I mean?
Where you least expect it!


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Old 12th November 2011, 12:07 PM   #18
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Well after a few hours asleep and some time with the wife, "if not she would fuss".

I have to say wow I am really happy with the results. Wish I would have done this a long time ago. I no longer feel the image is slightly off, it's really is solid and balanced. I was expecting the information retrieval to be improved because of the loss of distortion. But what I didn't expect was the feeling of naturalness. Sorry can't think of a better word.

I set both sides within the parameters in which SY provided. Here is where it sits.

Using 56k resistors "matched" I have about 155-162 dropped across the plate resistors.

152 to 155 volt differences between plate and grid and about 2.92-2.95 ma current each side.

I am still not comfortable with the BD139 voltage too high. I will try to source the parts very soon. Leaving for Guangzhou tomorrow and then will be busy for a few days.

tricomp I will read some more about the output impedance of the CCS and how to figure it out mathematically. I'm still learning and need to understand how it is calculated and how it effects the next stage and how to optimize it. I will do some reading.

By the way thanks you guys I learned something and I'm having fun.
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Old 12th November 2011, 12:18 PM   #19
SY is offline SY  United States
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The output impedance of the CCS is calculated by hoe + (hfe1hfe2)Re, and is likely to be well over 1M. hoe is rarely on the datasheets, but can be approximated by looking at the slopes of the collector current versus collector voltage curves. Generally, the second term dominates if the transistors have a high hfe.

As I mentioned before, in a cathode circuit, the CCS source impedance isn't terribly critical. If memory serves, Morgan Jones's book has a table of hoe for some common transistors, so you can come up with a pretty decent approximation.
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Old 12th November 2011, 04:48 PM   #20
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Ah, it really isn't very complicated.
I knew I had a document explaining the process of dimensioning a CCS in a very basic and understandable way with examples.
Just couldn't fint it ...until now.
Enjoy

rgds,

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