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Old 9th December 2011, 05:26 PM   #1171
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Back to basics.

Ive replaced all fancy biasing with resistors, and still get very varied anode voltages on the driver valve. So I guess theres more gremlins loose somewhere, I'll keep hunting.
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Old 9th December 2011, 06:07 PM   #1172
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Plus or minus 10% is OK !

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Old 9th December 2011, 06:43 PM   #1173
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Originally Posted by Soonerorlater View Post
If this was my amp I would like to know that all is well apart from the ECC803s ccs and the RLD bias scheme for the EL84's. I had similar problems with the ccs's in the basic BH version. I simplified the amp by replacing the preamp ccs with a 675R resistor ( IIRC ) and biased the EL84's separately with 270R resistors. Happily this gave me a working amp with all the proper voltage values. I then reintroduced the ccs's one by one solving any local faults as I went along.

HTH Bill
OK, what if there was a -55V supply to feed the cathodes of the input stage 12AX7? If I figure on 0.6 mA per triode, and use a 47k resistor for the tail resistor, do you think that arrangement would compromise the performance of the input stage by orders of magnitude compared to the CCS in the tail?

I know the CCS would be a better solution, just looking for something easy and super-reliable to start with, and this post gave me the idea...

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Old 9th December 2011, 06:56 PM   #1174
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Originally Posted by Yvesm View Post
Plus or minus 10% is OK !

Yves.
Everything seems to up and running. However i think there is a slight issue with one of the ccs' as I cant match the anodes.

As for the bias, one is pulling about 74mA, and the other about 82mA. What will be the best solution to match these? Should I be looking to change the screen resistors so both sides match up?

Cheers
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Old 9th December 2011, 07:08 PM   #1175
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One array is shining a lot brighter than the other and I just remember that the LEDs in each are from different batches, would that be an issue? Not if I can match the current through them I guess?
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Old 9th December 2011, 07:36 PM   #1176
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Not if I can match the current through them I guess?
Exactly then it should be no issue.
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Old 10th December 2011, 03:20 PM   #1177
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Ahh sweet music.

It's all up and running, still needs a few tweaks - need to lower the HT a little more as the el84s are running at about 40mA (too high?) and a little hum, think heater wiring maybe seeing as im using a choke for the HT.

Im not sure I can drag it away from my record player at the moment though...

As for the sound. It would be unfair to judge it against my other BH, becasue this one uses better transformers but I am very impressed. The sound is very detailed and clean. And the dynamics are far superior! I love it! As for the RLD bias, it looks like my amp has neons. Very cool.

Many thanks to all for helping me, ill post a picture here soon.

Cheers

Charlie
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Last edited by Brouilly; 10th December 2011 at 03:24 PM.
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Old 10th December 2011, 03:39 PM   #1178
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Old 10th December 2011, 04:24 PM   #1179
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Congratulations!
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Hi Rongon,


Quote:
OK, what if there was a -55V supply to feed the cathodes of the input stage 12AX7? If I figure on 0.6 mA per triode, and use a 47k resistor for the tail resistor, do you think that arrangement would compromise the performance of the input stage by orders of magnitude compared to the CCS in the tail?

I know the CCS would be a better solution, just looking for something easy and super-reliable to start with, and this post gave me the idea...


Hard to beat the simplicity of a resistor.

Can't see any point in deliberately increasing the resistance and then supplying the negative voltage to compensate. Others more knowledgable might disagree. No harm in trying out your idea to hear the difference.

In the past I've played about with cathode resistor substitutes. I started with ordinary diodes, led's. an LM314 regulator and finally Gingertubes ccs. To my ears Gingertubes ccs made the most noticeable difference even though I used non exotic caps.

The thrill of building your own gear is you get to tweak
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Old 10th December 2011, 04:35 PM   #1180
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.01 across the heater winding of power transformer and .01 from heater pin to chassis ground helps.

22 pf cap across the leds too.

great amp design

Last edited by DavesNotHere; 10th December 2011 at 04:41 PM.
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