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Old 14th April 2009, 09:49 AM   #1
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Default RH84 questions

Hi,

I am going to build RH84. I would like to use LED cathode bias for the EL84 - how do I calculate how much leds do I need. I will be using 10mA red leds.

Do I have to use 12AT7 for the input tube? Can I use ECC88 or the circuit demands just 12AT7?

Thank you.
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Old 14th April 2009, 10:43 AM   #2
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If you are going to build the RH84, build it in strict accordance with the given circuit if you want it to behave like the RH84 should. The configuration of the power supply is up to you, you can even not use a tube rectifier, although it is recommended for the best sound.

If cathode bias of the output tube is done by LEDS, or diodes for that matter, it will not function as it should. Frankly, I do not see why would you want to use LEDS to bias the output tube, maybe some light show?

The circuit is well balanced and calculated for ECC81 driver and EL84 output tube. Instead of the ECC81 you can use 5695 and E180CC with good results. ECC88 is not at all appropriate for this circuit.

For other attempts try other threads of the forum, as the amp was quite well discussed and documented by those who built it.
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Old 14th April 2009, 10:59 AM   #3
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Hi Alex

I understand that indeed the ECC81 can't be substituted for the ECC88 as the ECC81 spots a higher internal resistance, which is necessary for this circuit to work well...

But about the LED bias for the EL84... As I understand it, LEDs offer a low AC impedance, but provide a DC shift that biases the tube. This is actually the same function as provided by the bypassed 270R resistor in the original RH84, but the leds have the advantage that they don't suffer from blocking distortion due to overload, as thoroughly explained by SY in his "red light district" article. This way it is surely not recommended to use a led to bias the ECC81 (as the unbypassed resistor in the cathode does increase internal resistance), but I think that LEDs can't hurt in the EL84 cathode. Of course it won't be an original RH84 amplifier, but it still will have the local 'plate to plate' feedback and most certainly offer better performance when it comes to overload.
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Old 14th April 2009, 11:14 AM   #4
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Thank you for your replies.
Ok I will use ECC81/12AT7 for the input.
The cathode LED bias will work, but I dont know how much voltage drop do I need to make with LEDS.
I want to use the same LED biasing technique as in Red Light District.

When I will use 10mA red LEDs, then I will have to use at least 6 strings parallel, as EL84 is biased cca at 47mA. But how many LEDs in each string should I use?
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Old 14th April 2009, 11:30 AM   #5
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Kacernator

As you already mention the RLD you have probably also seen that SY uses a regulated and adjustable PS for the screens of the EL84's. When using a fixed PS and a output transformer with low RDC one should indeed spend special attention to the current flow, as there is no DC 'feedback': that is, if current increases there is no plate or cathode resistor that counteracts this increase by developing a higher voltage across it and therefore decreasing the voltage (and current) across the tube (the basic of auto-bias).

So, well, I would recommend you to use the same number of leds as used by SY in the RLD and play with the value of the screen resistor to adjust for sufficient, but not to much current. This setting will most certainly not be the best for another tube (as they are all different) so the best thing would be to use an adjustable regulator on the screens...and well, there goes the 'simplicity'

I hope this is more or less right...
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Old 14th April 2009, 11:45 AM   #6
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Hm, you are right about the simplicity. After all I think I will sacrifice little of performance for the simplicity, so will go with resistor cathode bias.

Lastly, what do you think of this PSU?...attached imaged.
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Old 14th April 2009, 11:49 AM   #7
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There is some overshoot (?) in that PS, try to fiddle a bit with the capacitor values to eliminate the 'bump'.

Also a small (220nF) capacitor at the input may make life of the input choke easier, while the PS still operates as a choke input one.
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Old 14th April 2009, 12:03 PM   #8
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Why is the overshot problem, when after less then a second there is none?

Placing 1uF before the first choke "flatten" the overshot...attached image.
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Old 14th April 2009, 12:07 PM   #9
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Hi Kacernator

Well, overshoot is not nice, I have been told so one should try to prevent it. Here is a nice article discussing it!

http://www.dhtrob.com/overige/pdf/dhtrob_psu.pdf
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Old 14th April 2009, 12:30 PM   #10
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Thank you very much.
I solved the overshot by changing the second choke for 100 ohm resistor.
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