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Old 26th January 2007, 04:48 PM   #1
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Default Rp / Rk on Aikido preamp

Have I done something stupid?? I'll need to check my Aikido Octal PCB for resistor values, but I was just going back over Broskie's notes for the PCB. Underneath the circuit diagram, he lists part values. I now think that I have misinterpreted his R2, R4 and R8, R11.

R2 and R4 are for the input tubes, while R8 and R11 are for the output tubes. I am running 6sn7s and the outputs are run hotter than the inputs.

So, what I did was to use 1K for input, and 680ohm for the output. HOWEVER, I think that while I did this but missed out something important:

I used 1K for both R2 and R4, and 680 ohm for R8 and R11.

On the final page of the PDF, Broskie gives a table of different values for running various tubes. He has a column for Rk, but there is also a column for Rp. The values for Rk and Rp for a given voltage can be quite different. From the circuit diagram, I can see that R2 and R11 are cathode resistors???? But what are R4 and R8??? I read them also as cathode resistors and put 1K and 680 ohms in them respectively. Maybe, these are the plate resistors (Rp) so maybe for 300V with 1K and 680 ohm Rk's, I should have used 9.8K and 9K. However, on the part vlue list, he has 2K as being the highest value.

Hmmmm.....Have I misread his layout and inserted incorrect resistor values for R4 and R8???? Or have I just had a brain-fart and got things correct anyway.

Thanks,

Charlie

"One of these days, I'll be able to give more advice than I take. Maybe this would be dangerous"
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Old 26th January 2007, 11:02 PM   #2
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Hi Charlie,

R2 and R4 will be the same value. R8 and R11 will be the same value. You should have about 5mA running thru R2 and R4 and 10mA running thru R8 and R11 if my memory is correct. Now, you can change the operating point to suit your taste and tubes and some people do this.

R2, R4, R8, and R11 are cathode resistors.
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Old 27th January 2007, 12:33 AM   #3
Sheldon is offline Sheldon  United States
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I assume that the rp (lower case r) in the table refers to the plate resistance of the output tube at the indicated operating point. See here: http://www.tubecad.com/2006/06/blog0071.htm

It's used to calculate the output impedance.

Sheldon
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Old 27th January 2007, 02:40 AM   #4
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Sheldon, BurnedFingers,

Yes, I do have my resistors correct.

More worrying is that I have been working on the B+ this evening. In recent measurements, I was only getting around 200VDC from B+. My PSU is 125-0-125; graetz bridge; 1.5uF cap; 10H choke; 60uF cap; 10H choke; 60uF cap.

This should give something like 300 - 340VDC.

So something is wrong. I totally unsoldered my transformer. When I measure voltage across the secondary (ignoring the center tap) I only get around 175VAC. Surely I should be getting more like 250VAC.

Maybe by transformer is busted, maybe it was always this way???

Any ideas? Did I measure it incorrectly???

Charlie
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Old 27th January 2007, 02:46 AM   #5
JoshK is offline JoshK  Canada
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What are you measuring at the wall?
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Old 27th January 2007, 03:16 AM   #6
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JoshK,

Funny thing.....my multimeter reads only 61V. I have it set on AC, but seems to be reading only half the cycle. I suppose I should have a read of the manual.

Charlie
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Old 27th January 2007, 04:42 AM   #7
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I believe that my multimeter is having issues. I have had it for four or five years, and it was one of the more expensive Radioshack ones. For some reason it only seems to be reading about half of what it should be reading.

Charlie
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Old 27th January 2007, 10:16 AM   #8
Stixx is offline Stixx  Germany
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Quote:
In recent measurements, I was only getting around 200VDC from B+. My PSU is 125-0-125; graetz bridge; 1.5uF cap; 10H choke; 60uF cap; 10H choke; 60uF cap.
I don't know how you are trying to arrive at 300-340V from a 125-0-125 trans...Isn't it rather a 250-0-250??
Even then , using such a small first cap, you are not going to get more than 300V (i simulated this very quickly...).
Plus you have a lot of overshoot...

Can you clarify??
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Old 27th January 2007, 10:25 AM   #9
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"I believe that my multimeter is having issues. I have had it for four or five years, and it was one of the more expensive Radioshack ones. For some reason it only seems to be reading about half of what it should be reading."

You should replace the battery in the meter and try again.

-- josť k.
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Old 27th January 2007, 02:55 PM   #10
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Stixx,

My claculations suggest that I should get somewhere around 300VDC, using the 125-0-125. When the secondaries (not the center-tap) are used, I should get 250VAC out and into a cap-input filter that should result in 250* sqrt2 which is around 353VDC. For a choke input, I should see something more like 250VAC * 0.9 = 225 VDC. The first cap is a cheater cap, which can be used to adjust the voltage of the final B+ to somewhere between 225VDC and 353VDC. However, at 300VDC, a 10H choke requires 300/10 = 30mA minimum current to work properly. My guess is that I am pretty close to that or just under.

Of course, I may have made a terrible mistake in my calculations and you may be absolutely correct. Please let me know, so that I can correct the errors of my ways.

Of course, my hum issues may simply be remaining ground-loop hum and it may clear if I up the 10ohm resistor to say 20 ohm.

Yes, I'll try a new battery before I buy a new multimeter!!!!!

Charlie
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