Assistance needed in finding B+ and correct Cathode Resistor, Simple SE - diyAudio
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Old 25th May 2010, 08:34 PM   #1
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Default Assistance needed in finding B+ and correct Cathode Resistor, Simple SE

Hello all, questions from the builder of a new Simple SE.

I am trying to determine the correct cathode resistors for use with EL34 Tubes. I am using the Edcor EXPR035 Power Transformer and the Edcor CXSE25-8-5K Output Transformers. Iím also using a GZ34 rectifier tube. I have looked over the manual, in particular, the Output Power Simulations from the Tubes And Transformers part of the manual. 1). How do I determine the B+ of the Power Transformer? I have looked over the manual and it isnít completely clear. Is there a particular section I should refer to? I have read the sections on multimeter use and electrical safety. I just want to make sure to take the measurements correctly so I don't destroy anything (or myself). Also, I am guessing that Rk is the resistor value for the given tube and B+ voltage. In addition, Is there any way to measure the Idle Current of the tube? My Dynaco Mark IIIís have a socket to measure; I realize these amps are different.

Thanks for your help.
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Old 25th May 2010, 09:09 PM   #2
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Quote:
Originally Posted by HTFAMOUS View Post
I am trying to determine the correct cathode resistors for use with EL34 Tubes. I am using the Edcor EXPR035 Power Transformer and the Edcor CXSE25-8-5K Output Transformers. Iím also using a GZ34 rectifier tube. I have looked over the manual, in particular, the Output Power Simulations from the Tubes And Transformers part of the manual.
Your SSE is very similar to mine, then. Same iron.

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1). How do I determine the B+ of the Power Transformer? I have looked over the manual and it isnít completely clear. Is there a particular section I should refer to? I have read the sections on multimeter use and electrical safety. I just want to make sure to take the measurements correctly so I don't destroy anything (or myself).
B+ is the main DC supply for this amp. You can measure it at the red OPT wires or at R1 (on the same side as C2). The power transformer supplies the raw AC to the power supply, but the final B+ depends on a number of factors. However, since your amp will be roughly the same as mine, I can tell you to expect a B+ of around 460V.

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Also, I am guessing that Rk is the resistor value for the given tube and B+ voltage.
Rk is the cathode resistor. Unlike your Dynaco, which is fixed bias, the SSE is cathode biased (or auto-bias). This means the tubes automatically fine-adjust their bias due to the inherent DC feedback there. You can read more about cathode bias elsewhere. However, the resistor value you choose sets up the "coarse" bias voltage and so that is why it tends to be different for different tube types and B+ voltages. Something like 680 ohms or larger might be appropriate for you.

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In addition, Is there any way to measure the Idle Current of the tube? My Dynaco Mark IIIís have a socket to measure; I realize these amps are different.
Measure the voltage across Rk and divide it by its resistance.
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Old 26th May 2010, 01:29 AM   #3
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Russ, thanks for your help; greatly appreciated.
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Old 5th June 2010, 01:58 AM   #4
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HTFAMOUS, what cathode resistor did you end up using? Can you provide me a link or something to place you bought? I'm going to use those Edcor TFs and EL34. I know I'm going to sound like a moron now but...I don't have a voltmeter or any other equipment so I'm trying to find a particular list of components someone used already so that I can put everything together without having to measure those values.
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Old 5th June 2010, 02:55 AM   #5
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Quote:
Originally Posted by imnewbie View Post
...I don't have a voltmeter
You absolutely must have at least one multimeter. Seriously. There's a Harbor Freight in virtually every state in the US. Go find one, give them four bucks, and come home with a multimeter.

7 Function Digital Multimeter
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Old 5th June 2010, 03:41 AM   #6
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Hello, I agree with Ty Bower, I think a multimeter is essential; in fact, it will save you money in the long run. Trust me, once you complete one DIY project you are likely to NEED to do many more. The multimeter is there to double check your work and it's there to double check many of the parts you buy. I'm using a Craftsman Multimeter 82139. I got it on sale for $30.00, it's auto ranging, it gets the reading correct the first time without guessing. I'm not an engineer, not even close and I'd hesitate to give any advice other than this...... I've built at least 15 DIY audio projects so far and I've used the multimeter (and a good Weller solder station) for every one. When I did my research for the Simple SE project I made sure to read all of the posts from George Anderson (Tubelab), Ty Bower and Russ Kinze. They have written extensively about this amp. Sorry if this isn't what you asked for but you can waste a lot of money (or hurt yourself) if you don't take some preliminary steps.
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Old 5th June 2010, 03:25 PM   #7
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That was a stupid noob question. I'll get some multimeters when I order new components for simple P-P.
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