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ptp zenkit1 with 5u4g rectifier-voltages off (Read 680 times)
Raduschka
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Posts: 239
ptp zenkit1 with 5u4g rectifier-voltages off
04/06/16 at 00:00:44
 
Hello,  I have built for the N-th time the zenkit1, this time with a tube rectifier in point-to-point form, following closely the youtube tutorial (the one in 12 parts). It has allen bradley 47k plate resistors on the input tube, silver/gold Mundorf 0.1 caps and i forget what else...plus a headphone out tapped from the binding posts. I'm also using the original power and output transformers from quite a few years back (maybe 6-7 years?? No Edcor in any case).
The problem is that if I use the specified cap values in the power section, I get horrible hum and off voltages. That is how I gradually replaced the first 1k resistor with a 500 ohm one and upped the first cap after the filter cap ( that one stays at 40uf, maximum for the 5u4g), that is C2, to over 100uf.
If i plug in my sensitive AKG k702 headphones (105db, 62 ohms) straight, without 100ohm resistors, the hum is very present. I measured the voltages today and they seem ...kind of...off...not quite... Here is where I need help!!!! Could these voltages be a direct cause for the hum?
here they are:
a. 334v (vs. 346v +/- 10v)

b.290v (vs. 309v +/- 10v) This one seems off by 10V!!!

c.12v (vs 10.25v +/- 2v)

d.3v (vs. 2.6v +/- 0.5v)

Should I replace the 6k8 resistor with a 5k one and remeasure point B?

I appreciate any kind of input, I'd love to keep my AKG rather than get less sensitive headphones. Just go gently on me, I'm learning big time!

pics:

https://www.flickr.com/photos/71648043@N02/25654886054/in/dateposted-public/

https://www.flickr.com/photos/71648043@N02/16495407031/in/dateposted-public/

https://www.flickr.com/photos/71648043@N02/14426339272/in/dateposted-public/

https://www.flickr.com/photos/71648043@N02/14336465633/in/dateposted-public/

https://www.flickr.com/photos/71648043@N02/15869114473/in/dateposted-public/
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« Last Edit: 04/06/16 at 00:14:33 by Raduschka »  

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Raduschka
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Posts: 239
Re: ptp zenkit1 with 5u4g rectifier-voltages off
Reply #1 - 04/11/16 at 02:50:41
 
Anyone?
I have just powered up the amp without the input tube, as per Dank's suggestion. Same hum Sad

I replaced the 2x20 uf in series caps with a huge solen 47uf cap... Nothing Sad

I replaced the 6k8 resistor with a 5k one to obtain 309V of B+. (I have a 500ohm resistor instead of 1k right ater C1)

C2 capacitance is up to almost 160uf...Nothing Sad

Would elevating filament circuit help? Does the input tube test mentioned above prove anything about input heaters?

Please help
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