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ptp zenkit1 with 5u4g rectifier-voltages off (Read 2613 times)
Raduschka
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Posts: 242
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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Raduschka
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Posts: 242
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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jdb996
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Re: ptp zenkit1 with 5u4g rectifier-voltages off
Reply #2 - 02/04/21 at 02:22:34
 
I stumble across this. I'm going to give it a try I think.

http://www.emissionlabs.com/Articles/APP-NOTES/AN-01-Designing-with-5U4G-and-274...
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Divasson
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Re: ptp zenkit1 with 5u4g rectifier-voltages off
Reply #3 - 02/04/21 at 12:14:30
 
Hi, in the first picture posted in your first post, there is a capacitor that seems to be shorted due to having been touched by the side of a burning soldering iron. May that be the issue?
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bobc
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Re: ptp zenkit1 with 5u4g rectifier-voltages off
Reply #4 - 02/04/21 at 12:48:33
 


Ripple.... In my experience, 1st cap = 47 uf. 2nd cap with diodes you can double this up for sure. I have even done this with a tube rectifier build, the current draw is such that many tube rectifiers can manage. The 2x20 doesn't impact ripple too much, 1st and 2nd caps.... Maybe the rectifier will have shorter life, but so far so good.

Try 47K/1K/47K for first PS section. You can experiment with the adding a second 47K for 2nd cap. We know for sure Smiley the current unit has a 47uf capacitor in this position from these forums and documentation.

Off voltages can be due to line voltage variations. I would not change the 6k8. More importantly, the 500K resistor vs. 1K will offer MUCH less filtering than the 1K. The 1K resistor also has some inductance as it is wirewound.

I have built up this circuit a few times. If you want to depart from the zen triode concept you can try chokes in the PS. A 10H choke in first stage and another RC stage will bring hum to near zero, but will depart from the Zen concept., and perhaps at expense of "speed".

Does the heater tap on transformer have a center tap? If not you can add one with 2x100ohm resistor to reference gnd (if not already). This has also helped.

AND as you may know.... a loose / weak solder joint in the HV supply will hum no matter how many capacitors. A loos weak solder joint on input jack will do the same. Good luck!
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bobc
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Re: ptp zenkit1 with 5u4g rectifier-voltages off
Reply #5 - 02/08/21 at 14:19:10
 
Also google the robinette box. This creates headphone interface for amps like se84.
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