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ptp zenkit with 5ug4 rectifier, voltages off (Read 13051 times)
Raduschka
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ptp zenkit with 5ug4 rectifier, voltages off
02/11/15 at 02:06:44
 
Hi, I've had this hum from the very beginning of the adventure, a few years ago, when I finished the first zenkit1. I did it over a few times since, this time I used silver gold Mundorfs and naked z Vishays etc and it would sound great if it wasn't for the hum. I've tried just about every suggestion I found on the forum, to no avail. I've upped the C2 to 110uf. C1 is 40uf, maximum allowed for the 5ug4 rectifier (NOS STROMBERG CARLSON Raytheon Made 5U4G,Coke Bottle). Tonight I finally measured the voltages, something I should have done from the start, but I have a family... Smiley  And they are off Sad  300v instead of 346v. 273v instead of 309v. Third checkpoint seems fine: 10.6v instead of 10.25. And then 5.3v on pins 3 and 8 of the input tube, but I have the bias switch and cannot tell which position it is in.
I am still using the original transformer the kit came with, the one that was later replaced with the blue Edcor.
Any ideas why my voltages are off? Please help
Raduschka


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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #1 - 02/11/15 at 12:36:36
 
Seems you are using a tube based rectifier instead of the the solid state rectifier shown in the schematics.
That will significantly drop all the voltages, starting at point A.
Using a quick PSU Designer II simulation I get about 40 volts lower using a 5U4G vs. 1N4007 in full wave mode.
Are you using the same series resistor values on the output of the rectifier that are shown in the ZKIT-1 schematic? 1K and 6.8K?
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #2 - 02/12/15 at 00:24:52
 
oh thank you Core32!!   I've been alone with this forever...   Yes, I'm using the same resistors, 1k/6w and 6k8/2w. I was starting to think that I had the wrong transformer. So the 5u4g would be responsible for the 'sag' ( I think it's called that)? I had used the supplied rectifier diodes in the first incarnation of the zenkit1 and had the exact amount of hum though. It only got better when I went from 33uf to 40uf on C1 (maximum allowed for the 5u4g) and up to 60uf or so on the C2 (instead of 33uf). Right now it is useless as a headphone amp without at least 150ohm resistors in series with the headphones........   Any and every suggestion is appreciated!!!!! I would do anything to get this amp to work with my headphones (AKG k702, 62 ohm, 105dB)
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #3 - 02/12/15 at 00:30:26
 
This time I built the zen exactly as shown in the Decware videos on youtube; same values throughout, tube rectifier and still had hum.

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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #4 - 02/12/15 at 01:09:00
 
I didn't have any expectation it would cure the hum, just the low voltage readings.
Sorry for the confusion.
So with the diodes instead of the 5U4G your voltages are right but the hum still exists?
If so, I will also assume you've read this:
Quote:
Could someone provide a link to where the Virtual CT mod is explained?  I've looked but can't find it.  I'm getting the impression its two 100 ohm resistors to chassis ground, wattage unknown, on the 6.3 vac heater wires and it is needed to reduce hum when using a transformer that does not have a center tap on the 6.3 vac.  On transformers with a center tap, just ground the center tap, the Virtual CT mod is not used/needed.  Is this correct? Anyone try just grounding one of the 6.3 vac wires?  Anyone try anything other than 100 ohms?  Old hum balance circuits I've seen had a 1k linear pot with center post tied to chassis ground and the 2 other posts tied to the 6.3 vac wires...makes me think a little tuning could make things even better.
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #5 - 02/12/15 at 01:25:50
 
yep, I had read and tried that, thanks! Even though the transformer has a center tap... I also installed all kinds of wires between the volume pot and the rca ground lugs.
With the diodes in place I had the same hum, however I did not measure the voltages for fear of electric shock Smiley I got over that fear yesterday.
I somehow take it for granted that low power supply from the psu WILL cause hum, am I wrong?
two quick questions: should I lower the 1k 6w to increase voltage and if so, by how much? I have other 1k 6w resistors on hand.
And second: I replaced the 2x 20uf electrolytic caps in series with one 15uf film cap. I do not understand the idea behind those 2 electrolytics, except perhaps to bridge a long distance inside the chassis????
Thanks
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #6 - 02/12/15 at 01:40:19
 
...the 15uf cap's power rating is of course more than double that of the electrolytics in series (650v vs. 2x250v)
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #7 - 02/12/15 at 01:40:52
 
Sorry but I do not have the tube experience to know whether the low voltage "would" cause the hum.
I suppose it's possible but I would expect high ripple on the B+ supply or noise on the inputs to be a more likely cause.
I do know that from my reading you should ground everything at one point, AND ONE POINT ONLY to the chassis, not have several places around the top plate grounding components. That includes the RCA connector mounting I believe. Possibly a common point of error.
The series electrolytics effectively double the voltage rating of each, while halving the capacitance to 10uF. A single high voltage 15uF cap will be fine there.
To raise the voltage levels you can reduce the series resistors values some but I would be careful not to go too far too quickly.
Placing another 1K 6W in parallel with the original one may raise the voltage too high and damage something.
I would need to run a simulation to see what is a good choice to try.
Do you have any other 6W resistors on hand?
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #8 - 02/12/15 at 01:44:41
 
I will have to go take a look, but I doubt it. I have a large quantity of 5w  1k kiwame resistors.
Everything is grounded at one point only, per instructions in the videos on youtube...
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #9 - 02/12/15 at 01:45:47
 
please run the simulation if you have a moment, it might mean a breakthrough in a cold case Smiley
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #10 - 02/12/15 at 01:56:47
 
I have:

1x 12w 100k mills resistor;  2x  5w   10k  mills resistors!
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #11 - 02/12/15 at 01:58:51
 
could go down to 800ohm, if I solder 1k and 2x 10k in parallel
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #12 - 02/12/15 at 02:08:54
 
So placing a 1K 5W in parallel with the original 1K 6W resistor gives you effectively 500 ohms.
Running a simulation with that value shows an increase to about 337VDC at the first test point.
If you want to try that and see if my sim. is set up close we can try to adjust it higher and adjust the second test point as well.
And be safe around these voltages. Deadly for sure!
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #13 - 02/12/15 at 02:12:04
 
I see, so after all 500ohm instead of 1k in that position seems ok? I can go straight for that. Yes, I loathe measuring voltages! I'm off to soldering those in then. I will use a kiwame 5w 1k, i think 5w should be ok? what do you think?
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #14 - 02/12/15 at 02:14:34
 
5W will be fine in parallel with the 6W.
Remember to drain the voltage off the power caps before touching with a soldering iron.
That's a pretty straight short to ground for some POP...
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #15 - 02/12/15 at 02:14:47
 
otherwise I have those 6w 1k mexican resistors that Steve uses, DALE I believe, they seem to be covered in tar Smiley
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #16 - 02/12/15 at 02:18:03
 
The power gets split when you parallel those so 5W is fine.
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #17 - 02/12/15 at 02:18:19
 
yes, i always discharge the caps, Even the itsy bitsy ones, just to be safe Smiley  I'm off to soldering then, will post results right after the headphone test. Your help is priceless, thank you again
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #18 - 02/12/15 at 02:28:42
 
I swear in your "guts" picture when I zoom in it appears there is some type of lug at the RCA connectors that appears to be touching the top plate.
Maybe.
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #19 - 02/12/15 at 02:37:10
 
I see what you mean, however I swear it does not Smiley   I have just added the 1k on piggy back, connecting cables
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #20 - 02/12/15 at 02:41:41
 
the noise is the same....:-(
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #21 - 02/12/15 at 02:45:37
 
So voltage is most likely not the issue.
You can check to see if the volts did go up some as I simulated if you want.
So I would look for grounding issues.
Where in the picture is your central "star" ground point?
And, have you pulled both inputs off the RCAs and tried running the volume up and down to see if the noise is coming from the inputs or after the preamp tube?
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #22 - 02/12/15 at 02:49:47
 
i will update the picture so it reflects the actual config, with the 47uf solen. The grounding happens right in the middle, where you see the red resistors. The voltages MUST have gone up, right?

Yes, I have just unplugged the rca cables and run the volumes up and down again... Absolutely no change.

Is it possible to find out whether the noise comes from the input or the output stage?
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #23 - 02/12/15 at 02:52:52
 
I have just swapped the input tube to 6922. same hum.

Could I have a faulty transformer?
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #24 - 02/12/15 at 02:56:44
 
Well if the hum had gotten louder/softer that would have told me the noise was entering before the volume control pots.
I also assume you have checked and the hum is about the same from both left and right speakers?
I think its safe enough to try this.
Pull the input tube and leave it out while running the amp.
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #25 - 02/12/15 at 02:58:13
 
hum is the same in both channels, I'm going to do the input tube trick now
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #26 - 02/12/15 at 02:59:44
 
hum is right there, unchanged, with the input tube absent. What does that tell us?
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #27 - 02/12/15 at 03:05:46
 
It's beginning to tell me I don't know what I am doing  :)
Unfortunately we have only eliminated a couple of possible sources and there are a few that are hard to isolate.
Sorry but I am running out of ideas.
You say this is the same on all your Zen amp builds?
How many would that be?
Seems possibly the problem is outside the amps.
Do your speakers have powered subs?
Is there any hum using a different amp but all other components and interconnects being exactly the same?
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #28 - 02/12/15 at 03:10:34
 
haha:-) Welcome to my world.
This is the 4th build I believe. The only thing that survived from the original build is the transformer. Interconnects and power cable are fine everywhere else. The hum is audible in my headphones (105dB). I have 10 inch audio nirvanas alnico and other fostex cabinets, no hum there if I recall correctly. But sound is a bit off, like the voltages. I have a minitorii, with Vcaps and obbligatos and whatnot. It only hummed when i lowered the reservoir caps of the 6x4's with 10uf. No hum whatsoever now that i went back to 47uf.
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #29 - 02/12/15 at 03:11:46
 
that should read: lowered to 10uf
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Core32
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #30 - 02/12/15 at 11:58:43
 
Ok. I did not realize you were only hearing this through headphones. Very sensitive ones at that.
I'm going to speculate that the kit is as good as you can get it if you can't hear the hum through decent speakers.
Getting the voltages correct might solve what you are missing through speakers when you say "But sound a bit off.." More adjustments to the series resistors can correct the low voltage for sure.
How do you have the headphones connected to the amp?
An impedance mismatch between the amp output and your phones might make the hum more audible.
Just postulating now.
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Raduschka
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Re: ptp zenkit with 5ug4 rectifier, voltages off
Reply #31 - 02/12/15 at 13:35:52
 
Hi Core32,

I have used resistance in parallel with the headphones to lower the impedance to 8ohm (headphone imp. is 62@). The hum remains the same. I have to use above 100ohm in series with the headphones to get rid of it. I don't know what that does in terms of mismatched impedance, but Steve seems to say it doesn't matter, as he suggested I use as much as I need to get rid of the noise.
So maybe you re right, this is as much silence as I can obtain from the amp. I am glad you helped me get the voltages on track, i will measure them this week end. Thank you very much!
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