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Nobsound 6P1 tube audio amp

96.42.56.28
Posted on May 23, 2020 at 20:58:25
sbjohnston
Audiophile

Posts: 4
Joined: May 23, 2020
Thanks for all the interesting forums and posts. Time to post something myself...

My Nobsound 6P1 tube audio amp from Amazon (inexpensive from China) arrived this week. I inspected it closely - construction is excellent. The components seem to be of high quality and the soldering looks good (or at least as good as lead-free joints can look). The tubes were of Soviet and Chinese manufacture.

I first listened to a favorite CD on my usual workshop system: CD player straight to a Crown D-75 amp and Klipsch R-15M speakers. Then I connected the Nobsound amp in place of the Crown and listened to the same tracks again. WOW - what an improvement! The sound was very clear and clean, an immediately obvious step-up in quality.

My only concerns are the unfortunate name and the lack of technical documentation. I traced the circuits and made a little webpage to post the schematic.

http://www.wd8das.net/Nobsound6P1.html


Steve

 

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Looks like a decent circuit. , posted on May 24, 2020 at 01:47:08
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000
 Nice to see a single ended amp at this price - I imagine most are PP.

Does it also have a 230v option? Says 110v.

 

RE: Nobsound 6P1 tube audio amp, posted on May 25, 2020 at 12:44:32
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 You really should raise the heater to 80v above ground or your input tube will have a short life. The circuit violates the max heater to cathode voltage rating. Also, get a 30uf Solen and use it before the choke, then put both your big caps after the choke or the rectifier will have a short life as well.

 

RE: Nobsound 6P1 tube audio amp, posted on May 25, 2020 at 15:55:45
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 I have to agree with you about the max heater to cathode voltage rating but I would bet that if you did the math the 150uf input cap is probably not causing the rectifier tube to exceed it's peak repetitive current rating.

The voltage is pretty low and the current for that circuit is also probably pretty low. Note, the "max" input cap value given in the data sheets is for the example that is shown in the data sheet which is always right up against max voltage and max current. That's why the "max" input cap values are always so low as shown in the data sheets. What is really important is not exceeding the PRC rating of the tube. Lower the voltage and current (and higher power transformer DCR) and you can increase the value of the input cap a lot.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Nobsound 6P1 tube audio amp, posted on May 26, 2020 at 07:23:02
FlaCharlie
Audiophile

Posts: 940
Location: Gville, FL
Joined: June 1, 2003
 I'm not so sure that using a 150uf first cap is as innocuous as you think.

Here's a link to the data sheet for the Chinese 5Z4. It seems to indicate that the value of the first cap is only 4uf. The linked RCA data sheet for American 5Z4 has a graph showing both 4uf and 8uf.

The use of 150uf is a huge jump.

https://tubedata.altanatubes.com.br/sheets/095/5/5Z4P.pdf

https://tubedata.altanatubes.com.br/sheets/021/5/5Z4.pdf

I don't think I've ever seen "peak repetitive current rating" listed on a data sheet. Is that the same as "maximum transient peak plate current each plate"?

Also, when you say "if you do the math", what's the formula?

I'm aware that rectifiers that run at lower than maximum current and voltage can safely use first caps that are a bit higher in value but it's always been a bit of a vague concept.

 

RE: Nobsound 6P1 tube audio amp, posted on May 26, 2020 at 10:26:57
sbjohnston
Audiophile

Posts: 4
Joined: May 23, 2020
Thank you for these insights. I was aware of the heater-to-cathode overvoltage that might be caused by the "stacking" of the input tube sections but not about the first filter cap stressing the rectifier. I don't see any color on the rectifier plates but it might be a more subtle impact than that.

Thanks...

Steve

 

RE: Nobsound 6P1 tube audio amp, posted on May 26, 2020 at 11:11:09
sbjohnston
Audiophile

Posts: 4
Joined: May 23, 2020
Thank you - very interesting points, especially about the way the rectifier and filter are interacting. I don't see any color on the rectifier but it may be a more subtle issue. I may well reduce the filter input cap value to be safe.

Steve

 

RE: Looks like a decent circuit. , posted on May 26, 2020 at 11:28:07
sbjohnston
Audiophile

Posts: 4
Joined: May 23, 2020
Yes, I believe I noticed 230 VAC versions offered somewhere online - direct from China? Or perhaps non-US Amazon sites?

The power transformer in mine appears to have a primary that is able to be wired for either voltage, so you could move one wire, reduce the rating of the fuse, and be operational on the higher line voltage.

Steve

 

RE: Nobsound 6P1 tube audio amp, posted on May 26, 2020 at 18:39:03
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002



" Is that the same as "maximum transient peak plate current each plate"?"

Yes.

This tube (5z3) has a PRC rating of .675 amps and a steady state plate current rating of .250 amps. Sorry, the 5z3 is not included in the sim or I would have simmed it.

You can see that the PRC (.220 amps) is less than the steady state plate current rating (.250 amps) and that's with a 150uf input cap and the data sheet says 4uf max but what people don't understand is that the 4uf max is for the operating condition given in the data sheet. The operating condition that goes with the max input cap value that the data sheet is talking about. Change the operating condition and you can change the max value for the input cap. The operating condition includes the voltage, current and the "series resistance per plate" which is usually the DCR of the power transformer although you can add resistor between each end of the secondary winding and each plate.

I can sim an operating condition where 4uf would be to much. I can and did sim a operating condition where 150uf is no where near too much. There are operating conditions everywhere in between.

Again, it's all about whether or not the PRC rating of the tube is being exceeded. It's not just about the value, in isolation, of the input cap.

Stop reading the "max input cap value" as some kind of absolute. It is not. The only absolute is whether or not the PRC rating of the tube is being exceeded.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

Here are some more examples this time using a 5ar4, posted on May 26, 2020 at 19:14:29
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002





The "max" input cap value is stated as being 60uf. Look at the data sheet. It shows 6 different operating conditions each using 60uf but as the current goes up the value of the "limiting resistor" goes up. If you lower the load current you can increase the value of the input cap. If you increase the value of the limiting resistor you can increase the value of the input cap.

Here's an example where 60uf is too much.






Here's an example where 220uf is close but not exceeding the PRC of the tube.




So it depends on the operating conditions not just the value of the input cap.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Here are some more examples this time using a 5ar4, posted on May 26, 2020 at 19:41:00
rivervalley817
 no images Tre` ... is it on my side?

 

RE: Here are some more examples this time using a 5ar4, posted on May 26, 2020 at 19:47:04
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 I show three pictures and can load them in isolation. Try this.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Here are some more examples this time using a 5ar4, posted on May 26, 2020 at 19:55:09
rivervalley817
 must be me ... turned off ad & script blockers & just see placeholders and your link returns an error ... odd, I'll revisit this manana

thanks for your reply

be well,




 

It's not just you, posted on May 27, 2020 at 04:33:13
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 There's no images showing for me either, and the link does give an error.

 

RE: Nobsound 6P1 tube audio amp, posted on May 27, 2020 at 07:55:54
FlaCharlie
Audiophile

Posts: 940
Location: Gville, FL
Joined: June 1, 2003
 Tre', thanks for the explanation.

I was able to duplicate your sim in PSUD although I can't get the graph section to work. I must have a setting wrong somewhere as I've never had much luck with the graphing function.

Not sure if I added it at some point but I do have the 5Z4PA available. The specs for it are:
PIV 1550 V
IFSM 99 A
IFRM 0.675 A

I'm also getting no images and a dead link on your second post, though.

 

I will try to post it again., posted on May 27, 2020 at 10:07:22
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Here are some more examples this time using a 5ar4, posted on May 27, 2020 at 10:11:52
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002



The "max" input cap value is stated as being 60uf. Look at the data sheet. It shows 6 different operating conditions each using 60uf but as the current goes up the value of the "limiting resistor" goes up. If you lower the load current you can increase the value of the input cap. If you increase the value of the limiting resistor you can increase the value of the input cap.

Here's an example where 60uf is too much.




Here's an example where 220uf is close but not exceeding the PRC of the tube.




So it depends on the operating conditions not just the value of the input cap.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Can you see the pictures now?, posted on May 27, 2020 at 10:12:27
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Can you see the pictures now?, posted on May 27, 2020 at 10:16:02
rivervalley817
 I'm good here ... what did you do differently?

with regards,


 

RE: Nobsound 6P1 tube audio amp, posted on May 27, 2020 at 10:19:34
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 You want to check the box next to "I(D1)". That will show the current flowing through one of the two diodes. You then want to click the "make it smaller button". then you use your mouse and a left to click and drag so you can get to see just what you want. Maybe just the middle part, left to right, with the top and bottom just showing.
That probably didn't make any sense at all. Sorry.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Nothing different., posted on May 27, 2020 at 10:20:03
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Nothing different., posted on May 27, 2020 at 10:25:16
rivervalley817
 oh great ... now I still have to root around in my system


with regards,


 

The reason the peak current is higher than the load current, posted on May 27, 2020 at 10:54:17
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 I got thinking that maybe an explanation is needed.

With a cap input filter following the rectifier tube each diode in the rectifier tube is only on for a short time.

Let's start at the beginning. Current will only flow through a diode one way, one direction. Current will only flow through a diode if the voltage at the input to the diode is higher than the voltage at the output of the diode.

First let's look at the power supply circuit as a whole. The secondary of the power transformer has a center tap that is grounded. Each end of the secondary winding is connected to the input of one of the two diodes in the rectifier tube. The output of those two diodes join together at the high side of the cap. The low side of the cap is grounded.

When the voltage between ground and one side of the winding is positive, the voltage between the other side of the winding and ground is negative. Negative voltage will not flow through a forward facing diode. So each diode can only be on/conducting half of the time.

With a critical inductance choke input filter connected to the output of the diodes, each diode (in turn) will be on/conducting for it's entire half cycle. (That is definition of "critical inductance". "the inductance value that causes each diode (in turn) to be on for it's entire half cycle".) The current through the diodes, when connected to a choke input filter, is not much more than the load current (the current the circuit is drawing).

With a cap input filter connected to the output of the two diodes, each diode (in turn) is only "on" (conducting) for a small part of their respective half cycles.

That being said, the power transformer and the diodes still need to deliver the same amount of power. To do this the diodes need to deliver a lot more current since each diode will only be on for a short time.

This is like only having one hour to do a project around the house vs. having all day. When you have all day you can work slow and steady and get the job done (low peak current) before the day is over. When you only have one hour to do the job you have to work your butt off (high peak current) to get the job done.

The reason the diodes act this way when connected to a cap input filter vs. a choke input filter is because the cap retains some of its charge during the time the diodes are off. The voltage from the power transformer secondary winding has to get above the voltage at the cap before the diode will start conducting. The higher the value is of the input cap, the less voltage it will lose between diode cycles so the shorter the time is when the diode will be "on" so the higher the peak current will need to be to deliver the needed power.

With a choke input filter this does not happen and the current is flowing through each diode (each in turn) all the time (that is to say, each diode is on for it's complete half cycle) and that is why the diode peak current is almost the same as the load current and there is no call (in the data sheet) for a limiting resistor.

Now, every rectifier tube has a limit as to how much peak current (the peak happens in each diode when each diode (in turn) turns on each cycle) it can handle.

So the voltage and the value of the limiting resistor and the value of the load current and the value of the first cap have to be taken into account.

I hope that helps.

Tre'



Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Nobsound 6P1 tube audio amp, posted on May 27, 2020 at 11:20:22
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002





View YouTube Video



Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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