Nobsound 6p1 Amp flaws/mods

StepheK

StepheK

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Hi all.

I wanted to post about my first tube amp and some issues I discovered reading online after I purchased it. This is a made in China, off Amazon, Nobsound 6P1, which has a major design flaw: the first cap after the rectifier tube being 150uf. This severely overworks the rectifier tube, causing them to fail and also possible arc if short cycled before this cap has drained down, which could take a while.

I swapped out that one cap for a 22uf 450v I had on hand, but plan to go down to a 4.7uf when I open it back up and see if it's still quiet. The tube sheet specs 5uf, but I don't think 22uf is going to kill it. The 150uf sure will! Mine came with a Soviet 5Ц4С slow rise rectifier, some came with a 5Z4, both have a 2a heater. I have no idea what the transformer is rated for on the 5V heater so be careful swapping tubes here.

The other issue, for use in the US, is a 110 mains tap which makes everything over-volt. I see in the Amazon reviews people saying they quit working after a few months and I have to believe this, along with the super oversized rectifier cap, is probably why. I put a 12.5V 3a transformer in a bucking configuration inline to resolve this voltage issue. The power trans also runs much cooler now.

I later changed the Chinese output tubes for 2 matched pairs of Soviet 6p1 tubes (install the matched pair in each strapped channel) and played with different 6SN7 tubes until I found a pair of driver tubes I liked, as the make/model of driver tubes make a big change in sound in this amp. Honestly the installed tubes sound pretty decent but I did find a pair of 6SN7, some old short bottle, side getter GEs, that helped the bass some.

I ordered a pair of 15w Edcor 3.5K OT with UL taps and plan some further mods like a bleeder resistor for the PS caps and separate bias resistors/caps for each output tube. I want to play with ultra linear mode and possibly some schade style feedback.

At this price point, for a real point to point wired tube amp, it's been a sweet sounding introduction into tube HiFi. I'm gathering parts to build one of Kegger's designs next, a KT88 amp he posted here:

https://www.audiokarma.org/forums/i...ct-single-ended-amp-seul.382162/#post-4823025

I do believe these kinds of home brew mods are just part of the deal with these made in China tube amps. It reminds me of shooting with Soviet era medium format Kiev cameras back in the 1990's. You could either pay $$$$ for a Hasselblad/Mamiya, or deal with upgrading/modifying the Soviet copies to make them usable.

Nobsound-6P1-schematic-edit-Stephe-new.gif
 
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The use of the 110v primary transformers are a real problem. And I don't know why they use such huge values for that first cap. I've seen heater circuit mods made for the heater circuits in various Chinese equipment. Mostly changing, or adding a resistor to bring the voltage down. Even swapping resistors and floating the ground to bring the heater voltage back down to 5v for those small rectifiers. Over the last year I have seen more of the Chinese stuff being built with 120v primaries, or switching supplies. But the bulk of the equipment still uses 110v transformers.. I took the chance on an all tube headphone amp with output transformers using a switcher, 90v to 230v to power it. I've had it for around 2 years now and have have had zero problems and the I don't hear any noise from the switcher.
 
Hi StepheK

Do you have a picture of the mods done inside the amp?
I want to do these to mine and wanted a road map to help with locations.
 
StepheK said:
I wanted to post about my first tube amp and some issues I discovered reading online after I purchased it. . . .
Click to expand...

EQ79 said:
Hi StepheK

Do you have a picture of the mods done inside the amp?
I want to do these to mine and wanted a road map to help with locations.
Click to expand...
Welcome to AK!

Just a word of practical advice . . . If you want someone to notice a post you've made and respond it's helpful to click on the 'quote' link and then the 'reply' link (in the bottom right corner of their post. That way they get a notice when they sign in.

Another way is to tag them like this: @StepheK
 
Hi, I saw this and was thinking I might make a short video showing these mods to this amp, stay tuned :)
 
OK here are some pictures. This is what the amp looks like stock with the oversize 150uf cap that needs to be replaced.

6P1 Amp Guts.jpg

Here is the amp with what I consider the bare minimum mods to correct this design flaws in the power supply, the 150uf cap replaced with a 22uf 450V cap, and then setup an external bucking transformer to get the input voltage @ 110V AC.

6P1-cap-1.jpg

Here is my amp now, with a bleeder resistor across the 150uf Cap, replaced the 2 cathode caps for the power tubes with audio quality 220uf 100V caps, and replaced both the coupling caps and the grid bypass caps with some Solen 0.22uF 630Vdc PPE Series film caps. These cap upgrades made a noticeable improvement for well under $20 including shipping for the parts from https://www.partsconnexion.com/

6P1-cap-2.jpg


I have some Edcor 15W UL tapped output transformers ordered and plan a rebuild with switchable UL/Triode modes, sperate cathode resistors/caps etc. in the future. That will be a little more expensive but I feel the tiny output transformers are likely the limiting factor with this amp after the cap upgrades.

I do highly recommend upgrading the output tubes to the Soviet 6P1P-EV versions, and playing around with different 6SN7 driver tubes as that does make a difference too.
 
I wonder why so many of these Chinese amps use such huge first caps in the power supply, especially with a tube rectifier?
 
StepheK said:
I do highly recommend upgrading the output tubes to the Soviet 6P1P-EV versions, and playing around with different 6SN7 driver tubes as that does make a difference too.
Click to expand...
I had to look up the 6P1 as I wasn't familiar with it. Looks like it's similar to a 6V6 but in a 9 pin miniature. Good thing that the Russian tubes sound good because, other than the Chinese originals, they seem to be the only other choice. Maybe I'm wrong, I just did a very quick search.

The only American tubes with similar characteristics are the 6AQ5, which should be electrically very similar but it's a 7 pin. As far as I know there's only one 9 pin that's similar and that's the 12AB5 but it has a different pinout and, obviously, a 12.6v heater.

I'd be tempted to rewire the sockets and switch to 6BQ5s. Slightly more power and lots more tube choices.

It's also unusual in that it's a PSE design. I see it's also wired in triode. How much power does it allegedly make?

I'm looking forward to your report on how much of an improvement the Edcors make.
 
StepheK said:
OK here are some pictures. This is what the amp looks like stock with the oversize 150uf cap that needs to be replaced.

View attachment 2101603

Here is the amp with what I consider the bare minimum mods to correct this design flaws in the power supply, the 150uf cap replaced with a 22uf 450V cap, and then setup an external bucking transformer to get the input voltage @ 110V AC.

View attachment 2101605

Here is my amp now, with a bleeder resistor across the 150uf Cap, replaced the 2 cathode caps for the power tubes with audio quality 220uf 100V caps, and replaced both the coupling caps and the grid bypass caps with some Solen 0.22uF 630Vdc PPE Series film caps. These cap upgrades made a noticeable improvement for well under $20 including shipping for the parts from https://www.partsconnexion.com/

View attachment 2101620


I have some Edcor 15W UL tapped output transformers ordered and plan a rebuild with switchable UL/Triode modes, sperate cathode resistors/caps etc. in the future. That will be a little more expensive but I feel the tiny output transformers are likely the limiting factor with this amp after the cap upgrades.

I do highly recommend upgrading the output tubes to the Soviet 6P1P-EV versions, and playing around with different 6SN7 driver tubes as that does make a difference too.
Click to expand...

Thank you so much for the pics, they helped me out alot.

I have some 6P1P-EB's I got from Riverstone Audio in it now and Tung-Sol 6SN7 and Genalex Gold Lion 5AR4 that will go in after these mods.
I also have an APC voltage regulator.
 
FlaCharlie said:
I had to look up the 6P1 as I wasn't familiar with it. Looks like it's similar to a 6V6 but in a 9 pin miniature. Good thing that the Russian tubes sound good because, other than the Chinese originals, they seem to be the only other choice. Maybe I'm wrong, I just did a very quick search.

The only American tubes with similar characteristics are the 6AQ5, which should be electrically very similar but it's a 7 pin. As far as I know there's only one 9 pin that's similar and that's the 12AB5 but it has a different pinout and, obviously, a 12.6v heater.

I'd be tempted to rewire the sockets and switch to 6BQ5s. Slightly more power and lots more tube choices.

It's also unusual in that it's a PSE design. I see it's also wired in triode. How much power does it allegedly make?

I'm looking forward to your report on how much of an improvement the Edcors make.
Click to expand...

I've been looking at adapters to use 6V6 in it so I'd have a variety to chose from.

Also thank you for pointing to the quote button.
 
While you are in there, pick up some new coupling caps. Those really helped a lot and those solen caps I used were $1.95 each.

The design is interesting and the way the 6SN7 is wired isn't something I have seen elsewhere but it seems to work. It drives my Klipsch bookshelf speakers pretty well.
 
EQ79 said:
I've been looking at adapters to use 6V6 in it so I'd have a variety to chose from.

Also thank you for pointing to the quote button.
Click to expand...
That would certainly be another way to do it. I'm not a fan of adaptors, simply because I don't like the look of tubes sticking up like that. Silly, I know.
 
StepheK said:
While you are in there, pick up some new coupling caps. Those really helped a lot and those solen caps I used were $1.95 each.
Click to expand...
It's not surprising that the parts used in these lower cost Chinese amps aren't the best. No doubt a coupling cap upgrade makes a big difference.

I'm not generally into playing the "what's the best coupling cap" game. But I am all about "bang for the buck" and I've had really nice results with some of the Russian caps, both the K40Y-9 PIOs and the FT-2 Teflons. A lot of people also like the Russian silver micas. I have some but I've only tried them briefly on a breadboard build.

The K40s are a little warmer but maybe not quite as warm as vintage American Vitamin Qs and others from that period. The FT-2s are incredibly clear, as are the silver micas. Many of the FT-2s are only rated at 200v but they have been proven to work fine in situations where caps rated at higher voltages are normally used.

Mike Samra, a highly experienced tech who used to post over at AA, tested them on a cap checker at 800v. Not for a few minutes either, for several days. And they didn't show any leakage. He doesn't recommend using them quite that high but they are tough.

It's my understanding that coupling cap voltages used in an amp are based on the amp's B+ even though they don't normally see voltages anywhere near that. Supposedly they're spec'd that high so that they can survive if there's a major failure in the amp. I've never had such a failure, but I suppose that's what would happen if all the tubes stopped conducting and the voltage soared. I guess the coupling caps would survive until the voltage dropped to zero after the PS caps blew up.

The 200v FT-2 teflons are physically large, though. There are also some rated at higher voltages but they're even larger. And there's also the FT-3s, which are much larger. They also don't have normal wire leads so you have to get creative to mount them.

Anyway, you should check out the Russian caps if you're not familiar with them. They've gone up in price some over the years but they're still probably the best "bang for the buck" around.

StepheK said:
The design is interesting and the way the 6SN7 is wired isn't something I have seen elsewhere but it seems to work.
Click to expand...
I've seen that before but I'm drawing an blank as to what it's called. Not sure what advantage it might offer over a more conventional design. I think the 6V6 and it's cousins are easy to drive.
 
FlaCharlie said:
I've seen that before but I'm drawing an blank as to what it's called. Not sure what advantage it might offer over a more conventional design. I think the 6V6 and it's cousins are easy to drive.
Click to expand...

Yeah it seems to work well in this amp, but may be because these 6P1 tubes are easy to drive. With no feedback and this design, this amp is very sensitive to what driver tube is used. That is a good thing if you like tube rolling :)

I don't know a lot about design yet, but I don't see many circuits using a 6SN7 to drive a KT88. Is that because they are harder to drive or what is the reason for that.
 
opnly bafld said:
6.8 watts
Click to expand...
Maybe that's for both channels. Who knows how they measure it. And a lot depends on the operating points and the primary impedance of the OT.

I do know that it's not uncommon for Chinese manufacturers, and probably manufacturers in general, to inflate the specs. There's a thread on another forum where one poster measured what looked to be a very well made Chinese amp. The manufacturer claims it makes 8 wpc at less than 1% distortion. His testing revealed that 8w measured at 1k (not across the whole audio spectrum) actually produced almost 25% distortion.

He was able to get a substantial refund from the seller after he submitted his results. Here's the thread with the full test results. It's interesting reading.

https://hifihaven.org/index.php?thr...t-it-is-about-economics-and-value.4569/page-2

My technical knowledge isn't very deep but, if you look at the data sheet, the 6AQ5 (the US equivalent) will produce 4.5w in Class A. That's one tube SE in pentode. Two tubes, in PP, is 10w but PSE, using two tubes, is less efficient than PP so maybe 7w. But that's in pentode. Wire it in triode and you'll probably lose 60% so that's maybe 4.2w in PSE triode mode. But that's at 8% distortion. So, probably under 2w at 2% and maybe 1.5w or less at 1%.

Hey, as long as it's got enough grunt for your use that's what matters. The value and effect of wattage is overrated, IMO. I don't listen at high volumes so I don't require much power.

I occasionally use a SS Yamaha M-60 power amp, which is 160 wpc into 8 ohms. It's got meters on the front and even when I'm listening at what is "loud for me", the meters (which I assume are fairly accurate) are peaking at under 2w.

Many people think that twice as much power is a big deal. But doubling the output power only gets you an extra 3db of sound output. If you want to hear what that sounds like, just move 1m (39") closer to your speakers. It takes 10x the output wattage to subjectively sound "twice as loud".
 
StepheK said:
Yeah it seems to work well in this amp, but may be because these 6P1 tubes are easy to drive. With no feedback and this design, this amp is very sensitive to what driver tube is used. That is a good thing if you like tube rolling :)

I don't know a lot about design yet, but I don't see many circuits using a 6SN7 to drive a KT88. Is that because they are harder to drive or what is the reason for that.
Click to expand...
I've got no experience with KT88s, and I've never looked at their specs closely, so I don't know what it takes to drive them. And drive requirements are probably different in pentode vs triode mode.

My understanding is that the 6SN7 is good at producing current but it's a low gain tube. It's claim to fame is that it's very linear, meaning very low distortion.

I did build a PP 6B4G amp that uses 2C22s (aka 7193) for inputs / phase inverters. They're single triodes that are equivalent to half of a 6SN7. Many people consider them to be the best sounding 6SN7 variant. They don't get as much attention because they have dual top caps. As a result they're cheap, the big dealer near me lists them for $7. All the ones I've seen are JAN produced for the military in WWII. I love to use oddball tubes.
 
FlaCharlie said:
I've got no experience with KT88s, and I've never looked at their specs closely, so I don't know what it takes to drive them. And drive requirements are probably different in pentode vs triode mode.
My understanding is that the 6SN7 is good at producing current but it's a low gain tube. It's claim to fame is that it's very linear, meaning very low distortion.
Click to expand...

They are nice sounding tubes but there is probably a reason people don't seem to use them with KT88 tubes.

As per the output of this 6P1 amp, I'm fairly certain at reasonable levels of distortion it's probably putting out a couple of watts as you said. It definitely starts distorting at higher levels, but with high efficiency speakers, it rarely sees over 1/2 volume for my use. Past 70% it does have audible distortion.
 
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