Mysterious SE OTL headphone amp from China?!

[piniu29]

Being a tube newbie but with some electronic repair experience I would like to mod or upgred my headphone amp with better parts, but before I put my "fingers" in I need to know more about the circuit.
I have bought a tube amp over one year ago from China it is a chinese product BTW very popular to DIY in China.
Attached drawing schema with my multimeter reading.

I roughly understand the power CF output section with the load big wirewound cathode resistor and the triode in parallel to reduce the output impedane to better match with lower impedance cans, but the pre-amp is a mystery to me? I've never seen anything like this, the pentode looks like is in triode conected but why pentode plate and the external shield is grounded?

[DF96]

Maybe it is a way to run it at lower current than normal. Might give lower noise.

[12E1]

Originally Posted by piniu29

I've never seen anything like this, the pentode looks like is in triode conected but why pentode plate and the external shield is grounded?

Just a thought - does the circuit as built actually match the schematic? It could just be that the pentode has been wired conventionally but the schematic has been drawn with errors.

[ErikdeBest]

This article may help

http://www.audiophool.cjb.net/Misc/6...ed%20plate.pdf

[12E1]

Originally Posted by ErikdeBest

This article may help

http://www.audiophool.cjb.net/Misc/6...ed%20plate.pdf

Interesting. Thanks - I was unaware of this technique.

[piniu29]

Thanks all for your Input!

I checked the circuit once again and no errors found with the schematic.
The pentode 6ж4 is connected like on attached photo.
The 6ж4 is a equivalent of more popular and the better known 6sj7,6sj7gt or RCA JAN 5693.
Thanks for the article it explains a lot. Therefore the grid 2 with grid 3 is use as a pentode plate by disabling anode plate by connecting with external shield (pin 1 of the socket) to ground. I was looking for any info in internet about the "rare" pentode configuration and couldn't find anything similar.
It's looks like the method for pentode with grounded plate in triode connection was forgotten in this part of world.

[oshifis]

How does it sound, and what headphones do you use with it?

[piniu29]

Originally Posted by oshifis

How does it sound, and what headphones do you use with it?

It's depend what tube are used, with the stock tubes Chinese 6N13P and russian 6ж4 the sound is good in both power and overall, but changing the tubes to RCA 6AS7 and EF800TFK (witch adapter) is remarkable! they make a great improvement to the sound BTW my favorite combo with Sennheiser HD650. This amp was developed for low impedance phones is build like tank, true dual mono.
Changing tube to some 5998 Tung or Centron 7236 instand the 6AS7 will provide more power which is neded to drive my orthos HiFiman HE-500 to it best.
Runing the amp with one of this combo centron 7236/tung-sol 6SJ7GT or Tung sol 5998/red hots RCA 5693 bring the sound to diferent level the HE-500 sound amazing, smooth, detailed, with deep and wide soundstage.
Is hard to believe it can be better but "never say never" I am actually willing to spend some USD on upgrade/mod this amp, even small improvement will be worth the extra effort.

[piniu29]

Based on the original design I modded my headphone amplifier with better parts and some small changes to the circuits.The effect of this mod exceeded my expectation! has improved all most everything, it's sound really good with low and high imp cans.

I got matched pair GEC CV1067 equivalent or better know as a 6J5 and just wondering how much better be use the triode instead of my current input tube 6sj7gt (tung-sol) which is wired on triode mode with grounded anode plate.

Just simply converted the wiring as shown in the attached schematic, the amp with the new input tube is running fine but the sound is not even good, lack of speed, flat, with less dynamic and floppy bass.

If I did not know that manufacturer of the amplifier does custom version called (modded / upgraded or reference) which exactly use the GEC tube instant of pentode I would thought that it have to be like this.

Guys I need your help to adjust the amp and get it sound as good as the previous version with the rear triode grounded plate mode.
Every clues are welcome.
Thx.

[DF96]

If you want it to sound like the original circuit then why not go back to the original circuit? You have completely changed the input stage. There is no overall feedback, so you are likely to hear any change.