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6J1 China preamp thoughts

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W

Whelkie D

Member
Joined 2018
Sadly no. I had the C3gs already which I use in a Little Dot MK2 (where they also sound great). And no, I wouldn't advise anyone to spend that sort of money just on an experiment, I just thought if anyone already had a set of C3Gs they might want to try them in one of these little devices.
 
pistollero

pistollero

Member
Joined 2016
gmcdonld said:
these mods will remove 100% of the hum in the fever 6j1 tube preamp. it took a little more than i originally thought. the first post had a pic but i am reposting it because you only have to add one cap to the 47 ohm resistor junctions and it is for the neg side of the supply because the neg supply bias's the grids and noise here becomes part of the signal and gets amplified. still need the extra heater supply cap. the second pic posted has the mod to fix the ac traces running too close to the grid pin 2 on the left channel tube which causes a lot of hum on the left channel.

the mods in red are required to remove the hum for normal use and is going to be good enough for most all situations. the mods in blue is for use in a situation where a very high gain amp is used after the preamp. also the bypass caps not only remove a little noise but it increases the gain from about 8.5 to 10.5 and gives a flat frequency response from 45 hz to 50 khz. the shields tied to pin 2 only works well if you use bypass caps. i had to do the full mod because i am running a compressor which uses a variable gain op amp with a max gain of x56 and now no noise at all.

i have built this preamp using this board in about a 6 x 7 x4 box it has tube inputs, 2 compressors, 2 pt2399 delays and 2 3band eq's 2 separate channels it works great. another thought the tube preamps will accept a 20 milli volt up to 2.5 volt signal with out clipping or distortion so you could take the right output and tie to the left input and get a higher gain 2 stage single channel tube amp. just a thought.
Click to expand...

Hello gmcdonld,

i just made you mods, except for the ground of the tubes on pin2. Just to confirm, is the connection of the shield of the tubes to pin2 of each tube correct?.

it sounds nice without the shield mod but i have hum...
 
pistollero

pistollero

Member
Joined 2016
aspringv said:
And there you go! Grid referenced to gnd and not -ve, as is the output node, plate load is gone, and the cathode load is now 4K7.

Works fine, and I really should have a play with some of the others. :)
Click to expand...

Hello, i did the mod as per your schematic on my second board, since i have bought 2 preamp boards.
the good part is there is no hiss or hum it is dead quiet, but the bad part is i don´t like the sound. I ear also some distortion on bass also in percussion etc.
is everything ok in your schematic?
anyone tried?
 
R

Raycarr00

Member
Joined 2013
did you try the 2x 2200uf caps in parallel and the 20ohm resistor subs.,in addition to the 470uf cap from the 2 resistors to RCA phono jack ground methods......... NO HUM!
Dead silent and no distortion. I've since soldered the power adapter (13v AC)to the board as apposed to plugging it in. Tubes are running cooler,but that resistor is running hot! I swapped the transistors for NTE373 and NTE374,as well.
 
pistollero

pistollero

Member
Joined 2016
Raycarr00 said:
did you try the 2x 2200uf caps in parallel and the 20ohm resistor subs.,in addition to the 470uf cap from the 2 resistors to RCA phono jack ground methods......... NO HUM!
Dead silent and no distortion. I've since soldered the power adapter (13v AC)to the board as apposed to plugging it in. Tubes are running cooler,but that resistor is running hot! I swapped the transistors for NTE373 and NTE374,as well.
Click to expand...
I will try your cap mod!
 
R

Raycarr00

Member
Joined 2013
Chinese pre

the 2x 2200uf caps in parallel and the 20ohm resistor subs.,in addition to the 470uf cap from the 2 resistors to RCA phono jack ground methods.
 

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jtore

jtore

Member
Joined 2021
Hi there, friend of mine is gonna give me his kit to assemble.
I read all the thread and found some posts very interesting.

I will surely proceed to modify it as per gmcdonld instructions on post#85.

Moreover I'm following avtech23 especially on post#148 and about this I have questions:
-Does it make any sense to have the heaters in parallel and feed them with 6.3V via zener or adjustable voltage circuit?
-Is it actually better to raise the voltage in the rest of the circuit? I'll probably trow in some 100V Panasonic capacitor that are reasonably cheap.
 
T

TasoTaso

Member
Joined 2018
Hi, I'm doing some tests with a 2x 6j1p buffer called Tube-01.
I changed the in-out caps with quality panasonic MKPs, 3.3uF / 400v.
I replaced all the electrolytic caps on the power supply with panasonic 25V.
I replaced all the electrolytic caps on the anode with 50v nichicon and panasonic.
I have built an adjustable linear power supply that goes up to 22v.
I did some tests and by increasing the input voltage I can get up to + - 40 / 45v which is equivalent to an anode of 80 / 90v instead of the original 60v.
I was wondering if you know a working point that can work with 80 / 90v.
I presume I will have to replace the Rs on the anode and cathode and maybe some more.
For heaters I always keep exactly 6.3v (12.6v because they are in series) ... by means of a resistor.

Thanks if you give me a point (polarization) of work so I can apply it and see how it goes and make some measurements, as well as listening.

I also made some measurements with FFT ... original version .. and it distorts a lot!
I think that at 80 / 90v of anodic you can have a better result than at 60v.
Thanks!
 
jtore

jtore

Member
Joined 2021
nofacemonster, given the price, trying it won't hurt much.

The one I had to assemble, has been anything but "deat silent", no matter the good mods here described.
Tried anything for the heater... looks like 12Vac directly from trafo, no diode, no resistor is the best option.

Removing LEDs has been a good improvement as well.

Tried different trafos , all 12Vac with different A ratings but the 50Hz hum was always there.

Bottom line, a nice unit to mess with, it can raise the volume of the source but personally I wouldn't use it in a listening system.
 
P

pako202

Member
Joined 2021
Hi guys, I bought a 6j1 preamplifier, I don't notice any noise, but I think it's because it doesn't amplify almost ... rather the sound is less than without the previous one. I connect the mobile to the input and connect headphones to the output and it sounds lower than if I connect the headphones directly to the mobile .... is this normal ????
 
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