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6J1 China preamp thoughts
6J1 China preamp thoughts
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Old 16th May 2018, 03:20 AM   #81
e_fortier is offline e_fortier  Canada
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6J1 China preamp thoughts
Just came across this thread, looking for a tube preamp with a gain of 10 (20db)

Looks like there is a more recent 6J1 preamp equipped with a step-up dc/dc converter, have a look below. Hope this is useful, I don’t know the seller so at your own risk.

Audio Mini 6J1 Valve Vacuum Tube Pre Amplifier Stereo HiFi Buffer Preamp Newest | eBay

BR
Eric
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Old 18th May 2018, 06:40 AM   #82
gmcdonld is offline gmcdonld
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Quote:
Originally Posted by Whelkie D View Post
Thanks for that - really helpful.
you are welcome Whelkie but there is still a problem with hum. as long as i have a fairly low impedance connected to the input i have no hum. with a high impedance like a guitar or nothing connected and the volume turned up the left channel hums quite a bit but i have found the problem. they ran ac voltage traces to the switch all aroung pin 1 of the left tube which is the grid or the input. really stupid. i am going to cut traces and mount an external switch. when i get it done i will post a pic
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Old 24th May 2018, 03:29 PM   #83
Whelkie D is offline Whelkie D
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Look forward to that. Hope it does the trick.
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Old 29th May 2018, 01:48 AM   #84
Montrealer is offline Montrealer  Canada
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Quote:
Originally Posted by gmcdonld View Post
i see you did the mod but the mod at the 47 ohm resistors originally posted by someone else is not entirely correct. i rechecked it to be for sure and i change it on my board. one of the caps has negative to ground and one has pos to ground.

here is the picture of the 6j1 hum removal modification
Thank you very much for your help. Your picture and notes make
it clear! I will do those mods!
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Old 2nd June 2018, 03:59 AM   #85
gmcdonld is offline gmcdonld
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Default re post 100% working fever 6j1 tube preamp hum removal

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.
Attached Images
File Type: jpg 6j1 tube preamp fix mod1 and 2.jpg (240.1 KB, 130 views)
File Type: jpg fever 6j1 mod 3 and 4.jpg (289.7 KB, 124 views)
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Old 2nd June 2018, 06:19 AM   #86
gmcdonld is offline gmcdonld
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made a mistake in the write up but not in the images, pin 1 is the grid not pin 2. pin 2 is the cathode
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Old 2nd June 2018, 07:43 AM   #87
Whelkie D is offline Whelkie D
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Quote:
Originally Posted by gmcdonld View Post
these mods will remove 100% of the hum in the fever 6j1 tube preamp. <snip>
That's brilliant. Thank you so much for all the work you've done explaining this. Gonna go and try these mods now. What sort of switch do you recommend for the cut ac traces?
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Old 2nd June 2018, 10:52 PM   #88
gmcdonld is offline gmcdonld
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the switch don't matter much. its only 12 v ac and a few hundred milliamps
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Old 8th June 2018, 04:43 AM   #89
gmcdonld is offline gmcdonld
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Quote:
Originally Posted by Montrealer View Post
Thank you very much for your help. Your picture and notes make
it clear! I will do those mods!
hey Montrealer, i reposted a new fix for the china 6j1 preamp. it is 100% working for complete hum removal. 6J1 China preamp thoughts
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