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6J1 China preamp thoughts
6J1 China preamp thoughts
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Old 26th July 2017, 07:18 PM   #21
colinmel is offline colinmel  United Kingdom
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I have built one of these Chinese pre-amps and I must say I quite like it. I'm new to valves/tubes so can anyone tell me why so much of the circuitry in these amps is taken up with providing current limiters on the + & - supplies? (The limiters kick in at 14ma but the valves only draw 4mA ish) Are they essential for a valve amp?
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Old 26th July 2017, 08:22 PM   #22
audiowize is offline audiowize  United States
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Quote:
Originally Posted by pcan View Post
This is the schematic dragram of the board on the picture at the beginning of the this thread.
Fever 6j1 Tube Pre-amp Circuit - CircuitLab
That's absolutely terrible. Such a low load on that poor little 6J1 is going to make all sorts of distortion and will have poor drive capabilities.

There's also a bit of an error in the schematic, since as its drawn, there'd be -28V present at the output of the preamp (why not ground the -28V rail and get rid of the coupling cap at the input?

It's also a little odd that the cathode resistor isn't bypassed; that isn't doing them any favors.

YGWYPF
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Old 27th July 2017, 06:58 AM   #23
colinmel is offline colinmel  United Kingdom
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The schematic is wrong. The 100K load resistor does in fact go to ground.

I guess the consensus is that these poor little tubes would be working in a more linear region when supplied with a higher plate voltage and higher cathode current? (more linear = less distortion?)

I'm still puzzled by the inclusion of the current limit circuits though.
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Old 27th July 2017, 07:09 AM   #24
6V6dude is offline 6V6dude  Australia
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No it's nothing serious but it's fun to experiment with it. I've just build another one and will try to put passive Baxandal between the two to see if it has enough gain for at least 1:1 output with it.
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Old 27th July 2017, 07:16 AM   #25
6V6dude is offline 6V6dude  Australia
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Quote:
Originally Posted by azazello View Post
Not headphone buffer. This can only work when connected to amplifier and headphones from the amplifier. I tried to connect headphones to mine and the output level is next to nothing. See the connectors ion photos - RCA.
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Old 27th July 2017, 08:06 AM   #26
colinmel is offline colinmel  United Kingdom
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I agree 6V6dude, these kits are a great way to get into experimenting with valves/tubes.

I too have another kit on the way on which I will experiment to try and improve the performance and learn a little on the way. (Although I'm really pleasantly surprised at the sound of this thing straight out of the box/jiffy bag!)

Any thoughts on the importance of the current limit circuits?
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Old 27th July 2017, 02:26 PM   #27
azazello is offline azazello  Bulgaria
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6J7 is not very good....I changed it to 6J5P with more current and sound is more better.
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Old 27th July 2017, 02:46 PM   #28
colinmel is offline colinmel  United Kingdom
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azazello, this amp uses a pair of 6J1's. The provided tubes sounded fine to me. I swapped for a pair of GE 5654W's (cost more than the whole kit) and I can't say I noticed a whole lot of improvement. (Maybe it's just me!) Having fun though.
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Old 29th July 2017, 09:08 AM   #29
6V6dude is offline 6V6dude  Australia
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Just trying things, I've put Baxandal between the two 6J1 boards. It makes one fine tone control. Sounds as good as my 300V 12AU7 tone control with about the same gain. I'm quiet impressed. Just to think I could have saved days building one. But than again what I've build will last for ever. The 6J1 one is just a board with everything cramped on it, transistors for voltage trippler and I don't like the series heater setup because it drifts over time or if tubes aren't well matched. But yes it does work very well for tone control regardless.
Click the image to open in full size.

Last edited by 6V6dude; 29th July 2017 at 09:17 AM.
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Old 24th August 2017, 06:16 PM   #30
colinmel is offline colinmel  United Kingdom
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Has anyone managed to remove the slight mains hum from these 12AC voltage doubler versions?
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