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6J1 tube buffer circuit diagram
6J1 tube buffer circuit diagram
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Old 3rd February 2016, 09:43 PM   #1
02GF74 is offline 02GF74  United Kingdom
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Default 6J1 tube buffer circuit diagram

I've got two different 6J1 tube buffers off ebay that I am looking to fit inside CD players and would like to have circuit diagram. Googling seems they are based on Musical Fidelity X10-D

The first board amplifies the signal so that it distorts - the workaround is a resistor divider to reduce the input (which is what the volume control on board 2 does) but I would like to redesign it so it has unity gain.

Secondly I may want to redo for a new layout so that the circuit fits inside the CD player (currently I would have to make two holes for the valves in the top cover).

Both boards have a 3x voltage multiplies proving +/- 35 V ; I may want to put that on a separate board.

Anyway here are the two boards: (not sure if they are identical)


board 1
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board 2
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Old 4th February 2016, 02:32 AM   #2
adason is offline adason  United States
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I have x10-d schematics somewhere. None of those boards look like it. Have you considered the amount of heat inside the cd player it generates?
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Old 4th February 2016, 10:24 AM   #3
02GF74 is offline 02GF74  United Kingdom
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Thanks for reply. I have seen the MF X10-D schematics. It has the voltage multiplier to give +/- 32 V DC supply from 12 VAC, like these two boards but the valve circuit differs. The MF has two valves whereas the ones I posted use one pentode.

I have traced the circuit diagram (for the first board, the rectangular one with 4 yellow capacitors) and it is as below.

Valves are a bit before my time so forgive my silly questions, hopefully someone can help.

1. voltage on grid (pin1) is very close to negative supply (1.4 V difference), is that ok? From memory the CD player output is 2 V pp so that gives little head room before the negative rail is hit.

2. info from web, gain of circuit is
-gm (Ra || Rp)
where Rp = plate resistance, Ra is R3 = 4.7 k;
is that correct?

3. I have seen similar circuit which has a bypass capacitor across R4 - what effect does this have on the gain?

4. how do I modify circuit to reduce gain?


re: heat - I plan to cut two holes in the top panel and have the valves poking through, may need to put louvres above the transformer.
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Old 4th February 2016, 11:39 AM   #4
Koonw is offline Koonw
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6J1 tube buffer circuit diagram
Hi, I think NFB will be the answer to all your questions, this is about unity gain buffer. However the gain can be changed if you want, by changing R6 value higher means more gain, have a look first.

Edit: it can take up to 10Vp input before clipping.
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File Type: png 6j1 unity gain buffer -1.png (208.8 KB, 586 views)
File Type: png 6j1 unity gain buffer -2.png (201.4 KB, 556 views)

Last edited by Koonw; 4th February 2016 at 11:44 AM.
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Old 4th February 2016, 01:35 PM   #5
02GF74 is offline 02GF74  United Kingdom
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Thanks for that. I see you changed value of R7 from 1.5 k to 100k. That alone would reduce gain as the input voltage is reduced by R7 and R2 divider.

And to confirm, you added negative feed back using C4 and R6.

What was the reason for changing value of R7? If I leave it at 1.5K and add the two feed back components, what will happen?

How do I calculate gain for the circuit you posted (I'm after gain of 1.0 or as close as is possible with minimal component changes).

Last edited by 02GF74; 4th February 2016 at 02:02 PM.
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Old 4th February 2016, 03:07 PM   #6
DF96 is offline DF96  England
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What is the purpose of this circuit? You need to be clear on that in order to design it.

It certainly is not a buffer, as it has far too high an output impedance. You say you don't want gain. You also imply that you don't want distortion. So what is it for?
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Old 4th February 2016, 04:16 PM   #7
02GF74 is offline 02GF74  United Kingdom
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I want to pass output from cd player through a valve without distortion or gain to pre amp. Just like musical fidelity do on their cd players and dm dac.
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Old 4th February 2016, 04:44 PM   #8
Koonw is offline Koonw
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6J1 tube buffer circuit diagram
Quote:
Originally Posted by 02GF74 View Post
Thanks for that. I see you changed value of R7 from 1.5 k to 100k. That alone would reduce gain as the input voltage is reduced by R7 and R2 divider.

And to confirm, you added negative feed back using C4 and R6.

What was the reason for changing value of R7? If I leave it at 1.5K and add the two feed back components, what will happen?

How do I calculate gain for the circuit you posted (I'm after gain of 1.0 or as close as is possible with minimal component changes).
R6 is 139K for unity gain, for calculation you can have a look in other post #24 But for this one is a little different I'll calculate later. R7 is part of NFB ratio. You can't not leave it as 1.5k is too low, but you can keep it as grid stopper resistor.
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Old 4th February 2016, 05:26 PM   #9
jazbo8 is offline jazbo8
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6J1 tube buffer circuit diagram
Quote:
Originally Posted by 02GF74 View Post
I want to pass output from cd player through a valve without distortion or gain to pre amp. Just like musical fidelity do on their cd players and dm dac.
If that's the case, why don't you just wire the tubes up as cathode followers? Or just replace the whole circuit with two pieces of wire and leave the filaments on for their decorative effects... Warm glow of the tubes without gain or distortion!

Last edited by jazbo8; 4th February 2016 at 05:35 PM.
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Old 4th February 2016, 06:29 PM   #10
adason is offline adason  United States
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just follow lampizator site...
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