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ebay 6J1 valve pre-amp - circuit diagram/gain
ebay 6J1 valve pre-amp - circuit diagram/gain
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Old 19th December 2017, 10:06 AM   #21
02GF74 is offline 02GF74  United Kingdom
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Quote:
Originally Posted by Bandersnatch View Post
As long as their PIV rating and current capacity is adequate.
cheers,
Douglas
With 12v ac supply I understand that the capacitors and diodes won't see more than 12v rms.

So to reduce the vf across the diodes, a 1n5817 should be fine.

Spec below.
Code:
              1N5817  1N58185  1N5819
Vr peak   	20       30   	   40
Vrms      	14   	  21   	   28

Vf @ 0.1A      0.2       0.3      0.6
Vf @ 0.2A      0.25      0.35     0.75
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Old 2nd January 2018, 05:23 PM   #22
02GF74 is offline 02GF74  United Kingdom
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Been busy modifying the board to use 7812 regulator for heaters (with diode to raise to 12.6v) and cutting tracks to make it cathode follower.

What effect will leaving r13/r18 (470k) tied to - 39 v instead of to 0v? Would save a fair bit of work.
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Old 2nd January 2018, 07:10 PM   #23
Ketje is offline Ketje  Belgium
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Practicaly no current in the cathode follower.
Mona
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Old 2nd January 2018, 09:14 PM   #24
02GF74 is offline 02GF74  United Kingdom
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^^^^ I take it that it will be OK.

I thought maybe it will pull down the input too far.
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Old 3rd January 2018, 09:56 AM   #25
DF96 is offline DF96  England
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Take it that it will not be OK.

There are essentially two ways to bias a cathode follower: apply the correct DC voltage to the grid, or use a small value cathode resistor. If you connect the grid resistor to -39V instead of 0V then you are applying the wrong DC voltage to the grid so the valve will conduct almost no current so create significant distortion.
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Old 3rd January 2018, 10:13 AM   #26
02GF74 is offline 02GF74  United Kingdom
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Many thanks for the reply, arrived in just in time as I'm about to continue with the modifications. Luckily I can avoid cutting the negative pcb tracks as there are two vacant pcb holes to 0v where the potentiometer used too.
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Old 3rd January 2018, 01:51 PM   #27
02GF74 is offline 02GF74  United Kingdom
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Completed mods and except for one mistake have it working, top and underside of board below.

The regulated supply to the heaters has eliminated buzz in L channel (was present with no input and pre-amp volume turned to 11) but when testing I could see on the 'scope there is a fair bit of crosstalk, which can be heard with signal applied to only 1 channel, (pre-amlp volume at 11).

I did same listening test on the original circuit - and there was similar cross talk on R channel.

So I am wondering as to the reason for this. My own opinion of the layout is not great - I would have kept inputs on the opposite side to the outputs.

1. there is pick up as the two input signal tracks run parallel to each other (see underside, lowest 2 horizontal tracks
2. the signal leaks across either through the 0 V rail (R13/18) or via the -ve rail (R14/19)
3. other?

update: even with valves removed and circuit not powered, seeing crosstalk. Resistance between both inputs as shown by dvm is ∞.

suggestions as to where to look?
Attached Images
File Type: jpg top 1.jpg (105.0 KB, 77 views)
File Type: jpg pcb 2.jpg (181.3 KB, 76 views)
File Type: jpg crosstalk.jpg (93.2 KB, 72 views)

Last edited by 02GF74; 3rd January 2018 at 02:11 PM.
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Old 3rd January 2018, 02:23 PM   #28
02GF74 is offline 02GF74  United Kingdom
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update 1: even with valves removed and circuit not powered, seeing crosstalk. Resistance between both inputs as shown by dvm is ∞.
update 2. as above; at about 100 Hz, I'm seeing the amplitude on the unused input start to rise and reach maximum at about 60 kHz - that to me seems to imply pick-up on the parallel tracks?

I'm wondering whether it is worth cutting the tracks for the inputs and replacing with screened cable?

Last edited by 02GF74; 3rd January 2018 at 02:36 PM.
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Old 3rd January 2018, 05:32 PM   #29
02GF74 is offline 02GF74  United Kingdom
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A quick rework to move input signals apart has massively reduced crosstalk. Also heatsink for the regulator.
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Old 4th January 2018, 10:27 AM   #30
DF96 is offline DF96  England
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Crosstalk which rises with frequency is due to either capacitance between high impedance points, or inductance in a common circuit loop (typically a ground connection).
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