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Old 11th October 2007, 10:49 AM   #1
tberd is offline tberd  United States
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Default VAL MPS mark II preamp gain reduction

I have an issue with my VAL MPS mark II preamp . It's a chinese copy of the Marantz 7 preamp . It uses (3) 12AX7 tubes . This unit sounds ok but has too much gain and slight hiss . Volume control is loud a 9 0' clock . I also tried 3 NOS JAN 5751 tubes . No gain reduction at all . I'm still using the 5751 tubes . Any help is greatly appreciated .

I bought some 12AU7 tubes . Looking for lower gain again . Had a fat bass roll off with them .


thanks guys
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Old 11th October 2007, 11:26 AM   #2
Tweeker is offline Tweeker  United States
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Most of the lower gain tubes than 5751 will want more current, rebias. You will no longer have a Marantz 7 clone, assuming the psu was even up to it.

The design just plain has way more gain than is typically needed these days.

You could try lowering the value of the first stages plate load (R1 and R20 in attached schematic) to say 150k (from 270k), 2nd H will raise though. It might not be so bad as 5751 does not need as much as a load as 12AX7. Use at least 1 watt resistor. Avoid carbon types here.
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File Type: pdf m7csch0822.pdf (21.2 KB, 128 views)
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Old 11th October 2007, 11:46 AM   #3
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I am not really a toob guy, but why can't you reduce the value of R19 (R33 in other channel) to increase feedback. As R1/R20 is inside the loop, reducing it changes the open loop gain, but doesn't alter the closed loop gain very much.

You may need to increase the coupling cap if you change R19 by more than a factor of 2.

You will also have to worry about stability at HF; check with a square wave for overshoot or ringing. If there is some you will need to play round with compensation, either a few pf across R19/R33 or maybe across R1/R20.
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Old 11th October 2007, 12:16 PM   #4
Tweeker is offline Tweeker  United States
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Quote:
I am not really a toob guy, but why can't you reduce the value of R19 (R33 in other channel) to increase feedback. As R1/R20 is inside the loop, reducing it changes the open loop gain, but doesn't alter the closed loop gain very much.
Which is why changing tubes didnt help much.

For some reason I just saw a tone control there rather than a feedback loop.
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Old 11th October 2007, 08:51 PM   #5
tberd is offline tberd  United States
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Here is some pics of the VAL MP L1S mark II . The left 12AX7 would be a buffer of some type . The right two are for left and right channels each .
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Old 11th October 2007, 08:51 PM   #6
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Old 12th October 2007, 05:43 PM   #7
kevinkr is offline kevinkr  United States
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How much hacking and whacking are you willing to do?

You can significantly reduce both the open loop and closed loop gains if you wish to do so.

One immediate idea which requires rework not for the faint hearted is to:

1. Disconnect R7 and R11 from ground , connect them together and then add a 10K resistor from that point to ground. Connect the end of R19 that was originally connected to the cathode of that first 12AX7 to that first node as well. This will reduce the gain to about 18dB.

2. You must add a good quality coupling cap between the grid of that 12AX7 and the volume control wiper.

To do these sorts of mods you will need to cut some etches, but it can be done neatly and won't be very noticeable if done carefully.

Further reductions are possible by increasing the value of that 10K resistor - note that you are reducing the supply voltage margin slightly, and this is highly dependent on the stage operating current, but in practice this is unlikely to be a real issue at the levels encountered in this first stage.

You can also fiddle with the cathode resistor and the plate resistor values to increase the current in the first stage if desired.

3. Switching to 5751 is also recommended in conjunction with the above mods.

4. Note that you need to fix the cathode follower circuit - that 100K resistor should not be there. Arguably a reconfiguration to dc couple to the preceeding stage would be better. With the arrangement I described above you can remove both coupling caps C1 and C2 and jumper across them, also remove R4 and R14. Change R15 to about 100K.
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