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Old 17th October 2011, 08:01 PM   #1
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Default Can anyone help with an ef86 tape preamp mod please?

Hi, can anyone help with what might a pretty naive question of mine please?
I've attached a schematic of the replay amp from a Brenell Mk 5, but I only really need the first couple of stages (ideally to output at the EXT AMP socket after the first half of the ecc83). Theoretically would it be as simple as removing the sections of the circuit I've marked out in red?
Any help with this wuld be very much appreciated, thanks

Ben
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Old 17th October 2011, 08:47 PM   #2
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Yes, it is so simple... but not quite: by removing the circuit marked in red the total current drawn will be much less and the (unregulated) B+ will rise considerably. The EL84 S.E output amp is taking about 90% of the total current, the input stage outside the red area only 1.1 mA. You'll need to change (rise) the value of R46 until you get the correct voltage back at C36C (260VDC). Then you must check if the voltage on the main filter capacitor in the power supply (not shown) is not exceeding the capacitor maximum ratings, or the 50+ years old cap will not survive and leakage (or explosion) will result ! You will probably need to change (or add) a resistor in the power supply too.

Last edited by Tubologic; 17th October 2011 at 08:51 PM.
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Old 17th October 2011, 09:38 PM   #3
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Thanks so much for the incredibly quick and helpful reply! I had a feeling that taking out a massive section of the circuit would create an imbalance but I wan't sure where. I've attached a gif of the power supply too.
I have a live and working one of these and a few dismantled variations (series 1's and 2's), the idea was to make some basic replay pre's based on this design.
Would you recommend getting a regulated power supply to avoid the potential fireworks?

Thanks again for your help

Ben
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Old 17th October 2011, 10:37 PM   #4
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Based on the values indicated in your diagrams I would start changing R66 (in the oscillator/PSU) from 470R to 10K and R46 (in the replay amp) from 10K to 39K
You should be close to the target values (260V at C36C) but final tweaking might be required depending on the actual circuit values and other unknow parameters. (capacitors leakage current,etc...) Of course, the circuit inside the red area must be disconected. You will still need to check if the voltage at C64C (the rectifier cathodes) doesn't exceed the maximum voltage ratings for this cap. A regulated power supply is not mandatory as fairly large voltage variations can be tolerated without impairing the circuit performances. However the supply must be perfectly filtered and ripple free. (changing the old electrolytics is allways recommended).
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