no i mean the one that i have a drawing of on page 1 there. that one is the one the designer designed for this phono preamp.
the one im using is the one that came with the zen preamp which had too much gain with 4 tubes and this one has too little voltage with the same tubes and too little gain. is it R1? the one that grounds the input single from the input jack?
there is 400v without anything connected to the B+ rail then when i connect the 4 plates it all sags down to 90 or so. the PS caps that are rated for 450v are charged up to over 400v. i increased the plate voltage with some paralleled resistors but the gain didnt improve none even though the voltage went up by 20v.
Chop, the possible answer could be a bit complicated to explain but here goes.
You are replacing a PS that used chokes for filtering with one that uses resistors. I ran the formula to calculate the resistance (reactance) the chokes provide and it came out to 15K and 3.75K. Your new power supply is using two 33K resistors. Without a load you will see full voltage but as soon as you hook it up it will drop like a rock.
You probably can't do a direct replacement with a 15K and a 3.75K resistor because the transformers probably start with different high voltage, so you should use this info as a guide to gradually lower the filter resistors to get your B+ when loaded up to what you need. I can post the formula I used to figure the reactance of the chokes if you want. It's not a difficult formula.
hey thanks a lot!
sure i would like to see that formula.
i did what wavebourn said about the plate resistors, i reduced the value of the plate resistor on the second triode and that brought the voltage up by 30 to about 80v but i saw no audible increase in gain.
do you think it has something to do with R1 the resistor that grounds the input jack? the instructions say that it should be adjusted for the cartridge.
how much voltage should i see from the turn table? any idea?
Here's the formula.. Using the formula of XL = (2Pi)x(Frequency)x(L) or 6.28 x 120hz x 20H = 15,072. For the 1st choke.
I would leave the preamp resistors as is. They are part of the original design and you don't want to change those if you have the right power supply.
Also, I would leave the first filter resistor as is, to start with, and only work with the second. The first resistor is there to buck the full supply ripple. Your powersupply is plenty adequate for current so start working with the second resistor first and monitor the voltage changes.
After all this, you may still have an input impedance mismatch, and or a low gain tube set compared to the other preamp...
The MM type cartridge wants to see about 47K input impedance to the preamp. That is 47K resistor to ground.
The turntable MM cartridge has very small signal. 4-5 millivolts. If it's a MC cartridge it's even smaller, 1/5 or less of a MM type. Those need a special preamp to use. MM= Moving Magnet, MC = Moving Coil.
You need to find out about the gain of your 6DJ8 as opposed to those Russkies... I couldn't find any info on the Russkies... but that figgers..
Good luck, keep us posted. I've run out of ideas now, as long as you've eliminated the possibility of any real circuit malfunctions.
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