I have a 3- 6SN7 tube preamp that was built for me a few years back that sounds really nice, wide deep soundstage, clean sound, zero noise, etc. It eventually had to be repaired, (burnt resistors and power supply cap failure), and eventually it had a power supply rebuild that is in a separate box with a 5AR4 rectifier. It has been very reliable, but lacks a bit of gain. I have to turn it up to 1-3pm to get any decent volume. I use a solid state power amp with 250 watts per side into 8 ohms to drive large Infinity RS4B's in my shop/garden area. Enclosed is a schematic of the preamp and power supply. Someone told me the power supply schematic was drawn incorrectly, but it has worked very well as it was built, which may not be exactly as is shown in the schematic.
Are there any resistor values in the preamp section that would increase gain if changed? I can live with it as is, I don't want to spend a lot of funds to do another design rebuild.
Thomas
Are there any resistor values in the preamp section that would increase gain if changed? I can live with it as is, I don't want to spend a lot of funds to do another design rebuild.
Thomas
Tell us about your source components (their output voltages if you know them) as well as what your power amplifier is (and it's gain if that's readily available).
At what percentage of the pot's rotation is 1-3PM? This is a pretty arbitrary standard, can you turn the preamp up to the point where your power amp clips or things get unbearably loud?
As far as the actual preamp goes, are you 100% sure about the accuracy of the preamp schematic? Using a 5K plate load on a 6SN7 isn't a particularly well thought out choice.
I attached an "estimated" plot of what a 5K load line looks like on 250V. The load line is so steep that it's hard to plot on the curves I found. This is absolute crap, take the preamp back to whoever designed it and ask for your money back or for a redesign.
At what percentage of the pot's rotation is 1-3PM? This is a pretty arbitrary standard, can you turn the preamp up to the point where your power amp clips or things get unbearably loud?
As far as the actual preamp goes, are you 100% sure about the accuracy of the preamp schematic? Using a 5K plate load on a 6SN7 isn't a particularly well thought out choice.
I attached an "estimated" plot of what a 5K load line looks like on 250V. The load line is so steep that it's hard to plot on the curves I found. This is absolute crap, take the preamp back to whoever designed it and ask for your money back or for a redesign.
Attachments
I don't know the output voltages, I use a Sansui T-717 Tuner and an old Denon 3520 CD player with adjustable gain turned all the way up. When I use a phono preamp, Hagerman Cornet, it works a bit better, there is more gain from the phono preamp. Amp is a rebuilt Hafler DH500. I have tried a few other high power amps, Adcom GFA 555, B & K Reference 200.2 with similar gain issue. Amp never clips and it never gets unbearably loud. It is loud enough for listening at about 80% of volume.
Thomas
Thomas
You could have a bit more gain without totally violating the designer's intent by increasing the value of R106. Try doubling it. There are many other possibilities, depending on your sense of adventure and available test gear.
The volume control is connected backwards. The wiper should go to the first grid.
Since the second stage is run at unity gain due to R106, you could get more gain
by just removing it. Also R105 heavily loads the first stage due to the virtual
ground action of the second stage. The R106 also heavily loads the second stage.
If you don't want to remove R106, you can increase the values of R105 and R106
to as much as 25k each, to unload the first and second stages. That alone
will give you several dB more gain and less distortion in each of the gain stages.
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Roughly around 20dB more overall gain with no R106.
If you do this and like it, then short the unneeded R105.
Do fix the volume control if it's actually wired like that.
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What I would do is to dump two of the 6SN7's and switch them to 6J5's setup as voltage amplifiers, then directly couple them to the remaining 6SN7 setup as a cathode follower.
The voltage amp could have a 650 Ohm cathode resistor and 47K plate load. Distortion will be way, way lower than the first stage you have now, and you'll magically pick up a lot of gain. Use another 47K resistor under each cathode of the 6SN7 for the cathode follower and move its plate connection to after the shunt regulator.
Most of the parts in the original design can just be removed...
The voltage amp could have a 650 Ohm cathode resistor and 47K plate load. Distortion will be way, way lower than the first stage you have now, and you'll magically pick up a lot of gain. Use another 47K resistor under each cathode of the 6SN7 for the cathode follower and move its plate connection to after the shunt regulator.
Most of the parts in the original design can just be removed...
That looks right for R106, and if it works it must be the right one.
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Try R106 approx. 25k. If that's still excessive gain, try 15k instead.
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I'll try 25k and the 15k resistors. Will I need to remove the R105 resistors and solder in a wire where they were?
Thanks, Thomas
Thanks, Thomas
Terrible design, so much so that it can´t be "improved", sort of stripping it and building something more "normal" inside that chassis.
Yes, by removing NFB you will get more gain.
Yes, by removing NFB you will get more gain.
No, the R105 must stay in place to complete the feedback network when using a R106.
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