I originally made this preamp for a 6AS7. The gain was as expected only 1.7. I ended up needing more gain, so simply swapped the 6AS7 for a 6BX7. Got plenty of gain, almost 20dB (18.8x), and that with a much too low value for the plate resistor.
This sounded fine and I was happy, but I knew it was not optimal with the 2200ohm plate resistor (which is ok for the 6AS7) and I finally replaced the resistor with a 3900ohm value. I also replaced the 10uF! metallized film coupling cap with a proper 470nF value foil cap. (I paralleled 4 x 3.9nF COGs just for good measure).
Much better!
The plate load should actually be higher for the 6BX7, and if the 6BL7 is to be used it really really should be higher, more close to 10kohm. But it's a compromise with the available voltage, the current I want high, and so forth. As it is it sounds plenty good.
But if done again, and not just using parts at hand, I would get a proper tube rectified supply giving at least 250V. Alternative is of course choke load the plates, which I think will be my fix, enabling me to keep the lowish 175V supply rail.
Nothing special at all, just another pre. This can take 6SN7, 6BL7, 6BX7, and even the 6AS7.
Edit: the load lines shown are with a plate voltage of 175V, so that will be either with a higher b+ rail, or with choke load.
This sounded fine and I was happy, but I knew it was not optimal with the 2200ohm plate resistor (which is ok for the 6AS7) and I finally replaced the resistor with a 3900ohm value. I also replaced the 10uF! metallized film coupling cap with a proper 470nF value foil cap. (I paralleled 4 x 3.9nF COGs just for good measure).
Much better!
The plate load should actually be higher for the 6BX7, and if the 6BL7 is to be used it really really should be higher, more close to 10kohm. But it's a compromise with the available voltage, the current I want high, and so forth. As it is it sounds plenty good.
But if done again, and not just using parts at hand, I would get a proper tube rectified supply giving at least 250V. Alternative is of course choke load the plates, which I think will be my fix, enabling me to keep the lowish 175V supply rail.
Nothing special at all, just another pre. This can take 6SN7, 6BL7, 6BX7, and even the 6AS7.
Edit: the load lines shown are with a plate voltage of 175V, so that will be either with a higher b+ rail, or with choke load.
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Nice looking little beastie. I suspect you are right, a bigger plate supply would likely help. Don't forget 5998; right between 6AS7 and 6BX7. And if you have 5A of 6.3, the type 6336.
cheers,
Dougls
cheers,
Dougls
Thanks. The 5998 does indeed look interesting. Almost 2A3-ish specs. But pricey! Must be getting rare.
My next pre will likely use 6W6... which means 6V6, 6L6 and the likes also can be rolled.
The 6BX7 is pretty similar to trioded 6V6...
My next pre will likely use 6W6... which means 6V6, 6L6 and the likes also can be rolled.
The 6BX7 is pretty similar to trioded 6V6...
That will work too. Try type 842.
cheers,
Douglas
ps-entirely kidding; type 842 prices make 5998 look like tubes from the nickel each pile.
Some other project. This one is using common and not expensive tubes.
How is the noise voltage of DHTs? Or microphonics? With 3A5 I had to mount them on vibration dampers to eliminate the microphonics. I think the directly heated cathodes are a little more finicky in those regards. I mean for the last power stage is one thing, but a preamp you need to think about the noise as more critical.
The 6BX7 is a nice tube for pre. I build one moons back experimenting with a couple of topologies but the best sound that I got was to parallel the tube & use 1 tube for each channel. Power supply is very important here, I ended up using CLCLCLC.
Cheers
"How is the noise voltage of DHTs? Or microphonics? With 3A5 I had to mount them on vibration dampers to eliminate the microphonics. I think the directly heated cathodes are a little more finicky in those regards. I mean for the last power stage is one thing, but a preamp you need to think about the noise as more critical."
Agree but DHT preamps are the best sounding devices.
01a Preamp (Gen2) – Bartola(R) Valves
You can start with resistor loaded plate and RC coupled filament/cathode. For DC heaters you can use LCL filters. Afterwards if you like it you may try CCS/chokes for plate load plus use filament bias.
Regards
Agree but DHT preamps are the best sounding devices.
01a Preamp (Gen2) – Bartola(R) Valves
You can start with resistor loaded plate and RC coupled filament/cathode. For DC heaters you can use LCL filters. Afterwards if you like it you may try CCS/chokes for plate load plus use filament bias.
Regards