I built some 2A3 monoblocks that I'd like to place near my speakers. Because of where things are placed, I need to run about 15 ft of cabling from the preamp to these.
I currently have a passive pre that obviously will not work.
I'd like to build a 12B4A preamp to drive the cables.
I want to use a preamp transformer (Sowter) that is 4:1 and I think a good match for the valve. I'd also like to reverse the output so phase is preserved.
Using an op point of 80V, 15mA, -10, ra = about 1200ohms. So with the 4:1 transformer, I expect that this will drive the cables at about 75ohms.
Does this make sense?
Thanks
Dan
I currently have a passive pre that obviously will not work.
I'd like to build a 12B4A preamp to drive the cables.
I want to use a preamp transformer (Sowter) that is 4:1 and I think a good match for the valve. I'd also like to reverse the output so phase is preserved.
Using an op point of 80V, 15mA, -10, ra = about 1200ohms. So with the 4:1 transformer, I expect that this will drive the cables at about 75ohms.
Does this make sense?
Thanks
Dan
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Hi Dan,
Why do you want to drive 75 ohms? That’s only good for video. IMO you don’t need 15 mA through that tube and a 4:1 transformer for a line driver (unless you want several volts into a 600 ohm load). 5 mA – 10 mA will suffice me think and the tube will last a lot longer. But you can experiment with that anyway.
Cheers
Why do you want to drive 75 ohms? That’s only good for video. IMO you don’t need 15 mA through that tube and a 4:1 transformer for a line driver (unless you want several volts into a 600 ohm load). 5 mA – 10 mA will suffice me think and the tube will last a lot longer. But you can experiment with that anyway.
Cheers
what kind of interconnect is that?
i have the same setup where my amps are 5 meters away from the rest of my gears. maybe i'm deaf but using a passive DACT fed by my clearaudio phono stage, i didn't have to worry bout the long run.
i also have a 12B4A pre and i just invert the speaker terminals which is free in fact sometimes i even forget
i've have a 5 meter DIY interconnect, OFC copper in teflon, hand braided and a commercial Quantum (it says made in Germany) 5 meter interconnect which is dirt cheap. both are terminated with Eichmann Bullet Plugs.
i have the same setup where my amps are 5 meters away from the rest of my gears. maybe i'm deaf but using a passive DACT fed by my clearaudio phono stage, i didn't have to worry bout the long run.
i also have a 12B4A pre and i just invert the speaker terminals which is free
in fact sometimes i even forget i've have a 5 meter DIY interconnect, OFC copper in teflon, hand braided and a commercial Quantum (it says made in Germany) 5 meter interconnect which is dirt cheap. both are terminated with Eichmann Bullet Plugs.
True, but the effect depends largely on the actual load line. And if the next amp has an input impedance of say 47K, I don’t see a problem.
But anyway most of the (AC) current is supped up by the magnetisation current of the transformer at low frequencies. So the difference can be noticeable at low frequencies and at high frequencies due to the cable capacitance. But IMO it is worth a try.
Cheers
But anyway most of the (AC) current is supped up by the magnetisation current of the transformer at low frequencies. So the difference can be noticeable at low frequencies and at high frequencies due to the cable capacitance. But IMO it is worth a try.
Cheers
help, this project's psu is going pearshaped fast
yes, my projects drag on and on.....
I know I haven't contributed much to this forum, but I have to ask for some help because I think I am about to pull my hair out.
I repackaged my phonostage (based on the WAD phonoII) with this line drive and also the separate phonostage psu combining the line drive's psu.
The phonostage is fine (as always).
My problem is that I can't get any B+ out of my EZ80 rectifier for my 12B4A line driver. And I mean any - 0v's!
The line drive B+ looks like this:
Hammond mains xformer 190-0-190 into
EZ80 into
.22uF
Hammond 20H choke into
100uF into
Hammond 5H choke into
33uF
into 12B4A anodes
I did up the op pt to run 20-25 mA
I measure the correct volts out of the Hammond mains transformer. 6.6V into the EZ80 heaters. But I get no DC out.
I've hooked the valve base up correctly. I've checked it a million times. Swapped in another EZ80 and a EZ81. No luck.
I run the phono stage psu like this with no problems
Sowter mains into
eZ81 into
.047uF into
Hammond 30H into
470uF into
30H into
470uF into
1st stage 120H into 10uf, and
2nd and 3rd stages 60H into 10uF
with no problems.
This is where my mother would say I'm not holding my mouth right...oh, that's fishing.
Any suggestions or sympathy most welcomed!
Dan
yes, my projects drag on and on.....
I know I haven't contributed much to this forum, but I have to ask for some help because I think I am about to pull my hair out.
I repackaged my phonostage (based on the WAD phonoII) with this line drive and also the separate phonostage psu combining the line drive's psu.
The phonostage is fine (as always).
My problem is that I can't get any B+ out of my EZ80 rectifier for my 12B4A line driver. And I mean any - 0v's!
The line drive B+ looks like this:
Hammond mains xformer 190-0-190 into
EZ80 into
.22uF
Hammond 20H choke into
100uF into
Hammond 5H choke into
33uF
into 12B4A anodes
I did up the op pt to run 20-25 mA
I measure the correct volts out of the Hammond mains transformer. 6.6V into the EZ80 heaters. But I get no DC out.
I've hooked the valve base up correctly. I've checked it a million times. Swapped in another EZ80 and a EZ81. No luck.
I run the phono stage psu like this with no problems
Sowter mains into
eZ81 into
.047uF into
Hammond 30H into
470uF into
30H into
470uF into
1st stage 120H into 10uf, and
2nd and 3rd stages 60H into 10uF
with no problems.
This is where my mother would say I'm not holding my mouth right...oh, that's fishing.
Any suggestions or sympathy most welcomed!
Dan
This is one of those humbling experiences where you have to be happy that the only thing that happened is that you've learned something.
My mains transformer has a 3.15-0-3.15 and a 190-0-190.
I wired the 3.15-0-3.15 to the EZ80 and a bridge rectifier for the 12B4A's heaters.
I earthed the centre tap on the 6.3 supply via a 47ohm resister.
Because I got strange voltage readings from the CT on the 190-0-190 supply I left it disconnected.
I have never before wired a rectifier tube and a rectified signal tube heater from the same secondary winding.
Fortunately, I didn't "experiment" but rather called on an expert, Phil at Bluebell who sold me Hammond iron.
I was told that the 190-0-190 CT had to be earthed to make the circuit funtion for the rectifier tube. And that because I had the rectifier and the signal tubes heaters wired off the same winding, that I could not connect the 3.15-0-3.15 CT, or else, well, I'll just say the word catastrophic was used.
Here's a lesson for me. I'm not an electrical engineer. I'm not working off a proven schematic. So, I'll stick to variations of what I've done, or get the advice of an expert before I try something new.
Thanks to Phil and thanks to the forum for your time.
Dan
My mains transformer has a 3.15-0-3.15 and a 190-0-190.
I wired the 3.15-0-3.15 to the EZ80 and a bridge rectifier for the 12B4A's heaters.
I earthed the centre tap on the 6.3 supply via a 47ohm resister.
Because I got strange voltage readings from the CT on the 190-0-190 supply I left it disconnected.
I have never before wired a rectifier tube and a rectified signal tube heater from the same secondary winding.
Fortunately, I didn't "experiment" but rather called on an expert, Phil at Bluebell who sold me Hammond iron.
I was told that the 190-0-190 CT had to be earthed to make the circuit funtion for the rectifier tube. And that because I had the rectifier and the signal tubes heaters wired off the same winding, that I could not connect the 3.15-0-3.15 CT, or else, well, I'll just say the word catastrophic was used.
Here's a lesson for me. I'm not an electrical engineer. I'm not working off a proven schematic. So, I'll stick to variations of what I've done, or get the advice of an expert before I try something new.
Thanks to Phil and thanks to the forum for your time.
Dan
The EZ80/81 cathode to filament insulation is designed to stand off at least 450V IIRC. You can use the single filament winding on your transformer to power all line stage filaments, just make sure that the cathode of the EZ80 is NOT connected to the filament winding, and that the rated current of the winding is greater than the sum of the 12B4's and EZ80's filament current. I would either float the filament center tap off of a resistive divider with cap bypass from the B+ or ground it. (Might hum this way.)
I would use about 68K/12K in the divider with a 100uF/25V cap connected to the center tap of your filament winding. This should bias it up 12V, more may be required depending on the cathode bias with your 12B4.
Yes the center tap of your 380VCT high voltage secondary needs to be grounded.
DC on the 12B4 filaments isn't strictly necessary and AC is actually preferable to poorly filtered DC due to the ripple content.
As an aside: The 6V4/EZ80 rectifier tube is used in some US tuners and pre-amps with a single filament winding which powers everything except in the case of some pre-amps with phono stages which may have a separate winding rectified and filtered for the phono stages specifically.
I would use about 68K/12K in the divider with a 100uF/25V cap connected to the center tap of your filament winding. This should bias it up 12V, more may be required depending on the cathode bias with your 12B4.
Yes the center tap of your 380VCT high voltage secondary needs to be grounded.
DC on the 12B4 filaments isn't strictly necessary and AC is actually preferable to poorly filtered DC due to the ripple content.
As an aside: The 6V4/EZ80 rectifier tube is used in some US tuners and pre-amps with a single filament winding which powers everything except in the case of some pre-amps with phono stages which may have a separate winding rectified and filtered for the phono stages specifically.
I was gonna say that.EC8010 said:
I've tried these tubes in a lot of configs over the years, and they consistently sound better with a bit of current running through them, ie 20mA of so. I used to leave mine on permanently and saw no measured degradation over extended timeframes.Pjotr said:
This all came together yesterday evening. The 12B4A operating point is around 20mA, 120v, -15v. Its dc heaters (5822 Schottky diodes, CRC, 4700uF’s and 1 ohm) are quiet and the HT psu seems okay.
I did not bypass the cathode and my output tranny is 4:1, so I figure gain is pretty much at unity, but Z out is a bit higher than recommended by Sowter, so compromised HF bandwidth.
Old setup: CD and phonostage directly into a 10K stepped attenuator into my (close by) 2A3 monoblocks. I want to do two things: 1) provide drive so I can move the monoblocks by the speakers (pls don’t tell my wife), and 2) get the phonostage output into something with muscle (its final stage is ECC83 CF with CCS active load).
Initial reaction:
Vinyl (NAS spacedeck source) came across with more power on the bottom end, better controlled, richer. Bit more slam. Vinyl sounded a bit meek compared to CD, not anymore. The highs are there but with not as much sparkle. I think I hear a bit of gain overall (on CD as well). Overall, pretty happy so far.
I think I’m happy to let it burn in a bit before I battery bias the 12B4A’s. And make some very long interconnects and move those mono’s. And reserve real judgement until then.
Dan
I did not bypass the cathode and my output tranny is 4:1, so I figure gain is pretty much at unity, but Z out is a bit higher than recommended by Sowter, so compromised HF bandwidth.
Old setup: CD and phonostage directly into a 10K stepped attenuator into my (close by) 2A3 monoblocks. I want to do two things: 1) provide drive so I can move the monoblocks by the speakers (pls don’t tell my wife), and 2) get the phonostage output into something with muscle (its final stage is ECC83 CF with CCS active load).
Initial reaction:
Vinyl (NAS spacedeck source) came across with more power on the bottom end, better controlled, richer. Bit more slam. Vinyl sounded a bit meek compared to CD, not anymore. The highs are there but with not as much sparkle. I think I hear a bit of gain overall (on CD as well). Overall, pretty happy so far.
I think I’m happy to let it burn in a bit before I battery bias the 12B4A’s. And make some very long interconnects and move those mono’s. And reserve real judgement until then.
Dan
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