I currently spend most of my working day around electricians and there is no way I would ever take it on faith that a power outlet is wired properly. I regularly watch as the sparkies do some very weird things ("hey, are you sure that's the way it's supposed to be wired...?")
On a vaguely grumpy note I can guarantee you that this is because most electricians I have worked with (UK, Ireland, Australia) have not really been taught anything about electricity. They are taught where to put the wires, not why. Present them with anything non-standard and watch what happens.
So no, don't trust your powerpoint...
On a vaguely grumpy note I can guarantee you that this is because most electricians I have worked with (UK, Ireland, Australia) have not really been taught anything about electricity. They are taught where to put the wires, not why. Present them with anything non-standard and watch what happens.
So no, don't trust your powerpoint...
As far as a nice little preamp, this design looks nice to me, from what you have posted above. I will use an isolation transformer and diode voltage doubler for the HV, though this HV may be about 250v or so, with a diode voltage doubler based on a straight isolation transformer going from line to line (approx 120V+ AC for me). should that be OK for the tubes, or do I need to throttle it down a bit?
Also if I use a small output transformer, like 28 or 40:1 as the output, and just a tiny 3 watt one, which I have here, would that be ok to drive my amp which needs 1/2 volt?
Technically, since we are just providing output in a clean manner, should the ratio really matter at all, as long as it's within the range of most output transformer specs as far as impedance ratio.
We are just offering the output to another pre-amp stage, and not a speaker.
Also if I use a small output transformer, like 28 or 40:1 as the output, and just a tiny 3 watt one, which I have here, would that be ok to drive my amp which needs 1/2 volt?
Technically, since we are just providing output in a clean manner, should the ratio really matter at all, as long as it's within the range of most output transformer specs as far as impedance ratio.
We are just offering the output to another pre-amp stage, and not a speaker.
This site seems to have an amazing amount of info, and lots of schematics old & new, though coming from a Czech domain.
A wealth of educational stuff too:
http://www.bonavolta.ch/hobby/en/audio/audioel.htm
I see a few choices for simple pre-amps here too.
One using a 5687 (an $8 tube):
http://www.bonavolta.ch/hobby/en/audio/5687line.htm
Another using both elements of a 6DJ8/ECC88 which is a $10 tube
(depending on brand), and could seemingly be done with an isolation trafo from 120V with a rectifier voltage doubler (to get 250vdc) arrangement and a low power filament supply:
http://www.bonavolta.ch/hobby/en/audio/6dj8linp.htm
Any comments on these?
A wealth of educational stuff too:
http://www.bonavolta.ch/hobby/en/audio/audioel.htm
I see a few choices for simple pre-amps here too.
One using a 5687 (an $8 tube):
http://www.bonavolta.ch/hobby/en/audio/5687line.htm
Another using both elements of a 6DJ8/ECC88 which is a $10 tube
(depending on brand), and could seemingly be done with an isolation trafo from 120V with a rectifier voltage doubler (to get 250vdc) arrangement and a low power filament supply:
http://www.bonavolta.ch/hobby/en/audio/6dj8linp.htm
Any comments on these?
As you only need 0.5V output, 100-150V B+ is plenty. The 6AU6 can give LOTs of gain, and if it's too much, you can reduce the plate load resistor for less, or triode connect it (tie screen to plate). Or you could use a 12AX7 and change to a lower gain tube if necessary (many choices with same pin connections). The back of an RCA tube manual has tables for most voltage amplifier tubes, telling you what gain and undistorted output will be for a range of component values - as does this GE data sheet: http://frank.pocnet.net/sheets/093/6/6AU6A.pdf
OK, so without getting too technical, I could then use the diagram from my earlier post with the 2 tubes, forget the 12B4 completely, and just get the line output from the 6AU6 (after the .22uF cap coming from the plate and at the exact point just before the 300 ohm resistor going into the 12B4) and the other end being at at signal ground?
And then be able to use that to another amp via an rca jack?
And then perhaps drop the values of the 20k and 47k resistors, so that if I only have 150v DC to start with from my power supply sceme the plate & grid would get enough voltage (maybe 0 and 20k?), it seems the resistor to the center grid is larger, though the schematic says it's getting the same voltage.
Anyway, if this is in the ballpark of decent functionality, I'll go for it, the main thing is a clear throughput without distortion.
Thanks Tom. 🙂
And then be able to use that to another amp via an rca jack?
And then perhaps drop the values of the 20k and 47k resistors, so that if I only have 150v DC to start with from my power supply sceme the plate & grid would get enough voltage (maybe 0 and 20k?), it seems the resistor to the center grid is larger, though the schematic says it's getting the same voltage.
Anyway, if this is in the ballpark of decent functionality, I'll go for it, the main thing is a clear throughput without distortion.
Thanks Tom. 🙂
Hi, that site is from Switzerland. I know, .ch sounds like Czech but it acutally stands for Confoederatio Helvetica - the latin name for Switzerland. Its the only name that the french, german and italian and rhaeto-romanic swiss could agree on. 😀
Those schematics are mostly window dressing in my opinion.
Check out Max Robinson's site for a nice little gain stage and a nice explanation of common amplifier types.
http://www.angelfire.com/electronic/funwithtubes/Amp-Gain_Block.html
Read this first though:
http://www.angelfire.com/electronic/funwithtubes/Amp-DB_Heart.html
I have a true one tube preamp 😀
I use a single tube, 2 triode sections, one for each channel for my preamp. The tube is a WE407A 😉
I use a single tube, 2 triode sections, one for each channel for my preamp. The tube is a WE407A 😉
An externally hosted image should be here but it was not working when we last tested it.
Thanks, I'd been to Max's site before, but overlooked this nice design, with the 12AX7. He seems to have gotten it perfect, so I'll probably go with that. Just have to figure out a power supply. If my B+ ends up being anywhere from 150 - 300v should that be fine too? I assume the circuit will just adjust itself power-wise and still give good performance if lower. I'll adjust the resistor coming from the B+ line as well.
If I design a power supply for example with a 250-0-250 transformer, my understanding is that this is RMS, so I will actully get 354 volts alter a full wave diode rectifier (using 1n4007's), and still a bit over 300v even with a choke input filter. Do you think a choke would help with the sound quality over a resistor in its place? And if so, what value choke would be acceptable for a small circuit like this?
If I design a power supply for example with a 250-0-250 transformer, my understanding is that this is RMS, so I will actully get 354 volts alter a full wave diode rectifier (using 1n4007's), and still a bit over 300v even with a choke input filter. Do you think a choke would help with the sound quality over a resistor in its place? And if so, what value choke would be acceptable for a small circuit like this?
frank754 said:
And those radios would be illegal if mde today. Put the plug in the wrong way and the chassis becomes hot -- and if someone manages to touch that chassis -- zap.
Use a transformer between all your circuit and the wall.
dave
Just to add a comment i haven't seen made yet... a mic needs something more akin to a phono preamp than a linestage preamp... a 6AU6, EF 86, or the 2 stages available in a 12AX7 would make sense to me.
dave
dave
arnoldc said:
How do you compare the WE407A to the 12
B4 that you built? Which one do you use in your system?
Kev
Decware ZSLA-1
http://www.decware.com/linestage/zp1.htm
And the schematic is available under the "Specs" tab.
Not widespread in use, but apparently good per the review:
http://www.decware.com/newsite/mainmenu.htm
http://www.decware.com/linestage/zp1.htm
And the schematic is available under the "Specs" tab.
Not widespread in use, but apparently good per the review:
http://www.decware.com/newsite/mainmenu.htm
Cool, thanks, I've pretty much got all my specs and parts, trafo, etc,. nailed down and will hopefully order parts today.
I'm going to use something similar to this power supply, but since the preamp is low power, just a small power transformer.
Instead of 2 diodes, I'll use four in a full wave bridge, and one
of these Hammond chokes (which one should I pick?)
156G 9H, 40ma, 300 ohms, 400VDC. EACH $11.
156L 5H, 75ma, 135 ohms, 400VDC. EACH $11.
156M 3H, 100ma, 86 ohms, 400VDC. EACH $11.
Also this power transformer 250-0-250 seems a good price
and compact, is 25ma enough for a 12AX7 pre-amp? (actually I
will use two 6AV6's for a 2-tube look, as they have the same spec, per element.
So my main question, is this Allied trafo a good choice?
http://www.alliedelec.com/Search/SearchResults.asp?N=0&Ntk=Primary&Ntt=6K1VF&sid=45B6A18043A0E17F
Also, what the heck are they doing in this power supply diagram adding the .01 caps and 10 ohm resistors, and the one in parallel with the choke?
Would they be helpful if I use a full wave bridge, or did someone just get carried away here?
I'm going to use something similar to this power supply, but since the preamp is low power, just a small power transformer.
Instead of 2 diodes, I'll use four in a full wave bridge, and one
of these Hammond chokes (which one should I pick?)
156G 9H, 40ma, 300 ohms, 400VDC. EACH $11.
156L 5H, 75ma, 135 ohms, 400VDC. EACH $11.
156M 3H, 100ma, 86 ohms, 400VDC. EACH $11.
Also this power transformer 250-0-250 seems a good price
and compact, is 25ma enough for a 12AX7 pre-amp? (actually I
will use two 6AV6's for a 2-tube look, as they have the same spec, per element.
So my main question, is this Allied trafo a good choice?
http://www.alliedelec.com/Search/SearchResults.asp?N=0&Ntk=Primary&Ntt=6K1VF&sid=45B6A18043A0E17F
Also, what the heck are they doing in this power supply diagram adding the .01 caps and 10 ohm resistors, and the one in parallel with the choke?
Would they be helpful if I use a full wave bridge, or did someone just get carried away here?
frank754 said:Cool, thanks, I've pretty much got all my specs and parts, trafo, etc,. nailed down and will hopefully order parts today.
I'm going to use something similar to this power supply, but since the preamp is low power, just a small power transformer.
Instead of 2 diodes, I'll use four in a full wave bridge, and one
of these Hammond chokes (which one should I pick?)
156G 9H, 40ma, 300 ohms, 400VDC. EACH $11.
156L 5H, 75ma, 135 ohms, 400VDC. EACH $11.
156M 3H, 100ma, 86 ohms, 400VDC. EACH $11.
Also this power transformer 250-0-250 seems a good price
and compact, is 25ma enough for a 12AX7 pre-amp? (actually I
will use two 6AV6's for a 2-tube look, as they have the same spec, per element.
So my main question, is this Allied trafo a good choice?
http://www.alliedelec.com/Search/SearchResults.asp?N=0&Ntk=Primary&Ntt=6K1VF&sid=45B6A18043A0E17F
Also, what the heck are they doing in this power supply diagram adding the .01 caps and 10 ohm resistors, and the one in parallel with the choke?
Would they be helpful if I use a full wave bridge, or did someone just get carried away here?
That transformer should be fine, but you'll have to use it full wave, not with a bridge. If you want to use a bridge, you'll need to a 125-0-125 or just a 250V without a CT.
The small caps and resistors are a form of filter for high frequency noise from the mains and diodes. Similar things can be done with a bridge.
If you are using 12AX7's, the current draw per section would be on the order of 1mA or so. So your total would be less than 10mA. You should choose a choke rated near the current range (a little higher rating than current) you'll be running.
Sheldon
KevinTams said:
I use the 407A in my system. Having said that, while the 12B4 sounds good (I've played with B+ 310V - 250V, Ip 28mA - 15mA) I still prefer the 407A with an LED on the cathode and R load.
My friend made a 407A based on mine, but uses choke load and it is even better than mine. And we haven't even played around with regulated supplies on 407A preamp...
Great, thanks, am working my orders now, will post back once the project is underway with pics, etc.
OK, I've finished it and it works, the only thing is that there is considerable AC hum so far. Here is a shot of the underside:
I used two 7-pin 6AV6's instead of a 12AX7, and the twisted green wires in the photo are the 6.3v heater supply, going from the first tube to the other, then to the pilot light (black wires, all twisted).
The power supply occupies about 75% of the underside, the 2 caps at the lower left are the main filter caps, the 2 black wires going over them come from the choke. The HV rail is just to the right where you see the 330k resistor, and to the right of the white cap, you will see the two 1N4007 diodes (the power supply was built per the schematic I posted above).
The top left 1/4 is the sensitive audio area, the top RCA jack is the input, the lower one the output. The input has shielded wire to the pot.
When I pull the plug, the preamp works beautifully with zero hum for almost a minute until the tubes cool and the power drains.
I'm looking for suggestions on what to try, I've thought of these ideas so far:
If it's not the filament supply - (1) maybe put a metal separation shield above the two filter caps to separate them from the jacks area (2) since the diode leads are long maybe put a shield under them to separate them from the power xfmr leads
If it is the filament supply, could I ground one side and add a diode to the other side of the 6.3 v just after it leaves the transformer, perhaps above the chassis so there is no AC underneath except at the far right?
I used two 7-pin 6AV6's instead of a 12AX7, and the twisted green wires in the photo are the 6.3v heater supply, going from the first tube to the other, then to the pilot light (black wires, all twisted).
The power supply occupies about 75% of the underside, the 2 caps at the lower left are the main filter caps, the 2 black wires going over them come from the choke. The HV rail is just to the right where you see the 330k resistor, and to the right of the white cap, you will see the two 1N4007 diodes (the power supply was built per the schematic I posted above).
The top left 1/4 is the sensitive audio area, the top RCA jack is the input, the lower one the output. The input has shielded wire to the pot.
When I pull the plug, the preamp works beautifully with zero hum for almost a minute until the tubes cool and the power drains.
I'm looking for suggestions on what to try, I've thought of these ideas so far:
If it's not the filament supply - (1) maybe put a metal separation shield above the two filter caps to separate them from the jacks area (2) since the diode leads are long maybe put a shield under them to separate them from the power xfmr leads
If it is the filament supply, could I ground one side and add a diode to the other side of the 6.3 v just after it leaves the transformer, perhaps above the chassis so there is no AC underneath except at the far right?
arnoldc WE407A
arnoldc,
do you have a schematic for this preamp you could share?
TIA and regards,
-- josé k.
arnoldc,
do you have a schematic for this preamp you could share?
TIA and regards,
-- josé k.
If you are really interested, I will put a schematic together in a few days. It's based on the info above starting at the beginning of the thread but with some mods.
As far as my hum problem goes, I tried something just a few minutes ago, and it solved 95% of the hum.
I ran a 100 ohm 1 watt resistor from one side of the 6.3v AC filament supply to a chassis ground. Actually it was from the farthest pilot light end of the 6.3 line.
This is akin to the hum bucking pot type of thing, and actually they say it can be reduced further with an exact resistor value, but it may vary when you replace tubes.
Here's a top view of the final result:
It's on a small 5x7x2 chassis, fully self powered, etc., and uses an Allied power xfmr that was only $13 and a choke from Angela that was only $11. The tubes are only about $6 each and real GE NOS.
You could probably build it for around $80.
As far as my hum problem goes, I tried something just a few minutes ago, and it solved 95% of the hum.
I ran a 100 ohm 1 watt resistor from one side of the 6.3v AC filament supply to a chassis ground. Actually it was from the farthest pilot light end of the 6.3 line.
This is akin to the hum bucking pot type of thing, and actually they say it can be reduced further with an exact resistor value, but it may vary when you replace tubes.
Here's a top view of the final result:
It's on a small 5x7x2 chassis, fully self powered, etc., and uses an Allied power xfmr that was only $13 and a choke from Angela that was only $11. The tubes are only about $6 each and real GE NOS.
You could probably build it for around $80.
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