Hi, I'm new to diy amps and everything, and I'm about to start the wiring of my first amp in a few days. I just have a few questions before I start.
1. My power cable has 3 wires, hot, neutral, chassis (safety) ground. When I was looking at grounding schemes, it seems that chassis ground is the only ground used, ie. all grounds connect to the chassis. How do I wire this? Hot and Neutral connect to the transformer in and chassis ground connects to chassis where all other grounds connect?
2. In the power supply, I need 250v and 180v. In the schematic, the resister isn't labeled that drops the voltage from 250 to 180. How large of a resister do need?
3. I need a volume knob. Where can I get a nice one? The knobs at radio shack were ugly as hell.
Thanks
Andy
1. My power cable has 3 wires, hot, neutral, chassis (safety) ground. When I was looking at grounding schemes, it seems that chassis ground is the only ground used, ie. all grounds connect to the chassis. How do I wire this? Hot and Neutral connect to the transformer in and chassis ground connects to chassis where all other grounds connect?
2. In the power supply, I need 250v and 180v. In the schematic, the resister isn't labeled that drops the voltage from 250 to 180. How large of a resister do need?
3. I need a volume knob. Where can I get a nice one? The knobs at radio shack were ugly as hell.
An externally hosted image should be here but it was not working when we last tested it.
Thanks
Andy
Hi Andy, and welcome to diyAudio 
As far as earthing is concerned, you are spot-on. With regard to Live and Neutral, it is safest to run the Live through a fuse, then a switch, then to the transformer primary. The Neutral can connect directly to the primary, if only a single pole switch is available.
The value of the resistor that drops the 250v to 180v will depend on the current taken. Do you know? Or, do you have the amplifier schematic?
Try a search of the forum for Volume Knobs. There have been a couple of threads about them.
Cheers,
As far as earthing is concerned, you are spot-on. With regard to Live and Neutral, it is safest to run the Live through a fuse, then a switch, then to the transformer primary. The Neutral can connect directly to the primary, if only a single pole switch is available.
The value of the resistor that drops the 250v to 180v will depend on the current taken. Do you know? Or, do you have the amplifier schematic?
Try a search of the forum for Volume Knobs. There have been a couple of threads about them.
Cheers,
here's the amp schematic.
thanks
Andy
An externally hosted image should be here but it was not working when we last tested it.
thanks
Andy
Your proposed grounding scheme will work fine.
It means that the neutral lead (white) and the safety ground lead(green) will be at the same potential, which is perfectly OK. That's how it's supposed to work.
As a point of good construction practice (schematics dont' always tell the whole story) What you'd be shooting for is to have the "audio ground" connect to the safety ground at only one place. Generally, that's very near the input signal connector.
The center tap of the high voltage secondary is the place to think of the "audio ground" returning to the transformer (it's sometimes called B minus for historical reasons, and B minus is usually at ground potential, but not necessarily). Ground is a slippery term, so hold it loosely, rather than in a mental grip.
As a more advanced noise reduction consideration, people sometimes put back to back "Ying Yang" diodes in between the audio ground buss and the safety ground point. Diodes drop about a volt across themselves, so the two "grounds" can differ by about that amount in either direction. Noise less than that volt can't get through, but any more serious imbalace can. Note that this sort of thing is NOT ok with the Underwriter's Labs, or your friendly home insurance firm. Mostly I mention it out of academic interest, not as a sugestion for a first timer.
I agree with dhaen's comment about the fuse and the switch - the fuse should always be the first thing attached, and it's also ok to put two fuses in both legs of the primary. The idea is that if it blows, everything goes cold, no matter how it's plugged in.
Volume knobs - you might stop by a thrift store, things fall off of old stereo gear all the time... (Hey, that's evil) Think about repurposing items - get a drawer pull from Home Depot and drill it out to 1/4" for the standard shaft size. Or glue the head of a GI Joe doll on a stick. My next project might use trumpet mouthpieces for knobs. The best part about DIY is that things don't have to be shiny black or brushed aluminum.
Have fun!
Andy N
It means that the neutral lead (white) and the safety ground lead(green) will be at the same potential, which is perfectly OK. That's how it's supposed to work.
As a point of good construction practice (schematics dont' always tell the whole story) What you'd be shooting for is to have the "audio ground" connect to the safety ground at only one place. Generally, that's very near the input signal connector.
The center tap of the high voltage secondary is the place to think of the "audio ground" returning to the transformer (it's sometimes called B minus for historical reasons, and B minus is usually at ground potential, but not necessarily). Ground is a slippery term, so hold it loosely, rather than in a mental grip.
As a more advanced noise reduction consideration, people sometimes put back to back "Ying Yang" diodes in between the audio ground buss and the safety ground point. Diodes drop about a volt across themselves, so the two "grounds" can differ by about that amount in either direction. Noise less than that volt can't get through, but any more serious imbalace can. Note that this sort of thing is NOT ok with the Underwriter's Labs, or your friendly home insurance firm. Mostly I mention it out of academic interest, not as a sugestion for a first timer.
I agree with dhaen's comment about the fuse and the switch - the fuse should always be the first thing attached, and it's also ok to put two fuses in both legs of the primary. The idea is that if it blows, everything goes cold, no matter how it's plugged in.
Volume knobs - you might stop by a thrift store, things fall off of old stereo gear all the time... (Hey, that's evil) Think about repurposing items - get a drawer pull from Home Depot and drill it out to 1/4" for the standard shaft size. Or glue the head of a GI Joe doll on a stick. My next project might use trumpet mouthpieces for knobs. The best part about DIY is that things don't have to be shiny black or brushed aluminum.
Have fun!
Andy N
Cool schematic, that looks nice and simple and should be good for a watt or two. I am building a similar thing at the moment, using 6SL7. You might want to add a capacitor (say, 100uF) across the 255 ohm (strange value BTW?!!) cathode resistor of the 6V6... Otherwise might be a bit anemic in the bass.
With regards to the power supply resistor, just use a bit of guesstimation (how much current is flowing, V=IR etc) and try... I am guessing 10K or less... Don't worry too much about getting the voltages exact, 6V6 can be run at up to 400V as long as the total plate dissipation is not exceeded. Adjust the cathode resistor to get the required value...
Also keep in mind that your earths should be separated, and joined only at one place- ie signal earths (input & volume) cathode resistors, power supply earths.
You should also ground one side of the filament.
With regards to the power supply resistor, just use a bit of guesstimation (how much current is flowing, V=IR etc) and try... I am guessing 10K or less... Don't worry too much about getting the voltages exact, 6V6 can be run at up to 400V as long as the total plate dissipation is not exceeded. Adjust the cathode resistor to get the required value...
Also keep in mind that your earths should be separated, and joined only at one place- ie signal earths (input & volume) cathode resistors, power supply earths.
You should also ground one side of the filament.
AndyN said:
Hold on
In Europe it is not OK to fuse the neutral side. The reason is that if the neural fuse blows first, it leaves the whole primary side "live".The only exception to this is medical instruments where other special precautions are taken.
Perhaps someone who is UL literate can advise whether the practice is allowed in the US on domestic equipment.
dhaen said:
I hasten to say that I am NOT in any way UL literate. The 2 legs fused business was something I picked up along the line from an old HAM years ago.
On further reflection, I think there's a distinction to be made between the modern US 3-prong cords, and the older US 2-prong unpolarized cords. In the day of unpolarized cords, it was a 50-50 chance that the unit would be plugged in with the nominal hot side attached to the unfused leg, and if the grounding were to fail, with the chassis were attached to the hot side, bad things could happen.
I think I ought to retract my statement about fusing the two legs, and leave it at "Use a polarized plug, and fuse the hot side. When in doubt, consult a professional."
Here's one reference:
http://home.comcast.net/~radiowarren/safety.html
I searched UL but they want $200 for pdf files of thier standards
Apologies for my Ameri-centrist assumptions...
Andy
The resistor:
Using the 6SN7 graphs and plotting in the 100k anode resistor at 180V, and the 2700 ohm cathode bias resistor, I get a cross at approx. 75V anode, -3V cathode and a 1,1mA plate current.
This is a pretty low operatingpoint for this tube!!!
Now, the 1,1mA would compute to an unlabaled PSU resistor of about (250V-180V)/1,1mA = 63,6k ohms. Closest E12 would be a 68k, 1W'er and would be fine. (this is a pretty high value for a resistor in this place.)
But from another standpoint, it is doing a very nice job in isolating the 2 amplifying stages, and lowering the ripple voltage from the 250V supply. This is nice, since the first stage needs low ripple on its B+ so as to not amplify any of it and causing hum in the loudspeaker.
But it also is making the PSU a less perfect one. I would guess that you could play around quite a bit with this resistor, moving down to 10k or less, as ShiFtY pointed out, without troubles. The lower the value the better, but too low will probably give some hum, depending on your speakers amogst other things. The thing is that with a high value, then the small change in platecurrent under dynamic (AC) conditions will cause the 180V's to vary relativle much, and thus not providing a constant voltage (as we would like it to be). So the smaller the value, the less the "sag" we get
hope this cleared anything
stigla
Using the 6SN7 graphs and plotting in the 100k anode resistor at 180V, and the 2700 ohm cathode bias resistor, I get a cross at approx. 75V anode, -3V cathode and a 1,1mA plate current.
This is a pretty low operatingpoint for this tube!!!
Now, the 1,1mA would compute to an unlabaled PSU resistor of about (250V-180V)/1,1mA = 63,6k ohms. Closest E12 would be a 68k, 1W'er and would be fine. (this is a pretty high value for a resistor in this place.)
But from another standpoint, it is doing a very nice job in isolating the 2 amplifying stages, and lowering the ripple voltage from the 250V supply. This is nice, since the first stage needs low ripple on its B+ so as to not amplify any of it and causing hum in the loudspeaker.
But it also is making the PSU a less perfect one. I would guess that you could play around quite a bit with this resistor, moving down to 10k or less, as ShiFtY pointed out, without troubles. The lower the value the better, but too low will probably give some hum, depending on your speakers amogst other things. The thing is that with a high value, then the small change in platecurrent under dynamic (AC) conditions will cause the 180V's to vary relativle much, and thus not providing a constant voltage (as we would like it to be). So the smaller the value, the less the "sag" we get
hope this cleared anything
stigla
Re: The resistor:
Absolutely!!!
Why take one of the best sounding and cleanest tubes in audio, and operate it down in the dirt?
Been playing on the simulator, and without a choke load, I can't find an op-point for the 6SN7 I like or would bother using myself. Try either a Hammond 156C choke ($12) or use an ECC99 instead ($13.50).
I'd also have about a 300V B+ and would suggest upping the filter cap for the 6V6 to about 100u, whatever B+ you use.
Best I could come up with what you presented was (for 6SN7)
B+ = 230V
Ia = 5.5mA
Ra = 22000
Rk = 400
Ck = 39 or 47uF
gain = 14
Zout = 5k4
R in PS =3k6 / 0.5W min
Too little power and too much HD for my tastes.
stigla said:
Absolutely!!!
Why take one of the best sounding and cleanest tubes in audio, and operate it down in the dirt?
Been playing on the simulator, and without a choke load, I can't find an op-point for the 6SN7 I like or would bother using myself. Try either a Hammond 156C choke ($12) or use an ECC99 instead ($13.50).
I'd also have about a 300V B+ and would suggest upping the filter cap for the 6V6 to about 100u, whatever B+ you use.
Best I could come up with what you presented was (for 6SN7)
B+ = 230V
Ia = 5.5mA
Ra = 22000
Rk = 400
Ck = 39 or 47uF
gain = 14
Zout = 5k4
R in PS =3k6 / 0.5W min
Too little power and too much HD for my tastes.
Thanks for the replies guys. So the 6v6 circuit is ok, but the 6sn7 is running way to conservative? I have a 3k5 that I can use in the ps, so I'll try that one first I guess.
I'm sorry if I sound stupid, but Brett, I don't know what a bunch of those values are. I know B+, Rk, and Ck, but I'm not sure what components Ra and Zout are. Ra I guess is the anode resister? Is Zout the resister after the coupling cap? And what is HD?
Does anybody have a better 6sn7 schematic they could show me? I can't really find anything online that fits what I'm doing with it. I'm really only limited to the voltage that I can get with my power transformer, anything else I can easily adjust. Thanks a lot.
Andy
I'm sorry if I sound stupid, but Brett, I don't know what a bunch of those values are. I know B+, Rk, and Ck, but I'm not sure what components Ra and Zout are. Ra I guess is the anode resister? Is Zout the resister after the coupling cap? And what is HD?
Does anybody have a better 6sn7 schematic they could show me? I can't really find anything online that fits what I'm doing with it. I'm really only limited to the voltage that I can get with my power transformer, anything else I can easily adjust. Thanks a lot.
Andy
ACRONYMS GALLORE...
Hi,
Hoping Brett won't mind:
Ra = Rp ( your anode/plate resistor)
Zoout AKA Zo = output impedance of that stage.
HD...errr....I'm guessing headache?
Yup, like running it from a straightjacket...
A high B+ would open up more interesting operating points on the loadline.
We like it hot.
Cheers,
Hi,
Hoping Brett won't mind:
Ra = Rp ( your anode/plate resistor)
Zoout AKA Zo = output impedance of that stage.
HD...errr....I'm guessing headache?
Yup, like running it from a straightjacket...
A high B+ would open up more interesting operating points on the loadline.
We like it hot.
Cheers,
Lico said:
The 6V6 stage will be fine with a bypass cap (about 300-400uF)
Not stupid at all, just my shorthand. For the 6SN7 section,
Ra = anode/plate resistor, ie change the 100k for 22k (about 3W)
Ia = anode current; yours is ~1mA, mine is 5.5
HD = harmonic distortion.
Zout is the output impedance of the 6SN7 stage (not counting the coupling cap Frank)
R in PS = the dropper resistor in the power supply between the last two caps.
Try the one I suggested (if we've made it clear for you now). I jiggled values around in my simulator this morning, and that's about the one I liked best based on the voltage limitations.
Hope that's clear now (if not please say so)
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