It is a way of nulling mains hum from heater supply.
the heaters require 6.3V , the supply is 12.6V - so you wire the heaters in series in pairs. you ground common and apply 12.6V to the other ends.
Another benefit is that should one tube blow up, it stops heater supply to the other side, and your board or transformer won't blow with it.
If you are bothered, you can connect all the filament grounds together, and then couple it to ground using a 100uF capacitor (though not necessary) try them not grounded, if there is hum, then ground it to see if it improves.
If it does not, try lifting your ground from your input's ground.
You can always do what I did after all else failed and supply DC to all your tubes, but your powertubes - keep them AC and away from your signal or driver tubes.
the heaters require 6.3V , the supply is 12.6V - so you wire the heaters in series in pairs. you ground common and apply 12.6V to the other ends.
Another benefit is that should one tube blow up, it stops heater supply to the other side, and your board or transformer won't blow with it.
If you are bothered, you can connect all the filament grounds together, and then couple it to ground using a 100uF capacitor (though not necessary) try them not grounded, if there is hum, then ground it to see if it improves.
If it does not, try lifting your ground from your input's ground.
You can always do what I did after all else failed and supply DC to all your tubes, but your powertubes - keep them AC and away from your signal or driver tubes.
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I'm not seeing anything unusual about the heater(filament)wiring .It is already in parallel, just using the chassis ground as return instead of using two wires to each socket. This is an old style arrangement, better done with a 2 wire arrangement.
You currently have a two prong plug arrangement shown, better to use a 3 prong with a safety earth conductor and eliminating the capacitor
You currently have a two prong plug arrangement shown, better to use a 3 prong with a safety earth conductor and eliminating the capacitor
So, I have a question or two about the way that the filaments are connected in this schematic. It is very strange to me, I have never seen it depicted that way before.
I have, but this is usually an RF technique. It spells hum problems for an audio frequency circuit. Grounding one side of the heater directly to a chassis ground serves to "short out" any RF that escaped across the stray capacitance between the heater and the cathode sleeve. Not a factor at audio frequencies.
For an audio amp, there is no need for a Lo-Z ground return, so the heaters are best connected to the heater secondary by two wires twisted together to reduce4 magnetic coupling.
A star type ground, or a semi-star, will help reduce hum greatly. You want the circulating currents from filter capacitor re-charge out of any commons. Even through a low resistance connection across a chassis that doubles as a ground plane can give enough voltage to be amplified as though it were a signal to make for clearly audible hum. Same applies for heater current that returns via a ground plane. That's not a consideration for RF circuits since any mains hum will be filtered off by LC tuners and/or BPFs. Having a Lo-Z return is the greater consideration in RF design.
You also don't want the AC mains connection shown here. The AC mains bypass capacitor is on the wrong side of the line (will make chassis somewhat hot even if switched off) poses a serious shock hazard if it fails short, and defeats the purpose of the PTX: removes isolation from the AC mains.
You're much better off using a three wire connection to the AC mains. You can include a ground loop breaker (attached) to help with hum reduction, and the chassis directly to ground will ensire that all metal will stay at ground potential even if there's a fault somewhere that would elevate the chassis above ground. Back in "the day" it wasn't done since three wire sockets weren't available. That AC mains bypass served as a hum reducer if the plug was in right. These days, that's neither necessary nor desirable.
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Ok, I am confused. if I put all the filaments in parallel, use the center tap and filter cap negative for voltage ground all going to one point that is isolated from the chassis, and put the third prong to the chassis, where do I ground my audio so that it doesnt get the cap recharge noise? Also how would I isolate my potentiometers and other audio components from the chassis and still have a complete circuit?
Are you doing a new build or fixing an old amp? The schematic shows the filaments in Parallel, just using grounding to the chassis instead of a separate return wire
There seem to be three options here:
1. leave it as it is
2. sort out the mains connection so it becomes safe
3. do 2, plus rewire all the grounds so the chassis becomes just safety ground instead of being an active part of the circuit
Unless you want a lot of work, including some debugging, I recommend option 2. This will bring it up to modern safety standards, while leaving the authentic hum of ancient audio items.
1. leave it as it is
2. sort out the mains connection so it becomes safe
3. do 2, plus rewire all the grounds so the chassis becomes just safety ground instead of being an active part of the circuit
Unless you want a lot of work, including some debugging, I recommend option 2. This will bring it up to modern safety standards, while leaving the authentic hum of ancient audio items.
I am sorry that I didn't make this clearer. I am building this amp, right now it is a pile of parts and a drawing. So what I think I need to do is this. Put the earth ground from the three prong plug to the chassis. Put the center tap of my HV, and my filter caps to the chassis, and make an isolated star ground for all my signal grounds.
Yes.stridor said:
No. Putting CT and filter caps negative to chassis is what people used to do 60 years ago and it is why old equipment always hums.
The power CT should go to the filter cap negative, then wire from there to the audio ground. Personally I would use a ground bus, not a star, but each to their own! At some point you can add a connection from the audio ground to the chassis.
Had I not seen that kooky way of hooking up the filaments I was going to ground it this way. HV center tap, filter cap negative, and every ground that is depicted here except those crazy filaments to the chassis, and use a two prong plug. This is the way I have always built amps because I have been cloning amps from the 30's and 40's, and hey it's rock n roll, expect a little hum. It hasn't been anything major, and certainly nothing that I couldn't drown out with humbucker pickups. Now that I have been told there is a better way, well I am open to try it, I just don't understand is all. If put my filter cap negative, and HV center tap together, and third prong earth ground to chassis where do I ground points 21, 22, 23, 14, 17, 28, 25 and so on on the above schematic without getting the cap recharge noise someone was mentioning? I have to have someplace to take potential difference from and it can't be the chassis either since that apparently creates the hum, so where is this point?
Well, if I'm doing a scratch build, I use a star/bus ground combo. Power transformer center tap to negative side of first two filter caps. That creates the first "star" ground. Then extend a wire to some other convenient point in the amp (the wire becomes the bus). This end of the bus becomes the star ground collection point for the audio circuit grounds.
Filter caps down stream of the first two are connected to ground along the bus, near the power supply end. Filament supply reference to ground is connected to the bus in there somewhere also--find the lowest noise location. Connect a wire from the star/bus ground to the chassis at one single point. Experiment to find the lowest noise location, probably near the input tube. Thus ALL grounds are essentially connected together, it's just a matter of connecting them in such a way that loop currents flow locally, not globally across the entire bus.
Filter caps down stream of the first two are connected to ground along the bus, near the power supply end. Filament supply reference to ground is connected to the bus in there somewhere also--find the lowest noise location. Connect a wire from the star/bus ground to the chassis at one single point. Experiment to find the lowest noise location, probably near the input tube. Thus ALL grounds are essentially connected together, it's just a matter of connecting them in such a way that loop currents flow locally, not globally across the entire bus.
Thanks to everybody for the advice and tips on this project. All of your responses have kind of shown me that I have a lot to learn. I know electronics pretty good, but this is more art than science it seems. I didn't realize it but the way i have decided to ground this is really more of a bus than a star ground. Jeffhigh, I am definitely going to use a three prong plug. I have been reading some threds on TDRPI about converting old two prong amps to three prong. Some of the reasons that people have decided to do it being mentioned in their posts were downright scary. Didn't Kieth Relf get electrocuted by an improperly grounded amp?
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An earth bus, if kept short, is also a star! best place for it is nearest to the input tube, on the chasis. get a thick single core wire shape it like a two inch staple, and connect both ends to your chassis. connect all other earths to this using individual wires, never connect two earth wires together and then bring one cable to your bus.
Also as far as your heater supply goes, twin triodes have twin heaters, with three wires. get a 12.6V supply and keep the heaters in series, and earth the middle wire, or use DC for heaters (not necessary) - if you do your job right, hum should be inaudible.
I would also suggest to get a second choke of high inductance but low currect, and a capacitor after R33 and connect the screens to this, and from here get a B+ to your small signal tubes. The cleaner the supply to your screens, the better it sounds.
Also use quality bypass capacitors (c9 c10) and paper in oil for coupling ones (c12 c13 c14 c15) I like K40y-9's - they are as good as any and cost little.
Also as far as your heater supply goes, twin triodes have twin heaters, with three wires. get a 12.6V supply and keep the heaters in series, and earth the middle wire, or use DC for heaters (not necessary) - if you do your job right, hum should be inaudible.
I would also suggest to get a second choke of high inductance but low currect, and a capacitor after R33 and connect the screens to this, and from here get a B+ to your small signal tubes. The cleaner the supply to your screens, the better it sounds.
Also use quality bypass capacitors (c9 c10) and paper in oil for coupling ones (c12 c13 c14 c15) I like K40y-9's - they are as good as any and cost little.
This is using 6SL7 twin triodes NOT 12AX7. There is only pin 7 and 8 for the heaters
Forget about the 12.6volts
Thank you for pointing that out, I learnt something.
HOWEVER,
aren't there TWO 6SL7's.
What are the disadvantages of connecting the heaters in series and grounding the common?
We all know, that would take care of hum, but are there any penalties?
Forget about the 12.6volts
Thank you for pointing that out, I learnt something.
HOWEVER,
aren't there TWO 6SL7's.
What are the disadvantages of connecting the heaters in series and grounding the common?
We all know, that would take care of hum, but are there any penalties?
The only penalty I can see is having to buy a 12 volt transformer, but I guess the next build I can do that and see how it works.
or use DC!
I just assumed since it was a new built, you hadn't purchased all the parts yet.
Frankly I would not earth one side of the heater supply.
I suggest you build it, without earthing the heater supply, use twisted wire from one tube to next to form a ring (from transformer to each tube, back to the transformer), and see if you have any induced hum from heaters. Test should be simple, run the amp, if there is any hum, just disconnect the heater supply, if hum immediately goes away, it is the heater supply!
Next step is to use two resistors, say 1W 47R connect then in series between heater supply rails and earth the middle.
Again, you may not have to, first build and see.
Well, I got done with this build, and I must say that I am thoroughly dissatisfied. When I was ignorant and connected each and every single ground to the most convenient place I could put a dollop of solder on the steel chassis I got better results than this meticulously planned pile of crap I have built. The hum is so bad there isn't any audio. It's like I connected a dc power supply, and a poorly filtered one at that, to a speaker. Obviously I have some issues to sort out. I will take some measurements and let you guys know the results.
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