Quick update, I cut out all the grounding and am doing a reassembly right now. I installed screened cable where it wasn't and took apart the umbilical and tightly twisted the heater wires. I also got the other 12vac transformer installed for the heater.
Now, Ill get this beast grounded! Is it okay if I tie all the grounds on the input/output terminals together then run a wire from there to the star setup? I hope so, because I am about to do it! lol
Evan
Now, Ill get this beast grounded! Is it okay if I tie all the grounds on the input/output terminals together then run a wire from there to the star setup? I hope so, because I am about to do it! lol
Evan
I would run screened cables from the inputs to the point where the inputs are selected. At this point the screens connect together. This means that the input wiring within the amp is merely an extension of the external wiring. The screen-joining-point should then be connected to the cathode circuit of the input stage, perhaps via the screen of the cable to the input grid circuit - you can do this with either star or bus grounding. The idea is that you don't have hum loops even inside an amplifier, and ultimately you want to apply your signal to g-k on the input stage.
Alright sounds good, that is basically exactly what I did. I did as you guys said, added screened cable where it wasn't and re-grounded. The hum that varies with volume is gone. There is a small 60hz hum across the spectrum now though.
I star grounded everything together, and left the "safety ground" grounded to the top plate. I had my hum still, so I tied the "safety ground" to the star ground I made and the hum that varied with volume setting is gone.
I did meter out my heaters at 17vac!! Seems too high, I have kind of high line voltage at my house combined with the +/-5% variation on the trafo has put it pretty high. Is there a reasonable way to tame this? I assume the tubes will not live long happy lives with heater voltage being so high. I am also hoping this is the source of the minor hum I still have (which is definitely a 60hz hum).
Thanks a lot for your help guys.
I star grounded everything together, and left the "safety ground" grounded to the top plate. I had my hum still, so I tied the "safety ground" to the star ground I made and the hum that varied with volume setting is gone.
I did meter out my heaters at 17vac!! Seems too high, I have kind of high line voltage at my house combined with the +/-5% variation on the trafo has put it pretty high. Is there a reasonable way to tame this? I assume the tubes will not live long happy lives with heater voltage being so high. I am also hoping this is the source of the minor hum I still have (which is definitely a 60hz hum).
Thanks a lot for your help guys.
17 Volts is way too high.
Calculate how much current the heaters are supposed to pull, then work out how many volts to drop across a series resistor in the supply to bring the voltage down to 12.6V. If your heater supply is centre tapped, you'll need two resistors, one in each leg to maintain balance.
Is you heater supply referenced to ground anywhere? This may be the cause of your slight hum. Inclusion of a "hum-dinger" pot may prove this, and may even solve your remaining hum... Or rectify it, regulate it!
Calculate how much current the heaters are supposed to pull, then work out how many volts to drop across a series resistor in the supply to bring the voltage down to 12.6V. If your heater supply is centre tapped, you'll need two resistors, one in each leg to maintain balance.
Is you heater supply referenced to ground anywhere? This may be the cause of your slight hum. Inclusion of a "hum-dinger" pot may prove this, and may even solve your remaining hum... Or rectify it, regulate it!
Actually, duh. I don't think I checked it with a load on the transformer. Ill check that right when I get home. The heaters do seem pretty bright though. Resistors I can handle, no problem.
Ill search 'hum-dinger".
I can't remember if the heater is referenced to ground honestly.
Evan
EDIT; Doz, could you either link me to a hum dinger explanation or explain it? I looked on here and didn't find anything.
Ill search 'hum-dinger".
I can't remember if the heater is referenced to ground honestly.
Evan
EDIT; Doz, could you either link me to a hum dinger explanation or explain it? I looked on here and didn't find anything.
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A hum-dinger is a pot, with the two ends connected to the heater voltage, and the wiper connected to ground. A couple of hundred ohms should do it. Adjust the pot for minimum hum. Bear in mind it will draw current itself, and therefore will load your heater voltage, as well as dissipate power itself, so will need to be appropriately rated.
Easy to check, pull the tubes, and measure each side of your secondary heater winding to ground. Lowish resistance would mean you did !
I've "got away with" just grounding one side of my heater transformer before now, but I couldn't stick my hand on my heart and tell you that's a good engineering solution
, and it certainly unbalances the heater circuit!Hello,
If you grounded Pin #3 and the hum got louder that raises my suspicions.
I looked sorta through the thread and did not read if the heaters are DC or regulated DC. My feeling is that a line stage should have well regulated DC heaters. Plus the heater should be at an elevated voltage with a bypass capacitor to ground. By elevated voltage I mean set up a voltage divider between B+ and ground made up of say a 300K resistor and a 100K the negative side of regulated heater DC then ties to the node between the resistors the bypass capacitor ties to the same node and ground. The heaters will be quiet!
DT
All just for fun!
AC heaters are what is being employed here. As a rookie, I can only assume DC would be better. The guys telling me AC is fine are people that know way more than me, and I can only hope to have that knowledge one day.
That being said, 5 ohms of resistance dropped my heater voltage to 11.85 (or something close to that). I had two 5 watt 10 ohm resistors, wired em' up in parallel and we were in business. I ended up using trial and error honestly.
The good news? There is no heater buzz until I am up into a very high area of gain. So high, I doubt the preamp ever gets turned up that high except for looking for noise.
I still have a low 60 cycle hum. I haven't ruled out ground loops, and the dynaco MKIIIs I have hum just a hair anyway.. which may be accentuating it. I have the center tap grounded now, along with everything else. I find if I don't connect the star ground to the top plate I get massive hum. If I could even cut the hum in half on this preamp, it would likely be fine.
I haven't ruled out the hum-dinger pot idea. I hate to ask this because I know you guys hate spoonfeeding but Ill go ahead anyway.
1) What wattage pot would be appropriate? Judging by how warm the resistors get that are dropping the voltage 2v I assumed 2 watt would be ok.
2) What resistance pot should I be using? 5 ohm? 50 ohm? 5k ohm?
3) Can I run both tubes through the same humdinger pot? Connect both heater CT's to the same terminal on the pot and a single ground from the pot to the star ground is what I was thinking.
Thanks again for all your help guys. Even though this is a basic preamp, it is very hard to diagnose and think logically with such little working knowledge about layout and wiring etc....
Evan
That being said, 5 ohms of resistance dropped my heater voltage to 11.85 (or something close to that). I had two 5 watt 10 ohm resistors, wired em' up in parallel and we were in business. I ended up using trial and error honestly.
The good news? There is no heater buzz until I am up into a very high area of gain. So high, I doubt the preamp ever gets turned up that high except for looking for noise.
I still have a low 60 cycle hum. I haven't ruled out ground loops, and the dynaco MKIIIs I have hum just a hair anyway.. which may be accentuating it. I have the center tap grounded now, along with everything else. I find if I don't connect the star ground to the top plate I get massive hum. If I could even cut the hum in half on this preamp, it would likely be fine.
I haven't ruled out the hum-dinger pot idea. I hate to ask this because I know you guys hate spoonfeeding but Ill go ahead anyway.
1) What wattage pot would be appropriate? Judging by how warm the resistors get that are dropping the voltage 2v I assumed 2 watt would be ok.
2) What resistance pot should I be using? 5 ohm? 50 ohm? 5k ohm?
3) Can I run both tubes through the same humdinger pot? Connect both heater CT's to the same terminal on the pot and a single ground from the pot to the star ground is what I was thinking.
Thanks again for all your help guys. Even though this is a basic preamp, it is very hard to diagnose and think logically with such little working knowledge about layout and wiring etc....
Evan
I agree that AC heaters can be relatively quiet. The valve data sheets do say AC or DC. As you say the wiring is key to keeping the AC out of the circuit. Even using DC heaters I twist the conductors tight and run them tight to the metal case.
DT
All just for fun!
<QUOTE> 3) Can I run both tubes through the same humdinger pot? Connect both heater CT's to the same terminal on the pot and a single ground from the pot to the star ground is what I was thinking. </QUOTE>
If you've grounded your centre-tap, there's no need for a hum-dinger. Where did you put your 5 ohms? On one side of the winding ? Or both? If you've put it on one side , you will have unbalanced the heater supply, and your centre tap ground will not be as sucessful at killing the hum. Stick 2x 2.5 ohms from each "hot"....
BTW My line stage has AC heaters , and there's no hum....
If you've grounded your centre-tap, there's no need for a hum-dinger. Where did you put your 5 ohms? On one side of the winding ? Or both? If you've put it on one side , you will have unbalanced the heater supply, and your centre tap ground will not be as sucessful at killing the hum. Stick 2x 2.5 ohms from each "hot"....
BTW My line stage has AC heaters , and there's no hum....
Aw man, I feel kinda dumb here 
DF; you are indicating that I need a seperate pot for each heater's center tap run to ground in the humdinger scenario right?
By heater winding, you mean the heater winding in each tube right? Or was I only supposed to ground the CT from one of the tubes and not both?
Thanks for your patience.
DF; you are indicating that I need a seperate pot for each heater's center tap run to ground in the humdinger scenario right?
By heater winding, you mean the heater winding in each tube right? Or was I only supposed to ground the CT from one of the tubes and not both?
Thanks for your patience.
No, winding=transformer secondary.
I forgot about heater centre-taps. So, a given heater circuit has one transformer winding and one or more valve heaters (usually wired in parallel). Every heater circuit has to have a reference voltage, which can either be ground or something else. The reference voltage can be simply connected to one side of the heater circuit, but for lower hum it is better to connect to a centre-tap. A centre-tap can be on the heater winding, a valve heater, the connection of a pair of resistors, or the slider of a humdinger. Each heater circuit should be connected to the reference voltage at one place only. A heater centre-tap is not usually a good place to do it as you lose the centre-tap if the valve is removed for testing.
I forgot about heater centre-taps. So, a given heater circuit has one transformer winding and one or more valve heaters (usually wired in parallel). Every heater circuit has to have a reference voltage, which can either be ground or something else. The reference voltage can be simply connected to one side of the heater circuit, but for lower hum it is better to connect to a centre-tap. A centre-tap can be on the heater winding, a valve heater, the connection of a pair of resistors, or the slider of a humdinger. Each heater circuit should be connected to the reference voltage at one place only. A heater centre-tap is not usually a good place to do it as you lose the centre-tap if the valve is removed for testing.
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