Hi,
The PT I am using in my latest project just happens to NOT have a CT on the 6.3V winding. So, The original Amp schematic I am building happens to show grounding 1 leg if the 6.3V.
The Next newer year shows having a CT, I thought It might be a good idea to put in a hum balance pot rather than mearly ground the single leg of the 6.3V.
My question is, Does it really matter where in the chain it is located?
The easiest drawing in my chassis is item Number 3 in this
Picture
The transformer donor unit had the pot located after the 6L6;s in the chain.
Gene
The PT I am using in my latest project just happens to NOT have a CT on the 6.3V winding. So, The original Amp schematic I am building happens to show grounding 1 leg if the 6.3V.
The Next newer year shows having a CT, I thought It might be a good idea to put in a hum balance pot rather than mearly ground the single leg of the 6.3V.
My question is, Does it really matter where in the chain it is located?
The easiest drawing in my chassis is item Number 3 in this
Picture
The transformer donor unit had the pot located after the 6L6;s in the chain.
Gene
Any of the arrangements in your diagram should work ok with indirectly heated tubes. I have never found the location of the pot to be critical.
The purpose of the balance pot is to provide an adjustable ground reference for the heaters. In theory there should be a setting which produces lowest levels of 100/120 Hz buzz.
In practice, I can't hear any difference! Another option is to create a non-adjustable centre tap by connecting an 100 ohm resistor from each side of the heater supply to ground.
The purpose of the balance pot is to provide an adjustable ground reference for the heaters. In theory there should be a setting which produces lowest levels of 100/120 Hz buzz.
In practice, I can't hear any difference! Another option is to create a non-adjustable centre tap by connecting an 100 ohm resistor from each side of the heater supply to ground.
martinab2 said:
OK Understood,
I have seen it done so many different ways that I felt it was not especially critical, But, Better to ask those who know rather than de-solder later.
Gene
Also from me a small contribution: I have very occasionally found that a pot gave a better hum null, especially where high-gain circuits (pre-amplifiers) were involved. But this was infrequent.
If cost is a factor you could always use a wire-wound resistor with a slider instead of a pot. If you have a supply of components, try this first and if no detectable difference, use fixed resistors or simply earth one side.
If cost is a factor you could always use a wire-wound resistor with a slider instead of a pot. If you have a supply of components, try this first and if no detectable difference, use fixed resistors or simply earth one side.
And keep in mind that for Leo Fender, "good enough" was the operating manufacturing philosophy. I even have a service bulletin from them regarding hum in the Champ. It tells us not to bother trying to get rid of the background hum, it is normal.
Grounding one side of the heaters is probably better than no hum cancelling, and a lot cheaper. SOme models have a hum null pot, but mostly the two 100 ohm resistors is what they used.
Grounding one side of the heaters is probably better than no hum cancelling, and a lot cheaper. SOme models have a hum null pot, but mostly the two 100 ohm resistors is what they used.
So If I ground 1 leg of the 6.3V, How can I determine which leg to ground? or Does it matter. The Color code on the 6.3V is 1-green, 1 black. All the CT units I built never had audible hum issues but this is the first time I have used a PT without a CT.
Gene
Oh BTW this is on a modded Pro 5E5 using 6L6's on output.
Gene
Oh BTW this is on a modded Pro 5E5 using 6L6's on output.
I doesn't matter which leg you ground. Remember that the heaters are insulated from the cathodes (up to the maximum heater to cathode voltage specified for that type of tube). By referencing the heater supply to a fixed voltage (in this case ground) we are just preventing it from acting as an aerial for hum and noise.
With directly heated tubes such as 2a3 etc, the heater is the cathode and the heater supply is part of the signal path. In this case you do need to use an adjustable hum cancelling pot or DC heaters supplies as hum is much more likely to be audible.
Luckily you don't have to worry about any of this in your current project.
With directly heated tubes such as 2a3 etc, the heater is the cathode and the heater supply is part of the signal path. In this case you do need to use an adjustable hum cancelling pot or DC heaters supplies as hum is much more likely to be audible.
Luckily you don't have to worry about any of this in your current project.
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