Reducing an Amp's Heater AC Voltage Supply

syscokid

syscokid

Ambassador of War & Peace
Country flag
Why would anybody do this? Answer: The heater AC voltage is too high for best tube life. Optimal voltage for heating up our tube's filaments is 3.15 vac per side, or 6.3 vac total. What is a safe operating range? Answer: 6.3 volts +/- 10% or 5.7 to 6.9v. Just so happens that one of my amps is putting out 6.8 vac. Not dangerous, but borderline high, and probably will reduce tube life. Now, would be a great opportunity to apply this mod that I just learned from the robrobinette.com website.

I needed to drop a total of a half of a volt with a couple of resistors. Calculating the proper value of the resistors involves knowing the current draw of each tube and how much voltage needs to be dropped. The results are .06 ohms per resistor with a 5 watt rating per resistor. The only source I found for these resistors was at Mouser.com. About $1.50 each.

I also wanted to install a tag strip board to secure the two new resistors. I bought a single piece strip with 8 lugs. 2 of the lugs are connected to a ground and mounting lug. I cut it in half, and grounded off the lugs that I wont be using:
Heater Current Voltage Droppers (1).JPG

Modified strips installed. Voltage dropping resistors installed. Heater wiring reconnected:
Heater Voltage Dropping Resistors (2).JPG

Just a broader look at this mod:
Heater Voltage Dropping Resistors (4).JPG

The final results: The heater voltage has been reduced to 6.28 vac, or 3.14 per side... Perfect... :woohoo:. No extra noise, too. In fact I think it's a tad bit quieter!
 
DonP said:
Yes. I try to choose a power rating at least three times the actual power they will dissipate in use. Cooler is always better in electronic stuff.
Click to expand...
Agree, but I'm used to a "rule of thumb" of at least doubling the power rating. I calculated 2.05 watts, therefore I chose a 5 watt rating. Also, notice those resistors have a lot of air space around them for cooling.
 
syscokid said:
Agree, but I'm used to a "rule of thumb" of at least doubling the power rating. I calculated 2.05 watts, therefore I chose a 5 watt rating. Also, notice those resistors have a lot of air space around them for cooling.
Click to expand...

Sure, I use a rule of three for PCB mounted components. In free air like you have, twice is plenty. The resistors are only going to see extra stress while the heaters are warming up and their resistance is low; and that isn't for long.
 
Mr Grumpy said:
Looks like a very neat job. Other than the pictures, I didn't really understand the other bits... :cautious:
Click to expand...
Ha ha... As long as you grant me a "Like", you don't need to understand! Just be amazed at what my silly-ness and boredom will conjure up... :cheers:

Robert Herndon said:
Very good and very detailed work. I'm amazed at people who are proficient at this sort of thing.
Click to expand...
Proficient? ... Me? ... :pound-hand: ... Must have been a good night's sleep:
 
syscokid said:
The heater voltage has been reduced to 6.28 vac, or 3.14 per side...
Click to expand...
It is not a per side. It is 6.3 V. The ECCs have two filaments specified as 6.3V/150mA which are put in series and have a central tap. Thus thei can be run with 12.6V/150mA or 6.3V/300mA.

I am also a bit surprised that You were using such large resistors: 0.6V*300mA=0.18W. Which means that You could safely use a pair of cheap metal film resistors of 1.2 Ohms/0.6W. Per Tube, of course. Power tube filaments need more, but You can alsways use a couple of smaller resistors in parallel which are readily available.
 
DonP said:
Did the heaters fail? As in, they stopped glowing?
Click to expand...

Hey, Don...power tubes would stop lighting up, rather frequently. I couldn't get more than 30-45 days out of power tubes, but I was also playing in a house band 6 days a week.
 
Combos played very loudly can suffer more than usually. The combination of high temperature and powerful vibration can shorten a heater's lifetime quite badly. It's worth introducing some kind of vibration absorption somewhere between speaker and valve.
 
You must log in or register to reply here.
Back
Top