45s running hot

[jimiclow]

I had the chance to measure the voltages in my amp with the 45s and these are the readings:
Line voltage: around 123 volts
Tube pins 2 = 360 volts
Tube pins 1/4 = 60 volts
So this means that the tubes are running at 300 volts which I think is the max for 45s.
Current is 37.5 ma (60/1600)
Plate dissipation = 11.25 watts

I hope my computations are correct.
We know that finding a 45 is very hard unless you want the more expensive new productions.
I want to run them at lower voltage/plate dissipation to extend their lives.
Will changing the power resistors to about 1.5k to drop the B+ work?
Do I need to change the cathode resistors too?
Thank you.

 

[Paul Joppa]

You will need to go to 1800 ohms to get the voltage down 25 volts. Figure 37.5mA for the 45, plus about 5mA for the driver, and use Ohm's Law Resistance = Voltage over Current; 25 volts at 42.5mA is 588 ohms added to the 1200.

An alternative would be to use a variac or make a step-down autotransformer to get the power line voltage to 120v or less.

The maximum voltage as specified by RCA was 275v, but there is usually a tolerance of 10% and you are within that.

Of course, if you want to be really cautious, you can always substitute a 2A3 while keeping the 45 configuration in the amplifier!

 

[jimiclow]

Thank you Paul!
I couldn't figure that calculation myself.
I will order a few resistors in the 1.6 to 2 k range so I can play around.
How is a 2a3 with a 275 volts 38ma? That should last a long time.
Will the low voltage affect the operation of the C4S?

 

[Paul Joppa]

The reduced voltage will have no effect on the C4S except for the reduced heat and dissipation - all good things.

The 2A3 should function very well at the 45 operating point, producing just a hair more power due to its lower plate resistance - and of course having some improved longevity. I imagine a modern single-plate 2A3 would sound more like a classic 45 - but I haven't done any careful listening to this combination.

 

[jimiclow]

Thanks again Paul!

 

[jimiclow]

I got a good deal on a used variac on ebay. I will try this first before resorting to changing the resistors.
My only concern is that dropping the line voltage will also lower the filament voltage. I don't know how low you can go on the filament voltage.

 

[jimiclow]

So I got the Staco "variac" today, adjusted the line voltage to 110volts. I got around 255 volts at 32.5ma. Filament voltage was about 2.2 volts.
In summary, my 45s now run at 255 volts, 32.5ma at 4k load. Perfect!

 

[Doc B]

That filament voltage is a bit low for best tube life. Ideally you want to try to stay within +/- 5% (i.e., 2.5V +/- 0.125V) and at the very most +/- 10%

 

[jimiclow]

Thanks Doc!
I will fine tune it to have at least 2.3 volts in the filaments.

 

[jimiclow]

After a few experimentation with resistor values, I've dropped the voltage to 260 using Mills MRA-12 2.7k.
The tubes are now dissipating at 8.25 watts from the stock 11.25.
If I lower the mains to 114 volts, I get 7.68 watts while still having 2.4 volts on the filaments.
I'm not noticing a drop in power nor sound quality.
I'm happy now.
Thanks to everyone who helped.

 

[Grainger49]

I think you have nailed it at 114V AC coming in.  The heater voltage is back up to where you want it to be. 

I would run them there.

 

[jimiclow]

Thanks Grainger.
That's what it's running right now. I now have 6 pairs of 45.

 

[Grainger49]

That should last you for a while.  Poster Paully, Paul, is running his 45s in much the same parameters.  He probably has a 20 year supply of tubes.  He doesn't listen as much as I do since he is 20 years younger and has a family, two daughters, who eat up much of his free time.