Heater voltage too high

[Bruce / Mission Amps]

Unless I'm just not thinking today (possible, LOL) it looks like two, .5 ohm 3 watt resistors, one in each leg of the filament supply would fix that....

[bob p]

> I never knew that if the load is too light the voltage would go higher than what its spec'd for.

That is true for all transformers that aren't actively regulated. Look at the voltage produced by a wall-wart that isn't regulated. It may be rated to deliver 9V @ 300mA. but if you measure voltage with no load connected, the voltage will be higher. And if you hook up too much load the voltage will sag below 9V.

As an extreme example, disconnect the red leads on your 270Fx secondary and measure them disconnected (no load). You'll see that they're higher than the spec for the transformer. Hook them back up and measure them connected (load) and you'll see that they've fallen. You can do the same test with the 6.3V leads. *

Matching the power supply to the load is important. Unless you have a regulated voltage supply I think that it's important to choose a transformer that closely matches the load presented by the circuit. If you've got too much transformer for the job the voltages won't be that far off, and you can tweak things as needed to bring the voltages in line. But something to consider is that buying too much transformer for the job just wastes money. If you add up the HT loads and the heater loads, your 270FX is capable of supplying more load than your circuit demands. The EX also has plenty. Some might even argue that because Hammonds are so conservatively rated, that you wouldn't be overloading a DX in that circuit. But for reasons that I don't really understand, everyone seems to choose an FX.

* Take a look at the spec sheeet for your transformer. I linked to it in a previous post. The spec sheet lists the NLV specs for each winding. with a 125VAC mains supply, under no-load conditions your 550VCT transformer will measure 590VCT +/- 2% on the HT lines, and 6.925VAC +/- 2% on the heater line. Those voltages will fall into the 550VCT and 6.3 VCT specs when the transformer is properly loaded.

The voltage issues are confounded because you're using the 115VAC primary tap on a line that has 120VAC mains. You can expect the secondary voltage specs to increase by 120/115 = 4.3%. Because your mains input is higher than your primary winding spec, all voltages on the secondary will increase. The combination of under-load and over-voltage in the input combine to give you 7.1 VAC with the heater lines "partially" loaded.

I guess that a 3rd option would be the bucking transformer that I mentioned in my first post. If you buck 6V off of the 120VAC supply that will decrease your mains supply by 4% to 114. that's pretty close to 115. I'd expect that your secondary voltages would all drop commensurately. Your heaters might go from 7.1 to 6.8.

My point is to say that you're dealing with two problems. The first is that your mains doesn't match the input to your transformer; the second is that your heater winding isn't fully loaded. both of these are contributing to the rise of your heaters to 7.1volts. The textbook approach would be to address each problem independently. Given that you like the HT voltages and don't care about the 5V winding, the most practical solution is to just dial-down your heater voltages by adding resistance.

[bob p]

Looks like Bruce and I did a simulpost.

Like Bruce said, the fix is pretty easy. The quick and dirty answer is indeed the 0.5R 3W resistors.

Here are the calcs:

You want to drop your heater voltage to drop from 7.1V to 6.3V and the load current is about 2 amps. You want to add a series resistor in each leg of the 6.3V supply line before your heaters.

Resistance needed:
V/I = R = (7.1 - 6.3)/1.8 = 0.444 ohms.
Call it 0.5 ohms.

Power dissipation in the resistors:
V^2 / R = W = (7.1-6.3)^2 / 0.5 = 1.28W.
Choose 3W resistors.

[Tom Phillips]

I say "not in each leg" or at least not .5 Ohms each.

If you add a 0.5 Ohm resistor in EACH side of the heater supply then, with the 1.8A current draw, there will be 0.9V drop across EACH resistor . This will give you a total 1.8Vac reduction and you will end up with a 5.3V heater voltage at the tubes. Right?

[bob p]

I say "not in each leg" or at least not .5 Ohms each.

If you add a 0.5 Ohm resistor in EACH side of the heater supply then, with the 1.8A current draw, there will be 0.9V drop across EACH resistor . This will give you a total 1.8Vac reduction and you will end up with a 5.3V heater voltage at the tubes. Right?

I've made mistakes before, so if I'm wrong this wouldn't be the first time. Instead of just telling us that our answer is wrong, could you do us a favor and tell us the right answer? And show us the math behind your work?

[J M Fahey]

Thanks God, easy to solve
Split that 0.5 ohms resistor into two .
0.22 ohms is a standard value, so there you have .
2W each will be fine, and also means a *very* common value.

FWIW: 1.8A*0.44r=0.792 so actual filament voltage will be 7.1-0.8=6.3V .

[Tom Phillips]

...could you do us a favor and tell us the right answer? And show us the math behind your work?

bob p,

I agree with the math you included in post #18. My comment was just that, for the circuit implementation, you recommended twice calculated resistance. Just a mistake. We all make them.

I thought the reason for my post was clear. However, if anyone would like to see a more detailed response from me after they read Juan's post #21, I'd be glad to write it up.

Regards,
Tom

[Gingertube]

KISS

I have just finished a repair on a MK3 Boogie - the fault was in the heater voltage dropping resistors which had been put into each heater line (and the accompanying PCB butchery).

I would advise:
Just use the White(0V) and Black (125V) primary wires instead of the White and Grey(115V) as suggested above.

115/125 x 7.1 = 6.53 Volts That will be just fine.

Ideally the specification for the 6.3V is +/- 5% (i.e. between 5.985 and 6.615) but keep in mind it will go up and down with the mains supply variations. In general +/-10% will be OK but use your brains in interpreting, if teh 6.3V is 10% high when the mains supply is 5% low then action is required, if however it is 10% high when the mains supply is unusually high then ignore it.

With these "traditional" tube amp transformers you can often use the secondary 5V winding (as long as its not used by a tube rectifier) to buck or boost the primary by 5V. Current rating and voltage withstand rating are applicable for this use.

Cheers,
Ian