Tube amp power supply reduce voltage

[Strawtles]

Hello, I have to realize a power supply for a tube amplifier which after the diode bridge should deliver 320 Volt, but I am using a transformer that I have at home with a higher voltage after the diode bridge I have 384 Volt.

I know that putting a resistor in series the voltage should decrease, but with my tester it always more or less the same.

It's possible that this is happening because the power supply isn't connected to the circuit, but I don't want to risk doing any damage.

Is there anyone who can tell me the right resistor value to use?
Many thanks in advance

 

[Khron]

a transformer that I have at home with a higher voltage after the diode bridge I have 384 Volt.

Is that unloaded, or with some dummy-load applied?

Is there anyone who can tell me the right resistor value to use?

That's right up there with "how long is a piece of string?"

 

[rock soderstrom]

Hello, I have to realize a power supply for a tube amplifier which after the diode bridge should deliver 320 Volt, but I am using a transformer that I have at home with a higher voltage after the diode bridge I have 384 Volt.

I know that putting a resistor in series the voltage should decrease, but with my tester it always more or less the same.

It's possible that this is happening because the power supply isn't connected to the circuit, but I don't want to risk doing any damage.

Is there anyone who can tell me the right resistor value to use?
Many thanks in advance

Your PSU is unloaded so the voltage will be higher. That is totally normal behavior. Best thing is to post the schematic of your amp and than we can help you better with your project.

 

[Strawtles]

Here is the schematic, thank you!

 

[rock soderstrom]

It's possible that this is happening because the power supply isn't connected to the circuit, but I don't want to risk doing any damage.

Yes, as just described, without a load your voltage will be too high after rectification.

Your amplifier is a medium guitar amplifier with approx. 15 watts (2x12AX7+2xEL84), which means that it already represents a considerable load that will significantly reduce your excessive voltage.

I would just try it out, it might be just right or it might be slightly off. Tubes are very tolerant under normal circumstances, but your EL84s are operating at the upper end, which is unfortunately normal for guitar amps. Note the maximum anode voltage of the EL84, that's the limit.

Try your power supply with the load connected, then measure your corresponding voltages. Normally nothing will break immediately, even with a little overvoltage.

 

[Strawtles]

The power supply in the schematic is for a transformer with a lower voltage (250 Volt) so the voltage with a 270 Volt trasformer it will be higher

shouldn't I use a resistor with a higher value instead of the 150 ohm one?

 

[rock soderstrom]

The power supply in the schematic is for a transformer with a lower voltage (250 Volt) so the voltage with a 270 Volt trasformer it will be higher

True, how much this changes in practice also depends on the power of the transformer.

shouldn't I use a resistor with a higher value instead of the 150 ohm one?

This would not change anything for the EL84 anode voltage, as this is taken from point A.

You may have to convert some voltage into heat before point A. You will see how much if you try your PSU with a load for a short time and measure the voltages accordingly.

 

[rock soderstrom]

As I said, some manufacturers operate their EL84s in similar circuits with anode voltages of 350V continuously. I don't want to defend or propagate this, but this is the reality in some well-known guitar amplifiers...what I want to say is that for a short test the EL84 can do it with higer anode voltages.

 

[Strawtles]

Isn't the maximum voltage supported by the EL84 300 volts?

 

[rock soderstrom]

Isn't the maximum voltage supported by the EL84 300 volts?

Correct, that's how I would interpret the corresponding datasheets.

Historically, however, the EL84 has been tortured with more, which has worked more or less well. You probably had to change the tubes more often, which was presumably the price for the "hot" sound! Everyone has to decide for themselves whether they should do it this way or not.

 

[rock soderstrom]

Your Tiny Terror also has too much anode voltage on the EL84s from the factory, hence the name!

 

[Strawtles]

 

[shabtek]

insert 100Ω of 10ish watts before 'A' and see what voltage that drops and infer the current draw, then you can determine a resistor value to achieve the desired voltagedrop

 

[Strawtles]

Yes, I'll do that. In this way, once I have obtained the right voltage before point A, following the schematic I'd get the right voltages

 

[scott2000]

Can you add up the current that's being drawn now to get an idea?Or is that not how it works?

Looking quick that would put that 150R at around 1.7k ish??

 

[EthanSpaulding]

Jet City 20 Watters are all using above 400V B+ with el84. Mesa dual caliber is around 410V with el84. You may only need to up the screen resistor value and use a bigger dropper resistor to the preamp supply (to keep the tone the same). Otherwise, a resistor and a zener diode will drop your B+ without giving your power amp a ton of sag with the very long time constants that large resistors and large capacitors would create.

 

[CJ]

Install an actual socket and put in a 5y3 rectumfrier.

 

[Strawtles]

Thanks everyone for the suggestions.

Now before to try the different solutions I have to complete the amplifier.

I hope to do this in the next few weeks.

Cheers

 

[rock soderstrom]

Good luck, keep us posted