Need help creating Bias Supply from HT

[lacrebob]

Hello,

I'm in the planning stage of building an amp with the foundation being the 5F6-A. I've got a great power transformer that I'm going to use, however it doesn't have a bias supply tap. I have Merlin's book on both preamps and power supplies, but I'm still struggling to understand the precise steps I need to take to create a bias supply that feeds the bias portion of the 5F6-A circuit.

Additionally, even though math isn't difficult for me, I've struggled with amplifier-related math, so I'm not sure how close or far away I am with my dropping resistor calculations - I'm trying to get the bias supply to 50V so that the remainder of the resistor values in the half wave bias circuit remains. I'm using Rob Robinette's adjustable bias modification as well as his power supply safety modifications.

I've drafted a half wave and a full wave and my understanding is that the resistor values need to be doubled for a full wave bias supply. Is that correct?

Is using UF4007 diodes correct and is their orientation correct?

Any help is much appreciated.

[ViperDoc]

Just looking at your drawings, I would suggest the following:

Place your 150K bias circuit drop resistor before the reverse diode to lower the voltage the diode sees. Even though the diodes are rated for 1000V, it'll be nicer. You can adjust the resistor value to arrive at the desired bias voltage.

You will only need one tap off one leg of the HT secondary to feed the bias circuit. Do not connect feeds to both legs. If the diodes fail closed, you will nuke your power transformer.

I would not recommend placing a fuse on the HT center tap. In failure mode, it removes the HT secondary ground path and causes standing voltages in the chassis. Alternatively, you can fuse each leg of the HT winding before the protection diodes. You don't have to, but I usually do. Use 500mA for each leg.

You do not need 4A fuses on the 5V winding. You can achieve the same protection by fusing the HOT 120V PT Primary winding before the mains power switch.

Do not place the bias pot wiper to ground. Place the tail resistor to ground instead. That way if the bias pot wiper fails, you at least have bias resistance in the circuit. It could save your tubes.

You could fuse the heaters, but generally don't need to.

Enjoy!

[Ten Over]

Hello,

I'm in the planning stage of building an amp with the foundation being the 5F6-A. I've got a great power transformer that I'm going to use, however it doesn't have a bias supply tap. I have Merlin's book on both preamps and power supplies, but I'm still struggling to understand the precise steps I need to take to create a bias supply that feeds the bias portion of the 5F6-A circuit.

Additionally, even though math isn't difficult for me, I've struggled with amplifier-related math, so I'm not sure how close or far away I am with my dropping resistor calculations - I'm trying to get the bias supply to 50V so that the remainder of the resistor values in the half wave bias circuit remains. I'm using Rob Robinette's adjustable bias modification as well as his power supply safety modifications.

I've drafted a half wave and a full wave and my understanding is that the resistor values need to be doubled for a full wave bias supply. Is that correct?

Is using UF4007 diodes correct and is their orientation correct?

Any help is much appreciated.

Here is my entry:

Bias for 5F6-A #3.png

There is no need for full wave rectification because the ripple voltage will be in the mV's with a two stage filter on half wave.

[lacrebob]

ViperDoc,
Thanks for covering many bases there. If I understand correctly, if I added a fuse on the HOT 120V PT Primary winding there would then be a fuse both before and after the main power switch.

Ten Over,
Thanks for the schematic. I see the capacitors are bumped-up and it looks like you made some calculations to arrive at the resistor values.
Your handle made me think of a lyric line that I wrote: "and they hang ten on the nose as they surf into the night."

Two quality replies - thank you again.

[sluckey]

Look at the Princeton Reverb schematic. Or a Plexi schematic. Or this...

https://sluckeyamps.com/phoenix/phoenix.pdf

[R.G.]

Place your 150K bias circuit drop resistor before the reverse diode to lower the voltage the diode sees. Even though the diodes are rated for 1000V, it'll be nicer. You can adjust the resistor value to arrive at the desired bias voltage.

Doesn't matter. The current through any series string without connections to the inner points is not changed by their order. Hence the voltage across any one component is not changed by their order.

You do not need 4A fuses on the 5V winding. You can achieve the same protection by fusing the HOT 120V PT Primary winding before the mains power switch.

A few years back I went on a math blitz about this issue. The upshot is that a fuse on the primary will not necessarily prevent transformer damage to a low voltage winding, especially in the case of a soft short. For hard shorts on 6.3V and 5V windings, the details of the primary winding resistance determine whether its fuse will blow before the wires over heat.

The mission of the primary fuse is to keep an already-failed transformer from starting a fire. It may, by lucky chance, stop damage to the transformer and secondary circuits, but that's not what it is intended for. If you want to protect transformers reliably, you need to fuse each winding with a fuse that passes the design max current, but blows before the wire in the trannie can overheat.

Yes, I know, we have gotten away with a single primary fuse for decades. Mostly it works. Sometimes not. Protecting a transformer requires fusing each independent winding section. Or just counting on luck. In the schematics here, you only need a fuse on one side of the 5V, as it doesn't have a CT. If there is no CT on the 6.3V, it can go with only one fuse; if it has a grounded CT, it needs one on each side.

You could fuse the heaters, but generally don't need to.

Yep - generally you don't run into the situations that would kill the trannie without blowing the primary fuse. But sometimes...

[lacrebob]

Slucky,
Your schematic helped me in a practical sense with my current project. Your amp is first one that I’ve seen with the parallel preamps - I’ve had that same idea for years. Too much for me to pull off at this time…but with this schematic I’ll be very tempted. It seems like it would provide tremendous tonal versatility.

R. G.,
Thanks for the additional info on your safety and longevity tips. I like your approach of building an amp that may never need servicing. I plan to use as many film capacitors as I can in this build - I think there’s enough room.

Thank you gentlemen.

[sluckey]

Slucky,
Your schematic helped me in a practical sense with my current project. Your amp is first one that I’ve seen with the parallel preamps - I’ve had that same idea for years. Too much for me to pull off at this time…but with this schematic I’ll be very tempted. It seems like it would provide tremendous tonal versatility.

More info on that amp here...

https://sluckeyamps.com/phoenix/phoenix.htm

[ViperDoc]

Doesn't matter. The current through any series string without connections to the inner points is not changed by their order. Hence the voltage across any one component is not changed by their order.

That's interesting. Why do I never see it that way on schematics?

A few years back I went on a math blitz about this issue. The upshot is that a fuse on the primary will not necessarily prevent transformer damage to a low voltage winding, especially in the case of a soft short. For hard shorts on 6.3V and 5V windings, the details of the primary winding resistance determine whether its fuse will blow before the wires over heat.

The mission of the primary fuse is to keep an already-failed transformer from starting a fire. It may, by lucky chance, stop damage to the transformer and secondary circuits, but that's not what it is intended for. If you want to protect transformers reliably, you need to fuse each winding with a fuse that passes the design max current, but blows before the wire in the trannie can overheat.

Yes, I know, we have gotten away with a single primary fuse for decades. Mostly it works. Sometimes not. Protecting a transformer requires fusing each independent winding section. Or just counting on luck. In the schematics here, you only need a fuse on one side of the 5V, as it doesn't have a CT. If there is no CT on the 6.3V, it can go with only one fuse; if it has a grounded CT, it needs one on each side.

If you were to place fuses on the 5V widing, would you choose 4A fuses?

Thanks for the input, R.G.

[R.G.]

That's interesting. Why do I never see it that way on schematics?

I don't know. I can assure you that it's true, and that many people are surprised by it. It's true of all series strings of whatever components. As long as you don't tap into the series string in the middle, the order makes no difference to any component's voltage across it or current through it. In this case, imagine that the diode is forward biased. The diode's resistance is nearly zero, compared to the resistor. So the current is essentially all determined by the resistor, almost like the diode isn't there. If the diode is reverse biased, the voltage across the diode is determined by it's leakage current, which is very, very small. It's so big that the resistor looks like a short circuit as long as the diode can support the voltage. This is true no matter which way you swap them.

If you were to place fuses on the 5V widing, would you choose 4A fuses?

I would probably start just a hair over the rated winding current, and make it a slow blow to cover the startup case where heaters are cold and pull more current until they heat up. This kind of fusing probably covers you for mild overloads too, as long as they don't go on too long.