"Redneck" B+ lifter

[martin manning]

Adding 50 ohms to each source the AC currents are 250 and 353 (reduced by 52%), so again 0.71 (or 1.41 if you prefer). I left it out originally so as not to assume anything wrt the different wire lengths and gauges. Looking at power dissipation in the coils with the 50 ohms in, they are 31.5W each, so both configurations are equalized wrt to heat generation. In both cases the secondaries would be generating 63W.

[cdemike]

Here you go. The transformers for FW and FWB would in reality have different cores and wire gauges.

This is great! It's also the kind of reminder I needed to not treat reference guides like the Hammond rectifier guide as gospel to pdf64's point. Seems like it'd be possible to get well over 800V at the reservoir node with a FWB based on Martin's simulation -- feels pretty safe to say a FWB is totally out of the question!

1A slow-blow for the HT fuse should be replaced with 500mA

Hmm, thanks for pointing that out. I see Marshall used a 500mA in the 1987/1986, so that would make sense. I up-rated since the big bottle tubes should be able to dissipate ~40% more power than the EL34's and 750mA isn't as easily available as 500mA or 1A. Doing the math over again, though, it seems that 500mA was probably more sensible (and maybe even pretty high for the 50w Marshalls).

Your hand-drawn schematic has a rectifier that uses only 1/2 of the high-voltage secondary at any moment. The other half is disconnected by a reverse-biased diode.


A full-wave bridge uses all of the high-voltage secondary, all the time.



If you mix the two approach without significant care, the center-tap of the high-voltage secondary will short-circuit your full-wave bridge and smoke the fuse (at least we hope it only burns a fuse).

Thanks! Yeah definitely don't want more at stake than a fuse. As I understand it, the reverse-biased diode in the proposed series rectifiers design in the initial post should prevent shorting the FWB rectifier the same way it does in the current configuration, right? Either way, it seems like Martin's suggestion involving placing the auxiliary PT's secondaries in series with the main PT's secondaries should avoid the problem since the overall rectifier topology would remain the same. Safety concerns seem like might shift to concerns regarding the main PT in that scenario based on:

Boosting the transformer voltage will increase output power as well as PT current and might exceed the 200mA limit.

As I understood it, though, the 200mA refers to the current at which the secondary voltage was measured rather than a maximum value? If that's wrong, then it seems I'll need to bite the bullet and rework the power supply overall, though I suspect the amp draws considerably more than that currently if it draws twice its idle current when actually amplifying a signal.

[Helmholtz]

As I understood it, though, the 200mA refers to the current at which the secondary voltage was measured rather than a maximum value? If that's wrong, then it seems I'll need to bite the bullet and rework the power supply overall, though I suspect the amp draws considerably more than that currently if it draws twice its idle current when actually amplifying a signal.

I assume all the specified winding currents are limits, as that's essential information for the cicuit designer.
200mA is about the full power winding current with a 35W to 40W amplifier.
PTs are not normally overspecified.
In case of doubt ask Mojotone.

[Helmholtz]

Adding 50 ohms to each source the AC currents are 250 and 353 (reduced by 52%), so again 0.71 (or 1.41 if you prefer).

If you add 2 identical but non-overlapping half-cycle currents, the resultant RMS current is 1.414 times the RMS current of one of the half-cycle currents.

[martin manning]

On the transformer side of the rectifiers the secondary currents are independent, so I’m thinking the RMS currents would be added.

[Helmholtz]

On the transformer side of the rectifiers the secondary currents are independent, so I’m thinking the RMS currents would be added.

The RMS currents would only add directly (or fully) if the partial currents were in phase.
To check just measure the combined current after the diodes or in the CT with a FW circuit.

[martin manning]

The transformer doesn’t see the combined current waveform. All it knows is that a half wave rectifier current is drawn from each half of its secondary.

[Helmholtz]

The transformer doesn’t see the combined current waveform. All it knows is that a half wave rectifier current is drawn from each half of its secondary.

Correct, but the combined current can be measured in the CT wire.

Each half of the winding supplies half the total power.
As P = I²R, the RMS value of the combined current must be sqrt(2) times the RMS of each partial current.

[martin manning]

Doesn’t that make the FW and FWB RMS current identical? (Give or take a diode drop)

[Helmholtz]

Yes, that's what I meant.
Output currents are identical but winding (wire) currents differ by sqrt(2).

[cdemike]

Just placed the order for that small toroidal which I plan to wire up per Martin's series secondary proposal early into this thread. The Mojotone PT secondary wires are both color-coded red, so I'm trying to figure out the best way to get the PTs to work in-phase. I could always use a DPDT switch to reverse polarity in a test setup if needed, but is there a way to test it before potentially running the transformers working against each other? Given the startup differences between EI and toroidal PTs, it seems like that might be pretty hard on both of the PTs if they were wired out-of-phase though ideally those conditions would only occur once. I'm on the hunt for a scope and wave generator, but I'm surprised to find there aren't a ton coming up for sale locally.

[sluckey]

I'm on the hunt for a scope and wave generator, but I'm surprised to find there aren't a ton coming up for sale locally.

Where are you located in Alabama?

[cdemike]

Birmingham, so unfortunately some distance from Mobile. Some for sale around there?

[sluckey]

I have a Hewlett Packard 3310B Function Generator and a Tektronix 465B scope that I'll probably never use again. They've been in my bedroom closet for several years. Excellent condition. $300 for both.

[Helmholtz]

Just placed the order for that small toroidal which I plan to wire up per Martin's series secondary proposal early into this thread. The Mojotone PT secondary wires are both color-coded red, so I'm trying to figure out the best way to get the PTs to work in-phase. I could always use a DPDT switch to reverse polarity in a test setup if needed, but is there a way to test it before potentially running the transformers working against each other? Given the startup differences between EI and toroidal PTs, it seems like that might be pretty hard on both of the PTs if they were wired out-of-phase though ideally those conditions would only occur once. I'm on the hunt for a scope and wave generator, but I'm surprised to find there aren't a ton coming up for sale locally.

You don't need a scope to find the correct winding phasing.
When wiring the windings in series total voltage will either increase or lower depending on relative polarity.
So just connect and measure the AC voltage in each leg.
Wrong wiring doesn't stress the transformers.