Questionable Output Transformer Ratio. Maybe.

[sluckey]

The Bandmaster uses a capacitor input filter, not a choke input filter.

A choke input filter will have much less DCV output than the cap input filter when a load is applied. Much less than you're thinking. The only reason the DCV is so high now is because there is no load connected. With no load current flowing, the choke can do nothing. By definition, a choke (inductor) opposes a change in current. Since there is no current there is no change in current, and the choke has nothing to oppose.

[dtbradio]

The Bandmaster uses a capacitor input filter, not a choke input filter.

A choke input filter will have much less DCV output than the cap input filter when a load is applied. Much less than you're thinking. The only reason the DCV is so high now is because there is no load connected. With no load current flowing, the choke can do nothing. By definition, a choke (inductor) opposes a change in current. Since there is no current there is no change in current, and the choke has nothing to oppose.

I will see if I can put together a test load of around 2K ohms to see what the drop ends up being. The bandmaster shows 435 volts after the choke, but I know that's only with the 6L6 screen loading and the small amount of draw from the assorted preamp tubes.

[dtbradio]

I connected a 3.18K test load, and found about 287V after about 5 seconds powered on. Will test it with solid state rectifier and see what it drops to later.

[TUBEDUDE]

That 35uF before the choke is a filter capacitor also.

[sluckey]

I connected a 3.18K test load, and found about 287V after about 5 seconds powered on. Will test it with solid state rectifier and see what it drops to later.

Hmm, not working out like you had hoped. I wonder what could be wrong? The answer has already been provided. If only you could believe.

[dtbradio]

That 35uF before the choke is a filter capacitor also.

Yes, used as the filter/reservoir for the power tube plates. They are not as susceptible to the power supply noise in this case since they run in push-pull configuration which tends to cancel those kinds of common-mode signals.

[dtbradio]

The Bandmaster uses a capacitor input filter, not a choke input filter.

A choke input filter will have much less DCV output than the cap input filter when a load is applied. Much less than you're thinking. The only reason the DCV is so high now is because there is no load connected. With no load current flowing, the choke can do nothing. By definition, a choke (inductor) opposes a change in current. Since there is no current there is no change in current, and the choke has nothing to oppose.

The bandmaster schematic to which I linked shows both cap and choke filters. As for your statement of "Since there is no current there is no change in current", I think you meant to say no change in voltage. And you are correct, an inductor opposes current change by causing a voltage shift either up or down (inductive kick-back, which can cause problems in other types of circuitry not relevant here) whereas a capacitor opposes a voltage change with a current shift. Another way to look at it is that an inductor opposes an AC voltage whereas a capacitor opposes a DC voltage. The reason the choke makes the DC filtering so much more effective is that it forms a low-pass filter in conjunction with the capacitor to ground that follows the choke. Any AC ripple or other noise present in the rectified DC sees a high impedance when it hits the high-value choke, and what little makes it through the choke then sees a low impedance to ground, effectively removing the rest of it from the DC component.

I connected a 3.18K test load, and found about 287V after about 5 seconds powered on. Will test it with solid state rectifier and see what it drops to later.

Hmm, not working out like you had hoped. I wonder what could be wrong? The answer has already been provided. If only you could believe.

Setting aside your condescension, I never said I didn't believe. Being fairly new to tube circuitry, and having never worked with a high-value filter choke in a high-voltage tube circuit before, I was not familiar with how much they can effect the steady-state voltage level under differing loads. I sometimes work with small filter chokes in low-voltage DC (12V) circuits in my profession as a radio technician, and those chokes are low enough in value that they don't affect the DC voltage by very much.

As to my circuit, I found that under the same load with solid-state rectification I saw 310 volts with the choke, and about 470 volts without it. This is a great example of not only the voltage drop under load through the choke, but also of the voltage sag seen through the 5U4 rectifier when compared with the solid-state rectifier.

Since my original question of transformer impedance was nicely answered early on in the thread by the quote below, I will discontinue hijacking my own thread. Thanks to all who provided useful information!

19.1 squared is 365. An 8 ohm speaker would give only 2.9k, so I would suspect a 16 ohm speaker, which would give you a 5k8. That would look pretty good with 6L6 at 450V plate, 350V screen.

[sluckey]

As for your statement of "Since there is no current there is no change in current", I think you meant to say no change in voltage.

No, current is what I meant to say. We were talking about a choke and the point is that a choke does nothing unless there is current flowing through it. In that circuit, when current flows through the choke due to a load, that current will be changing due to having to continually pump up the charge on the filter caps. The inductance properties kick in to oppose that changing/charging current. The result is a much smoother DC voltage.

My other point was that a choke input filter will have a much smaller DC voltage output whenever a load is connected to the filter. Your experiment proved that quite well.

[martin manning]

I believe Steve is talking about load current, which if near zero results in little rate of change in the current passing through the choke, and little voltage drop. Voltage drop is proportional to the rate of change of current, L di/dt. If the load impedance is low, the charging pulses through the choke are much larger, and have a much larger maximum rate of change, therefor a large voltage drop appears. It's the rate of change in the current that matters.

[TUBEDUDE]

That 35uF before the choke is a filter capacitor also.

Yes, used as the filter/reservoir for the power tube plates. They are not as susceptible to the power supply noise in this case since they run in push-pull configuration which tends to cancel those kinds of common-mode signals.

I know what it's for.
My point was your repeated statements that the rectifier fed the choke directly, with no cap first muddied the waters, delaying any useful
responses. Always post with schematics and/or pictures.