Adding a choke to Ampeg V-4 power supply?

[Steve Conner]

If you need a 6k resistor in series with the choke to get the voltages down... What do you suppose is the impedance of a 10 henry choke at 120Hz? 7.5k. So the choke will only reduce ripple by about 3dB compared to the resistor alone.

Chokes are for when you want a high AC impedance with low DC resistance. If you need to lose DC voltage too, then a resistor is a more cost-effective solution.

[pdf64]

'I think that a properly sized swinging choke feeding the screens is a beautiful thing'

How to actually implement that? It seems to be either/or, I can't see how to have the existing cap input system and a swinging choke input system.
Or would the swinging choke be fed from its own rectifier?
Pete

[Steve Conner]

Yes, a rectifier is either capacitor-input or choke-input, it can't be both. And the swinging choke only works as part of a choke-input filter.

[trobbins]

If there was no bypass capacitance on the screens - just a dropper R, or in comparison series LR - then the screen voltage would instantaneously modulate more with the signal frequency for the LR case. If there was some bypass capacitance on the screens then a large signal transient step response on the screens would give a bit different screen voltage sag response between the two cases.

[pdf64]

Could a single B+ winding (eg center tapped) on the power transformer supply 2 full wave (ie 2 diodes each) rectifiers / smoothing systems, one FW rectifier with a reservoir cap for the plate supply, the other FW rectifier with a swinging choke for the screens?
If so, what are the benefits and drawbacks?
It seems a good way of achieving a stable screen supply that's nicely lower than the plate supply.
Pete

[trobbins]

There is no problem with using double rectification off the same winding for a typical AC-0-AC CT HT winding configuration, where the CT is common to both B+ supplies - a separate set of diodes is needed to feed each filtered output. I'm planning to use such a setup for an amp restoration using a quad 807 with a 410-0-410 PT secondary, as I need to get the screen down towards 300V. I've come across a few amps that use a doubler where the the screen supply comes from the mid-point of the series output capacitors via a subsequent choke filter.

[jamesmafyew]

I've come across a few amps that use a doubler where the the screen supply comes from the mid-point of the series output capacitors via a subsequent choke filter.

The Musicman HD130 is a good example of this. 725V and 360V outputs from a single HT winding with only two diodes and no center tap.

[hakantoz]

Does anyone have drawn the circuit diagram for this? It may help a beginner to understand your comments. It will give brief description about the circuit.

[trobbins]

Yes, a rectifier is either capacitor-input or choke-input, it can't be both. And the swinging choke only works as part of a choke-input filter.

The output filter to a rectifier can blend from pure choke (constant current), to pure capacitor (constant voltage). Those pure filters are used in the typical text-book derivations of rectification design equations and parameters. A blend is achieved simply by having non-ideal components. A swinging choke is one type of non-ideal filter, but a little difficult to interpret the current swing that may occur. A 'soft' capacitor input is easy to interpret by most due to an understanding of ripple voltage. A soft capacitor input to a choke input (ie. CLC) is what I have used a few times to 'tune' the idle voltage for an output stage - and this is fairly easy to understand as a large voltage swing input to the choke, but not as large as just choke input, whereby there is an increasing 'dc' level component on the input side of the choke.

A good reference on many power supply topics, although for an industrial technical audience, is:
http://www.introni.it/pdf/Motorola%2...20Handbook.pdf