Explain this Mesa power supply?

[Randall]

Mesa claims the Maverick has both solid state and tube rectified power supplies. I can see from this schematic that with the rectifier select switch open as drawn, that pin 8 feeds the reservoir caps as well as plates and screens, while the diodes feed nodes C and D, a hybrid situation. What I don't understand is, when the switch is closed and pin 8 is connected to the anodes of the diodes, how is this now tube rectified? I would think the diodes would need to be disconnected. What is happening with both supplies at the same point?

http://tubefreak.com/mav4.gif

[cerrem]

Mesa claims the Maverick has both solid state and tube rectified power supplies. I can see from this schematic that with the rectifier select switch open as drawn, that pin 8 feeds the reservoir caps as well as plates and screens, while the diodes feed nodes C and D, a hybrid situation. What I don't understand is, when the switch is closed and pin 8 is connected to the anodes of the diodes, how is this now tube rectified? I would think the diodes would need to be disconnected. What is happening with both supplies at the same point?

http://tubefreak.com/mav4.gif

The silicon diodes would be in parallel with the tube diode when the switch is CLOSED, thus the silicon diodes have a lower forward voltage drop than the vacuum diode.....so the vacuum diode is not conducting during the charging angle..
This is an old "mickey mouse" trick done many times over the last 50 years.... however Mesa will go after you and legally threaten you if you have this in your amplifier...since somehow they got this patented along with a bunch of other common circuit mods...
I know of only one person who did not back down and went to court with MESA.... In the first 10 minutes they showed this is prior used ciruitry and cited examples used in the 60's ...case thrown out... But for a small amp builder with limited budget, this tactic is design to scare you and cost you money you cant afford to defend yourself and derail your business....

[Old Tele man]

Gotta pay attention to where the HV-center tap and filter capacitor grounds are located...then things make sense.

With the switch OPEN the SS-diodes power only the upper voltage string and the rectifier powers the lower voltage string.

With the switch CLOSED the upper and lower voltage strings are shorted together and the SS-diodes power both strings simultaneously, as cerrem described above.

[Justin Thomas]

Possible they also fudged the schematic on purpose to foil cloners?

Justin

[Enzo]

I would tend to doubt that for a couple reasons. First the schematic as drawn works. Second, anyone with the remotest knowledge of tube circuits wanting to clone the circuit could figure out a simple power supply. I find it easy to overlook schematic errors, because in most cases the circuit is already familiar.

[Randall]

"when the switch is CLOSED, thus the silicon diodes have a lower forward voltage drop than the vacuum diode.....so the vacuum diode is not conducting during the charging angle."

This I do not understand. Why do they not both conduct?

[Enzo]

Current takes the easiest path. You know that tube rectifiers drop 20-30-50 volts across them, and silicon diodes drop maybe 1 volt. Think of the tube as having a big resistor inside. The current won't fight its way through the tube when the diode path is there.

Imagine a 10k resistor and a 100 ohm resistor in parallel. Now apply some voltage to them. Most of the current flows through the 100 ohm resistor.


I suppose we shouldn't say the tube is not conducting, but it is safe to say the current through it is tiny.

[Randall]

Makes sense. I understood this with resistance, but not with rectifiers I guess. So, can we think of this as the tube rectifier is for all practical purposes at idle when the switch is closed?

[J M Fahey]

Yes.
It neither passes current nor drops voltage, hence not dissipating power.
I would also call that "idle"

[Pedro Vecino]

But curiously in the first versions of Dual / Triple Rectifier (two channels) the rectifier switching system was this.



I consider it less practical. In my case it makes the automation of the bias circuit more difficult. I have had to change many times the old circuit to the new one to use a double switch (one circuit for the rectifier selection and another synchronized to adjust the bias).
Later developments of Rectifiers that associate channels to rectification modes (Roadster and Roadking) also use the "modern" system with a single circuit.

[Old Tele man]

Just noticed the 'spongy' and 'bold' input power switching...cute.

[nickb]

This is an old "mickey mouse" trick done many times over the last 50 years...
\..In the first 10 minutes they showed this is prior used ciruitry and cited examples used in the 60's ...\

Out of interest, can you list some examples?