Best way to drop B+ 150 VDC?

[ViperDoc]

I have a Silver Jubilee-based project that has an unloaded B+ of 430 VDC at the first filter cap. I need to lower that down to approximately 300 VDC or lower to power a Metro ZL effects loop. What would be the best design for that? I can calculate a single dropping resistor value, but if that single resistor were to fail, the board would fry. My thoughts are that a drop of 150 VDC is considerable. An alternative I thought about would be to dedicate the 20% lower-output PT secondaries to this supply, but that would require an additional rectifier...

What would you do? Thanks as always.

[luix]

As I can see the ZL loop use the LND150/250 so the current should be less than 30mA, I suppose much less since 300V*0.03A = 9W and the device dissipate only 0.7W in the right condition (thermal resistance is fairly high!).

Let suppose that the dissipation is 0.6W the current can only be 2mA.

With this current you can use the classic mosfet source follower (remember the gate resistance otherwise it will oscillate) with no heatsink.

I think that resistive drop is the best practice for such low current, remember that the AC signal current should be fournished by the capacitor so the bigger is better but always considering the charge time...

[didit]

Perhaps simply read the instructions
?

Decoupling (Dropping) resistor chart
B+ at PI supply: 2 watt resistor needed
400-386V -------------------------------------------- 56K
385-366V --------------------------------------- 47
K365-345V ---------------------------------- 33K
344-326V ----------------------------- 22K
325-305V ------------------------ 10K
304- 280V ------------------ 4.7K

Loaded, expect approximately 400VDC off your plate supply.

Best .. Ian

[martin manning]

I would tap the screen or PI node, since the plate has very high ripple voltage.

Edit: From the instructions Ian linked:

1:Measuring the B+ supply node to power your loop board.
The power for the loop board is tapped directly from the amplifier PI (phase inverter) supply node normally. In some cases, the power tube screen supply node may be used instead, typically in low/lower B+ amplifiers (with 400V or under B+ max).

[ViperDoc]

Perhaps simply read the instructions
?

Decoupling (Dropping) resistor chart
B+ at PI supply: 2 watt resistor needed
400-386V -------------------------------------------- 56K
385-366V --------------------------------------- 47
K365-345V ---------------------------------- 33K
344-326V ----------------------------- 22K
325-305V ------------------------ 10K
304- 280V ------------------ 4.7K

Loaded, expect approximately 400VDC off your plate supply.

Best .. Ian

Aah, many thanks, Ian. I am aware of those instructions. I am
mainly concerned about voltage dropping from such a high B+, since 400+ is higher than the other amps I’ve put these loops into. If all I need is a large enough dropping resistor, I’m good as gold.

[didit]

Pay heed to Martin’s additional thoughts. You need to measure loaded and properly biased. And connect to PI node on the supply chain. Expect plate voltage dropping around 400V and PI another substantial amount.

[ViperDoc]

Copy that, thanks guys!

[ChopSauce]

I have a Silver Jubilee-based project that has an unloaded B+ of 430 VDC at the first filter cap. I need to lower that down to approximately 300 VDC or lower to power a Metro ZL effects loop. What would be the best design for that? I can calculate a single dropping resistor value, but if that single resistor were to fail, the board would fry. My thoughts are that a drop of 150 VDC is considerable. An alternative I thought about would be to dedicate the 20% lower-output PT secondaries to this supply, but that would require an additional rectifier...

What would you do? Thanks as always.

Sorry for deviating from the main stream (again), but an additional (solid state) rectifier dedicated to the FX board does not look like an expensive (in any meaning of the term) solution to me...

[martin manning]

An alternative I thought about would be to dedicate the 20% lower-output PT secondaries to this supply, but that would require an additional rectifier...

...an additional (solid state) rectifier dedicated to the FX board does not look like an expensive (in any meaning of the term) solution to me... :?

You would need a couple of diodes, a resistor, a couple of high voltage electrolytics, and space for a small board to hold them.

[ChopSauce]

Now I'm sure I have been missing something, this time: filtering! No less...

I'm going back to my workshop to drill some pedal stuff...

[didit]

Now I'm sure I have been missing something, this time: filtering! No less...

I'm going back to my workshop to drill some pedal stuff...

Tapping in off the main B+ chain after choke and a few stages of filter caps is free, and nearly ripple-free. Why worry about alternatives?

Best .. Ian

[ChopSauce]

Are you talking to me?



That was mostly out of curiosity.

I posted in doubt. Maybe I shouldn't rely so much on Martin and peers to help me learn about the matter of this forum?

[martin manning]

That was mostly out of curiosity.

Keep asking questions and floating ideas. Imagine a forum where everybody knows everything... It would quickly die!

[didit]

Twas never an intention to oppress curiosity. Simply adding a bit reinforcing Martin's wisdom.

If you want solutions looking for problems this recent nearly topical thread offers a few viable approaches to a well-regulated voltage drop.
https://ampgarage.com/forum/viewtopic.php?f=6&t=34010

Best .. Ian

[ChopSauce]

Simply adding a bit reinforcing Martin's wisdom.

That was my intention too...