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Generate a negative bias voltage?
Generate a negative bias voltage?
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Old 14th June 2021, 09:25 AM   #21
Friston is offline Friston
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Yes, through the reactance of the caps via the centre tap to earth. You don't need the caps if you just wanted equivalent B-.
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Old 14th June 2021, 09:27 AM   #22
Depanatoru is offline Depanatoru  Romania
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The cap C1 ( and C2 ) can charge when the top of the winding is positive and that charge is transfered through the second bridge rectifier . That's why the current consumption must be low enough .
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Old 14th June 2021, 10:45 AM   #23
JMFahey is offline JMFahey  Argentina
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Thread as old as the Pyramids but since it was resurrected ....
A floting winding feeding a bridge rectifier will never give you a negative voltage by itself because its ends are *always* positive relative to Ground, best case 0 V, so no diode or combination of them will do.

You can see it as "an AC winding with a strong DC component added" and therein lies the solution.

You can block the DC component with a capacitor and rectify the AC component, which NOW can be referred to Ground and give you a (low current) negative bias voltage.

Marshall does that since at least the 90s.

The capacitive-resistive divider is made out of C15: 47nF Class X and R7 56k.
Half wave rectified by D1, the rest of the components (R30 - R31 - Pr1 - C13) is typical Guitar Amp Bias adjustment.

Crude as it looks, has worked faithfully for dedcades in tens of thousands Guitar amps.

Click the image to open in full size.
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Old 14th June 2021, 10:46 AM   #24
Chris Hornbeck is offline Chris Hornbeck  United States
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Y'all are certainly welcome to try it.

Good luck,
Chris


update: see R7 above
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Old 14th June 2021, 01:40 PM   #25
oemcar is offline oemcar  United States
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Generate a negative bias voltage?
I had same issue-
Since I'm using SS rectifiers, I put voltage tripler on my 5vac winding to get bias voltage needed (-19vdc). Tied the + side to ground... has been working fine. So can you use one of the three 6.3V windings for this? Hammond spec sheet should show if one winding is lower current than the others...that's the one I would use.
Jim

Last edited by oemcar; 14th June 2021 at 01:43 PM.
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Old 14th June 2021, 05:28 PM   #26
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by kevinkr View Post
Use this at your own risk.. I am not a fan of doing it this way, but it is an option. You will need to determine the value of the capacitors empirically to get the bias voltage you require. I'd start at 0.1F and work your way from there.
Actually, Kevin, that is an excellent circuit. One must choose capacitor voltage ratings high enough, as they have to withstand almost VP-P differential voltage. I like 600 V capacitors, in general.

The observation of needing to choose values empirically is OK, but on the other hand kind of flimsy. Better (in my mind at least) is just to strap a resistor-Zener load to the C- supply, and regulate the heck out of it. Cheap, easy, works, no hassle. Since the C- supply doesn't need to supply much current (ever! if done right!), then the Zener doesn't need to be particularly muscular.

I have always loved the joint-responsibility-to-do-what-they-are-best-at approach in designing electronics. Chips of brilliantly refined sand for some bits, pretty light-show glowing valves and gas regulators for other bits, ridiculous but glorious 803 ... 805 ... 806 ... 811 ... 8 valves for the heavy lifting stages, in pyrex envelopes and well muffled very-lightly-forced-air cooling sleeves. Whisper quiet, but 5+ ft/min. Keep em Kool(er).

So yah capacitor-coupled trickle supplies are awesome. And cheap.

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Old 14th June 2021, 07:30 PM   #27
petertub is offline petertub  Sweden
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Generate a negative bias voltage?
Whatever means the negative bias MUST be present before B+ is applies. One way is to make it as simple as possible and use a tap on the B+ winding.


If one has to make a more complicated circuit, with additional transformer or similar, one could use a relay triggered by the bias to enable the B+ or mains transformer.
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Old 14th June 2021, 08:30 PM   #28
oemcar is offline oemcar  United States
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Generate a negative bias voltage?
The OP hasn't logged in for over 6 years
Jim
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Old 14th June 2021, 09:26 PM   #29
GoatGuy is offline GoatGuy  United States
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On one amp, oh maybe 15 years back, after the advent of cheap-by-the-gross lithium 3 V pancake batteries, I got an 8 pancake holder thing, and used it to generate –24 V nominal. The fellow informed me that the batteries lasted roughly 6 years before needing replacing.

It used to be commonplace for many valve radios to have a 'C' supply consisting of dry cells.

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