amp power supply - fur geekz only

cronnin

cronnin

New member
I'm interested if anyone can explain me, why do they use grounded cener tap in power transformers with SS?

I can see that it suits better if there's a tube rectifier afterwards, but why do they use like 330-0-330 secondary output with solid state rectifiers?
Why don't they put a simple 330-0 one coil secondary?
 
Re: amp power supply - fur geekz only

The center tapped (ie... 330-0-330) power supply allows for full wave rectification while a single sided supply only rectifies half the wave. Full wave rectification (120 cycles) is easier to filter than half wave (60 cycles), allowing for more reasonable capacitor values in the circuit and ultimately a quieter amplifier.
 
Re: amp power supply - fur geekz only

cronnin said:
Yes, but why not a single ended with a bridge rectifier?
Click to expand...

Those are actually quite common...and they usually have an additional secondary winding to supply the bias circuit, unless they are cathode biased. There is no easy way to pull negative voltage from a bridge rectifier that is set up to produce a positive voltage. The center tapped supplies simply pull a tap from one side to supply the required bias voltage.

By the way, it is possible to add a tube rectifier to an amp with a bridge recifier. I've modded several JCM800s & 900s with this mod and the result is beyond cool.
 
Re: amp power supply - fur geekz only

Quite common? Didn't realise that. On all schematics with SS rectifier I've seen till now there were only center grounded PS. That's why I asked. Time to read more stuff on the net I guess:)

So..you added tube in series with SS, or you have a switch so you can choose both ways?
 
Re: amp power supply - fur geekz only

cronnin said:
Quite common? Didn't realise that. On all schematics with SS rectifier I've seen till now there were only center grounded PS. That's why I asked. Time to read more stuff on the net I guess:)

So..you added tube in series with SS, or you have a switch so you can choose both ways?
Click to expand...

The Marshall JCM900s have bridges as an example.

The tube replaces the top "half" of the bridge. Both SS and tube can be switched but there must be a large value resistor between the two to reference the voltage difference. The amp needs to be biased so that it is running hot when in tube rect mode.
 
Re: amp power supply - fur geekz only

I'm no electrician, by any means, but would a 3-prong plug offer better overload/circuit protection having a dedicated ground prong rather than a polarized 2-prong?
 
Re: amp power supply - fur geekz only

DrNewcenstein said:
I'm no electrician, by any means, but would a 3-prong plug offer better overload/circuit protection having a dedicated ground prong rather than a polarized 2-prong?
Click to expand...

No; the neutral and ground are tied together in the breaker box (at least in the US) The three prong plug does enforce proper indexing; way back in the day, there was no "standard" of which side of the recepticle was hot and which was "neutral". That is why older amps have ground switches...you never knew what you were plugging into. By the way, a GFCI works just fine with a 2 prong.

If you have a vintage amp and the power lead is in good shape, in the interest of keeping it as original as possible, I would keep it that way.
 
Re: amp power supply - fur geekz only

Bridge indeed, but again center-grounded :)
I'm totally loosing it with double zerO:)


Considering the plugs - I thought I saw some improvement in hum reduction (when plugged into 3-prong) but I can't say it's a rule. It just may be just the position I usually hold my guitar when plugged into bedroom or basement socket. May be even the wiring thing. Dunno.
 
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Re: amp power supply - fur geekz only

Will this work? Better view :)

http://www.drtube.com/schematics/marshall/cd0192-iss7.pdf

So, the bias is separated before stand-by switch, and we have Red and Black giving B+ voltage, if I get it right?

No ground on the 'main' secondary coil here?

The upper secondary that is center grounded is for heaters as I see (FS6.7) but I see both A and B+ marked here...something's wrong.
 
Re: amp power supply - fur geekz only

cronnin said:
Will this work? Better view :)

http://www.drtube.com/schematics/marshall/cd0192-iss7.pdf

So, the bias is separated before stand-by switch, and we have Red and Black giving B+ voltage, if I get it right?

No ground on the 'main' secondary coil here?

The upper secondary that is center grounded is for heaters as I see (FS6.7) but I see both A and B+ marked here...something's wrong.
Click to expand...

The bias (in all fixed bias amps) should be energized in standby. If it weren't, the power tubes would briefly go into runaway while the bias voltage builds in its circuit. IIRC, both B+ leads are red.

The B+ is not center tapped and grounded as in a full wave rectifier. The negative side (DC) of the bridge rectifier is ground referenced; by referencing to ground at this point, what would have been negative voltage in a center tapped bridge rectifier, becomes positive in relation to ground and then has the output of the positive side of the bridge rectifier stacked on top. This allows the designer to wind fewer turns to achieve the desired voltage.

The "A" and "B" designations for the filament design aren't refering to A+ or B+; thier only purpose is to indicate points on the poweramp/power supply that correspond with point on the separate pre-amp schematic.
 
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