Don't understand this transformer

[sotownsend]

**broken link removed**

The power transformer on the left, could someone explain the outputs on it? I know the 6.3v and 5v are just simple X-former coils, but whats with the weird 350/50v blu/GND coil? Is that a 350V coil P2P or 350V from ground??

Thanks for any help!

 

[BrownOut]

The top and bottom wires are 350 RMS with respect to the tap in the center. Looks like the blu is a 50 volts tap WRT the same center point.

 

[colin55]

The output is called "350v - 0v - 350v" with a 50v tap. You connect the 0v (called the centre-tap) to chassis and when diodes are placed on the top 350v output and lower 350v output, you can get a full-wave rectifier by using just two diodes. The circuit uses four 1N4007 but it only needs two as these diodes are 1,000v PIV

 

[MikeMl]

Let me ask? Are the 5U4s supposed to sound "better" than the 1N4007s?

Can anyone explain the funky power input switching? Spongy? Bold? Gnd?

 

[sotownsend]

I believe the diodes are faster at reacting which imposes a tighter sound

 

[MikeMl]

OooooK, so can you explain what "tighter" means?

 

[sotownsend]

Actually, I think it's more a function of stability not speed since diodes will give a cleaner signal. Anyway, any noise of the power signal will amplify into the output sages and fluctuate the sound. Tightness is just how much the output signal fluctuates beyond it's
input signal.

I'm no audio or electronics exper however

 

[BrownOut]

OooooK, so can you explain what "tighter" means?

It's the opposite of "looser"

 

[Mr RB]

MesaBoogie make arguably THE best sounding guitar amps in the world. I've repaired a few and played through a few. If I was richer I would probably own one. There's nothing in that circuit done from incompetence.

One of the things that sound best about tube amps is the "sag" ie poor voltage regulation of the main PSU. When playing rythm guitar the audio is like a repetitive series of spikes as the sound is very percussive with a lot of attack.

So the time constant of the PSU sag is tuned so when playing rythm guitar the peaks of sound come through louder but the psu voltage sags within mS for the sustained part of the guitar sound envelope. It would be bad for playing the sound of an orchestra and classical music, but for a single guitar repetitively hitting the strings it makes the sound much more percussive and rythmic.

The tube rectifiers have a much higher impedance than the silicon diodes and will give a very different sound. High-end guitar amps have switches for features like this so the guitarist can choose the type of sound they want. Guitar amps are not the same as audio amps. Guitar amps deliberately colour the sound in a number of ways, and they are part of the sound of the "instrument" the same way that effects pedals are part of the instrument too.

 

[sotownsend]

I'll second that, the sound is increadible and the design is increadible. I'm hoping to construct a clone of the pre and power supply

 

[Boncuk]

I'll second that, the sound is increadible and the design is increadible. I'm hoping to construct a clone of the pre and power supply

If you build the power supply I suggest to leave the 1N4007 (two in series) as they are.

I guess the design engineers have spent some thoughts why to use them this way. I'm also sure they know that an 1N4007 can handle 1,000V.

Boncuk

 

[Nigel Goodwin]

If you build the power supply I suggest to leave the 1N4007 (two in series) as they are.

I guess the design engineers have spent some thoughts why to use them this way. I'm also sure they know that an 1N4007 can handle 1,000V.

Yes, but that's 1000V PIV - which in that circuit is probably been exceeded.

As suggested, the valve rectifiers are there to lower the sound quality still futher, which many guitarists seem to like.

 

[Boncuk]

The circuit uses two 1N4007 but it only needs two as these diodes are 1,000v PIV

Where is the difference? (uses two but it only needs two? )

 

[Nigel Goodwin]

Where is the difference? (uses two but it only needs two? )

I suspect he meant 'uses four'

 

[colin55]

The power supply has only got 100u on the smoothing and this will have the greatest effect on allowing the power rail to dip during high current demands.

The 5U4 will simpy produce a lower voltage on the power rail.

 

[MikeMl]

So why not just switch a resistor in-series with the 4007s to simulate the plate-resistance of the 5U4s.? You can also control the degree of sag by switching in/out more/less filter capacitors.

 

[Nigel Goodwin]

So why not just switch a resistor in-series with the 4007s to simulate the plate-resistance of the 5U4s.? You can also control the degree of sag by switching in/out more/less filter capacitors.

The sag doesn't happen in a linear fashion as a resistor would, it gives a particular kind of distortion that some guitarists like.

For those who remember?, we used to replace faulty metal (selinium) rectifiers with a silicon diode and a series resistor - the resistor is needed to drop the voltage to the same as the metal rectifier provided.

 

[colin55]

The voltage drop produced by the valve is more like a zener diode drop. It is a fairly constant drop and the valve can supply a current of about 300mA.
Valves were used in the old days, simply because high voltage diodes were not available.

 

[Mr RB]

[QUOTE = sotownsend]I'll second that, the sound is increadible and the design is increadible. I'm hoping to construct a clone of the pre and power supply [/QUOTE]

Have you got more schematics to put up?

MesaBoogie have been making top of the line guitar amps for over 30 years and have some real smart engineers there. It's like a treat to see the way they have done things.

 

[Nigel Goodwin]

[QUOTE = sotownsend]I'll second that, the sound is increadible and the design is increadible. I'm hoping to construct a clone of the pre and power supply

Have you got more schematics to put up?

MesaBoogie have been making top of the line guitar amps for over 30 years and have some real smart engineers there. It's like a treat to see the way they have done things.[/QUOTE]

In what way are they any different to anything else?, Marshall are probably one of the most highly rated guitar amps, and certainly by far the highest in rock music.