multitap PT - switching for power amp and preamp separate possible?

[Bieworm]

Gentlemen...

I have built an amp with this power transformer with succes. I can switch the B+ between 18W, 9W and 2W by means of a rotary switch. As expected the tremolo don't work at 2W switching, so I was wondering if I can split the power amp and the preamp with this transformer. Is this problematic with 2 FWB rectifiers? Normally the 18W has a single 32+32uf/500V can cap filtering. Now I'm going double with the splitting.. is it too much?
The power transformer has 160mA.

[kagliostro]

In the Ampmaker Double Six (1W - 6W - 12W)  the PS is arranged this way



Franco

[Bieworm]

In the Ampmaker Double Six (1W - 6W - 12W)  the PS is arranged this way



Franco

Thx Franco
But that scales down Power amp and Preamp. I need the preamp voltages to be at max for the tremolo to swing.. I just built one with that PTx and scaled down the whole amp with the 3 way switch, but the trem is weaker at 9W and gone at 2W

[kagliostro]

OK, I understand

Some time ago I studied something that seems to be what you have draw, some time is past and actually I don't remember well

it seems to me to remember that what you propose is feasible but I don't remember so well to be sure 100% (say I'm 98-99% sure)

I'm trying to find in my archive some examples I've draw few years ago and see if they correspond with your proposal

Franco

[PRR]

> Is this problematic with 2 FWB rectifiers?

Yes.

[Bieworm]

> Is this problematic with 2 FWB rectifiers?

Yes.

Thanks.. so I will have to rest my case and settle for the full amp scaling. That 2W setting is just a gimmick for me anyway.. won't be using it a lot. I can live with not having tremolo then..

[WiderGates]

Yes, it's possible.
This is from the Marshall DSL20hr schematic for auto bias:

HT1, HT2, HT5 = Anodes, Screen, PI
HT3, HT4, HT6 = Preamp

[PRR]

Yes, it's possible.

That has two separate windings: CN9-CN12 and CN11-CN10.

That works just like two 9V batteries, no other connections, can be stacked to make a 18V battery.

OP is looking at a single winding with a tap. I say he needs a long extension cord and a fireproof bench on the lawn, because I predict fire. (at least heat, maybe smoke...)

[WiderGates]

Thanks a lot PRR, I understand what you mean.

I know this is not exactly what was questioned,
but I have found a different solution for a PT with center tap, this is from a Fender "The Twin".
It bring down the voltage to one half of Bias, B+ and G2.
Power is then a quarter.
A, B and C remain at full voltage.

Perhaps that can help anywhere.
Ciao

[Bieworm]

Thanks a lot PRR, I understand what you mean.

I know this is not exactly what was questioned,
but I have found a different solution for a PT with center tap, this is from a Fender "The Twin".
It bring down the voltage to one half of Bias, B+ and G2.
Power is then a quarter.
A, B and C remain at full voltage.

Perhaps that can help anywhere.
Ciao

Thx for the effort.  But that switching is very different from what I am to achieve.. 👍

[kagliostro]

It is a compromise but seems it can be used (proposal from a friend in an italian forum)


May be it will be noisy ??? (Double de value of the capacitors and number of RC filters on pre PS ??)





Franco

[pdf64]

I’m not sure that the HT supply for the preamp there would work in all 3 switch settings?

[shooter]

looks to be only 2 possibilities in 3 choices


1 - 2 - 1(3)

[PRR]

Just a little preamp? Stop faffing around. Get a 6V:240V heater transformer, wire it backward to the heater winding. Get 230VAC and make about 320V of DC, which will filter-out to like 280V or 250V, enough to jiggle your tremolo.

[HotBluePlates]

... I know this is not exactly what was questioned,
but I have found a different solution for a PT with center tap ...

The challenge is this is fundamentally different than what Bieworm has now.

   -  Bieworm's plan has a switch selecting among 3 choices of which taps to connect to the Main Bridge rectifier.  He's comtemplating adding another bridge across the two taps with the highest voltage solely to power the preamp.  The whole point is to make his lowest-power setting work.


Look at which diodes conduct in each bridge:
Assume the 0v tap is relatively-negative, the 190v tap is "a little positive" and the 275v tap is "very positive."
Assume the power amp's bridge is set for 2 watts (the precise need for this plan); the switch is now drawn below for this.
The preamp's bridge will connect the 0v tap to ground.
The power amp's bridge will connect 190v to ground.

So 190v tap is shorted to the 0v tap, and the transformer burns up.


The Other Plans with bridges connected to a center-tap transformer don't really matter, since they don't address Bieworm's need with his present lowest-power setting.

That makes PRR's suggestion of an (cheap-ish) auxiliary transformer the shortest path to a working 2 watt setting.
(Otherwise, just use a JFET as your oscillator, and low-voltage doesn't matter anymore)

[kagliostro]

That will be a nice, sure, simply, low cost solution

+1 for PRR




Franco

[Bieworm]

... I know this is not exactly what was questioned,
but I have found a different solution for a PT with center tap ...

The challenge is this is fundamentally different than what Bieworm has now.

   -  Bieworm's plan has a switch selecting among 3 choices of which taps to connect to the Main Bridge rectifier.  He's comtemplating adding another bridge across the two taps with the highest voltage solely to power the preamp.  The whole point is to make his lowest-power setting work.


Look at which diodes conduct in each bridge:
Assume the 0v tap is relatively-negative, the 190v tap is "a little positive" and the 275v tap is "very positive."
Assume the power amp's bridge is set for 2 watts (the precise need for this plan); the switch is now drawn below for this.
The preamp's bridge will connect the 0v tap to ground.
The power amp's bridge will connect 190v to ground.

So 190v tap is shorted to the 0v tap, and the transformer burns up.


The Other Plans with bridges connected to a center-tap transformer don't really matter, since they don't address Bieworm's need with his present lowest-power setting.

That makes PRR's suggestion of an (cheap-ish) auxiliary transformer the shortest path to a working 2 watt setting.
(Otherwise, just use a JFET as your oscillator, and low-voltage doesn't matter anymore)

Thanks HBP! I've used an IRF840 before for this. Now that's an idea! That trem worked tremendously!!

Just off the record, how about a voltage doubler for the preamp B+, that only switches on when the 2W path is selected? more complicated, but should work, no?

[HotBluePlates]

... how about a voltage doubler for the preamp B+, that only switches on when the 2W path is selected? ...

Draw an accurate schematic for this plan.  Assume a voltage-state for the winding taps as I did.  Identify each forward-biased diode.

Is anything shorted together?


Even if it works, I would still just use the MOSFET oscillator for simplicity, or use PRR's heater transformer solution.

[HotBluePlates]

... (proposal from a friend in an italian forum)


May be it will be noisy ???

The tap in the middle of the full-wave rectifiers must always be connected to the negative end of the preamp supply's filter cap to work.

     -  In some switch positions it is connected to the power amp supply's B+ half of the time, so be prepared for fireworks.

     -  In another position it is disconnected entirely, so the preamp supply gets no voltage.

[kagliostro]

The tap in the middle of the full-wave rectifiers must always be connected to the negative end of the preamp supply's filter cap to work.

This will happen via the diodes of the FWB (but the tap isn't exactly on the middle of the winding)

In some switch positions it is connected to the power amp supply's B+ half of the time, so be prepared for fireworks

In some position there is only one half sine wave rectified, no full wave

-  In another position it is disconnected entirely, so the preamp supply gets no voltage.

  When  I don't see 




Ahem ... no, I see, but that way the intermediate tap connection seems unnecessary 


As told, + 1 for PRR suggestion

Franco

[HotBluePlates]

     -  In another position it is disconnected entirely, so the preamp supply gets no voltage.

When  I don't see 

Switch in the Upper position.  The rectifier running to the preamp supply cannot conduct.

The Full-Wave rectifier for the Preamp relies on the assumption that the middle-tap is an unchanging reference voltage.
But instead of 275/2 ---> 137.5v - 0 - 137.5v, we get a "center" tap that moves from +52.5v during one half-cycle to -52.5v during the other half-cycle.  In other words, if the outer ends of the winding are consistent voltages when the Preamp rectifiers conduct, the "ground" swings by 105v.

Let's look at it a whole other way, and say the middle-tap is always "0v."  To understand what might happen, you need to diagram each half-cycle:

Switch in the Middle position.  Positive half-cycle:

The ends of the PT are now +85v and -190v.  The Power amp gets -190v at the negative side of its power supply, and 0v at the positive side.  190v AC is rectified.

The Preamp power supply has -190v at its negative terminal, so the upper Full-Wave diode conducts.  We're assuming the middle-tap is "0v" so this delivers +85v to the positive side of the Preamp supply.  +85v to -190v yields 275v AC across the Preamp power supply.


Now switch half-cycles:

The lower Full-wave diode conducts +190v.  It tries to pull the Preamp supply positive by that 105v yielding 190v --> -190v = 380v across the Preamp supply.  You get 105v of ripple, which will not be fully filtered, and is certain to swamp any signal in the amplifier with hum.

Meanwhile the situation is worse at the Bridge:
The lower right diode is forward-biased and passes +190v to the positive side of the supply which used to be 0v.  The negative side of the supply stays at -190v, so this tries to create 190v AC of ripple.  The total voltage of the Power Amp supply tries to rise to 380v, defeating the purpose of the power-switching.

What happens with the middle-tap is unclear.  The upper-right Bridge diode would seem to have 0v on both side of the diode, which may or may not conduct.
     -  If the upper-right diode (circled in Green) can conduct, it short-circuits 0v to 190v. BOOM!!
     -  If the upper-right diode cannot conduct, then in fact the lower-right diode passes no current; Power Amp supply is half-wave rectified.
     - if the upper-right diode does not conduct, then the Preamp diode does not conduct; Preamp supply is half-wave rectified.

It's a coin-toss whether the result is insurmountable hum, "just doesn't work," or a burns up the PT.  Clearly this is a bad plan, but it's not obvious this is true unless you move beyond the usual assumptions and diagram the exact voltages that will occur each half-cycle.

[kagliostro]

Ciao HBP

Many Thanks

Franco

[Bieworm]

Wow HBP!!! You clearly took the time to clear this out. Thanks!👍