Power transformer parallel usage

[anotherlin]

Hi everyone,

Just have a quick question regarding main/power transformer usage.

Suppose I have a simple 5W single ended tube guitar amp with tube for the preamp (let's say a 12ax7) and 1 power amp tube (el84).
It has a main transformer with the following windings.

0V-6,3V for filament;
0V-190V-250V for high voltage.

Because the high voltage has a 190V and a 250V taps, I can have a low/high power switch : Using 190V (low power) or 250V (high/full power) for high voltage supply.

My question is as following : Can I wire in parallel two supplies on the HV secondaries ?

1) The preamp supply (for the 12ax7 low current) has rectifier (diodes) connected to 0V-250V, followed by caps filtering etc.
2) The power amp supply (for the el84 which use much more current) is connected to 0V but has a switch to select 190V or 250V taps (low/high mode), followed by caps filtering etc.

The idea is that the preamp "stays the same", always using highest (250V tap) high voltage for maximum headroom.
But the power amp can be supplied with lower voltage.

Of course, the transformer is built at the rated maximum 250V tap current usage.

So far, I don't see why it wouldn't work. But I'm no expert, that's why I'm looking for your advice.

Thank you.

[Jebnor]

Could you draw us a block diagram? I'm a little confused as to exactly what you mean.  Draw and label all the transformer taps too that helps.

[anotherlin]

See the attached picture. Hope it makes thing clearer

The idea is to always have the preamp supplied at let's say 300V. But to be able to have the poweramp supplied at 200V (low power) or 300V (high).

I'm just wondering if it's possible to have that kind of "parallel" usage of a transformer secondary.
Especially considering the fact that this can be quite "unbalanced" : The preamp always uses at most 6mA at the 250V. Whereas the poweramp should use around 20mA at the 190V tap or 250V tap.
In the case we are in "low power mode", most of the current goes through the 190V tap.

[Richard Crowley]

Yes, you can have "parallel" (or perhaps "redundant" or duplicate) power supplies like that.
However, I don't see how you plan to use "bridge rectifiers" as you have shown.
Perhaps when you draw the full diagram without shortcuts, you will discover the problem.

[anotherlin]

Ok, I will draw the complete intended PSU tonight, when I'm back from the office !
Thank you.

[anotherlin]

Ok, here's a more detailed schematic. I didn't include the values of the filter cap but that should be ok.

My question is : What happen if I source lot of current on the 190V tap and only a few mA on the 250V one while they are on the same secondary. Is it ok ? I see no reason why it wouldn't be ok to do so. But I just don't want risk frying a transformer trying to make my point

[Richard Crowley]

No, you cannot do that.  Not with bridge rectifiers like that.
When the switch is in the low power position, the grounds are not the same and will damage the diodes and transformer.
If you want to switch like that, you will have to commit the "0V" lead to ground as shown in your first diagram.
And use half-wave simple rectificataion.

[ciccio]

No, you cannot do that...... And use half-wave simple rectificataion.

It's true: with bridges a loud bang will be the result.
Another point: with dual voltage the power amp will clip way before the preamp, and this will result in unexpected distortion. But this is a guitar amp, so who knows: maybe it is will give a better sound
Best regards