question about transformers

[whomeno]

I'm looking to build a real mctube. I have a 12v -a 16 volt and a 18 volt transformer.
Can I use the 16 volt and drop it to 12 some how to use it?
The mctube calls for 2- 12v

[BetterOffShred]

If you just need 12VDC you can use a LM7812 with some pre and post filtering..  I guess I'm not sure what a real McTube is

[whomeno]

here is schmatic also my 12 v died and 18 volt no good. can i do this with just 1 transformer some how?

[Rob Strand]

here is schmatic also my 12 v died and 18 volt no good.

It might have died because the circuit is pulling too much current for the wall-wart.

I'm looking to build a real mctube. I have a 12v -a 16 volt and a 18 volt transformer.
Can I use the 16 volt and drop it to 12 some how to use it?
The mctube calls for 2- 12v

Is the 16V marked on the device or is that what you measured?
If you measured 16V with no load you might find it drops a lot when connected to the circuit.
You can tweak the 12ohm resistor to get 12V on the filaments, or use a regulator.
You might need a small heatsink on the regulator.

It depends very much on the specifics of the transformers/wall-warts.

[whomeno]

ok If I would go with 1 Transformer. instead of two. what would i need to get

would this one work to do it all, instead of two      https://www.tubesandmore.com/products/transformer-power-125-v-15-ma

[thermionix]

It might work, what are your current requirements?

[whomeno]

2-120v/12v 0.5a
want to go to 1 transformer

[Rob Strand]

ok If I would go with 1 Transformer.

It's a much cleaner solution if it works.

The regulation of the transformer is unknown but it's likely to be a bit worse than a 500mA transformer.
You may have to tweak R2 to get the heater voltage right.  The heater current for the ckt is 150mA but you probably will need a 300mA transformer because of the losses associated with rectifying and filtering the rail to DC.

The +B is going to be a bit high, perhaps 125 * 1.4 = 175VDC.  Loading the 12V winding will cause it to drop.
However the initial voltage could be upto 20% on 175VDC depending on the regulation and how the loading on the 12V winding affects the 125V rail.     If the +B rail is high I recommend adding a resistor between the diode bridge and the C1.    This reduces the voltage on C1, C2 and +B.   You don't want to go over 150V or so for C1 and C2.  Increasing R1 would drop the voltage on C2 but not C1, so that why an extra resistor is necessary.   The current on the +B is very low.  A 1mA winding would cover it.

[thermionix]

If you want simpler, since it's a 12AX7, you can run the heaters at 6.3VAC humbucking.  It won't be as quiet as DC, but in many cases quiet enough.  More power transformers to choose from.  DC heaters has become fairly popular with high gain amps due to the low noise, but I've had to repair a lot of failures.

[PRR]

You want line isolation with *TWO* output voltages, 6V/12V and 140V.

If you can't find a dual-secondary transformer (ones like this are not common), then two transformers is THE way to go.

The original design is as cheap as possible. Don't try to be cheaper.

[iefes]

You could also go with a SMPS based charge pump. This would allow you to run the whole thing off a single 12V transformer (or wallwart). I've used one of those with success in a blackface-style tube preamp with one 12AX7.

[amptramp]

There are a number of transformers available that have two primaries that can be connected in parallel for 120 VAC input or in series for 240 VAC input.  If you use one of these, you can use only one primary winding as the 120 VAC input and the other as the HV secondary.  The normal output for the filament would give less than half the power it is rated for but that may be enough.  I see a lot of these transformers as low-profile PC-mount types.  The primaries and secondaries are usually widely separated so there is little capacitive feedthrough from the power line to the supply.

In most cases, the improvement from DC filaments is negligible.  Rectifying and filtering the filament supply introduces harmonics of the 50/60 Hz input so unless you are prepared to use a regulator, you may be better off with AC, especially for a guitar amp where the power supply input frequency may be below the flat part of the passband.  All the harmonics would be within the passband.  Run the heater wiring as a twisted pair and make the average heater voltage high - biasing the heater at +20 to +30 volts above ground keeps filament emission from affecting the sound because the grid is near ground and repels any electrons.