A 6SN7 direct coupled preamp & regulated supply for everyone's perusal.

[G]

Hi all. I wanted some opinions and vetting of a preamp & supply. Here ya go:

[G]

Here is the regulated supply:

[ray_moth]

Suggestions for the preamp:

I'd connect a resistor of, say, 1Meg between the slider of the volume control and ground, to prevent unpleasant noises in case the track has imperfections that could cause momentary open circuit.

With the resistor values and B+ voltage shown on your schematic, the plate current in each triode will be only about 2.5mA, which is rather low. As you say, the plate voltage of the first triode will be 75v, which I also think is too low.

If you can increase B+ to 300v, using the same resistor values you will have a plate current of 5mA in each triode and a plate voltage on the first triode of 150v. This is a better operating point and both triodes will be dissipating only 750mW, which is well within their ratings.

Of course, there are all sorts of clever things you could do with a 6SN7 to achieve similar ends, including SRPP and mu-follower. My suggestions are just aimed at improving the results with your own perfectly good topology.

Suggestions for the PSU:

If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA. This give 350v with a cap inout filter but will only let you draw 30mA (you can only use 60% of the transformer's AC current rating with a cap input filter). That's not going to be enough. I would get a 250v tranny with 100mA rating.

You will need two OD3 VR tubes in series in each regulator. Driving the VR tubes from a constant current source, as you have in your schematic, is a nice refinement but probably not really necessary unless your line voltage is highly variable.

I can't comment much on the CCS, since I don't know the device you're using, except to say that you need to set it to deliver 20mA for each channel. That allows 10mA for the preamp and another 10mA for the VR tubes in each channel. 5mA is the absolute minimum current for an OD3 to work so, to allow for low line voltage, you need to feed it more than that under 'average' conditions. It is happy with up to 40mA, so 10mA is a reasonable figure. That's why you need a bigger tranny.

[hotbottle]

If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA. This give 350v with a cap inout filter but will only let you draw 30mA (you can only use 60% of the transformer's AC current rating with a cap input filter). That's not going to be enough. I would get a 250v tranny with 100mA rating.

I was just interested in this calculation. Is it just a rule of thumb? and is it dependent on the type of rectifier? Also, what causes the limitation, inrush current of the cap?

[stoolpigeon]

How is it direct coupled?

sp

[hotbottle]

No coupling cap between U1 and U2

[zigzagflux]

Originally posted by ray_moth

If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA.

Hello Ray:

I have to disagree with you on this one. The transformer is rated 40VA, and comes with a 6.3V 2A aux winding. That leaves 27.4VA left for the secondary, which at 250V is 110mA. It looks like its rated for a full bridge operation.

Also, he actually doesn't have a cap input filter, it's more of a hybrid, riding the fence between choke and cap input. His transformer current is about as nice as you can get without the harsh turn-off of the choke input supply. So the transformer looks pretty perfect to me.

Of course, if he pursues a 300V supply instead, it would be useless, but looks good for 150V. PSUD confirms, at least.

Cheers.

[ray_moth]

Sorry, zigzag, I misread the schematic! I now see that the cap is, indeed, 0.33uF, not 33uF. In that case he has a choke-input filter, to all intents and puposes, and he must be using the full winding already. Becasue it is a choke-input filter, he can probably get 225vDC @50mA.

A pair of OC3 VR tubes in series would give 210v B+, allowing 15v to be dropped across the CCS and providing for a somewhat better op point for the 6SN7s than a 150v B+.

[G]

Originally posted by ray_moth
Sorry, zigzag, I misread the schematic! I now see that the cap is, indeed, 0.33uF, not 33uF. In that case he has a choke-input filter, to all intents and puposes, and he must be using the full winding already. Becasue it is a choke-input filter, he can probably get 225vDC @50mA.

A pair of OC3 VR tubes in series would give 210v B+, allowing 15v to be dropped across the CCS and providing for a somewhat better op point for the 6SN7s than a 150v B+.

I don't understand what you are saying Ray. The transformer is 125 - 0 - 125 @ 100mA not 50 mA. The transformer does not have two 50mA windings on the secondary. It has one center tapped 100mA winding. Am I missing something?

[ray_moth]

What it is, G, is that the 100mA rating assumes that you willuse the tranny as it was designed to be used, with the center tap as negative and a diode on each end of the winding to give the positive. This will give 125v @ 100mA.

Using the full winding with a bridge rectifier, as in your schematic, will give twice the voltage but only half the current, so you get 250v @ 50mA.