Calculating Plate Voltage

[minions]

How do you calculate the plate voltage of the power tubes in a given circuit?

Thanks.

[sumgai]

minions,

Short answer - you don't.

Long answer - you start with the tube manufacturer's recommendations, and work from there. If you want/need lower power than the tube can supply, then it's a safe bet that you can lower the B+ voltage. If you want/need more power, then you'll have to take (or more) of four paths:

- raise the voltage beyond the tube's design spec;
- use more tubes;
- use more efficient speakers; or
- give up and be satisifed with what you've got.

Seriously.

What happens is we start with a desired output wattage, say 50 watts, just for drill. We then decide what tube to use, and how many. For our purposes, the book says that a 6L6 or an EL34 (6CA7) can give us 25 watts, each, in a certain configuration. Let's choose the EL34, and take a pair of them.

OK, the book says that in order to get the desired wattage, we'll need to feed the tube about 420 to 450vDC (that's the B+ voltage). At this point, we select a transformer that can handle the power we expect, we select components to give us the right bias voltage and so on, and we're all set.

Obviously there's a lot more to it, but in a nutshell, that's it. We can experiment if we want to, but that will be from our known starting point, not just from out of the clear blue.

Now, what did you have in mind?

HTH




sumgai

[minions]

Okay, so how do you raise the voltage?

[stevo58]

Okay, so how do you raise the voltage?

You either change the rectifier - to one that drops less voltage - or go from a tube to a solid-state rectifier, or change the power transformer.

HOWEVER, even changing to a different rectifier tube, you better know what you are doing. The power tube anode voltage is usually taken from the first power supply node (sometimes the second) so in raising that voltage, you will raise the voltage of every node down the line, which means every tube down the line. Which potentially means you will have to change all the dropping resistors, too.

FURTHERMORE, some rectifiers (for example, the commonly-used 5U4) use significantly more heater current than others (say, the commonly-used 5Y3 and 5AR4) and the power transformer may not be built to deliver it. And, you don't want to exceed the voltage rating on the filter caps, remembering that when you first switch on the amp, there is a voltage spike up to about twice the rated voltage of the transformer.

THEN AGAIN, some amps (for example, the Deluxe Reverb) already drive the tubes way past spec, and putting even more voltage on them is of questionable wisdom.

So, why do you want to do this? To get more clean headroom, or more volume? Then it makes more sense to go to a more efficient speaker, and leave the guts of the amp alone. Otherwise, imho, you need a different amp.

steven