Working on a power amp schematic

[mort]

Yeah I meant the grid, like what shows in the drawing.



Forgive my ignorance, but what do the grid and screen resistors provide to the circuit?

[Dave H]

Corrections.

The schematic will look better if you flip the bottom 6V6 symbol. Right click on its outline and select 'Flip component' ' Right to left'

[mort]

^^ that does clean it up a little. thanks.


I've got the model set up enough to read unloaded DC, PT output, and power the filaments for the output tubes.


Any input on how to choose the 9 pin tube? 12ax7, 12au7, 12at7, 12bh7a or 6bk7b

[J M Fahey]

Mort, just build a

5C3 deluxe

and call it a day.
YOU ARE NOT LEARNING ANYTHING THIS WAY.
Walk before you run.
I shudder when I follow what you are trying to do.
Unfortunately you know *just enough* to be dangerous to yourself.
Plus your reckless attitude.

You don't even know the parts *names*, let alone what they are for, even less parts values, etc.
Don't even ground the filter caps, how's that?

[jazbo8]

You may get more comments if you could post the pictures instead of linking to them. Schematics without part values are also not very useful for discussion.

[mort]

Actually I've learned quite a bit in the last few days.

Including how to use a schematic program, thanks for the link btw. I'm really enjoying it.

Sure I'm ignorant to many many aspects of electronics. But keep in mind that 95% of this endeavor is taking place on paper or on the computer. It's more about the studying and asking questions. I do have the partially assembled model but I'm not implementing parts until I've done the studying to do it proper. So I don't agree with the statement that it's "dangerous"

And as for the cap and resistor values, I'll be adding that data as I've learned and studied how to calculate what they should be for the desired effect, and why.



I can touch more on this later. Gotta get back to work for now....

[mort]

You may get more comments if you could post the pictures instead of linking to them. Schematics without part values are also not very useful for discussion.

Does this work? I'm not sure if it expands enough to see it well.





So I've discovered that the 6x5 recto won't supply enough current for 2 6y6's so I've chosen the 6ax5 instead. It draws twice the heater current though, putting me right up to the 4a rating of the 6.3v supply of the PT.



The 6ax5gt will supply up to 150mA of DC current.

Let me see if the math works out. Correct me if I've skipped anything.

6y6 plate diss. = 12.5w
let's use max voltage of 200v
that's 62.5mA (X2) = 125mA

6y6 screen diss. = 1.75W
Let's say 200v
that's 8.75mA (X2) = 17.5mA

12at7 plate diss. = 2.5w
@ 300v
that's 8.3mA (X2) = 16.6mA


Total is 159.1mA

With the output tubes biased down to about 55mA, that would put the total at around 144mA and 11W per tube. Sound reasonable?



Here's a photo of the test model as it sits right now.

[jazbo8]

Not quite, even though the number may come out close to what you came up with... First, since this is a push-pull amp, you need to estimate the maximum current, which occurs at the lowest plate voltage, so say 150mA. The screen current is usually a small portion of that, say 20%, or 30mA. For preamp tubes, you never run them close to the maximum plate dissipation, so figure 2mA per section, so the total is about 184mA, round it up to 190mA.

Also as I posted over at diyaudio, Eli mentioned the 12AT7 would not give you enough gain, so you may need to add another gain stage before it.

[mort]

I don't want to get too ahead of myself worrying about the 12at7, right now anyway. It's basically there as a marker for now, giving me data to make calculations from.


I've been playing around with that PSUD program, trying to familiarize myself with it. I'm trying to target <200v to the 6y6 screens, and 200-300v to the plates of the 12at7.

I added in an RC filter between the rail and the 6y6's to get that screen voltage down. Hopefully that's a reasonable way to do it. But how do I go about manipulating the plate voltage to the 6y6's?

[jazbo8]

Did you read tubelab's comment on diyaudio? He mentioned that the screen voltage needed to be kept under-control - so 135V is what you should shoot for, as for the plate, it is usually the first node on the PS, so you typically just select the power transformer with the correct secondary voltage and current ratings (PSUII is great for that), to hit the desire plate voltage. The RC filters just knock down and filter the voltage for the following nodes.