Power tube theory - I'm failing at it.

[Urumiko]

HI all,

I was looking for some help in plugging a gap in my knowledge.
For some time now i've been struggling to get beyond just following suggested designs and data sheets and fully understand the interplay of values when designing a power amp. I have at my disposal Merlin Blencows book on preamps, and Igor S popovich, how tube amps work.

The concept is - If i have an idea of the output wattage I want from an amplifier. and i select an apropriate tube and transformer.
How do i estimate/manipulate the wattage of the resulting amp.

In this case i'm looking to make a single ended amp with roughly 2-3w output.

I'm told runing a 6v6 or el84 at 180v in to a 5k traf will get me there.
But im failing to find the calculatons that id need to make to confirm this.

Any doodling arround the plate voltage and plate current come out with 5-6 watts by my calculation. but im guessing that would refer to DC current where as id need AC current?

Can anyone point me in the right direction in terms of what id need to consider to estimate outputs in this way when designing an amp?
I'm guessing i need to calculate the AC voltage and variance in current at the annode somehow?

[alexradium]

once you choose the right bias value for the tube at your plate voltage and primary impedance,you're done,you have the maximum clean power allowed to a class A amp,which,despite all that you can read everywhere,is up to a third of the tube power.

[eschertron]

http://bmamps.com/ivds.html

using this online calculator, I'm getting just under 2W audio power, and about 6W dissipation. As Alex suggested. Is there a reason that you are limited to a B+ of 180V?

And all this begs the question: is this a HiFi amp that needs plenty of headroom, or a low-wattage guitar amp designed to be pushed into clipping?

[Urumiko]

http://bmamps.com/ivds.html

using this online calculator, I'm getting just under 2W audio power,

If its not too much trouble Could you possibly walk me through the values you have selected,
and where you are getting the resulting audio power figure from?

The Goal is to construct a small characterful but not high gain guitar amp inside a vintage radio cabinet for low volume use.
I want to gift it to my dad, we often jam at low volumes, my 5w blackstar typically does just fine on about 1/3 volume.
I'd like to put together something that tlooks nice sat on the shelf and will serve as a basic guitar/mic amplifier.
The rough plan was to take my femiliar champ layout, limit the power stage output down to 2-4w, then add a 2nd preamp channel that would handle a mic.
If all that goes well i was considereing trying to jimmy in the gubbins from a cheap digital reverb on a paralel send from the preamp channels too.. but first things 1st

So far I have only rebuilt/tweaked existing amps with basic fender champ esq designs. I'm always fine with understanding what is in front of me but when it comes to designing from scratch i get pretty lost pretty quick.

Hammond 125BSE & 260C transformers already en route, will be rectifying with diodes.

[Dave H]

Can anyone point me in the right direction in terms of what id need to consider to estimate outputs in this way when designing an amp?
I'm guessing i need to calculate the AC voltage and variance in current at the annode somehow?

The maximum peak voltage swing is 180V = 127V RMS so power is 127^2 /5000 = 3.2W (Ignoring losses)

The current for centre bias is Vpk/Rload = 180/5 = 36mA

Allowing 40V for losses (tube 'knee' voltage, OT resistance etc) reduces the peak voltage swing to 140V and the power to 1.96W.

[Urumiko]

The maximum peak voltage swing is 180V = 127V RMS so power is 127^2 /5000 = 3.2W (Ignoring losses)

The current for centre bias is Vpk/Rload = 180/5 = 36mA

Allowing 40V for losses (tube 'knee' voltage, OT resistance etc) reduces the peak voltage swing to 140V and the power to 1.96W.

Thats magic dave thank you.
you are not the dave who goes by the handle tubeman61 are you?

cheers

[Dave H]

you are not the dave who goes by the handle tubeman61 are you?

Not guilty