PP triode transformer load

[Thomasdj]

Hi, I'm currently trying to design af push pull triode amp with inspiration from ZVex's IMPamp. That mean that I am using the dual triode 6021 as output tube and LED cathode bias. This is my first time built from scratch and I am trying to figure out what output transformer I need.

I recently bought a pair of OT's rated 5W with a 4ohm secondary and a 10kCT primary winding. I plan to use 8ohm speakers and guees that this will reflect back an 20kCT load.

The sylvania 6021 datasheet shows 165 maximum plate voltage - I plan to use 160V. I've read at valvewizard.co.uk that a blue LED drops 3 voltage at the cathode.

I've attached the plate curves with my load lines - will this setup work or am I pushing the tube to hard?

Best regards

Thomas

[rman]

Assuming the red line is maximum plate dissipation, I would say back off a bit.

Cheers.

[Thomasdj]

Originally Posted by rman

Assuming the red line is maximum plate dissipation, I would say back off a bit.

Cheers.

Hi rman, thank you for your reply

yeah, the red line is the maximum plate dissipation. I plan to operate the triodes in class A - should I lower the plate voltage?

[Arnulf]

Originally Posted by Thomasdj

Hi rman, thank you for your reply

yeah, the red line is the maximum plate dissipation. I plan to operate the triodes in class A - should I lower the plate voltage?

There are so many things wrong with your picture (considering your description) that I don't know where to start

First of all, what kind of load do you intend to drive ? Unless it's some extremely efficient speaker or curiously low impedance headphones you're probably going to be dissapointed by the sub-1W output power.

If you intend to pursue this idea further, calculate correct load impedance first, plot correct loadline (above it says class A, pictures shows class AB), figure out how you're going to drive your tubes (whether you're going to dip into positive grid voltage area or not) and then determine your operating point (which mostlikely isn't going to be anywhere near 160V cathode-anode voltage if you want any kind of signal out, considering dissipation limit).

[Thomasdj]

Originally Posted by Arnulf

There are so many things wrong with your picture (considering your description) that I don't know where to start

First of all, what kind of load do you intend to drive ? Unless it's some extremely efficient speaker or curiously low impedance headphones you're probably going to be dissapointed by the sub-1W output power.

If you intend to pursue this idea further, calculate correct load impedance first, plot correct loadline (above it says class A, pictures shows class AB), figure out how you're going to drive your tubes (whether you're going to dip into positive grid voltage area or not) and then determine your operating point (which mostlikely isn't going to be anywhere near 160V cathode-anode voltage if you want any kind of signal out, considering dissipation limit).

Hi Arnulf, thank you very much for your reply!

The class A/AB are just me trying to understand

I've read the push pull article at valvewizard.co.uk - he use an example ..if we bias hot enough (i.e., push the Class A load line up far enough) it will become pure ClassA. What have I misunderstood here?

I don't want to use any extremely efficient speaker I just want to built a push pull amp with the 6021 tube and my already bought transformer.

However, I like the idea with LED biasing but I am not sure if i can use my transformer.

Best regards

Thomas

[Thomasdj]

Originally Posted by Arnulf

First of all, what kind of load do you intend to drive ? Unless it's some extremely efficient speaker or curiously low impedance headphones you're probably going to be dissapointed by the sub-1W output power.

oh..btw, I'm not expecting a huge sound and I plan to use some desktop speakers....

[Arnulf]

Originally Posted by Thomasdj

I've read the push pull article at valvewizard.co.uk - he use an example ..if we bias hot enough (i.e., push the Class A load line up far enough) it will become pure ClassA. What have I misunderstood here?

That article is a great starting point !

Your picture shows "broken" loadline, indicating that one device cuts off at some point. This is class AB operation (one device cutting off after certain output excursion). As the line you quoted says: if you bias it hotter, both devices will conduct 100% of the time, hence no break in the loadline.

I don't want to use any extremely efficient speaker I just want to built a push pull amp with the 6021 tube and my already bought transformer.

Well but you must have some sort of speaker that you intend to hook up to it ? Depending on its efficiency you might not be happy with the result ... I mean less than 1W into average non-efficient speaker really isn't much power unless you are in a very quiet room and always listen to content of roughly same level of volume (like radio).

However, I like the idea with LED biasing but I am not sure if i can use my transformer.

LED is just a nonlinear resistance (unlike real resistor). If you understand cathode biasing with resistor, then you understand LED biasing as well

Figure out what kind of laod you intend to drive and what kind of power level you want, then plot the laodline correctly and see whether you can get desired result with selected parts. It's impossible to guess whether your transformer can work out to your satisfaction without actually knowing what you're trying to accomplish.

You also have to consider your preamplification stage (or input level) and phase splitter.

[Thomasdj]

Originally Posted by Arnulf

That article is a great starting point !

Well but you must have some sort of speaker that you intend to hook up to it ? Depending on its efficiency you might not be happy with the result ... I mean less than 1W into average non-efficient speaker really isn't much power unless you are in a very quiet room and always listen to content of roughly same level of volume (like radio).

Figure out what kind of laod you intend to drive and what kind of power level you want, then plot the laodline correctly and see whether you can get desired result with selected parts. It's impossible to guess whether your transformer can work out to your satisfaction without actually knowing what you're trying to accomplish.

You also have to consider your preamplification stage (or input level) and phase splitter.

I'm not sure I understand anything of this

The amp is intended to stand in my little kitchen with small speakers..so i guess 1 watt will suit perfectly..I plan to use small average 8ohm speakers, - that will give 20k plate to plate load with my transformer.

The blue line is the 1/2 x 20k loadline at 160V.
The purple is the same loadline "LED-biased" at -3v at 160voltage.

Am I wrong??

[Thomasdj]

Originally Posted by Arnulf

That article is a great starting point !
You also have to consider your preamplification stage (or input level) and phase splitter.

I plan to use a mp3 player as a preamp

[Arnulf]

Reflected load impedance doesn't scale proportionally with the number of turns ratio - it scales with ratio of number of turns squared.

MP3 players usually put out less than 1V RMS (my player outputs around 0.5V RMS) so it won't be much of a preamp. With Vg1 = -3V you'll want at least 2V RMS in. Assuming your phase splitter (which we still haven't seen mentioned) doesn't provide any gain you're looking at less than 0.5W otherwise.