12AT7 Headphone amp

[J_Hart_14]

So i was doing some back of the paper calculations and needed a little help. See the pdf. I was thinking of just doing a transformer coupled 12AT7 headphone amp. I was having a bit of trouble with the Turns ratio on the transformer.

Also any comments would be helpful. I know that I have this tube running as hot as possible but I was going to adjust but I was having problems with the transformer ratio.

[DF96]

Use the load line, not plate/anode resistance. Your aim is to present the load line resistance you have selected. This may be lower or higher than the anode resistance.

[J_Hart_14]

Has anyone made such an amplifier with any success? I was thinking I could get about .42 watts out of it minus transformer losses. Do you think a standard input will be able to drive it just fine? Using the load line a 2 v peak to peak would produce the .42 watts. what about a transformer with a 21.9 TR for that low a wattage?

[dsavitsk]

12AT7 is a terrible choice here. You want the primary impedance of the transformer to be several times higher than the rp of the tube, so if the tube's rp is 11K, the transformer should be ~50K. A 50K OPT is not really possible.

If you want to transformer couple a dual triode, try something like a ECC99, 6H30, 6N6p, 5687, or possible ECC88 instead.

[MrCurwen]

It's not a dual triode, but my favourite miniature is the 6C45P, although it needs a bit more current (20mA to 30mA) to sound its best. It has a mu of at least 40, and Rp of only a kilo ohm or so, so a 5K primary will do fine, 10K even better.

[DualTriode]

Hello,
I have done several versions of what you are discussing, SET headphone amplifier. The two I liked the best were the 6BX7 and triode connected 6BQ6GTB. The transformer is an impedance matching device the turn ratio will depend on the rp of the tube and headphones. I tried an ElectraPrint 8000 ohm to 300 ohm 3 watt transformer (40ma) and an Edcor 5000 to 300 3 watt transformer.
If you are interested I will look up the Edcor transformer model number. After paying the one custom fee they were about $20 a piece. The B+ voltage does not need to be too high to drive the limited voltage swing required to produce a half a watt output. For a power transformer I used a Triad N 51X isolation transformer.
DT
All just for fun!

[J_Hart_14]

Well I contacted edcor to see if they had a transformer to match what i was looking for. Also if they have the design on the shelf then they get rid of the custom fee (which is about $40) I will see about the 6BX7 and 6BQ6TB. I guess to reduce the primary impedance of the transformer i could also parallel a couple of the 12at7 triodes. Surly they have a transformer with the proper turns ratio. As long as the Tr is correct then it should produce the proper primary impedance.

Would you mind looking up the Edcor part number? Did you perhaps try the 12AT7, the only reason is i ask is I have about a dozen on hand. Thanks for the comments.

Also, why dsavitsk do you say that the primary impedance should be in the 50K range? Did i miss a rule of thumb or something?

[ikoflexer]

Seems to me awkward to go to such lengths to get a custom transformer when you can so much easier choose an adequate triode. I build a headphone amp around the 6c45p triode, and it sounds great. Not that many tubes fit the one tube amp scenario. To each his own though.

[DualTriode]

Hello,
That Edcor transformer is in my hand it is GXSE3-300-5K it has a 40% UL tap if you want to use it.
DT
All just for fun!

[dsavitsk]

Originally Posted by J_Hart_14

Also, why dsavitsk do you say that the primary impedance should be in the 50K range? Did i miss a rule of thumb or something?

Originally Posted by dsavitsk

You want the primary impedance of the transformer to be several times higher than the rp of the tube

That one. Same reason the plate resistor is 10x rp, or you use a CCS. The higher the load the better. If it is too low, you will get a lot of distortion. But, as I said, it is not really possible to wind a (good) transformer with a 50K primary Z.

Tubes are cheap. The fact that you have some 12AT7's should not limit you to them. You are going to spend hundreds of dollars to save $6 and end up with an amp that performs poorly.