DIY Audio Projects Forum

K-12G Tube Amp Headphone Jack Output
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Author:  DisasterArea [ 04 Nov 2010, 19:20 ]
Post subject:  K-12G Tube Amp Headphone Jack Output

is there a good way to wire a headphone jack into this amp. I've seen some people split it off from the speaker terminals with a 100K resistor, is this a decent way to do it, or is there a better way?

Author:  Gio [ 05 Nov 2010, 18:27 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

You need to properly load the amp. A 10W, 8 ohm resistor across the speakers outputs will do and this properly loads the tube amp. Then run a ~120 ohm 2W resistor from the + speaker output and to the headphones.

Essentially it will be like a dummy speaker / amplifier load.


Author:  DisasterArea [ 06 Nov 2010, 00:13 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output


Author:  jerkin409 [ 25 Dec 2015, 09:01 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

Greetings folks and thank you for this great resource. I am just now getting into audio stuff and know nothing but how to solder, build one tube radios, arch top guitars and cabinets for radios and now amps. I did manage to build some 4-tube stereo kits from Antique Supply that gave me great problems until I discovered that I had ruined the output transformers by disconnecting the speakers while the units were on. To make things worse the company that makes these kits will not sell any replacement transformers for these Christmas presents. Real find folks at S-5 (?).

Anyway I good guy at another retailer that sells these kits did get a post from this forum that is no doubt the answer to my problem of using headphones with these little amps. Many Thanks to Gio!!!

But to prove just what level of novice I am, I must admit that I STILL have questions. When you say place a 8 watt 10 ohm 'across' the speaker outputs does this mean that I solder that resistor from the (+) from one side speaker output over to the (+) of the second speaker output? I know, I know, pretty stupid question but I have already had to buy 3 new kits and have run out of time and money!!

And the second stupid question relates to your second comment about running a 120 ohm 2 watt resistor from the (+) speaker output to the headphones. I am guessing that this is a simple "inline" solder from each speaker output individually to the two headset (+)'s?

I'll attach pics to see what you think of the cabinets.

Author:  laurie54 [ 26 Dec 2015, 18:25 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

jerkin. In answer to your last post. NO.
As you have found out, (the hard way) , The output tranys must have a load across them at all times. Since you want to use headphones instead of spkrs you must load the tranys, as the phones will not be enough of a load by themselves. So, you place the 8W 10-ohm res from spkr + to spkr -. One for each chnl. That satisfies the trany loading problem.
Next dealing with the phones.
The spkr outs will output way more signal than the phones can deal with so you need to reduce the signal getting to the phones. This is done with the 120-ohm res in series with each headphone spkr.
What you will have is one 10-ohm res from spkr+ to spkr- (per chnl). and
one 120-ohm res from spkr+ to headphone input for each chnl.
This will allow headphones to be used with this amp. When switching back to spkrs however you must remove all theses items and replace them with the spkrs.
You should now realize that it is very important (and cheeper) to "NOT" do any of this switching while the amp is turned on.

Author:  jerkin409 [ 27 Dec 2015, 10:16 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

Well, I guess one couldn't help but understand the instructions now! I really appreciate the help and your time taken to explain these things to me. It is strange how helpless ignorance can render a person....especially after he had just lost $600+ by not understanding the notion of "keeping a load on those output transformers".

I did install the 120 ohm resistor inline to the headphone jack last night (BUT WITHOUT DISCONNECTING THE SPEAKERS!!) and the headphones sounded incredible! Thank you so very much. But I had to order the 8 watt resistors before I can deal with the speaker project as you described it for me. But now I have another question about that. Will the speakers function with the 2 ohm 8 watt resistors in place? My word!, this really does expose my ignorance does it not?

Author:  laurie54 [ 27 Dec 2015, 20:59 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

My oh my how muddled up things can get> (me included) LOLLL So you are 1/2 way there to done.
I can only hope you have bought 8-ohm or 10-ohm resistors not 2-ohm. I suppose the best would be 8-ohm which take the place of the spkrs. To answer your last question, again NO. Only the resistors (8-ohm each) OR the spkrs would be connected at one time. Remember, the res are taking the place of the spkrs, presenting the appropriate load for the tranys. And "do not" change from the spkrs to the res/headphones while the amp is operating.
Sooner or later, surfing threw diff threads on this and other DIY sights you will see amps with 1/4 phone headphone jacks which are the "locking type". This type of jack will not allow the phones to be unplugged accidentally. You now know why they are used.

Author:  laurie54 [ 27 Dec 2015, 22:06 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

I'm sorry for your loss jerkin. That's a steep pill to swallow. Little comfort but also know your are not alone. I lost count years ago with even the smaller (but they all add up). Coming home with $40Can in mono jacks thinking i had stereo. No returns. Dam these eyes. Thankfully it's still more than worth the fun, pride, excitement, ingenuity, achievement, the desire, and birth of knowledge, on looking down at that square box with hot glass and a glow passing on the song that keeps us all going. Merry Christmas everyone.

Author:  jerkin409 [ 02 Jan 2016, 23:50 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

So can you switch to the 10 ohm 8 watt circuit between the speaker leads and then plug in the headphones or did someone up there just say not to do that? Boy this is confusing. I should have stayed with designer radios....much simpler!

Not only that but thus far I have been completely unable to locate 10ohm 8 watt resistors in any supply house I can think of. Any suggestions?

This information, by the way, is a God-send for me.....I'd be in much worse shape than I already am had it not been for you unselfish folks.

Author:  mike567 [ 03 Jan 2016, 09:30 ]
Post subject:  Re: K-12G Tube Amp Headphone Jack Output

Use 10 ohm 10 watt if you cant find 8 ohm. Will be close enough

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