Convert Super Twin To Headphone Amp?

[tbryanh]

I want to remove the power tubes from the super twin and install an output transformer at the the splitter to drive headphones.

Any ideas how to do it e.g. what transformer to use, how to wire it, etc.?

[olddawg]

I want to remove the power tubes from the super twin and install an output transformer at the the splitter to drive headphones.
Any ideas how to do it e.g. what transformer to use, how to wire it, etc.?

Wow...talk about hitting a nat with a sledge hammer. I know there are some learned people here that can probably tell you how, but just from a time, expense, and functionality perspective, you might be better served just getting a small moddling amp with a headphone out.

[tbryanh]

Everybody has their quirks.

[tbryanh]

After removing the power tubes, it looks like I can remove the lead from the circuit on one of the 0.22 uf coupling capacitors and connect it to one end of the primary of an OT and connect the other end of the primary to ground.

The 0.22 uf capacitor is the one between the splitter and power tubes. The lead to be lifted is the one connecting the capacitor to the 1500 ohm blocking resistor.

Valvewizard says the ideal load for a cathode follower is twice ra. The Valve Wizard -Cathode Follower

A data sheet for the 12AT7 says ra is about 11K http://frank.pocnet.net/sheets/093/1/12AT7.pdf, so the primary impedance ot the OT should be about 22K.

Looks like the Hammond 1750A transformer will work. It has a primary of 22,800 ohms and a secondary of 8 ohms. Hammond Mfg. - REPLACEMENT - Tube Guitar Amplifier Transformers & Chokes

[tbryanh]

Valvewizard shows the other lead of the coupling capacitor as being connected to the cathode. Maybe the other lead of the coupling capacitor in the twin should be connected this way too?

[Ptron]

Looks like the Hammond 1750A transformer will work. It has a primary of 22,800 ohms and a secondary of 8 ohms.

I don't know the proper way to make a headphone out but I believe headphones, unlike speakers, are generally high impedance, like 10k or something, so I think you're barking up the wrong tree trying to get an 8ohm secondary.

[tbryanh]

I had a vintage pair of headphones that were 8 ohms.

Now I have an AKG pair that are 55 ohms.

[Ptron]

I believe headphones, unlike speakers, are generally high impedance, like 10k or something

Heh, heh. I was a little bit off on the numbers (must've been thinking of pickups or something) but headphones apparently are generally higher impedance.

From the first site that came up on Google:

Low impedance headsets may vary from 75 ohms up to about 150 ohms. Phones in this impedance range may be directly plugged into the headphone jack routinely found on recording and playback equipment.

Higher impedances sets normally have an impedance of around 600 ohms

[Albert Kreuzer]

Higher impedances sets normally have an impedance of around 600 ohms[/I]

So, what about leaving the power tubes in and driving 150 pairs of headphones with them?

[olddawg]

When I did it with a smaller amp I simply used an 8 ohm load resistor across the output (with the finals in) and used a voltage divider in parallel like a line out set up for the headphone load, although that may not work so well with this beast. I did it to use a largish amp to drive multiple headphones in a low rent studio. You would need a beefy resistor (or bank of resistors/ if you cranked it. You could also take the line out or effects send (you were planning on pulling the tubes, right?) of the amp and Y cord it into a stereo amp with a headphone jack.

[tbryanh]

So, what about leaving the power tubes in and driving 150 pairs of headphones with them?

Might work in a movie theater that has front row seating reserved for the hearing impaired.

[tbryanh]

You could also take the line out or effects send (you were planning on pulling the tubes, right?) of the amp and Y cord it into a stereo amp with a headphone jack.

It ain't got no stinking line outer or effects sender when the power tubers are pulled.

[olddawg]

It ain't got no stinking line outer or effects sender when the power tubers are pulled.

Sorry, didn't look at the print until now. Just tap it off of the Master Volume. Should be a lot easier than re-engineering the power amp.

[tbryanh]

Sorry, didn't look at the print until now. Just tap it off of the Master Volume. Should be a lot easier than re-engineering the power amp.

Yeah that looks better and with the different tube impedance and the 55 ohm headphones, it looks like the 1750A transformer will work.

[Steve Conner]

This badboy will turn anything into a headphone amp, no pulling of power tubes required.