KOSS A1220 - Vintage Tube Headphone Amp - Diagnose and Mod questions...

[Pano]

I agree with Eli, best would be to tear it apart and rewire it better. It's a small, simple amp, so not a huge chore. Worthwhile as a learning exercise, too. Adding a mains ground to the chassis should certainly help. It makes a big difference to my vintage 2 prong tube amps.

I do worry about the full bridge rectifier. Isn't that going to double the B+ voltage?
What is the B+ voltage now? Have you measured it?

[Eli Duttman]

Switching to full wave rectification doubles the ripple freq., not the nominal rail voltage. Remember, it's a cap. I/P filter. Rail regulation and ripple level improve, tremendously.

Look at the OEM parts list. The filter caps. are shown as 150 WVDC. The OP did well to use higher rated stuff for replacement.

[the fuhz]

Originally Posted by Don Hills

The hum might be coupling from the power transformer to the output transformers. Is the hum still present if you run the amp with the tube unplugged?

Yes, the hum is still present without the tube. Is that a sure sign of anything?

[the fuhz]

Originally Posted by TubeHead Johnny

The power supply resistor R5 is 1.5K on the schematic, but in the picture it's 4.7K. That might be part of the bass problem. Although bringing up the voltage might also bring up the hum.

Good eye, I'll swap that out and report back.

[the fuhz]

Originally Posted by Eli Duttman

Switching to full wave rectification doubles the ripple freq., not the nominal rail voltage. Remember, it's a cap. I/P filter. Rail regulation and ripple level improve, tremendously.

Look at the OEM parts list. The filter caps. are shown as 150 WVDC. The OP did well to use higher rated stuff for replacement.

Originally Posted by Eli Duttman

Switching to full wave rectification doubles the ripple freq., not the nominal rail voltage. Remember, it's a cap. I/P filter. Rail regulation and ripple level improve, tremendously.

Look at the OEM parts list. The filter caps. are shown as 150 WVDC. The OP did well to use higher rated stuff for replacement.

Thanks for all of the suggestions, however I am little intimidated because I don't know nearly half of the changes you are talking about. I'm new to this field.

First things first. How do you recommend properly upgrading to a three prong plug? I assume that I clip out the 'death cap' and connect the new ground wire to the chassis? Should the positive lead then go to the on/off switch? and the neg go to the power transformer?

[Eli Duttman]

Thanks for all of the suggestions, however I am little intimidated because I don't know nearly half of the changes you are talking about. I'm new to this field.

If you "go under the hood", a knowledge of how things work is important. Google NEETS, which is U.S. Navy training material and available gratis on the WWW. Tubes or SS, some grounding in theory is needed to be an effective tweaker, let alone designing from scratch.

[Don Hills]

Originally Posted by the fuhz

Yes, the hum is still present without the tube. Is that a sure sign of anything?

It's a pretty good sign that the magnetic field of the mains transformer is coupling to the output transformers. Headphones matching the amp in age tended to have poor bass response compared with modern phones, so it probably wasn't a problem back then. It's hard to tell from the picture but it looks like the mains transformer is on the top of the amp. Is it aligned the same as the output transformers? Look at the lines formed by the metal cores. The output transformer cores are aligned "front to back". If the mains transformer is aligned the same way, you can try turning it 90 degrees it so that its core is at right angles to those of the output transformers. You may be able to remove the mounting screws and carefully turn it to see if it makes a difference.

But certainly do as the others suggest - FIRST, remove the "death cap" and replace the mains cable with a 3-core one before you make ANY other changes.

[Eli Duttman]

Don, from the photo, it looks like the power trafo lam stack is at 90^o to the O/P trafo lam stacks.

Fuhz, tell us what the sheet metal is made of. If it's steel, that could be coupling the magnetics' windings

[Don Hills]

Huh? Oh yeah, you're right. I couldn't see it under the other components. Thanks.
Looks like a case for installing some humbucking coils... the transformers are unshielded, hopefully there's enough room between the windings and the cores for another layer of winding.

Humbuckers were quite common back in the days of electro-dynamic (field coil) speakers when the field coil was also used as the choke in the B+ filter. The humbucking winding was on the same bobbin as the field coil. It picked up some of the ripple signal and was connected in series with the voice coil, so that it opposed the ripple signal.
In this case, if the hum is definitely coming via magnetic coupling from the mains transformer, I'd try a few turns around the power transformer feeding a few turns around each output transformer. Definitely a "suck it and see" situation, though.

[the fuhz]

Thanks again for the suggestions guys, Im going to add a 3 prong plug this week and report back...