High voltage values at startup in headphone jack

[Spinifex]

Hey, I have just finished building the speedball upgraded Crack, but the voltage at the tip and ring of the heaphone jack jumps up to 15-20 V with a cold start. I've tried resoldering several joints, especially all the capacitors, but no luck.

The original crack worked, though I had a few weird readings: all the 90V read as 75V, and the 206V reading was at 21 (last one before the screw), not 20. The cold start heaphone read at 9V and everything else worked, and everything seemed to be in place, so I just kept it as is.

With Speedball installed, the cold start headphone jack jumps up to 15-20V. The 75V read at 68 and the 206V reading is stil at 21. I've tried re-using the resistance values from the crack manual, and they seem unchanged except terminals 7, 9, B3 and B6, which are over the limit of my voltmeter, but I assume it is due to the changes of the Speedball.

If anyone can help, it would be appreciated.

[Doc B.]

206V at 21 is correct. You may want to look over the two posts in the thread called Crack Manual - corrections.

Is this with the supplied tubes, and did you run the amp for very many hours before you upgraded to Speedball? I kind of sounds like your tubes may need some more run time to form (or reform if they are old stock) their cathodes. If it has already been running for 50 or more hours call or email us and we can send out some different tubes to try. In the meantime I would suggest monitoring the headphone jack voltage to see how long it takes to drop down. Don't plug your headphones in before it gets down below 9V.

[Paul Birkeland]

Did you, by chance, swap out the coupling caps?

[Spinifex]

The amp is brand new, I built it sunday, and speedballed it today. The tubes are the ones supplied, I will let them burn and report back.

By swapping the capacitors, you mean switched them for something non-kit? If so then no: they're kit.

[Paul Birkeland]

Ok, good info. Building the kit stock first is a very intelligent decision. Can you verify that the 2.49k resistors are still well attached on the headphone jack, especially to ground?

The charging current and time for the cap to energize should change very little after the upgrade.

[Spinifex]

Yes, the resistors are still there. Even re-soldered them to be sure.

[Jim R.]

So, basic question here: should it be a matter of practice to turn an O
TL amp before plugging the headphones in?

The last headamp I built, a Millet minimax, had a protection relay circuit in it, and before that my ASL MG Head DT OTL could use either tx or otl coupling and there were no warnings about turning the unit on before plugging in the headphones.  Also, does this apply to the s.e.x. amp as well?

Sorry for the newbie question, but I'd rather not fry my new cans right out of the box.

-- Jim

[Spinifex]

Frankly, I think it should become a reflex for all DIY tube headphone amp without muting circuits (like the one in the Minimax). I am always waiting 30 seconds before plugging the headphones in with the Crack, the trick is to remove them when you are turning it off, so you'll never leave them in.

[Paul Birkeland]

The SEX doesn't need any wait time, as the auto-former is at ground potential by design.

In the Crack, having headphones plugged in during start up reduces the resistance under the coupling capacitors, reducing the total voltage during start up.

Since I now have three Cracks sitting in my listening room, I will do some testing today with various headphones.  My suspicion is that the parallel resistance of even 600 ohms and the 2.49k resistors will drastically reduce the start up voltage. 

The one troubling situation will be the folks that put a 330 or 470uf cap in to reduce the roll-off with low impedance cans.  In those cases, the 2.49k resistors really need to be swapped out with 1k resistors. 

-PB

[Spinifex]

... Forgot to mention it in the first post, but my brand new SRH840's left driver popped upon one of the first startup with the Crack. There was a sharp sound, and upon inspection, the plastic cover of the driver was like a balloon. The sound was nearly completely gone. Touching the plastic cover, it snapped back to its original position and the sound returned, but the plastic remained wrinkled over the magnet. I tried several other times, but it never happened again.

I chalked it up to a headphone defect (being brand new) and returned it for replacement. From now on I will not keep my headphones in the jack on startup.