OT- dumb question

[lifer]

we all know a powered up amp w/o a speaker load will eat an OT.

i don't get it.....what is there to "break"? there's no moving parts......

just windings- so what happens?

[novosibir]

w/o load you'll get very large voltage peaks caused by the primary's inductivity, which is interacting with the capacity of the 1-st filter cap. Peaks, which might be as big as 3kV (=3,000V) in tube amps w/ B+ about 450-500V

Think about -> ignition coil in cars - up to 40,000V

Since the secondary is ~nearly~ ground potential, the peaks punch through the insulation between prim & sec, leaving a conductive burn path..... which becomes more and more conductive, the longer you try to play the toasted OT..... until the HT fuse blows.

If you don't have a HT fuse in the amp, then next you need might be also a new PT

Larry

[joshwilson3]

Are those setups any good where they put something on the output jack. So if you turn the amp on with nothing plugged in the output jack, it doesn't damage the OT? I think it is a ground or resistor thing they put on the output jack.

[novosibir]

The resistors across the speaker jacks - 47R, 150R or even higher - are more or less senseless, because nonetheless at a too high load the peaks occur, maybe a bit lower in voltage.

BF and SF Fenders have shorting speaker jacks. Much better than an open speaker jack, peaks are avoided, but lots of current flows through the primary with a shorted speaker jack (zero ohms), what might overload the primary, whereby it might burn open.

The only what protects an OT and the output tubes w/o load are transient suppressing diodes, as found in Trainwrecks.

Check here: http://forum.metroamp.com/viewtopic.php?f=4&t=25832" onclick="window.open(this.href);return false;

I've used each one BY509 (5kV) instead of 3 * 1N4007 in series, but don't use it anymore. They influence the sound, especially when the amp is dimed.

Larry

[lifer]

Thanks for your input Larry...we can always count on you for excellent info!

[Roe]

...

The only what protects an OT and the output tubes w/o load are transient suppressing diodes, as found in Trainwrecks.

...

I've used each one BY509 (5kV) instead of 3 * 1N4007 in series, but don't use it anymore. They influence the sound, especially when the amp is dimed.

Larry

Larry, do you feel that the 3 1n4007s influence the sound as well?

[novosibir]

Larry, do you feel that the 3 1n4007s influence the sound as well?

I can't give you an extensive expertise in this concern, because I haven't experimented too much with these diodes.

First I've built the BY509's into my DINO amps and into a few of my Marshall rebuilts for a while at a time, as my British Purist still wasn't born. Then again & again I was wondering about a strange nasty tone especially with the amp at high volume or dimed. I've experimented with different bias settings and found out, that you can minimize it with extremely hot bias - but it's always there and only disappears with the diodes removed.

I absolutely don't like tinkering and three diodes soldered together and then in a bow across the tube socket IMO doesn't look very professional, hence I've never tried it with 3 1N4007 in a row - and only the existing possibility, that diodes there might influence the sound convinced me, to leave it at all.

Tube amps aren't uncomplicated consumer good, they're rather wild animals. So the 'lion tamer' has to take care to his animals, double & triple check all his connections before playing on - and if he's careless or unwarily with his sensible stuff, then the next repair bill will treat him straight

Fortunately it's not been the case yet, that I've had to swap a toasted OT of a DINO or BP

Larry

[joshwilson3]

I would be cautious buying a tube amp from say Guitar Center. As back when I didn't know any better. I'd go to try out some tube amps, turn them on and wait for the sound. After a while no sound, I'd look an notice it wasn't connected to a cab. I probably done that several times.

Now that I know better, I always make sure it is connected to a cab before I turn it on. Though I don't fool with the ohm setting as there is no room to pull everything out and check it. So, I just assume the Guitar Center techs have the amp ohmed up for the cab it is sitting on. Haha.

There is no telling how many people turn those amps on with no cab connection. Though, I put the blame on Guitar Center as they'd have a double row of tube amps sitting on cabs, and only 2 or 3 of them would have speaker wires connected to cabs. They should have all of them connected to cabs.