Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bakerlite on March 13, 2011, 06:24:50 pm

Title: why is is essential to have G S T output jack socket?
Post by: bakerlite on March 13, 2011, 06:24:50 pm

Wiring up my otput jacks here...

I dont have any 3 terminal switching  switchcrafts left. i just have the tip and ground type on the bench.

The layout calls for a G S T switching type for the main speaker out and a non switching  tip and ground type for the extra speaker connection.

Can anyone explain why? or know of a clear place on the internet to grasp it better?

I must say despite googling they still confuse me. even wen i soldered up the the input section i was wondering why the need for the swithes on the tips? just to ground when not in use?

but surely the main speaker out should always be in use.

anyway - if someone can help me out i'd really appreciate it!

Title: Re: why is is essential to have G S T output jack socket?
Post by: sluckey on March 13, 2011, 07:39:01 pm

The switching jack is wired to provide a short circuit (zero ohm) load to the OT in the event the speaker cord is not plugged in. The alternative would be an open circuit (infinite ohms) if the speaker cord is not plugged in. Infinite ohms would reflect a high voltage back to the primary that could cause damage to tubes, tube sockets, or OT. A short circuit would reflect zero volts and that's a much safer option. That's a choice between the better of two evils. This applies to tube amps.

The opposite is true of modern transistor amps.

Title: Re: why is is essential to have G S T output jack socket?
Post by: stingray_65 on March 13, 2011, 11:06:29 pm

I usually will wire a cement power resistor from switch to ground

anything close will do 10, 20 even 50 ohm, and as many watts as the amp, or close.

Hopefully if you're playing and no noise is coming out, you'll investigate.

The OT wants to see some sort of load, dead shorted as Sluckey mentions is better than wide open; but I feel a resistive load is even better.

Ray

Title: Re: why is is essential to have G S T output jack socket?
Post by: 38Super on March 14, 2011, 09:23:59 am

Instead of using a (shorted when empty) output jack, you could add a 150 or 200 Ohm power resistor from 16 Ohm - if you have one - OT tap to ground.  It will only siphon off a little current and will be a failsafe in case you forget to plug in the speaker.  I suggest adding it to the highest impedance output tap on your OT.


cheers,

rob