FAILSAFE

[TIMBO]

Hi guys, This turned up on one of my resto's.
I can see the idea behind it but not sure if it is a good idea.

[sluckey]

I like it.

[2deaf]

Wouldn't that run the power tube cathodes through your speaker if the plug was only inserted half way?  I'll pass.

[HotBluePlates]

Or maybe do it like Ampeg did with a 4-pin XLR where 2 pins are the speaker connection and 2 pins are the connection between cathode & ground (these 2 pins are shorted together at the speaker cabinet's matching XLR jack).

[2deaf]

A Switchcraft #13 jack with the isolated "make" circuit looks like it would achieve the desired goal and still use a standard 1/4" phone plug.

[John]

In the schematic,  that is labeled a stereo jack. The 12B stereo jack that I have has that isolated make pin, which I plan on connecting to the PI cathode (like the Ampeg)

[PRR]

> Wouldn't that run the power tube cathodes through your speaker if the plug was only inserted half way?

So?

This is a Guitar Amp. It must have a TOUGH speaker to stand normal duty.

Round numbers: a 50 Watt amp at 8 Ohms is 20 Volts and 2.5 Amps. Cathode current will be about 200mA or 0.2 Amps. Cathode current is a *tenth* of normal abuse.

Look at another way. We know the OT does a 20:1 voltage step-down from plate to speaker. It also does a 1:20 *current* step-UP from plate to speaker. Cathode current is sure to be much less than audio abuse.

Yes, loosely suspended small-coil Hi-Fi speakers are easier to blow. They are not expected to carry CLIPPED! signal. They are not expected to get a steady push for more than the peak of a 50Hz wave; any longer and the coil moves out of the gap and possibly rips the weak suspension.

It will thump as it passes the finger, but not even like the POP of plugging a guitar in while the amp is live.

[2deaf]

Well, I guess you set me straight.  Still, I ain't fixin' to run any appreciable DC through MY speakers any time soon.

Also, with a stereo cliff jack in the half-inserted mode, the amp can be operated with no load on the OT which is what they are trying to avoid.  It seems to me that this is more likely to occur than having the speaker plug(s) lying on the ground with the amp operating.  But then again, I'm probably talking foolproof as opposed to failsafe.

I tried a Switchcraft #12B stereo jack with a plug inserted half-way and found that the tip made contact with both sides of the stereo.  So the cathodes would be connected to the hot side of the OT output with the speaker connected.

Incidentally, the 12B has no switches either isolated or otherwise.       

[John]

You'll never fool proof anything. They keep making better fools.


The switch I have, there is a pin that is not connected to anything until the plug is in, it's then connected to the sleeve (grd). If that's not actually isolated, my bad. I think the point of the circuit is, to keep someone from hammering a power chord with no speaker plugged in.

[HotBluePlates]

You'll never fool proof anything. They keep making better fools. ...

I think that was the gripe about Ampeg's foolproof 4-pin cable which connected output tube drive only when the speaker cabinet was plugged into the amp: the new & improved fools kept losing the special 4-conductor cable to make their amp work...

[2deaf]

The switch I have, there is a pin that is not connected to anything until the plug is in, it's then connected to the sleeve (grd).

That sounds like a #12B stereo jack which is what was shown in the OP.  The #13 has a little plastic button that closes an entirely isolated switch when the plug is inserted.  This isolated switch could be used to connect the standby switch (or the cathodes directly if no cathode standby switch) to ground.  It's still not foolproof because the fool can leave the plug lying on the ground at the speaker end.  HBP's description of the Ampeg system sounds like it takes this possibility into account and it should thwart a fool.  However, those fools can be quite ingenious sometimes with objects as simple as a paper clip.

[John]

However, those fools can be quite ingenious sometimes with objects as simple as a paper clip.

Surely, you can't be talking about the ol' paper clip in the wall socket trick!

[HotBluePlates]

...  It's still not foolproof because the fool can leave the plug lying on the ground at the speaker end.  HBP's description of the Ampeg system sounds like it takes this possibility into account and it should thwart a fool.  However, those fools can be quite ingenious sometimes with objects as simple as a paper clip.

IIRC, Ampeg installed male XLR at the amp and the speaker, so the cord was female-female. No way to use the paperclip to connect the points to get the output stage working without actually using the cable (and if you have the cable, you'd deserve what you got if you jam a paperclip in the speaker end of the cord instead of just plugging it into the speaker cab).

[2deaf]

It was just a joke.

[John]

Shirley you're not serious!

[PRR]

> ain't fixin' to run any appreciable DC through MY speakers

Agree it is rude.

From the amp-maker's POV, amp run UN-loaded is liable to arc-over inside the OT. Customer is majorly unhappy. Whereas a speaker that thumps only part power is not going to die, and is unlikely to be damaged enough to notice. So the amp maker's decision is reasonable, and may be "forced" by a load of weak OTs. Whether this is the best all-round choice is, of course, debatable.

[xm52]

The Ampeg safety scheme initially had the center tap of the high voltage winding looping to the speaker cabinet (see below). The circuit wasn't completed unless the speaker cable was connected to the cabinet. Ampeg changed it because of a request from the regulatory agency. A safety circuit that carries B+ was deemed unsafe. If you touched the middle two pins and were grounded, you could receive a shock. There were safer ways to achieve the same thing.


Their next approach was to disable the phase inverter via the cabinet loop (see below). This was better because there was only a couple of volts in the safety loop. Although never advertised, with the PI disabled, the power amp is shut down. Since the preamp was still active, you could send that signal out via the EXT AMP jack on the back of the chassis.


The third safety revision was to hang a 250 ohm, 10W resistor from the 16 ohm output transformer secondary tap to ground. This provided a load on the output that protected the amp in the event that it was powered up without a cabinet connected. This doesn't serve as a dummy load but it does allow some time for you to realize that you've done something wrong. This is a common safety circuit found in other amps. The 250 ohm resistor does not affect the sound of the amp. A large resistor in parallel with a small impedance such as that provided by an 8 ohm speaker is close to that of the speaker.

[TIMBO]

Thanks guys, I was able the fire up the amp today, after a few pot issues and dirty sockets the failsafe was put to the test.
WELL, I don't like it.
It does work as it should, but as it relies on a minute amount of surface contact between plug shaft and jack contact and can be a bit noisy from cabinet vibrations, corroded jacks and plugs.

As shown on the schem there is a standby sw.
How good of an idea is it to have a the standby here????
This switch was disconnected and had no standby in place.
The configuration of the power supply won't allow a switch in the PSU.
Again the debate of the usefulness of the "STANDBY"