1964 Fender Princeton Ext. Speaker Jack

[Cathode Ray]

When I was a kid, I once made the mistake of plugging the speaker into the ext. speaker jack instead of the "speaker" jack.

Noticed the amp didn't sound right, thought something was broken, so I turned the volume all the way up. Could hear the guitar, but low volume and distorted.

Back then, I used to sit on the little combo amp while playing, and so I also noticed the top of the cabinet getting unusually warm.

Shut everything off, poked around and eventually figured out what I had done wrong. Once I plugged the speaker into the speaker jack, everything worked normally - no damage.

Fast forward 30 years or so, and I read the Ed Van Halen account where he used to intentionally do the exact same thing with his '63 Bandmaster.


Can anyone explain electronically why this works, why it sounds the way it does, and what you are actually doing to the output transformer when you connect a speaker this way ?


Thanks

[sluckey]

It sounds that way because when there is nothing plugged into the (main) speaker jack, the OT secondary is shorted to ground by the switch contact on the main speaker jack. Theoretically you should not get any sound but the switch contact on the jack usually has a very tiny resistance so it's not a perfect ground. EVH did a lot of stupid stuff.

[Cathode Ray]

It sounds that way because when there is nothing plugged into the (main) speaker jack, the OT secondary is shorted to ground by the switch contact on the main speaker jack. Theoretically you should not get any sound but the switch contact on the jack usually has a very tiny resistance so it's not a perfect ground. EVH did a lot of stupid stuff.

Looking at the schematic of both amps, I totally get this.

Theoretically, it shouldn't put out any sound and the OT should burn up...

but neither of these occur.

[sluckey]

If you clean the contact on the main speaker jack (or replace it with a good one) you won't get any sound. Having a short on the speaker jack is much easier on the amp than having an open circuit. How long are you gonna play thru an amp that's making no sound before you start investigating?

[sirbergs]

Don’t do what Eddie did! He could afford it. lol


Sent from my iPhone using Tapatalk

[Cathode Ray]

If you clean the contact on the main speaker jack (or replace it with a good one) you won't get any sound. Having a short on the speaker jack is much easier on the amp than having an open circuit. How long are you gonna play thru an amp that's making no sound before you start investigating?

So the reason this works (loosely defined here) is due to a small amount of resistance from the unused "speaker" jack?



so we can redraw this scenario thusly:



Would this be correct ?

I'm not sure...

[Tony Bones]

EVH did a lot of stupid stuff.

This is true. I think it's also true that he said he did a lot of things that he really didn't do. Popular lore is that he deliberately lied to be deceptive. A less common theory is that he doesn't really remeber what he actually did!

[Cathode Ray]

EVH did a lot of stupid stuff.

This is true. I think it's also true that he said he did a lot of things that he really didn't do. Popular lore is that he deliberately lied to be deceptive. A less common theory is that he doesn't really remeber what he actually did!

He probably doesn't even really exist.

I still wanna know why this does what it does.

Guess I'm as stupid as Ed.

[sluckey]

I don't know how to say it any simpler. The switch contact on the jack is dirty and not making good contact. So, instead of being a perfect switch with zero ohms resistance and totally killing the signal to the speaker, the dirty contacts may have a resistance of .5 ohms. This resistance is in parallel with the speaker that's plugged into the ext jack and allows a bit of the signal current to flow through the speaker. The dirtier the contacts, the higher the resistance and the more signal can reach the speaker that's plugged into the ext jack.

[Cathode Ray]

I don't know how to say it any simpler. The switch contact on the jack is dirty and not making good contact. So, instead of being a perfect switch with zero ohms resistance and totally killing the signal to the speaker, the dirty contacts may have a resistance of .5 ohms. This resistance is in parallel with the speaker that's plugged into the ext jack and allows a bit of the signal current to flow through the speaker. The dirtier the contacts, the higher the resistance and the more signal can reach the speaker that's plugged into the ext jack.

So, schematically, is the second image I posted correct ?

Alternately, if you were to replace the "speaker" jack with a variable resistor would that work as a volume control for the speaker connected to the ext. speaker jack. (I think what you're telling me is yes)

And back to my original query, what is all of this jackassery doing to the OT ?


Thanks

[sluckey]

The whole purpose of that switching jack is to prevent big damage to your OT, output tubes, and sockets in the event that the speaker is disconnected. An open speaker load is very dangerous to a tube amp. A zero ohm load is not nearly so dangerous.

[Cathode Ray]

The whole purpose of that switching jack is to prevent big damage to your OT, output tubes, and sockets in the event that the speaker is disconnected. An open speaker load is very dangerous to a tube amp. A zero ohm load is not nearly so dangerous.

I appreciate your patience, and your answers.

I actually have an idea I'm working on.

I'm sure as with everything I could think of with a tube amp - it's been done or tried before.

[Tony Bones]

So, schematically, is the second image I posted correct ?

Yes

Alternately, if you were to replace the "speaker" jack with a variable resistor would that work as a volume control for the speaker connected to the ext. speaker jack. (I think what you're telling me is yes)

Well, yeah, but it would radically change the load seen by the tubes. That would necessarily change the sound. Maybe better, but probably not. And, the variable resistor would need to dissipate most of the power delivered by the amp. An L-Pad is basically a variable resistor that is designed to maintain the same load on the amp at all positions and handle the power. There's probably a way to wire up an L-Pad in the extension speaker jack on your amp to do what you're thinking about.

FWIW, I ran one of these on my 66 Princeton Reverb in the early 80's. There's a mark on top where it melted the tolex as proof. I thought it sounded cool at the time.

[Cathode Ray]

So, schematically, is the second image I posted correct ?

Yes

Alternately, if you were to replace the "speaker" jack with a variable resistor would that work as a volume control for the speaker connected to the ext. speaker jack. (I think what you're telling me is yes)

Well, yeah, but it would radically change the load seen by the tubes. That would necessarily change the sound. Maybe better, but probably not. And, the variable resistor would need to dissipate most of the power delivered by the amp. An L-Pad is basically a variable resistor that is designed to maintain the same load on the amp at all positions and handle the power. There's probably a way to wire up an L-Pad in the extension speaker jack on your amp to do what you're thinking about.

FWIW, I ran one of these on my 66 Princeton Reverb in the early 80's. There's a mark on top where it melted the tolex as proof. I thought it sounded cool at the time.

Yep.

Was just curious about how/why this weird scenario even worked.

I think I understand it now. And I agree, a reactive or resistive load/line-out is the way to go with this sort of thing.

[Stevem]

To be on the safe side of things with a amp your building like in regards to your speaker blowing while your playing you can place a 250 ohm 7 watt resistor across the amps speaker jack.
Should the output circuit go open then the 250 ohm resistor is the load and when everything is working fine the output stage does not even see 250 ohm resisto, so no output power loss is seen!