Fender accessory jack

[Diverted]

Hi,
Could use some advice. I built a AB763 85W Fender Twin circuit for a friend, and stuffed it into a small Princeton-Reverb-style cabinet because he said "I want it small small small."
He loves it but is now finding out that with only one 12" speaker, it's not as loud as it could be. So I am going to build him a speaker cabinet loaded with another 12". Both speakers are 8 ohms, so if I run them in parallel itll be a 4 ohm load.

My question is about the best way to modify the amp. Im going to put in an extra jack and run it in parallel with the existing speaker jack. However, the speaker in there now is being fed by the OT 8 ohm tap, so if I wire in a second speaker in parallel, there's going to be an impediance mismatch.

This is how Fender wires its asccessory jacks and I'm wondering why that is, since it seems to me that it would create the same mismatch.

My question is: Should I just put the OT secondary 4 and 8 ohm leads on a beefy impedance switch, and tell him to switch it to 4 ohm every time he uses the extra speaker?

[stratomaster]

I've been tasked with a similar objective. A single channel pro reverb in a Princeton cab/chassis.  I'd love to see photos of the build to see how others have successfully stuffed 10 lbs into this 5lb bag. 

As for your question, you may be able to get clever with a switched accessory jack and have it break the connection to the 8 ohm tap when a 2nd speaker is connected.

[shooter]

it's not as loud as it could be.

I've worked on a few twins, i ONLY take the chassis usually.  I hook it up to a bench 15" and i'm deaf, my neighbors are starting to shoot guns my way
so there might be something off that you missed?

[tdvt]

I'd love to see photos of the build to see how others have successfully stuffed 10 lbs into this 5lb bag.

I would advocate for using a Bassman chassis instead of a Princeton chassis, as it is not considerably longer (2")  but it is deeper front-to-back, making it easier to fit the larger transformer set. Also, the slightly bigger cabinet is far more 12" speaker-friendly.

Much easier to fit everything in, while ending up with a very similar form-factor to a Princeton.

I am currently doing the cabinet for a second Bassman chassis-ed 12" combo as I find it a happy medium.

Mojo offers a combo cab for a Bassman chassis, but it is taller than necessary & I don't care for the proportions, so I have built my own.

[Diverted]

it's not as loud as it could be.

Hahaha

[Diverted]

it's not as loud as it could be.

I've worked on a few twins, i ONLY take the chassis usually.  I hook it up to a bench 15" and i'm deaf, my neighbors are starting to shoot guns my way
so there might be something off that you missed?

Trust me, it's crazy loud. I think he's talking more about the presence of it ... with twice the number of speakers there will be a lot more air moving. Anyway, I'm going for it.

As for Stratomaster's question, here are a few photos. I'm sorry I don't have better at the moment, these are all I saved after building it four or five years ago. When I get it back to build the new cab and install the extra jack, I'll take more.
I built it in a Princeton Reverb-size (17"w) chassis. To do this I put the power transformer on a separate chassis at bottom left. Without that I don't know how I would have been able to do it.

[sluckey]

As for your question, you may be able to get clever with a switched accessory jack and have it break the connection to the 8 ohm tap when a 2nd speaker is connected.

Sunn figured it out a long time ago. This will do exactly what is needed.

[Diverted]

Brilliant! Thank you Sluckey!

[PRR]

> Sunn figured it out

When some (tired, stoned, drunk, distracted) fool plugs one speaker into just the EXT jack, it mismatches.

I agree it is probably mild.

And if the Main speaker were hard-wired (instead of jacked) there is less to go wrong. That's how I've usually seen this scheme.

[sluckey]

When some (tired, stoned, drunk, distracted) fool plugs one speaker into just the EXT jack, it mismatches.

You can't protect against stupid.   

Here's how Sunn actually labeled those jacks. A little better but it's still not clear what happens when you have an 8Ω speaker plugged into both jacks.

[Diverted]

So, someone elsewhere just noted that the Switchcraft 13A is rated for 1A. I looked up the datasheet and this is true. I told the guy I really don't see this switch failing; it's a standard high quality Switchcraft switch that's been used in amps that are running at way more than 1A.
But just to quiet that voice in the back of my mind, I'm good, right?

Thanks.

[sluckey]

So, someone elsewhere just noted that the Switchcraft 13A is rated for 1A. I looked up the datasheet and this is true.

Did you also notice that the very popular #11 and #12-A used in millions of amps are also rated at 1 amp?

[Diverted]

Yes, and I told him exactly that in my response.

This is what I got back:

"Its your decision but 80W to 4 ohm the RMS current is 4.5A and for sine wave the peak current come 6.3A"

[HotBluePlates]

This is what I got back:

"Its your decision but 80W to 4 ohm the RMS current is 4.5A and for sine wave the peak current come 6.3A"

Fender's been "getting away with it" for the past 65 years.

If you have a lot of extra room you can use the technically-better Speakon jacks.  But no one is gonna notice the difference for at least the next 65 years.

[sluckey]

He's pretty smart. Let him work it out.