Bassman 135 ext speaker jack

[pontiacpete]

I’m trying to restore a bassman 135 that had been badly altered/ modified. The ext speaker jack was removed and replaced with a regular switching jack. These amps originally had that strange elongated jack as shown in the picture at this linkhttps://www.google.com/search?q=bassman+135+layout&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari#imgrc=IH9pGLB8cEqIUM:
Is there a substitute for this jack?
What do these jacks do?
Does anyone know where I can find one?

Thanks

[Leo_Gnardo]

The original switching jack in ext. speaker position selected a lower impedance tap to run 2 cabs in parallel. Fender used it on their bigger amps in the 70's, also it's found on Ampeg SVT of that period. There's a simplified version Fender has been using on some of their Blues & HotRod series amps over the last 15 years or so, but I don't have a model number for it. And I haven't noticed it for sale at any of the usual suspects. The good part, it's a Switchcraft jack. Not so good, those are really small switch contacts - on both the older & newer models and can be damaged by trying to pass lots of current. If I'm not mistook, they're rate-d at 5 amps. That's only 100W at 4 ohms. If you play at "polite levels" you could get away with it. Also the metal on the contacts tends to corrode with age and you wind up with an intermittent or open connection. I put in a 15A rated toggle switch so the user can select correct impedance, and not have to worry about sizzling contacts on those jack switches.

[Chuck H]

Looking at the switch diagram on the schematic it's not clear (because of non standard graphics) what the switch is actually doing. I think the sleeve of the weird jack is isolated (not indicated) Since the switch is made out of unobtanium, and probably isn't a good, reliable way to switch secondaries anyhow, especially for something as critical as the load for the amp, I would probably default to a switch. A single jack and an impedance switch in the other hole or parallel two non switching jacks and drill another hole for the switch. It's not like it's a BF Twin Reverb or anything so I don't think an extra hole is a big deal. Besides, the original jack failed and can't be replaced.

EDIT: As Leo indicates, there are other jacks out there that may or may not be just right for the job. There are small signal jacks that WILL do the job, but I wouldn't use them for 100+W speaker terminals. I still vote impedance switch.

[pontiacpete]

I’m wondering where the connection to the line out goes. ?

[Dave H]

What do these jacks do?

From looking at the schematic Chuck posted

Connecting a speaker to the bottom jack (top jack unused) connects it to the 4ohm OT tap.

Connecting a speaker to the top jack (bottom jack unused) connects it to the 8ohm OT tap.

Connecting speakers to both jacks puts the speakers in series and connects them to the 8ohm OT tap.

[Chuck H]

I thought that was the case also. I edited my post and removed that observation because I couldn't be sure and Leo has a lot more experience with some of these amps than I do. But that's what I thought. I would even suggest testing the OT ratios to be sure and then, probably use a switch.

[bob p]

There's a simplified version Fender has been using on some of their Blues & HotRod series amps over the last 15 years or so, but I don't have a model number for it. .

The Blues Deluxe Reissue service manual shows two potential candidates:

Part # 48356000 Jack phone transfer 13A
Part # 21550000 Jack phone tip shunt 12A

Here's the Gold Mine:

Jack Schematics page from Switchcraft:

http://www.switchcraft.com/Documents...Schematics.pdf

[bob p]

Not so good, those are really small switch contacts - on both the older & newer models and can be damaged by trying to pass lots of current.

those small switch contacts remind me of working on an old points & condenser ignition. If you have the switch and the contacts are oxidized, sometimes a quick pass with points-file or an emery board is all that it takes to clean them up.

[Chuck H]

I've always loved that jack schematic/diagram page. Unfortunately, as mentioned on my edit in post #3, it's been my experience that most of the trick switching jacks are only suitable for signal level connection and would never be durable enough for a speaker jack. YMMV

[Pedro Vecino]

From looking at the schematic Chuck posted

Connecting a speaker to the bottom jack (top jack unused) connects it to the 4ohm OT tap.

Connecting a speaker to the top jack (bottom jack unused) connects it to the 8ohm OT tap.

Connecting speakers to both jacks puts the speakers in series and connects them to the 8ohm OT tap.

And... keeping contacts closed when nothing is connected!
It´s definitely a very smart circuit. The switching plates of the jack are really thick and with a lot of tension. I have not seen problems in them having been running for 30 or 40 years.

I’m wondering where the connection to the line out goes. ?

http://www.thetubestore.com/lib/thet...-Schematic.pdf

It´s connected to the 4 Ohms secondary.

[bob p]

I've never liked the Fender switching system. It seems to be designed to be user friendly (in a world where every cabinet presents an 8R load) but it's not all that robust, the switch being the weakest link. I agree, if I were building an amp I would not do it that way.

IMO the only reason to preserve the original switching paradigm is if you're rebuilding the amp for the general public, as some users expect every Fender amp to work like the original and are easily confused by change. If you're rebuilding the amp for your own use then I wouldn't bother chasing down the jacks, I'd just build it in a way that suits you.

For many people the Marshall switching system works best. Being a cheepskate that doesn't like to buy expensive switches, I'll just hook a set of jacks up to the 8R tap and a set of parallel jacks to the 4R tap and plug my load into the proper jacks. That way the switching occurs in the brain rather than in the hardware and no switch gets stressed beyond it's rating. I got used to this paradigm with the early Mesa amps and I've been hooked on it ever since.

I might also add that I try to safeguard against the no-load condition by hard wiring a resistor across the transformer secondary. In many amps Fender will short the secondary if no speaker cable is plugged in. I use a parallel high value resistor instead, which protects against the situation where an unconnected cable is plugged into the output jack. I picked up an ammo box of 270R / 5W wirewounds that were almost free on clearance, so the cost to do this is near zero.

[Pedro Vecino]

The capacity of the circuit to connect outputs jacks in series and assign them to 8 Ohms suggests the use of 4 Ohms cabinets as more common. Marshall has always handled impedances of 16 and 8 in their cabinets, which makes the pair of parallel jacks and impedance selector more logical.
I would not use Fender's method at the present time in a new amp but I recognize that it makes sense in relation to how oriented his offer then.

[Chuck H]

Parallel jacks and a switch (may need to add Dymo labels)

The jacks are there, the switch tells you what the total load should be. MOST amps are done similar to this so it's probably the least misunderstood by musicians.?. It's certain that the current standard switching jack that's there can't execute the proper function, so the user wouldn't be familiar with "normal" operation for that amp anyway. That, or they've been doing it wrong already.

[Leo_Gnardo]

I thought that was the case also. I edited my post and removed that observation because I couldn't be sure and Leo has a lot more experience with some of these amps than I do. But that's what I thought. I would even suggest testing the OT ratios to be sure and then, probably use a switch.

After peering at your schematic I'm in agreement with Dave H: plugging into the ext jack puts the speakers in series. Now the choice is "do I want to copy the Fender wiring, or do something that will make sense for the speakers I'm using now (or customer or future buyer of the amp.)"

Some other Fenders parallel the speakers, so does the SVT.

Also, if the series switching is used, if there's a fault that causes a disconnection of the speaker while the amp's in use, that would be a bad scene for the output tubes.

Thanks bob p for the part numbers. Switchcraft 13A would be the jack I mentioned, used to select output taps in the ext. speaker of Fender's HotRod & Blues series amps.

[bob p]

Pedro is right -- it is a series switcher.

The Fender system makes sense for amps like the Twins, where you've already got a 4R load coming from two 8R speakers, and the only cabinet that you're likely to add is another 4R cabinet with two 8R speakers in it. In that case it makes sense to have the switch to a series setup when you add another cabinet... sort of ...

The problem being that speakers that get wired in series only sound 'right' if they're the same kind of speakers wired in series. IMO different types of speakers sound better wired in parallel than in series. I think that switching to 2R in parallel would have been a smarter decision. Maybe Fender did what they did because they got a deal on 4R / 8R transformers. 2R / 4R and a parallel connection would have made more sense to me.

Back in the day the Fender paradigm made sense. Today, not so much. Fender built their amps to be used with other Fender products, and since they standardized on 8R speakers the match-ups were simple. but in an era where nothing is like it used to be, we have cabs with loads that can be anywhere from 2R to 16R, and that series jack doesn't make much sense any more. More flexibility is needed, especially with a head. Having any method of Z selection seems like a much better solution than being forced to go up.

If you don't have that special vintage Fender jack, I wouldn't waste a lot of time looking for it.