Dual tap OT quandry

[Randall]

Customer brings me a 2016 Blues Jr and wants every mod known to man done to it. One of which is to install a David Allen OT he supplied with 4 and 8 ohm taps. He wants to be able to run just the stock speaker, but wants an additional speaker jack installed so he can plug an extension speaker in if he wants. What would be the best way to achieve this? I'm thinking wiring two jacks in parallel and wiring a SPDT switch between them and the two OT taps?

[Chuck H]

I think this is the safest way to go because it's the most easily understood. Mesa does a thing with some of their amps where the jacks are labeled for the ohm load. It's stupid. Another system I've seen would be switching jacks where the main speaker is always in the designated jack and plugging in the extension jack halves the secondary ohms. But then what if you ONLY want the extension cab? Will the guy know or remember to unplug the main speaker and lug the extension cab into THAT jack? Too much to go wrong in the hands of most guitar players.

I vote parallel with a switch. Then it's on the customer. Niether jack is "wrong" and the switch should be adjusted for the load. They KNOW they're changing the load when they fuss with the speakers, so...

[SoulFetish]

I think this is the safest way to go because it's the most easily understood. Mesa does a thing with some of their amps where the jacks are labeled for the ohm load. It's stupid. Another system I've seen would be switching jacks where the main speaker is always in the designated jack and plugging in the extension jack halves the secondary ohms. But then what if you ONLY want the extension cab? Will the guy know or remember to unplug the main speaker and lug the extension cab into THAT jack? Too much to go wrong in the hands of most guitar players.

I vote parallel with a switch. Then it's on the customer. Niether jack is "wrong" and the switch should be adjusted for the load. They KNOW they're changing the load when they fuss with the speakers, so...

This is how Marshall does it. I personally like the speaker out to be the speaker out, and using a switch to set the desire impedance as well.
But i prefer the "main/extension" switching jack set up because you can use the main jack as a shorting jack when there is no external load plugged in. While not ideal, a short across the output of a tube amp can save the output transformer and tube sockets if there is no load plugged it and the output goes open. This protects the player (and... sometimes,.. the technician) from screwing up and not connecting the speaker for whatever reason.
Not my favorite method of protection, but if I had to choose between the two.
Multiple designated output jacks like Mesa, and others do it makes no sense to me. With Mesa, it doesn't really matter though. Those jacks save mesa customers from having to deal with the 32 switches and 12 knobs that would be there if the speaker jacks weren't.

[Randall]

My idea is to put the switch between the jacks so that it points to the jack. One side points the main jack, and the other side points to the jack when you plug in an extension cab.

[Chuck H]

Eh. It easier to wire parallel jacks when you don't jam the impedance switch between them. For any difference it makes I don't think it's worth the trouble. JM2C. Whatever you decide I think SoulFetish is right to mark the main jack and the extension jack and make the main jack shorting.

[SoulFetish]

It helps to have the back panel silkscreened, labeling each accordingly. But who the hell has time (or equipment) for that. You could put a red washer on the main just so they know. But, the other great thing about wiring the main speaker for shorting, is that if they plug into the extension jack, they don't get any sound.
If that happens, we can only hope the wind is at their back that day, and by process of elimination, the guitarist figures out that it's the other jack.

[pdf64]

I prefer a safety resistor across the secondary, about 20-30 X the nominal load value.
The low value of the feedback resistor network in most BF/SF amps might act as such a safety resistor. Shorting jacks are no use if the speaker blows, cable breaks etc, but combined with a safety resistor, they seem to provide a resilient ‘belt and braces’.
With only 2 impedances, I prefer to use 3 jacks, rather than 2 parallel jacks and a switch; on the basis that the fewer things to go wrong in the secondary circuit the better (prime example being the dodgy removable impedance selector Marshall used to use). That excludes the shorting jack option but I’m doubtful that ‘ELxx’ tubes in fixed bias can cope with a shorted load any better than an open load.

[Helmholtz]

Early Marshall JTM45s (RS OT, no imp. selector) had a 1k safety resistor.

[pdf64]

Wow, that's interesting, I've not seen that reported before; do you think they left the factory like that?

[Helmholtz]

do you think they left the factory like that?

Yes, I think so. They look original. I have a collection of pictures from Plexi Palace and from http://marstran.com/Historic%20Data%20Base.htm (print-outs) and the 1k CC safety resistor can be seen on several amps. My '66 JTM50 with Drake OT and impedance selector doesn't have it. Older schematics don't show it either but I have never seen a schematic with the RS OT.

[Randall]

This is a blues Jr, and I don't think (or see a reason) why the main speaker would be disconnected. This guy is savvy and just wants to be able to add a cab in certain rooms. I know the switch will work, but I have also seen reference on billmaudio,com that there is a way to wire up an aux speaker jack so that it switches to 4 ohms automatically. Anyone know how that works?

The main speaker jack is wired for shorted when unplugged.

[Jazz P Bass]

I have attached the Fender HR series jack switching setup.
Hot_Rod_DeVille_Spkr Jack.pdf

[Randall]

Thanks Jazz, I suspected it was something along those lines. That's what I will do. Need to figure out the correct switching jack, preferably an open switchcraft type.

This should do the trick. http://switchcraft.com/Product.aspx?ID=2701

[Chuck H]

Alright. Assuming Randall has worked out how to proceed...

What is the purpose of bypassing the 1 ohm resistor on the power tube cathodes in the schematic with a diode.?. Is that to limit the affect AC standing on top of the 1 ohm resistance may have on the mV reading? Since the measurement here is usually taken when the amp is at idle there shouldn't be any to speak of, right? Is it even significant enough for this extra diode?

[The Dude]

That's a conundrum for me also. I've seen diodes used for clamping or constant gm circuit/purposes (with a larger value cathode resistor), but the 1 ohm parallel resistor seems to make the diode a moot point. Maybe it's not intended for normal circuit operation, but rather if the resistor starts to open?