4/8 ohm output on OT

[bryankloos]

Hello All,

Help me understand multiple outputs on an Output Transformer.
I am building a Champ style amp, with 6l6/6ca7 as the output tube.
The transformer I purchased has both 4ohm and 8 ohm taps op the output side.

I'm planning on using an 8ohm speaker. That said, I would like to have an external speaker jack on the amp and have it switchable between the 4/8 taps, should I want to use a cab with 1 or 2 speakers...

Is this as simple as wiring the two taps to a SPDT switch and selecting which tap feeds the output jacks?

And what's the consensus on shorting jacks for the speaker? I understand a shorted load is better than open load, so it makes sense to use shorting jack. How do you wire up shorting jacks when there is an external speaker jack?

Thanks!

Bryan

[pdf64]

The simplest, most reliable / resilient method would be to have 1 x 8ohm jack and 2 x 4ohm jacks.
A shorting jack is no good for several failure modes, including the shorting contacts going bad, which they a liable to do.
All that's required is a reasonable degree of resistance across the secondary, to make sure that secondary current can always flow, come what may.
eg 220ohm 1W hardwired across the winding, ie between the 8ohm and common wires from the OT.

[ChopSauce]

I wish I could get it. You mean a 220R/1W resistor in series?

[pompeiisneaks]

The 220ohm is always in circuit, but once you put a speaker on it, that is much lower resistance/impedance (8ohm or 4ohm) so the majority of the current will flow through the speaker. It's a way to protect against no load by always having a backup load path.

~Phil

[pdf64]

I wish I could get it. You mean a 220R/1W resistor in series?

No, in parallel with.
See Vox AC50 (470ohm across 16ohm winding) https://el34world.com/charts/Schematics ... ac50_1.pdf

[pompeiisneaks]

Additionally, as was mentioned with resistance in parallel with two different resistors (speaker and resistor) you get slighly less total resistance than the lowest one, something like 7.7 ohms with a 220 ohm resistor and an 8 ohm speaker. Impedance mostly works similarly with the exception that impedance has a resistive, capacitance and inductive portion of the load that adds phase and other such complexities to the calculations but still the end 'load' to the transformer and thus reflected to the power tube(s) is basically very nominally changed.

~Phil

[ChopSauce]

Thanks for clarifying! The "between" was somewhat ambiguous, yet that makes sense to put it in parallel.

Now I know what to do with my non shorting jacks...

EDIT: so that https://www.tube-town.net/ttstore/xicon ... -watt.html (Xicon Carbon Film Resistor 220R / 1 Watt) would be enough for even more than a small SE El84 powered amp...

The fact that a small resistor like that can handle the output power isn't that much intuitive. Might it be that not all the power is delivered when no speaker is plugged in?

[fredouille]

hi I was wondering also , as I'm building an amp I ordered 4ohm and 8ohm secondary and as I planned to use only 1 at the time, leaving the second winding unused and unloaded could be an issue
so looks like the 220 Ohm is a solution

[fredouille]

I wish I could get it. You mean a 220R/1W resistor in series?

in parallel to prevent no load on the winding

[pdf64]

hi I was wondering also , as I'm building an amp I ordered 4ohm and 8ohm secondary and as I planned to use only 1 at the time, leaving the second winding unused and unloaded could be an issue
...

No, sorry if I've misunderstood, that's not an issue at all.

Loading every secondary output simultaneously is totally unnecessary.

The issue we attempt to mitigate (by using a resistor or shorting jack) is accidentally / inadvertently leaving the secondary open circuit. The load presented to the valves then becomes the OT primary inductance, which can cause oscillation and / or enormous back emf voltage spikes, which can damage the anode circuit insulation.

Provided some reasonable degree of impedance gets reflected back to the primary, such issues as that should be avoided.