Twin Reverb - Speaker load with two 6L6 tube pulled and using both output jacks

[lacrebob]

I’m going to pull the inner two 6L6 power tubes from my Twin Reverb and I’m trying to figure out a way that I might still use the two 8 ohm speakers in the combo cabinet to arrive at a final speaker load of 8 ohms.

My understanding is that when both the Speaker and Ext. Speaker connections are used the appropriate speaker load with 4x 6l6 tubes is halved from 4 ohms to 2 ohms. Here, I’m hoping to halve a 16 ohm load (two 8 ohm speakers in series) to arrive at an 8 ohm load.

Do you think the speakers could be wired both in series and parallel while using both the Speaker jack and the Ext Speaker jack as shown in the attached diagram to get my desired result?[attachment=0]Twin Reverb - speakers seeking total 8 ohm load.jpeg[/attachment]

[sluckey]

There's no way to connect two 8Ω speakers and end up with 8Ω total impedance. There are only two choices... parallel for total of 4Ω or series for total of 16Ω.

I think you misunderstand how the speaker and ext speaker jacks are connected. They are hard wired together and connected to the one and only 4Ω OT secondary. Your drawing shows the two speakers connected in series resulting in a total of 16Ω load. When you pull two tubes that will look like 32Ω.

[lacrebob]

Put another way, it seems that because the positive and negative ends of each plug are connecting to the same point at the transformer secondary, then there isn't a parallel connection.

[Carlsoti77]

Just use one speaker to see if you generally like the change in the power amp. A pair of 6L6s at high voltage into a single 12" is still going to be loud. If it gets you to the sweet spot, then you can start looking at getting appropriate speaker replacements.

[solderhead]

My recommendation for the quick and dirty way to adapt the amp for using a pair vs a quad of 6L6 would be to install a switch that allows you to bypass one speaker with a resulting 8R load on the OT.

This may not be immediately obvious, but the amount of SPL reduction that you get by pulling two output tubes isn't all that meaningful on it's own -- the amp still makes quite a bit of power and two speakers continue to move a lot of air. The real gain that is made in SPL reduction comes from cutting the operating acoustic efficiency of the amp in half -- by using one speaker in the speaker/air interface rather than two. A single speaker will make the amp quieter. IME pulling tubes without pulling a speaker doesn't accomplish all that much.

Pay attention to the fact that as you decrease the load on the PSU by pulling tubes, all of your rail voltages will increase. If you really want to address this problem the right way, one might consider placing a switchable load on all of the rails to keep voltages from rising when the tubes are pulled. Optimally, I'd put in a switchable substitute load when pulling tubes. If your heater rail rises out of spec you will experience shorter tube life. This can be important as tubes are getting more expensive than they used to be.