I have a 135 watt Twin that has a pair of power tubes yanked; I had it biased afterwards at Nashville Amp Repair, a well-regarded place. When I had it there, the guy told me that I had to have an 8 ohm load with the two tubes yanked, as opposed to the normal four ohms. He also said that I should have 8 ohms on the external jack, too, were I to use it. I was very specific in my questions and he was very clear when he said that.

9 months later:

I called them about another issue and the conversation wandered onto the external speaker jack. I asked about it because right now I'm using a very low efficiency 75 watt 8 ohm 1x12 for practice but I'm getting involved with a project that may end up gigging and I want the option of more speakers if I need some more live oomph. Well, they now say that I should have 16 ohm loads on each if I'm using both jacks, that I should treat it like a 2x12 wired in parallel. Since I was trying to get earlier breakup all along, he even suggested one lone 16 ohm load on the normal jack because the volume would be slightly less.

I am totally at a loss now. If I'm using both jacks simultaneously, do I need 8 or 16 ohms in each? Also, I read somewhere that I need to halve the value of the main fuse because it's underfused if a pair of tubes is yanked and the original fuse is left in. Is this true? I really don't want to melt my baby.

Thank you for any help.

Remove V1 & V6. Put the 12AX7 from V1 into V6 and leave V1 empty. Try the vibrato channel.
Last edited by woad_yurt at Dec 14, 2012,
Ok! I think I got it.

When you pull a pair of power tubes, the output impedance is doubled. So if your amp was 4 ohms before it's 8 now, and you should (ideally) use an 8 ohm load.

If you're using two outputs from your amp, they're in parallel. So any speaker impedances are halved. So if you use two 16 ohm speakers in parallel it's an 8 ohm load.

SO when the guy told you to use the 16 ohm out on your 8 (formerly 4) ohm amp, he was having you mismatch the impedance so the amp worked less efficiently. Using an 8 ohm amp at 16 ohms gives you about a 10% efficiency reduction and is generally safe. Plus you're using one less speaker so you're getting a rather significant volume reduction.

I wouldn't worry about the fuse. You can get a lower one if you want but a short is going to blow it either way.
Maybe I should've said that I'm not concerned with having it run less efficiently with a 16 ohm load. I'm getting as much gain as I'll ever use now because the V1 tube is pulled and there's a 12AX7 in V6. It sounds great for what I'm doing.

If the amp came stock with 4 tubes and two 8 ohm speakers wired in parallel for a load of 4 ohms, are you saying that, to use the aux output simultaneously with the regular output in stock form, I would have to have two 8 ohm loads? Wouldn't that entail having to change out the original speakers in the combo cab? They made it like that?

Remove V1 & V6. Put the 12AX7 from V1 into V6 and leave V1 empty. Try the vibrato channel.
Last edited by woad_yurt at Dec 14, 2012,
With an aux cab you'd either have a mismatch or disconnect the internal speaker and use two cabs. The use is limited since there's no impedance selector but most people don't need or use an aux cab with a Twin.
Some guy on another forum says that the earlier Twins had those jacks in parallel but not the utralinear ones like mine. He said that they are wired in series and that, with a stock 135 watter, using the regular output jack taps into the 4 ohm tap while using the aux output jack taps into the 8 ohm tap.*

That seems like it would make sense because one could hook up another cab without fiddling around with the original speakers. Do you have a source to quote? I'm not discarding what you say but, when I have two opposed opinions, I check them out. Fender doesn''t have a downloadable owner's manual for my amp.

* One has to double the impedance values with mine because of the yanked power tube pair.

Remove V1 & V6. Put the 12AX7 from V1 into V6 and leave V1 empty. Try the vibrato channel.
Last edited by woad_yurt at Dec 15, 2012,
It can be, yeah. They're in parallel so you can use one or both as you please.
I don't understand? What can be?

Remove V1 & V6. Put the 12AX7 from V1 into V6 and leave V1 empty. Try the vibrato channel.
Before the edit your post asked if it could be run with either output jack connected.

I didn't notice that you had the UL. Every other Fender Twin has a parallel jack but yours does indeed have that different wiring. Our opinions aren't opposing, I just didn't notice that you had a UL.
Oh, sorry about that. My A.D.D. strikes again.

So, in a nutshell I use an 8 ohm load on the regular output by itself or a 16 ohm load in the aux output by itself?

That'd be perfect because I can use the two JBLs in series in my 2x12.

Remove V1 & V6. Put the 12AX7 from V1 into V6 and leave V1 empty. Try the vibrato channel.
The 16 ohm load on the external jack with no load on the normal jack doesn't do well. I just tried it with a 16 ohm cab and it's like 1/4 volume, max. So, I stopped. I hooked the cab up to another head on the 16 ohm output and the thing screamed so I know it's not the cab. The Twin works perfectly with an 8 ohm load on each speaker out.
I thought you understood the implications of what you posted before - guess not. Your amp doesn't have parallel jacks so you can't plug into just the external jack. You're lucky you didn't blow the output transformer. The connections are in series with a switching jack on the external speaker out, that prevents an open circuit when nothing is plugged into it. By unplugging the primary jack you caused an open circuit (or partially open - Fender puts a resistor on that jack to prevent your amp from blowing up when you do what you did) which is why you got little volume.

You must, must, must have something plugged into the regular output when the amp is on. You can plug into the external load as well, but the primary speaker out needs to be connected.