Mixing Cab Impedances

Hey all,

I know this is a well discussed topic and I have gone through the archives to see if there's any answer pertaining to my question specifically, but have not had too much luck.

I know it is not generally recommended to mix cab/amp impedances, for example, an 8ohm cab with a 4ohm amp. I also know that pulling tubes on a Twin is a route one can take to create an 8ohm load, but some advise against such methods.

I recently purchased a HT style 2x12 cab that had two E120-16s in it and I've loved the E120s. My previous rig consisted of 2 8ohm D120s and that's what I intended on running, but I've had a serious change of heart in favor of the E120s. I have an unmodified 72 Master Volume Dual Showman Reverb, which as you all know likes to see a 4 ohm load. Would it be possible, or reasonable at all to run the two E120-16s in parallel and also a separate 1x12 D120-8 in parallel (to the 16s) to create a 4ohm total load? Forgive me if my impedance math is poor and/or flawed. I've debated just ditching the Dual Showman and using my SF Deluxe(for the 8ohm impedance), but I very much prefer the tone of my DS. I just would like to find any way possible to use the E120s safely with my Dual Showman without having to spend more dough, if possible!

Side question, I'd be curious to learn how Jerry ran is 3x12 cab, I wouldn't be surprised to learn if Macs could handle impedance differences.

Thank you all for the help!

Re: Mixing Cab Impedances

#175507  by BoogieMK2

Yes, that works. You can find resistor calculators on line, one is

https://www.omnicalculator.com/physics/ ... l-resistor

The formula is 1/Rt = 1/R1 + 1/R2 + 1/R3 + …….

1/16 + 1/16 = 2/16 = 1/8 ohms. Invert and you get 8 ohms
1/8 + 1/8 = 1/4 which gives 4 ohms

Re: Mixing Cab Impedances

#175509  by franklins_timmy

This may not work for what you're going for, but seems like it might. Can't say whether it's good or not, don't have one. Something I have thought about getting off & on the last few years, just never have.

https://reverb.com/item/35225130-palmer ... net-merger

Re: Mixing Cab Impedances

#175511  by strumminsix

Think you need to read up more as these are complicated topics and think your conflating important things. To help:

1) Output impedance is controlled by your output transformer. Pulling a pair of tubes will reduce the wattage output not change impedance.

2) Your mismatched cabs look like this:
Cab 1 = 2 16ohm speakers in parallel = 8 ohms.
Cab 2 = 1 8ohm speaker
Combined, in parallel , the amp will see a 4ohm load which is fine. A DSR could even see a 2.7 and be fine as long as you aren't running it wide open nightly
As your amp is a 72 DSR is 4ohm with 80W (I think, maybe 100), each cab will see the same wattage as they're both 8 ohm load cabs:
Cab 1 will get 50% of the power so 20w to each speaker
Cab 2 will get 50% of the power so 40w to the speaker

You'll be pushing that D120 to it's max while the E120s will still be idling and may sound sterile.

^^^ updated as my brain was backwards with series and parallel even tho my math was right?!

Re: Mixing Cab Impedances

Strumminsix,

Thank you for the response/help!

I was previously of the mind that wiring in parallel would half the overall ohms while series would add them. So being that Cab 1 features two 16ohm E120s in parallel, they make an 8ohm load, then running that entire cab in parallel to the 8ohm D120 would then make for a 4ohm total load.

I previously have run two 8ohm D120s in (what to my knowledge is…) parallel for the 4ohm load that the DS likes to see.

Thank you!
strumminsix liked this

Re: Mixing Cab Impedances

#175513  by BoogieMK2

Two 8 ohm loads in series will give you 16 ohms. Series resistance adds. Two 8 ohm loads in parallel gives 4 ohms per the formula provided in a previous post.

Power = Volts squared / resistance = current squared x resistance.

You will have the speakers handling different power. In the 2x16 and 1x8 ohm cabs in parallel, each cab will consume the same amount of power. If the amp is putting out 100W, speaker in the 1x8 ohm cab will drop 50W and each speaker in the 2x16 ohm cab will drop 25W

Re: Mixing Cab Impedances

Thank you, Boogiemk2!

What would your thoughts be on running an 8ohm load on a 4ohm amp? I've heard many varying opinions, I would love to use the E120s with the Dual Showman in any way possible, without potentially causing harm to the amp/speakers. I've heard flyback voltage is a possible phenomenon that can occur with mismatched impedances.

I hate to shelve such awesome speakers, but it doesn't make sense in my mind to risk ruining them, especially being they have the original cones.

Re: Mixing Cab Impedances

#175516  by strumminsix

Thank you for the response/help!

I was previously of the mind that wiring in parallel would half the overall ohms while series would add them. So being that Cab 1 features two 16ohm E120s in parallel, they make an 8ohm load, then running that entire cab in parallel to the 8ohm D120 would then make for a 4ohm total load.

I previously have run two 8ohm D120s in (what to my knowledge is…) parallel for the 4ohm load that the DS likes to see.

Thank you!
Sorry, I fucked up and had the terms backwards. Bad day, edited my post.

Yes, 2 8ohms, parallel to 4ohms is exactly what the DSR likes to see

The LOAD running that way would be ideal. But the D120 would be get twice the wattage that each of the E120s would get. And the issue there is the E's are high wattage speakers when the D is low wattage.

Re: Mixing Cab Impedances

#175517  by BoogieMK2

What would your thoughts be on running an 8ohm load on a 4ohm amp? I've heard many varying opinions, I would love to use the E120s with the Dual Showman in any way possible, without potentially causing harm to the amp/speakers. I've heard flyback voltage is a possible phenomenon that can occur with mismatched impedances.

I hate to shelve such awesome speakers, but it doesn't make sense in my mind to risk ruining them, especially being they have the original cones.
Normally, a higher load on the output transformer is not a problem. You will get less output power. Power = current squared x resistance. I have not heard of fly back voltage in an amplifier output circuit, will look it up. There is a fly back voltage in an old fashioned cathode ray tube, not anything like an audio amp.

Re: Mixing Cab Impedances

#175527  by Chocol8

strumminsix wrote: Mon Apr 01, 2024 2:19 pm 1) Output impedance is controlled by your output transformer. Pulling a pair of tubes will reduce the wattage output not change impedance.
This is very wrong! The OT sets a ratio but does NOT set the output impedance independent of the tube type or number of tubes! If you pull 2 of 4 tubes you need to double the load impedance to maintain a match. Think of it this way, the voltages don't change (much) and the current capability of the individual tubes didn't change, so if you have half as many tubes, you have half as much current and the same voltage. By Ohm's law, you need to have twice the resistance (impedance) to stay in balance. If not you are asking the tubes to push out twice as much current. A Twin or DSR (exactly the same chassis) will usually survive this but it isn't ideal. If you use the internal speakers and an external cab like Jerry did up until the introduction of the Mac's in 73, you will end up with a double mismatch. The amp will be expecting an 8 ohm load and seeing 2 ohms! That wouldn't be good.

There is nothing wrong or dangerous about pulling 2 power tubes to match an 8 ohm cab. You will get slightly higher B+ voltages because you are putting less load on the power supply, but the Twin Reverb/DSR is robustly powered and SS rectified, so there is little impact on the voltages from pulling a pair of tubes (one from each side!) compared to some more marginal designs. You probably want to check bias, but other than that, they will run great on 2 power tubes.
Last edited by Chocol8 on Sun Apr 14, 2024 8:59 am, edited 1 time in total.

Re: Mixing Cab Impedances

#175528  by Chocol8

BoogieMK2 wrote: Tue Apr 02, 2024 5:53 pm Normally, a higher load on the output transformer is not a problem. You will get less output power. Power = current squared x resistance. I have not heard of fly back voltage in an amplifier output circuit, will look it up. There is a fly back voltage in an old fashioned cathode ray tube, not anything like an audio amp.
Be careful! A solid state amp is usually happier with a higher impedance load. Most designs are even OK with an open circuit/no load attached, which is basically infinite resistance.

Tube amps are the opposite! There is more danger of damaging a tube amp with high impedance than too low. This is why Fender and most builders wire switching speaker jacks to fail to a short if the speakers are unplugged. That is like a 0 ohm load which isn't great, but is more likely to blow a fuse than damage your OT. No load or super high resistance is very bad with a tube amp! The worst part about running to high a load on a tube amp is it will usually work fine for a while, but the damage is slowly cumulative and will eventually bite you.

A Fender Twin Reverb or the Dual Showman Reverb has a robust OT that can generally handle a "one step" miss match. I still wouldn't push it if you don't have to, and lean towards lower not higher. When running with an extension cab as they were designed to do, the load would go from 4 ohms to 2 ohms. Pull a pair of tubes and they are equally safe with 8 ohms or 4 ohms.