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I have a Twin Reverb with the Custom 15" speaker. Been recovering from surgery and wrote parts that can evolve into a few new songs where it would just be guitar using some octave effects, and percussion. The issue is the octave effects with drive can get sloppy and lose some tightness which I would prefer to keep since its a single mail instrument. I remember how good octave down and ring mod stuff would sound with Vox pathfinder and its 8" speaker and was thinking of finding or making a cheap cab with 8s or 10s in it to get that definition/tightness back. The amp lists on the back:

Parallel Speaker jacks
85w
4 ohm total

I have never considered this before so wanted to gather some info of people who have done this.

Would adding more speakers "soak up" headroom as well coax some drive from this clean amp?

Because it is parallel jacks, what ohms should the external jack be seeing? 4, 8, 16?

Thanks!

What is the impedance of the current speaker?

Is this an old tubed Twin?

I think they tolerate a 100% mismatch pretty well, which means you can get away with any final load between 2 & 8 ohms. The output is pretty solid, if you want to shift headroom/breakup I'd look at rolling input tubes or modification - or inefficient speakers so you have more loss at the domain change between electrical power and air pressure.

JonnyAngle wrote:What is the impedance of the current speaker?

4 ohms on the internal 15” speaker

So because the jacks are parallel, I should shoot to make (or find) an 8 ohm cab to show the amp a “6” ohm load? Or am I off and need a 16 ohm or 2 ohm. It’s weird to see the 4 ohm on the amp next to the jacks with the word “total” having an internal 4 ohm speaker already...

I should specify that this fender is a reissue. I think the full designation is “Twin reverb custom 15” and it came out in the last 10-15 years?

I also have an old Flot A Tone A600RT that has an ext speaker out jack, but the speaker is wired into a tube socket/hub looking thing and there is no jack to measure and I can’t get an ohms reading off that thing easily (could I take off the back panel and try to Digital multi meter the speaker tabs?) but if I’m hearing that Fenders are more unique that they can handle miss-match easily maybe it’s best not to take chances with this old amp (amazing surf reverb in this Flot, better than the twin reverb even)

If your internal speaker is 4 ohms and you hang an 8 ohm load in parallel you will be showing the amplifier a 2.66 ohm load (I can show the math if you're interested). Still in the realm of safety with that 100% mismatch tolerance.

That said, if this were my amp I might also consider replacing the stock 15 with an 8 ohm speaker and then building out an 8 ohm extension cab. The problem with uneven loads in parallel is that the power is not divided evenly between speaker systems, and so unless you have a lower efficiency speaker (system) on the low ohm leg and a higher efficiency speaker (system) on the higher ohm leg (that is therefore passing less current) you might run into the "feature" of one speaker being louder than the other.

It really can devolve into a dark art, depending on how picky the end user is.

Sometimes it works right out of the box though, it just sucks when it doesn't and you've spent the last three weeks building a cab/etc.. so my advice is to hedge your bets and have another amp on hand that can be volume trimmed to match the Fender, etc..

I have a hoarder mentality though, so take my blathering with a grain of salt.

hangwire wrote:I’m hearing that Fenders are more unique that they can handle miss-match easily maybe it’s best not to take chances with this old amp (amazing surf reverb in this Flot, better than the twin reverb even)

I wouldn't say they are unique, it's just that some amps are more sensitive than others to load changes and with the exception of the 400PS I think all Fenders just connect the output of the transformer direct in parallel to the pair of speaker jacks without providing user faculty to set nominal impedance.

For example, the output transformer in my Quad is the same as you'd find in a Twin, and the four 8 ohm speakers are wired in parallel resulting in 2 ohms. I don't use the extension cab output they thoughtfully provided though, since I don't want to replace my tubes more often than necessary.

Some companies employ different designs that are less forgiving, but everything is using electrons to push paper which is not a stable load (there's slop in the real world), so unless something is tuned to get the absolute maximum out of a given active component you're generally free to have fun without too much worry.

That Flot sounds like a cool amp!

crochambeau wrote:If your internal speaker is 4 ohms and you hang an 8 ohm load in parallel you will be showing the amplifier a 2.66 ohm load (I can show the math if you're interested). Still in the realm of safety with that 100% mismatch tolerance.

That said, if this were my amp I might also consider replacing the stock 15 with an 8 ohm speaker and then building out an 8 ohm extension cab. The problem with uneven loads in parallel is that the power is not divided evenly between speaker systems, and so unless you have a lower efficiency speaker (system) on the low ohm leg and a higher efficiency speaker (system) on the higher ohm leg (that is therefore passing less current) you might run into the "feature" of one speaker being louder than the other.

It really can devolve into a dark art, depending on how picky the end user is.

Sometimes it works right out of the box though, it just sucks when it doesn't and you've spent the last three weeks building a cab/etc.. so my advice is to hedge your bets and have another amp on hand that can be volume trimmed to match the Fender, etc..

I have a hoarder mentality though, so take my blathering with a grain of salt.

Oh the ideal solution would be to get one of those vibrolux custom silver face things that came out a few years ago, but money wise the $800-900 and time to find a used one is way more than finding an ugly 2x10 or 8”/10” speakers in the many shops around town. I believe Sonic is/was a speaker cab company that is based near here so finding junky 80s-90s grey fur or too much grill aesthetic cabs for $50-75 is easy.

I’m wondering, if I have to cobble together my own cab from used speakers/unloaded box, should I make it a 16 ohm cab to get the impedance above that above quoted 2.66 for any tech benefit for the amp/tubes?

hangwire wrote:I’m wondering, if I have to cobble together my own cab from used speakers/unloaded box, should I make it a 16 ohm cab to get the impedance above that above quoted 2.66 for any tech benefit for the amp/tubes?

I would recommend steering toward keeping your loads more balanced than shooting for an end number as close to 4 as possible.

Observe:

1/4 (the divisor represents your four ohm load) = 0.25
1/16 = 0.0625

0.25 + 0.0625 = 0.3125

1 divided by 0.3125 is 3.2, which is the (nominal) load seen by the amplifier.

So, you have gained about a half of an ohm by sizing out to 16 on the extension.

Let's look at how the split appears to the speaker level signal path:

You've a 3.2 ohm load, achieved by supplying a 4 and 16 ohm path.

3.2/4 = 0.8 (or 80% of 1, or the whole)
3.2/16 = 0.2 (20%)

So your 4 ohm load is seeing four times as much power as the 16 ohm load.

Not an issue if the 4 ohm speaker produces the same or similar sound pressure levels at say, 40 watts as the extension cab does at 10, but as you veer farther apart this can become more of a potential issue. Or not, there's a ton of shit at play, speaker efficiency, actual surface area of drivers, etc.

In the end use your ears, I only took to this tirade of mathematics to punctuate the point that should you come across a 4 ohm extension cab it will probably be safe to plug in and listen to as well, though if you're playing loud it's likely you'll be putting harder miles on your tubes, which ultimately are designed to be replaceable.

crochambeau wrote:

hangwire wrote:I’m wondering, if I have to cobble together my own cab from used speakers/unloaded box, should I make it a 16 ohm cab to get the impedance above that above quoted 2.66 for any tech benefit for the amp/tubes?

I would recommend steering toward keeping your loads more balanced than shooting for an end number as close to 4 as possible.

Observe:

1/4 (the divisor represents your four ohm load) = 0.25
1/16 = 0.0625

0.25 + 0.0625 = 0.3125

1 divided by 0.3125 is 3.2, which is the (nominal) load seen by the amplifier.

So, you have gained about a half of an ohm by sizing out to 16 on the extension.

Let's look at how the split appears to the speaker level signal path:

You've a 3.2 ohm load, achieved by supplying a 4 and 16 ohm path.

3.2/4 = 0.8 (or 80% of 1, or the whole)
3.2/16 = 0.2 (20%)

So your 4 ohm load is seeing four times as much power as the 16 ohm load.

Not an issue if the 4 ohm speaker produces the same or similar sound pressure levels at say, 40 watts as the extension cab does at 10, but as you veer farther apart this can become more of a potential issue. Or not, there's a ton of shit at play, speaker efficiency, actual surface area of drivers, etc.

In the end use your ears, I only took to this tirade of mathematics to punctuate the point that should you come across a 4 ohm extension cab it will probably be safe to plug in and listen to as well, though if you're playing loud it's likely you'll be putting harder miles on your tubes, which ultimately are designed to be replaceable.

I appreciate the math! Helps me understand what is at play.

I have never replaced any amp tubes in any amp, so at my peak I wasn’t playing out enough to cause significant wear

hangwire wrote:I have never replaced any amp tubes in any amp, so at my peak I wasn’t playing out enough to cause significant wear

I go ages between replacement as well, that said, sometimes the difference between worn and fresh tubes is like upgrading your amp. The longer you go, the better the change feels.

crochambeau wrote:

hangwire wrote:I have never replaced any amp tubes in any amp, so at my peak I wasn’t playing out enough to cause significant wear

I go ages between replacement as well, that said, sometimes the difference between worn and fresh tubes is like upgrading your amp. The longer you go, the better the change feels.

quick math follow up:

so with an 8 ohm cab that would be 66% from amp internal speaker and 33% from cab using your math above [2.66/4 and 2.66/8]. That actually sounds in theory a good ratio

Just a follow up, was able to get a loaded 2x10 peavey bass cab for $50. I have a way to sell the speakers for $20. Found a celestial g10 16ohm locally. I can be patient and wait for a second 16ohm to pop up or get a cheap Jensen one shipped off reverb.

Not to take a step down the dark rabbit hole, but open to knowing more about wattage of each speaker and what to consider for a better balance if 66% is still going to the internal 15” speaker on a 85w tube amp. Would there be a speaker wattage too low or too high that would immediately raise eyebrows to avoid so I have a range to limit myself?

Thanks again for all the info, this is very helpful in guiding me