Is it safe to run extension cabs off a BF Deluxe Reverb?

Marc Stone · From: Louisiana, USA

What ohm load can the extension jack on an original BFDR handle, and is it safe to run extension cabs off these amps? A friend of mine who is a tech thinks that might put too much strain on the aging transformers.

Thanks

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

If you don't push the hell out of it, it's generally considered safe to run an old Fender BF/SF amp with load impedanced mismatched +- 100% - that is, if the output transformer is not somehow weakened already. But a weakened transformer - one that's been abused somehow in the past - might have problems. DR transformers tend to run hot when pushed, and they can be trouble. I had one smoke on me, and I wasn't really doing anything 'wrong'. I was running it straight into an extension cab with a JBL-130F, 8 Ohms total load, at a not-so-loud gig - when smoke started coming out the open Vibrato Channel input jack. I imagine that long years of pushing it hard took their toll.

IMO, if one wants to run mismatched, it's better to run higher-load (4 Ohms - e.g., main speaker 8 Ohms in parallel with an 8 Ohm extension speaker) rather than lower-load (16 Ohms) - the flyback voltage spikes from a too-low load can damage the OT. Again, if the transformer is robust in the first place, it may not be a problem, but I wouldn't run 16 Ohms.

My experience is that the silverface output transformers I've had were a bit more robust - that might influence my decision somewhat. The one I smoked was a BF, and I had a devil of a time finding a period-correct replacement.

For more info than you probably want to know, look here - http://www.geofex.com/tubeampfaq/tubefaq.htm#transformers

Mitch Druckman · From: Arizona, USA

I've run an 8 ohm extension cab with both my '66 and reissue Deluxe Reverbs without any problems. They sound great with the additional cab.

Marc Stone · From: Louisiana, USA

I'm going to run the amp pretty hard, prob about 7.5 on the vol. Blues/rock guitar thing, outdoor stages. I guess I better think twice about running an ext cab if there is a chance of smoking one of the transformers.

Bob Hamilton · From: California Central Coast

I wouldn't risk it with my '64..

Dave O'Brien · From: Florida and New Jersey

What Mitch said...

Jim Sliff · From: Lawndale California, USA

What Dave said, especially about flyback. A common misconception is that it's safer to run a higher *numbered* impedance extension than a lower one, which is simply wrong - but regardless, the impedance of the extension speaker should match that of the stock one (and NEVER run an extension without a speaker plugged into the normal speaker jack!).

All that being said, whether a '64 or '74 if the amp has been serviced it'll be fine. But if it's a 20+ year old amp and it's either never seen a shop for normal servicing (checking caps for leakage and probable replacement of filter and bias caps, checking resistor values for drift, tube checkout and rebias etc) or it's at for an extended period (5 years+ without being turned on, which can result in he filter caps crystallizing - any amp that sits that long needs to be checked for safety of the transformers and other components) shouldn't even be switched on unless it's at a qualified tech's shop. It's a time bomb. Many old amps will run fine with no service, but they can suddenly blow a cap without warning, taking out a power or output transformer. That not only kills the amp but kills the vintage value as well.

I've run (and seen plenty of others...) Fender BF & SF amps with extension cabinets of proper impedance with no problem. They were built to handle it and in 45 or so years of messing with the things I've never seen one blow due to a proper extension speaker.

One last note, sort of off the OP's topic (Deluxe Reverbs) but on the same subject - be careful just picking a *cabinet* of the same impedance as the stock speaker (or total impedance of stock multiple speakers). You need to know how the stock ones are wired and how the extension cabinet is wired - a single 8 ohm speaker and a 2x12 8-ohm cabinet (consisting of two speakers of other values, depending on whether they are wired in series or parallel) are NOT "equal".
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

T. C. Furlong · From: Lake County, Illinois, USA

Jim Sliff -

Rich Gibson · From: Pittsburgh Pa.

I ran my 71 deluxe with a Marshall 4X10 cab out of the extension jack for 5 years of steady weekends.No problems and it sounded great.Still have the amp and use it regularly.

Jim Sliff · From: Lawndale California, USA

TC - if the speakers AND jack are wired in parallel you need to factor the impedance of each speaker as an individual unit - the 2-speaker cabinet, when everything is parallel, is just "seen" by the output transformer as THREE items - not two.

There are several other extension jack wiring schemes that can affect the total, and if the cabinet is in series but the jacks are in parallel it's another scenario.

If you are mixing a single with multiples, or two multiples, OR parallel and series wiring you must know how each piece is wired - and after years of doing this, if it's a really complicated/screwy setup I go to one of the audiophile sites that cover it in detail and have on-line calculators rather than my ancient electronics texts (I think they were written in Sanskrit)...

Wink

As far as plugging in a 4x10 Marshall cab - sure, it might work, and depending on the impedance it might sound great...and be right on the edge of blowing your amp up...or sound weak and tinny. If it sounded good, didn'5t blow and the total impedance was outside the normally-accepted 100% "safety zone" of Fender tube amps, you simply got lucky.

Also - FWIW Marshalls are generally not nearly as forgiving as Fenders - that's why many have an impedance switch and multiple tap OT.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Earnest Bovine · From: Los Angeles CA USA

T. C. Furlong · From: Lake County, Illinois, USA

Jim Sliff,

The only possible thing that I can imagine you are suggesting is that the jack has an impedance that needs to be factored in.

-Or- are you talking about the speaker in the combo amp plus the speakers in the extension cabinet. If that's what you mean by all three - I agree. And you're right, it can get tricky when you are figuring series and parallel connections.

The formula for parallel is: load = impedance of driver one X impedance of driver two divided by impedance of driver one + impedance of driver two. Take the example of two eight ohm speakers in parallel. 8X8=64 8+8=16 64/16=4 Two eight ohm speakers in parallel will appear to the amp as a four ohm load.

The formula for series is: load = impedance of driver one + impedance of driver two.

Two eight ohm speakers in series 8+8=16

No?

TC

Earnest Bovine · From: Los Angeles CA USA

Some cables and connectors might have significant impedance compared to the ~8ohms of a speaker, so we don't use those with speakers.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

A single nominal 8-Ohm speaker cab, a cab with two 16-Ohm speakers wired in parallel for a nominal 8 Ohms, and a cab with two 4-ohm speakers wired in series for a nominal 8 Ohms do not necessarily have the same complex impedance. The nominal impedance is 8 Ohms, but the individual speaker reactive impedances and the way they combine may be quite different. There can be similar types of differences for more complex speaker topologies.

For example, if the inductive and capacitive impedances of the speakers were the same (or similar), the capacitive impedance of a nominal 4-Ohm series pair would be about half of a 8-Ohm speaker, which would be about half of a 16-Ohm parallel pair. Similarly, the inductive impedance of a 4-Ohm series pair would be around double of a 8-Ohm speaker, which would be about double a 16-Ohm parallel pair.

Of course, even if they're exactly the same type of speaker, the inductance and capacitance of each different impedance speaker may be different. That doesn't get into the differences in reactive impedance from completely different types of speakers with the same nominal impedance. The point is that the frequency response can be quite different for different speaker configurations depending on the number and type of speakers and how they're wired to get the nominal impedance. They'd be the same if each speaker in the 4-Ohm series pair had double the capacitive and half the inductive impedance of the 8-Ohm speaker, and similarly between the 8-Ohm speaker and the 16-Ohm parallel pair. But I doubt most speakers of different nominal impedance satisfy this type of relationship - I think those parameters depend heavily on how the voice coil is wound and a bunch of other things, and aren't necessarily linearly related to the DC resistance.

My attitude is that when combining speakers like this, I want to at least listen to the different speakers and topologies, and maybe even measure complex impedance. I used to have a General Radio 650-A impedance bridge for this purpose, but it got jettisoned some years ago.

Jim Sliff · From: Lawndale California, USA

Richard Sinkler · From: aka: Rusty Strings -- Missoula, Montana

Dave Grafe · From: Hudson River Valley NY

Richard, you are now explaining the cutting edge of musical performance science. Back in the day when the band played too awful to listen to they just turned it up real loud. Nowadays when the band is too ugly to look at they just fill the stage with smoke. Somtimes they also shine a lot of really bright lights in the audience's eyes just in case the smoke isn't enough....

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