Another Fender amp question

Dave Zirbel · Post by **Dave Zirbel** » 12 Aug 2002 8:19 pm

My early to mid 1970s Deluxe Reverb is designed to take an 8 ohm speaker. Was the extension speaker jack designed to take an additional 8 ohm speaker, or does the total load need to stay at 8 ohms?

Thanks, Dave Z

Ricky Davis · Post by **Ricky Davis** » 13 Aug 2002 2:03 am

Dave it doesn't matter what ohm load you got going on that Deluxe.
You can do that with a tube amp but not a solid state......so your safe if you run a 8ohm ext...or 4 ohm ext.....even with a 8 ohm speaker already in it.
Back when I had my deluxe I put all kinds of different speakers in it and ext. too.....and the only thing I had to do at any time at all was to turn the volume up maybe a number or down a number...
Ricky

Michael Brebes · Post by **Michael Brebes** » 13 Aug 2002 9:43 am

It's actually the other way around, solid state is not particular about impedance but Tubes Are. Because of the transformer replacing the plate resistor, the impedance of the speaker does make a difference. I wouldn't put an external speaker of smaller than 8 ohms on the Deluxe. The transformer is designed for an 8 ohm load.

Ken Fox · Post by **Ken Fox** » 13 Aug 2002 12:57 pm

Solid state is very particular about being ran into a lower impedance than it is rated into. This can take out the output transistors and set fire to speaker in a hurry. An impedance higher than rated is wonderful, even an open circuit!
Most tube amps are forgiving about an impedance mismatch one way up or down. However the flyback voltage created by the speaker back thru the output transformer has long term effects on the plates of the tubes and possibly tube sockets. I posted a good article about this, from Weber VST, last week here on the Forum. We have probably all done and got by with it, that don't make it right!
In the case of Fender amps, I would not run an extension speaker on an amp with a mere parallel jack, except at lower volumes. Some amps (such as the 135 watt Twin) are designed for a true external speaker by using a different tranformer tap that is brought into the circuit via a switch on the external speaker jack. The same thing is done on the Hotrod series of amps.

Craig A Davidson · Post by **Craig A Davidson** » 14 Aug 2002 3:34 am

Two 8ohm speakers would make that a 4ohm load then or does the transformer account for that?

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1985 Emmons push-pull, Session 500, Nashville400, 65 re-issue Fender Twin, Fender Tele

Ken Fox · Post by **Ken Fox** » 14 Aug 2002 9:37 am

Generally 2-6V6 tubes need to looks at about an 8000 ohm load. The transformer is usually 7600 to 8500 to 8 ohms. As impedance is directly proportional to the square of the turns ratio that would imply a turns ratio of about 32.5 to 1 and impedance ratio of 1062.5 to 1, for an 8500 ohm primary. For a 4 ohm load the plates of the tubes would be looking at 4250 ohms. According to my GE tube characteristics chart a 6v6 in a class AB1 circuit would like to see 8000 ohms plate to plate. I would tend to think the Deluxe was designed with that in mind! Looking in my Hammond replacement transformers chart, I see a 7600 ohm primary transformer recommended for a 25 watt 6V6 application. Like I said, you can probably get by with an extension speaker, but you can't prove it to be technically correct. [This message was edited by Ken Fox on 14 August 2002 at 10:58 AM.]

Eric Jaeger · Post by **Eric Jaeger** » 14 Aug 2002 2:34 pm

Hmmm, it the extension output jack series or parallel? If series, then the load sums,
if parallel it's a little more complicated but basically two 8-ohm speakers in parallel give you a 4 ohm load.

Ken's absolutely right, most transistor amps
hate low impedance loads, and many can become unstable into low loads. Unfortunately, many modern high fidelity speakers (ribbons especially), can go under 1 ohm once in while, and it varies with frequency.

Ken Fox · Post by **Ken Fox** » 14 Aug 2002 2:38 pm

The 135 watt Twin is one rare bird that puts the extension speaker in series with the internal speakers and changes from a 4 ohm tap to an 8 ohm tap via a switch contact on the jack! That way a 4 ohm external load and the internal 4 ohm load are summed at 8 ohms and have the correct tap on the OT. Interesting, they did not just put the jack in parallel like on the old Twins.

Craig A Davidson · Post by **Craig A Davidson** » 14 Aug 2002 5:47 pm

What about a 4ohm speaker extended out of a twin reverb(the 65 re-issue). That gives me two ohms? Is that ok?

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1985 Emmons push-pull, Session 500, Nashville400, 65 re-issue Fender Twin, Fender Tele

Bob Metzger · Post by **Bob Metzger** » 15 Aug 2002 11:46 am

What Ricky, Micheal and Ken have to say is true.

Tube amps run best with the proper load matching the output transformer and the power tube complement. However, ask any guitarist who has been thru the wars and you'll hear time and again that Fender amps withstand impedance mismatch very well, within reasonable limits.

The question is however - not can you do it - but does it sound better?

Bob M.

Donny Hinson · Post by **Donny Hinson** » 15 Aug 2002 4:01 pm

No Craig, I wouldn't run a 4-ohm speaker out of your Twin Reverb, at least at any significant volume level. You see, none of this "impedance stuff" becomes significant (in terms of amp reliability, i.e. possible damage) until the volume goes up. The more you push the amp, the more significant the "problem" becomes. I'd use an 8-ohm speaker as an external with the T/R, and feel pretty safe with that. But when you start with the "Travis Tritt" volume levels, or when you start punching them "Chalker chords" on C6th, you gotta be aware that an amp is just like a car. The harder you "push it", the less time it's lkely to last. Prudence is cheaper than repairs.

Ken Fox · Post by **Ken Fox** » 15 Aug 2002 7:51 pm

Right on, Donny. That's what I been trying to say, too!

Bobby Lee · Post by **Bobby Lee** » 16 Aug 2002 9:04 am

I'm not familiar enough with the Deluxe to answer your question, Dave, but it really all boils down to the output transformer's configuration. If the external speaker jack comes from a separate transformer lead, it would probably handle a 4 ohm load easily. If it's wired directly to same transformer lead as the internal speaker, you wouldn't want the total to go below 4 ohms.

The only time I've ever seen a transformer blow in a Fender amp was when someone did something really stupid, like wiring six speakers in parallel. Wiring any speaker system to less than 2 ohms is just asking for trouble, no matter what kind of amp you're using.

My Boogie has two 4 ohm jacks and an 8 ohm jack. The 4 ohm jacks are paralleled on one transformer winding, and the 8 ohm jack is on another. I have, on occassion, accidentally run an 8 ohm load (the internal speaker) from one of the 4 ohm jacks. The amp sounded a little squeezed, but it still performed well. I was probably the only one who noticed the difference in headroom.

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<img align=right src="http://b0b.com/b0b.gif" width="64" height="64">Bobby Lee - email: quasar@b0b.com - gigs - CDs
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Craig A Davidson · Post by **Craig A Davidson** » 18 Aug 2002 6:48 am

Donny I guess my question is this: I am at four ohms in a stock situation, so, if I extended an eight ohm speaker I would then have a six ohm load or does the transformer adapt to the odd load? Right now I hook up the extra cabinent and unhook the two-twelves. I just wondered about running them all at once.

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1985 Emmons push-pull, Session 500, Nashville400, 65 re-issue Fender Twin, Fender Tele

Ken Fox · Post by **Ken Fox** » 18 Aug 2002 9:10 am

Anytime you parallel a resistance with another resistance the total is always less than the least of the two. An 8 ohm and 4 ohm load will be less than 4 Ohms:

1/8 + 1/4 + 1/resistance total= .375/resistance total

R/total= 1/.375= 2.67 ohms

Hamilton Barnard · Post by **Hamilton Barnard** » 23 Aug 2002 7:29 am

If solid state amps hate lower impedance loads why do manufacturers like Crown and QSC advertise increased power outputs with lower impedance loads, like all the way down to 2 ohm? I thought they built the 'ideal' impedance load near zero in those things...

Sheeesh, by the time we get all our monitors hooked up in parallel, that circuit would seem like it would be the next thing to a dead short; yet the amps have never protested.

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My Marshalls.

Ken Fox · Post by **Ken Fox** » 23 Aug 2002 9:36 am

Because they are designed to handle the power at two ohms! Many amps are not designed for a two ohm load, as the power transistors cannot dissipate the heat and are not rated for the increase in power that results from the amp seeing a lower impedance. Some amps use current limiting circuitry (some older Peaveys, for one) and actually put out less power at 2 ohms than at 4 ohms.

Bill Terry · Post by **Bill Terry** » 23 Aug 2002 9:53 am

I worked for Peavey dealer in the 80's. We regularly got a complaints about a monitor system, e.g. "the amp goes off, but after a while it comes back on, then goes off..."

You'd find out that the band had about a half dozen monitors daisy chained around the stage to one little monitor amp rated at 130 W at 4 ohms. It seems that a lot of those guys figured as long as you had a speaker, a cable and somewhere to plug it in you were good to go.

Those little Peavey 130 monitor heads were almost bullet-proof, thermal cycling all nite on a bandstand, hooked to an ohm or so load, and just kept working. Pretty amazing... Hartley must have carefully considered his intended market

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Bill Crook · Post by **Bill Crook** » 23 Aug 2002 11:54 am

It's really hard to kill a "CS-800"

Kudo's to "PEAVEY"

Ken Fox · Post by **Ken Fox** » 23 Aug 2002 2:35 pm

I was a Peavey dealer in the late 70's and early 80's myself! I saw that same scene over and over! They made then and now some pretty tuff stuff!
I have seen more than one amp cycling because someone though a guitar cable would make a great speaker cable!