8 ohm speaker on a 16 ohm secondary - is it safe

[dbishopbliss]

I just bought a replacement speaker for my guitar amp (trace elliot velocette).  Generally, this amp has a 8 ohm speaker, so I didn't pay attention when I bought a new one for it.  Ends up, it has a 16 ohm speaker and I bought an 8 ohm speaker because I didn't bother checking first. 

After doing some more research I discovered the output transformer has 3 secondaries, 5.3 ohm, 8 ohm and 16 ohm.  Apparently, the circuit board (gasp) has jumpers so the manufacturer wasn't tied to a particular model speaker.  That said, its a real pain to pull the amp out of the enclosure.  I want some instant gratification.

What will happen if I use a 8 ohm speaker on a 16 ohm secondary?  The transformers are known for being undersized and blowing up even when the speakers are matched correctly.  Should I be concerned?

[Paul Joppa]

Being a push-pull amp, probably with feedback, you must understand that this is not the most experienced forum!  :^)

Being a guitar amp, it is likely to be overdriven. With a low load impedance, that makes it easier to drive it into current overload. If that drives one tube into cutoff, there are some issues with inductive kick - I remember reading that some guitar amp technicians add diodes somewhere to limit this effect, which can be quite destructive. I think it was in Pittman's book.

Finally, you said "The transformers are known for ... blowing up even when the speakers are matched correctly.  Should I be concerned?" Well, yes, I would be. Even with a matched speaker!  :^)

[dbishopbliss]

Being a push-pull amp, probably with feedback, you must understand that this is not the most experienced forum!  :^)

I realize that my guitar amp questions are off topic, but I haven't found a forum I trust when it comes to guitar amps.  There seems to be a very different approach to building amps in the guitar world.  I have had people tell me that there are several people who make a living at selling "modification" kits that are actually unsafe.  I suppose they cannot be that unsafe because some of them have been in business for 20 years I know of. 

The other thing I notice, and maybe this is just because of when the amps, cables, etc were built, is that the construction leaves a little to be desired.  Huge blobs of solder, pools of flux, stray wires. 

Audio guys seem much neater and see to know the theory and best practices better than what I have found elsewhere.  Again, maybe I haven't found the right place to look yet.

Back to your answer... are you saying, "No, doesn't seem safe"

[Paul Joppa]

Haha! No, I am being careful to make no comment on the issue on the grounds that I don't have enough relevant experience. I just shared the few things I have heard, and gave my only reference for more.

[porcupunctis]

David, that's a pretty "iffy" move.  If you got the taps for 16 ohm, I would definitely take the time to switch them over.  If you were going the other way (8 ohm tap to 16 ohm speaker) you would not risk burning anything up but you wouldn't get the most from the amp either.

You're right, most of the guitar amp guys are different.  Many are musicians who hear a rumor that some mod will make their amp sound (insert just about anything here).  They go out and buy a soldering iron and the results are all over the board.

There are some out there that really know their stuff, though.  Aspen Pittman and Dan Torres come to mind but there are others. 

I restored a '61 Fender Princeton a few years back and I treated it like top audio gear.  I fussed over the tubes, the wiring, the soldering, everything.  I stayed away from mods altogether and kept it as close to factory stock as possible.  It sings, and I love it.

Stay fussy.

[dbishopbliss]

I opened it up tonight and swapped the jumper to the 8 ohm speaker.  The jumper was actually soldered and made to look like a resistor.  I tested it, and it definitely has no resistance.  In addition, it has a single black stripe on a tan body. 

Fired up the amp and it sounds pretty good with the new speaker - an Eminence Red Fang Alnico.  I didn't have a lot of time to play yet... family going to bed and its a school night.

[JC]

I'll weigh in with Mr. Massey here: If the output transformer has a reputation for being on the delicate side, I'd just change the taps.  You can always drive a higher load with a lower impedance output, but doing it the other way around is dicey.  Some of the vintage iron was abused regularly in such a way every time a player borrowed another cabinet to add to his rig.  It held  up somehow, probably because the manufacturers suspected that's what would be happen and had their transformers over-engineered.  But, I've had to tell many an owner of newer big-name tube guitar amps that they needed to have transformers replaced, output and power transformers both. For no apparent reason but that they just failed.

EDIT:  Oh, good, I see you already did it while I was delayed in posting.  Enjoy!

[dbishopbliss]

With regard to the delicate transformer, I'm wondering if it is really the transformer's being undersized or if there were just poor manufacturing standards.  I found a page out there where a person complained about the amp being too quiet (mine is very loud for 15 watts).  The amp tech discovered that the leads to the transformer were mis-labeled (actually mis-colored if that's a word).  Hopefully, mine are not this way.  Not what I want to do, but I know that mercury makes replacements if I need it.

[Grainger49]

David,

Run to the RS and buy yourself a few 10 ohm 10W resistors.  They still sell them, I think!  Put one in series with the 8 ohm speaker.  It wastes power but allows you to test the circuit. 

Then you can use the other resistors for breaking in capacitors.  I have two cooking now.