Impedance. Got me wondrin'......

purpleplexi

Well-Known Member
Joined
Feb 14, 2011
Messages
1,793
Reaction score
3,483
A while back I was struggling with the brightness of my SV20C and I moved the speaker lead from the 16 ohm output to the 8. This smoothed things out quite a bit but after a while I got bored with it. I wanted my old Marshall 'bite' back. So I plugged back into the 16 and there it was.
In the meantime I had seen an alnico gold 10 8 ohm for sale used. I then wondered if I used an 8ohm speaker with the 8 ohm output would I still have the 'bite' in my sound. Then I wondered what makes amp designers choose a particalular impedance when designing an amp.
As you can see my thirst for knowledge is limitless. Does anyone know?
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
3,014
Reaction score
6,107
Well, as to why specific output impedance taps are made available, it has to do with parts. Transformers that already exist (readily), cabs and speakers pretty much all using 8 and 16 Ohms nowadays and a for a long time, too. When there's a 'standard', it makes sense for manufacturers to keep adhering to it, often. If your amp has an 8 Ohm tap, you can practically connect pretty much any cab/speaker to it.

Attaching a 16 Ohm speaker/load to an 8 Ohm tap is just fine, but don't do it the other way around (lower impedence load on higher rated tap).

Mismatching the safe way (as you did, putting a 16 Ohm speaker on your 8 Ohm tap) will result in a loss of Wattage (put very simply). Many artists have come up with all kinds of combinations that gets them "their mojo tone whatnot".

The Gold 10 will (apart from its specific tonal characteristics) react the same on the 8 Ohm tap as your 16 Ohm speaker will on the 16 Ohm tap. Of course, no two models of speaker are entirely equal and you might love it or hate it. I have no first hand experience with the Golds.

Now, here's something to wrap your head around; if you decide to somehow connect the 8 and the 16 Ohm speaker, you'll want the 8 Ohm on your 4 Ohm tap and the 16 Ohm speaker on the 8 Ohm tap. In turn, this balances the resistance for the output valves as the OT's primary has no "notion" of what's connected. This is put very simply, and somebody might flame me for that, but if you want more info feel free to do some digging, and I'm sure some people here will chip in shortly...
 

junk notes

Well-Known Member
Joined
Dec 10, 2018
Messages
4,121
Reaction score
5,748
Studio Series are self biasing. Just read that there are certain individuals here on MF that have the schematic and swore to secrecy. :coffee:

I am wondering if the G10-35L that are in the 1965 cabs or the 5005 combo, sound good?
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
3,014
Reaction score
6,107
Studio Series are self biasing.

Uhm, yes, they are, indeed. I'm missing the point, maybe... ?

Just read that there are certain individuals here on MF that have the schematic and swore to secrecy. :coffee:

Well, we have the schematic for the SV20, and perhaps a detailed overview of the DI is on its way. I'll be ripping my SC20 apart soon and use the SV20 schematic as a base for the output stage, FX loop and DI. The preamp is pretty much a stock 2203, just the component labels need correcting, of course.

Not so surprising that techs have to sign an NDA for that stuff. I've had to sign an NDA just to have a product demo'd once...
 

mark123

Well-Known Member
Joined
Apr 2, 2017
Messages
649
Reaction score
1,279
From http://www.valvewizard.co.uk/se.html

Remember though, the load line drawn is only correct provided the right impedance speaker is plugged in. Connecting a higher impedance speaker will cause the load line to rotate anti-clockwise around the bias point, possibly causing screen-grid failure due to passing below the knee of the grid curves (although if you're lucky, the screen resistor will fail open first). It can also cause arcing in the transformer due to much higher anode voltages being developed when the valve is overdriven. Connecting a lower impedance speaker will have the opposite effect; the load line will become more steep, pushing the valve into cold Class A operation which may or may not cause over dissipation of the anode (thankfully it usually doesn't). It is therefore always safer to plug in a lower impedance speaker than a higher one, if you have to.

This is specifically speaking about SE but I’m thinking a mismatch is a no-go all around.
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
3,014
Reaction score
6,107
From http://www.valvewizard.co.uk/se.html



This is specifically speaking about SE but I’m thinking a mismatch is a no-go all around.

Well, mismatching is messing with the fidelity of the sound, but so is introducing any kind of distortion or having an unbalanced PI - and those are staples of electric guitar amplification. Note that while people go out and buy balanced ECC83's for their PI, Marshall sabotaged this by using different resistor values for each leg. In fact, [enter rabbithole], using an output transformer at all is messing with your fidelity.

Attaching a higher impedence load to a tap rated for less will not induce any damage, within reason. I recommend going no higher than maybe 2x or 2.5x the rating. If you're pushing an amp to its absolute limits, make that maybe 1.5x to 2x.

Attaching a lower impedence load to a tap rated for more can easily start to damage things. Here I would use a 0.75x rule of thumb, though personally I avoid ever doing this, certainly if I'm going to pound the amp hard for a prolongued period of time.
 

mark123

Well-Known Member
Joined
Apr 2, 2017
Messages
649
Reaction score
1,279
We’re talking about reflected load to the primary in this case and tubes can handle a range but I’m not suggesting anyone try it and see because you can either fry your tubes or your output transformer.

$$$$$
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
3,014
Reaction score
6,107
We’re talking about reflected load to the primary in this case and tubes can handle a range but I’m not suggesting anyone try it and see because you can either fry your tubes or your output transformer.

$$$$$

I don't mean to be nasty here, but it might help if you get your facts straight. Now, what you're saying is not "wrong", but to imply that mismatching of the type I outlined earlier as 'safe' is likely to cause damage is a falsehood. OP in fact already did just that (16 Ohm speaker on an 8 Ohm tap) with no ill effects. Of course, people get away with doing things wrong all the time, but I've yet to encounter anyone who actually managed to damage anything this way. Wearing the output valves out a little faster; sure - but actual "damage"... not in my book yet.

Again, I mean no harm or disrespect, I merely beg to differ. :)
 

purpleplexi

Well-Known Member
Joined
Feb 14, 2011
Messages
1,793
Reaction score
3,483
There's a YT vid I've seen where Dr Z of expensive amps fame suggests that we all try mismatching impedances to get the tone we want. I guess he ought to know. After all he knows how to get people to pay a lot of money for amps. Not saying they're not good mind.
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
3,014
Reaction score
6,107
There's a YT vid I've seen where Dr Z of expensive amps fame suggests that we all try mismatching impedances to get the tone we want. I guess he ought to know. After all he knows how to get people to pay a lot of money for amps. Not saying they're not good mind.

I'm an engineer and I've encountered a fair deal of salesmen. People who are good at both are very, very uncommon. :shrug:
 

mark123

Well-Known Member
Joined
Apr 2, 2017
Messages
649
Reaction score
1,279
I don't mean to be nasty here, but it might help if you get your facts straight. Now, what you're saying is not "wrong", but to imply that mismatching of the type I outlined earlier as 'safe' is likely to cause damage is a falsehood. OP in fact already did just that (16 Ohm speaker on an 8 Ohm tap) with no ill effects. Of course, people get away with doing things wrong all the time, but I've yet to encounter anyone who actually managed to damage anything this way. Wearing the output valves out a little faster; sure - but actual "damage"... not in my book yet.

Again, I mean no harm or disrespect, I merely beg to differ. :)
I don’t mind. You can do what you want with your stuff. 👍

In my reading is been stated that a lower load is safer than a higher load because the higher load can take you outside of the tubes impedance range. I’ve also read the opposite with no explanation beyond anecdote. I match my impedance because in my experience an imagined mojo boost from mismatching doesn’t really exist.
 


Top