Check your stock speaker ohms Rm50

Just a heads up, I was moving speakers around and putting the stock g12-T75
in my 50, always have run it at 8 ohms from the stock amp position. For whatever reason, I looked at the small printed info near the wire leads and it says 16 ohms. son of a. I have had that speaker mixed with all kinds of 8 ohm speakers in other spots and always assumed it was a match. jeez..
2 things to get from this, always check the fine print, and mismatching speaker ohms might not matter as much as I thought. :shock:

Been ages since I studied basic electronics so I might be wrong on my knowledge but,
a 16 Ohms speaker will put more load on the output of an 8 Ohm amp
but it'll only start to get serious when you start working the amp beyond half power

A 16 Ohm speaker will put LESS load, I=V/R. Of course, this is the DC formula but, close enough. The lower the ohm load across the OT, the higher the current. If you're going to mismatch, better to go into a "higher ohm" load which is a lower current draw.
I.E. Better to run the amp with the selector on 8 ohms into a 16 ohm speaker than a 4 ohm load. (two 8 ohm speakers in parallel.)

All I know is that was supposed to be a 8 ohm speaker from randall when I bought the amp new. I really should have checked. I haven't had that speaker in a the combo for years, and just decided to put it back in for a change. I actually ended up leaving the lead 80 in there after all. What a great speaker that one is.

I agree. The original speaker in my RM50 is also 16 ohm.

Good to know! I just bought an Ampeg GVT52-112 this weekend and had the same thing happen: 16 ohm speaker plugged into the 8 ohm jack. They had 3 of these amps so I've been wondering if I'd be better off returning it for another one, just in case it did any transformer damage. This one sounded the best of the 3 though so I don't really want to return it, but also don't want it blowing up anytime soon. Luckily I read a user review online about this Ampeg combo coming with a 16 ohm speaker, so I checked right when I got home and switched it.

Moral of the story: always check the impedance of a stock speaker.

Actually, running the amp with the impedance switch at 8 ohms into a 16 ohm speaker puts a lighter load on the OT. I think the confusion comes from thinking a 16 ohm load is "more" than an 8 ohm load.
Think of it this way: a 0 (zero) ohm load is a DEAD SHORT.

kc2eeb said:

Actually, running the amp with the impedance switch at 8 ohms into a 16 ohm speaker puts a lighter load on the OT. I think the confusion comes from thinking a 16 ohm load is "more" than an 8 ohm load.
Think of it this way: a 0 (zero) ohm load is a DEAD SHORT.

Click to expand...

I think of it like in hydraulics
decrease the valveopening (= increase Ohms) and increase water pressure (= Watts) and something is gonna blow

Sorry, but I have to correct you. (OMG I sound like a school teacher)
To use the plumbing analogy:
Current is the volume of water
Voltage is the pressure
Resistance you might say is the diameter of the pipe
So, think of a high resistance as a pin hole in a dam, it's only a drip,
but, lower resistance, you've got a hole in the dam and the valley is flooding.
High resistance =less current

Wrong analogy, since this is a static power provided by gravity
A speaker does not 'pull' Watts, it gets them delivered

You're forgetting the pump that seizes because it can't get over the resistance, or maybe before the pump dies, the pipes crack or the valve is blown to pieces

Ok, let's try this.
I did not say the speaker "pulls" watts.
The most fundamental formula in electronics and electricity is current =voltage/resistance. So, if you have 80 volts/8 ohms = 10 Amps
80 volts/16 0hms = 5 Amps
The higher resistance, with everything else remaining the same, produces less current.
This is for DC. For the purpose of audio amplifiers the resistance goes up as frequency increases so a guitar speaker actually has higher resistance as the frequency goes up.
Think of a car battery with jumper cables attached. If you touch the cables together and hold them you'll probably blow the top off the battery because the ohm resistance is very low, but, if you connect it to a headlight, (higher resistance) lower current and the bulb lights up.

Lol, can't help myself here...

The valve analogy is applicable, as a resistor. The smaller pipe analogy is applicable as a wire/conductor with a higher resistance (ohm/ft for example). In both cases, pressure (and voltage) decreases across the resistor, and flow rate (current) is equal, throughout the discreet circuit.

I'm a mechanical engineer, so I have to analogize electrical stuff as such, lol.

LOL, I know. I've never been comfortable with the water analogy.
That's the problem with "texting" it's really not a conversation. Each party tries to "guess" what the other is thinking.
All the best to one and all! Pat

kc2eeb said:

Ok, let's try this.
I did not say the speaker "pulls" watts.
The most fundamental formula in electronics and electricity is current =voltage/resistance. So, if you have 80 volts/8 ohms = 10 Amps
80 volts/16 0hms = 5 Amps
The higher resistance, with everything else remaining the same, produces less current.
This is for DC. For the purpose of audio amplifiers the resistance goes up as frequency increases so a guitar speaker actually has higher resistance as the frequency goes up.
Think of a car battery with jumper cables attached. If you touch the cables together and hold them you'll probably blow the top off the battery because the ohm resistance is very low, but, if you connect it to a headlight, (higher resistance) lower current and the bulb lights up.

Click to expand...

So if I connect a 1000 Ohm cab to a amp rated for 8Ohms, It'll save on the electric bill?

Yep, and it won't be very loud.

Just checked mine -- it's 16 ohms. All along I had thought it was 8 ohms...

For awhile I have used my RM50 combo with the matching Randall 1x12 cab, which has an 8-ohm Greenback in it. I have the impedance switch on the RM50 set to 4 ohms, so I'm safe in that regard given that my the total parallel resistance is 5.33 ohms, but I still have a mis-match...

As long as the impedance switch on the amp is equal to or LOWER than the impedance of the actual speaker, you're OK.
For example, a 4 or 8 Ohm setting on the amp is OK going into an 8 Ohm speaker.

Think about it like this... I read most of, but not all of, these silly back and forth posts about water and all that **** LoL. If you set the amp at 16 ohms (ohms is resistance, hopefully everyone already knows this) and you put a 4ohm speaker in front of it... The amp is going to send a lot of pressure (which would be a better word than some of the ones I've read here) to overcome a 16 ohm resistance. If there's only a 4 ohm speaker and a 16ohm signal, the amp will release too much power, and then you may have some damage. If you do the reverse... send out a signal expecting a 4ohm resistance, to a 16ohm speaker... the signal will be too low and the volume won't be very loud. ...there won't be any damage in the 2nd case. Your volume control will just be turned up higher. It's pretty simple.

kc2eeb said:

As long as the impedance switch on the amp is equal to or LOWER than the impedance of the actual speaker, you're OK.
For example, a 4 or 8 Ohm setting on the amp is OK going into an 8 Ohm speaker.

Click to expand...

To an extent. From what I've read, a 16 ohm speaker into a 4 ohm tap is a bad idea and can cause flyback voltage.