Ohm help for WEM Control ER 15 vintage valve amp

[sohosteve]

Does anyone know what ohm speaker cabinet I need for this amp?

I have a Marshall 2 x 12 with 2 x 16 ohm speakers which I've been told requires 8 ohms

Can I use this cabinet safely with the WEM Control ER 15?

Thanks

[Jazz P Bass]

The WEM ER15 is made for a 15 ohm load.
16 is close enough.

That is according to this literature.


Judging by the large size of the OT & the fact that it is a 15.5 watt amp, I would have to say you will not 'hurt' the OT with an 8 ohm load.

[Jazz P Bass]

Oh, and here is the schematic.

WEM ER15.pdf

[DRH1958]

Am I missing something here? Two 16 ohm speakers cannot be wired any possible way to get to 16 ohms. In series, you get 32 ohms and parallel you get 8 ohms. Sure, you can use 1 speaker and be fine but not both. If it has been your experience that an 8 ohm load will work, then OK. You have been a great source of knowledge on this forum, so I'll believe you. I have never used one of these, so I don't know about the mismatch of 15 vs. 8 ohms. 15 vs.16, yes no problem. Some OTs handle mismatches well, others no so much.

[Jazz P Bass]

Agree.

I was not stating anything.
Mearly putting the information that I found out there for the OP to decide.

Of coarse the cabinet cannot be rewired to meet the desired specs.
That was not even implied.

As a tech, I can only go by the rules.
The specs call for 15 (sic).
Then the OT 'requires' that load.

But then again, musicians have been abusing there tube gear forever with mismatched loads.

[DRH1958]

Absolutely correct. One person says one degree of mismatch(8 to 16 ohms) is fine and the next says you will be abusing the output tubes,tranny etc. I do know I have had several instances where I have read Marshall OTs don't handle mismatches as well as Fender. Watkins and Marshall are very similar. in fact, isn't the Marshall 18 watt amp a copy of a Watkins circuit, so that made me wonder about the mismatch. You're right, put the info out there and let the OP decide. We can't tell others what to do, only give info and our experience to help them make their own choices. Just thought I would bring up the mismatch issue in case the OP took it as an endorsement to go ahead to do it.

[sohosteve]

Hi there
Thanks for all your kind replies.
However I'm still somewhat confused!
As I mentioned, in my initial post above, the cab I wish to use contains TWO 16 ohm speakers that require an 8 ohm input.
What I was trying to find out is if I can safely run the Wem into those speakers without damaging either?

[olddawg]

i wouldn't worry about it. If you aren't diming the amp (because you will use some volume), you can always put a 16 ohm resistor in parallel with the cab or use two 16 ohm cabs.

[Jazz P Bass]

The only way to know for certain is to investigate the output transformer(OT).

The secondary (the speaker) reflects that load INTO the primary winding of the OT.
This will change the primary load expectation into the EL84 plates.

The two EL84s want to see a certain load.
You pick an OT that , with a specified secondary load, reflects this through the primary into the plates.
With me. Everybody is happy.

You want to lower the load that the OT (designed for a 15 ohm load into (2) EL84 tubes) wants to see.

Some amp designers pick a middle of the road OT that can go down with no harm, in case something like this comes up.

Is your OT like that?

You will never know until you determine the turns ratio.

[DRH1958]

The short answer is it can,but it might not. As Jazz explained, all a transformer does is reflect one impedance(secondary) into another(primary) according to the turns ratio of the transformer. Yours says it is designed for 15 ohms on the secondary. Your 8 ohm cab will reflect a lower than designed for impedance to the primary. This presents a smaller load to your output tubes than the amp was designed for. Some amps are OK with this and others are not. As olddawg pointed out, if you don't run your amp hard, probably no worries. If you crank it up, it might not be OK. Any time you don't match the speaker with the stated load on the amp, you run the risk of doing damage. The question is will it do damage? Is the amp's primary and output tube circuit going to handle this mismatch? There is no way to really tell for a 100% correct answer. Will it work? yes, it will. Just not as intended. It's up to you if you want to run the risk. You can hook it up to one speaker and it will be fine. 15 to 16 ohms is not enough to worry about.
Your amp only puts out about 15-18 watts, so unless your speakers are only designed to handle 18 watts, they will be OK.

[Jazz P Bass]

Hi there
Thanks for all your kind replies.
However I'm still somewhat confused!
As I mentioned, in my initial post above, the cab I wish to use contains TWO 16 ohm speakers that require an 8 ohm input.
What I was trying to find out is if I can safely run the Wem into those speakers without damaging either?

Well, the cabinet doesn't require an 8 ohm input.
It presents an 8 ohm load.

The OT/speaker load mismatch will not hurt the speaker.
Honestly, I would not worry about it.

[The Dude]

i wouldn't worry about it. If you aren't diming the amp (because you will use some volume), you can always put a 16 ohm resistor in parallel with the cab or use two 16 ohm cabs.

I think you may have misread the OP. The optimal target impedance for the amp here is 16 ohms- not 8. The OP has an 8 ohm cabinet already. Only pointing out for clarity.

[DRH1958]

The ratings are designed to protect different items. The ohm rating of the speaker is designed to protect the amp form large mismatches. It presents its rated impedance to the amp.
The wattage rating of the speaker is designed to protect it from too much power from the amp. An eight ohm speaker doesn't care what ohm output it is connected to, just as long as the power it receives is not higher than the wattage rating. Most amps will handle a one step mismatch(4 to 8 ohms or 8 to 16 ohms). Agree with Jazz, unless you are running your amp wide open for extended periods, probably no worries. 15 to 8 ohms mismatch should be fine.

[olddawg]

My thinking is usually this.... Most amps are designed to run 2 speaker cabinets. With one stock cabinet or the speaker load in a combo, the load is usually the optimum load for the OT. Generally the extension cabinet is the same load in parallel. So the minute you plug in an extension cabinet the load is halfed in most cases. I've never seen it hurt anything as as long as the loads are what the amp was intended to drive. Even a Super Reverb has an extension cab option and they are a 4 ohm load. With an extension cab its a 2 ohm total load. If this amp is designed to drive whatever ohms and has an extension speaker jack, I wouldn't think twice about driving half the load.

[The Dude]

Oh, I completely agree. I wouldn't worry much about running an 8 ohm load on a tube amp's 16 ohm tap as long as you're not consistently diming it. Solid state, a different story. The only reason I pontificated as I did in the post above was to avoid confusion for the OP.