Technical Question

jbrown013

Member
Messages
387
I have an old head with 2 output jacks for a speaker cab, I cannot recall which is what ohms. Does anyone know how to check this before I go plugging it in and potentially hurting something.

Thanks,

Joe
 

hammersig

Silver Supporting Member
Messages
1,425
What kind of amp is it? As far as I know, there's not a way to test with a multimeter. I'm sure someone on here has the same amp.
 

jchan

Member
Messages
1,684
The 2 jacks may be in parallel with an ohm selector switch or they may be for 2 different impedance outputs. If you reply back with the manufacturer and model, someone may be able to help.
 

jbrown013

Member
Messages
387
The amp was built for me by someone I had met on HC. He built it for me around 2003 iirc. He soon after that left for the military lol. I'm pretty certain im one of the only people that have an amp of his, he was going to start up a company called Chapel Amps. He did a great job, although no knobs were labeled lol. Is there anyway to open it up and see how they are wired or what not and tell that way?
 

Deaj

Member
Messages
4,674
The lower impedance OT tap will very likely to be louder than the higher impedance tap. So if one of the speaker outs is a 4ohm tap and the other is an 8ohm tap you could plug your cab up to each speaker out and play. If one of them sounds louder that's going to be the 4ohm.
 

kimock

Member
Messages
12,524
The lower impedance OT tap will very likely to be louder than the higher impedance tap. So if one of the speaker outs is a 4ohm tap and the other is an 8ohm tap you could plug your cab up to each speaker out and play. If one of them sounds louder that's going to be the 4ohm.
That depends on the speaker impedance though, right?
If it's a 4 ohm speaker, the amp will make more power with that speaker on the 4 ohm tap, yes.
If it's a 16 ohm speaker, the amp will be louder on the 8 ohm tap than the 4, but you still won't be at the correct load, etc.

The lower impedance tap would normally make more power on a solid state amp though for sure.
 

StompBoxBlues

Member
Messages
19,943
I'm just thinking here...not recommending anything directly to you, but...

Generally, you get 4, 8, 16 ohm speaker outs on amps.

Generally, you can get away with a slight mismatch...one "notch"...so
if you use 8 ohm speakers...if it was designed for 4, you are one notch away, if it was designed for 8 you are on the money, if 16 one notch away.

Either of the other ones, and you could be mismatching too far away.
I think your best options are...

1) find that person that made the amp, google search, etc. and ask.
2) Open it up, look very closely at the output transformer, and see if there is some identifying number-company on it, google that..etc
3) Tell US MORE INFO....like what kind of amp (I mean is it totally unique or modelled after a Fender, or Marshall, or other amp? What kind (and how many) output tubes are there...it might not help, but there is very little info to go on. Marshalls often go into 16 ohms, I think, the Marshall cabs...Fenders, I think often are 8 ohms, but not totally sure, but other folks here might have a broader experience to go from IF they know what the amps lineage (what it was supposed to emulate if anything)...

4) Take it in to a tech and pay them to check?
5) Look for old pics of the amp, if you have any, etc. and see what bottom you used with them....find out what the impedance of that was.
Did you sell the cab and just keep the head?

Or just use the 8 ohms out...don't use both outputs. Or try each and verify that it is parallel in that it sounds the same no matter which one you plug into. Here though I think you CAN ohm it out at least to find out if the outs are parallel....they should have common points if so.
 

jbrown013

Member
Messages
387
Ill post some pictures after work today of external / internals it may help idk. From what I was told when I bout the amp it was modeled after a vox ac 15. It has 2 el34s, an 23at7 and a ef86 for tubes. I am sorry for the lack of information that I am providing, and thanks for all the helpful posts. Ive tried to search this guy. I even found my transaction from paypal but no luck as of yet in regards to contacting him.

Thanks,

Joe
 

hasserl

Member
Messages
4,711
Look at the wires going to the output jacks. If the jacks are wired in parallel there will be one colored wired running from the output transformer to the jacks, it will connect to one jack first and then looped to the second jack. If the jacks have separate impedance ratios then there will be separate wires running to the from the output transformer.

Note there may (probably?) also be a black wire running to the jacks, that would be the ground for the OT.
 

jchan

Member
Messages
1,684
I believe a good amp tech should be able to approximate it. You should be able to measure the output unloaded voltage V1 of the amp. Then connect a variable output load to the speaker output. Vary the load output until the measured voltage is 1/2 of V1. The resistance then is the output impedance (or close to) of the amp output. That is only an approximation but should give an idea. You should check this for both output jacks. They may be 2 separate connections or in parallel. Note: the measurement for unloaded output should be done quickly as you don't want to leave the amp on unloaded for anytime longer than it takes to make the measurement and then turn it off. If you are not familiar with safe electronics techniques I suggest you take it to a qualified technician.
 

StompBoxBlues

Member
Messages
19,943
I believe a good amp tech should be able to approximate it. You should be able to measure the output unloaded voltage V1 of the amp. Then connect a variable output load to the speaker output. Vary the load output until the measured voltage is 1/2 of V1. The resistance then is the output impedance (or close to) of the amp output. That is only an approximation but should give an idea. You should check this for both output jacks. They may be 2 separate connections or in parallel. Note: the measurement for unloaded output should be done quickly as you don't want to leave the amp on unloaded for anytime longer than it takes to make the measurement and then turn it off. If you are not familiar with safe electronics techniques I suggest you take it to a qualified technician.
First, to the OP, just to say, I wasn't giving you a hard time, just trying to help you get the info you need. Now that you mention the Vox AC15 I think and hope others can give you some insight. I hope you get a difinitive answer.

Second, To this poster, yep...this is the way to find out. I did a similar thing with pedals, built an "impedance tester" that works on the principle that when you measure with no load, then adjust and halve the voltage you have found the impedance (by measuring the resistance it took to halve it...in series, since it should match the impedance) but I don't know how dangerous this would be with an amp....but I bet a good amp tech could do it!

I still think using an 8 ohm nominal value would work okay. It seems so difficult to pay money to someone to find out something that should be labeled on the amp. (and to the OP, IF you find out for sure, write it in magic marker on the amp itself!) but maybe the best way.

It's a real pain in the you-know....
 

jbrown013

Member
Messages
387
I didnt think you were giving me a hard time, I just feel bad that Im not really providing as much info that could be needed. I really do appreciate the help that is being posted too! Wonder how hard it is to locate a tech to look at this. If any one knows of one I live in the jacksonville florida area.


Thanks,

Joe
 

jchan

Member
Messages
1,684
I didnt think you were giving me a hard time, I just feel bad that Im not really providing as much info that could be needed. I really do appreciate the help that is being posted too! Wonder how hard it is to locate a tech to look at this. If any one knows of one I live in the jacksonville florida area.


Thanks,

Joe
How about this guy: http://schenkeinsteinaudio.com/
 

cap47

Member
Messages
2,273
Put a cable in each jack with power off. Measure with ohm meter across the tip and body of plug. 16 ohm should be about 1.2 or 1.3 ohm, 8 ohm should be about 1 or 1.1 ohm and 4 ohm at about 0.9 ohm. Let the meter settle each reading , read until it is at lowest. I recently put an OT and selector switch in my VK and these were reading I had found when changing selector.
 




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