The Guitar Refinishing and Restoration Forum Forum Index The Guitar Refinishing and Restoration Forum
This discussion forum is hosted by The Guitar ReRanch and was created to serve those interested in the arcane art/science of guitar refinishing and restoration. Those with all levels of experience are welcomed to participate.
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Is there a way to Check the ohms of a amp?

 
Post new topic   Reply to topic    The Guitar Refinishing and Restoration Forum Forum Index -> Pick-ups, Amps and Electronics
View previous topic :: View next topic  
Author Message
Seanwimer



Joined: 29 Jun 2012
Posts: 1110
Location: Western Mass

PostPosted: Tue Mar 11, 2014 5:08 pm    Post subject: Is there a way to Check the ohms of a amp? Reply with quote

Just by happen chance that amp was labeled backwards is there a way to check the ohm on a amp not the speaker but the amp ?
_________________
...
Back to top
View user's profile Send private message
tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Tue Mar 11, 2014 6:47 pm    Post subject: Reply with quote

There is, but it's very involved and involves disconnecting the output transformer, taking some readings, and consulting a table... I'm not sure if there's a web link, but I think I've seen such a table on the net before.
_________________
proud endorser of Curt Mangan strings, K&K acoustic guitar pickups, and Latch Lake Music slides
Back to top
View user's profile Send private message Visit poster's website
tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Tue Mar 11, 2014 6:48 pm    Post subject: Reply with quote

here you go. This outlines the whole deal.

http://radioremembered.org/outimp.htm
_________________
proud endorser of Curt Mangan strings, K&K acoustic guitar pickups, and Latch Lake Music slides
Back to top
View user's profile Send private message Visit poster's website
greysun



Joined: 31 Mar 2011
Posts: 421
Location: Chicago

PostPosted: Wed Mar 12, 2014 7:56 am    Post subject: Reply with quote

just playing devil's advocate a bit here - but... 90% or more of the time, wouldn't the amp be matched to the speaker? Shouldn't it be a pretty good estimate to measure the speaker and then back up into the ratios for the amp?
_________________
this is the beginning...
Back to top
View user's profile Send private message
BluesmanDave



Joined: 17 Nov 2008
Posts: 2825
Location: Sacramento CA

PostPosted: Sat Mar 15, 2014 2:29 pm    Post subject: Reply with quote

greysun wrote:
just playing devil's advocate a bit here - but... 90% or more of the time, wouldn't the amp be matched to the speaker? Shouldn't it be a pretty good estimate to measure the speaker and then back up into the ratios for the amp?


Well, yeah... unless it's a head with no speaker attached. Wink
_________________
www.davesegalmusic.com
Back to top
View user's profile Send private message Visit poster's website
Mr. Clevername



Joined: 08 Dec 2006
Posts: 2457
Location: San Francisco, CA

PostPosted: Sat Mar 15, 2014 6:45 pm    Post subject: Reply with quote

I sure wish somebody would figure out some way an end user like me could easily find out.

As a charter member of OBACG (Off-brand Amp Collectors Guild), I've often found myself with an orphan amp and no idea what speaker to use. The advice I usually get is to just hook something up and see what happens . . .
Back to top
View user's profile Send private message
tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Sun Mar 16, 2014 5:31 am    Post subject: Reply with quote

Here's how I would proceed with a tube amp of unknown output impedance, when Google and other sources don't manage to turn up any documentation (usually it will, if you persist enough).

Find a 4 ohm cabinet and plug it in.

Does the amp sound really really quiet and low output-- lower than it should be (extremely so)? Then there's a high likelihood that you have a pretty far mismatch and it's wanting to see a 16 ohm load.

If it sounds like it's maybe in the range of normal, or if you're unsure, find an 8 ohm cab with the same size speaker(s) of roughly the same sensitivity (ballpark is fine). Connect the head to both 8 ohm and 4 ohm cabs. Which one is louder? There's your match. If still unsure, then it's probably of little consequence.

in TUBE amps, "output impedance is higher than the speaker impedance" (i.e. high impedance source into low impedance load) is generally safe for the amp and transformers, at least for short periods. A 16 ohm output into a 4 ohm speaker isn't likely to cause permanent damage to anything.

in TUBE amps "output impedance is lower than speaker impedance" (i.e. lower impedance source into higher impedance load) is more dangerous. A 4 ohm output into a 16 ohm speaker starts to get dicey for your output transformer, with the possibility of permanent damage.

Again, the above rules are for TUBE amps. For solid-state, it's somewhat the opposite.

One thing to remember-- MATCHING impedances maximizes power transfer (efficiency). Power is the ability to do work, so the matched impedance will always do more work for the same amount of amp power output. More work means the speakers move more, which means louder. But to make a gauge, you have to keep other variables (speaker size, cab size, speaker sensitivity, number of speakers, etc) pretty close to equal.

Not entirely related, but for completeness-- BRIDGING impedances maximizes voltage transfer at the expense of some power. Bridging impedance is "lowest possible source impedance into highest possible load impedance", usually a load of 5X or more the source. We want this for things like low-level inputs where we want to keep as much voltage as possible... this is why most guitar amps have something like 1M input impedance. However, while the voltage stays as high as possible, it's very inefficient and can do very little work (i.e. there's very little current moving).
_________________
proud endorser of Curt Mangan strings, K&K acoustic guitar pickups, and Latch Lake Music slides
Back to top
View user's profile Send private message Visit poster's website
Seanwimer



Joined: 29 Jun 2012
Posts: 1110
Location: Western Mass

PostPosted: Sun Mar 16, 2014 6:40 am    Post subject: Reply with quote

Quote:

in TUBE amps "output impedance is lower than speaker impedance" (i.e. lower impedance source into higher impedance load) is more dangerous. A 4 ohm output into a 16 ohm speaker starts to get dicey for your output transformer, with the possibility of permanent damage.


This is what I'm scared of the most. Cause this would be my setup if the ohms on the head are mixed up.
_________________
...
Back to top
View user's profile Send private message
tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Sun Mar 16, 2014 8:55 am    Post subject: Reply with quote

So your cab is 16 ohms?

If the amp is wanting to see 8 ohms, short-term experimentation is unlikely to fry anything, particularly at moderate volumes. If your amp wants to see 4 ohms, it starts to get pretty perilous.

What type of amp? BF/SF Fenders tend to hold up to OT abuse pretty well. Some less-known makes and many Marshalls are not quite as forgiving.

What makes you suspect there's a mixup?

If you could get a make and model number of the OT as well as list the output tube complement (4x EL34, 2x 6V6, etc) we might be able to do some sleuthing.
_________________
proud endorser of Curt Mangan strings, K&K acoustic guitar pickups, and Latch Lake Music slides
Back to top
View user's profile Send private message Visit poster's website
patrickbrose



Joined: 29 Aug 2012
Posts: 107

PostPosted: Mon Mar 17, 2014 12:47 pm    Post subject: Reply with quote

Any Markings on the OT that would identify it (model number, make etc)? That way you can look up the data sheet for it which would have the secondary impedance.
-P
Back to top
View user's profile Send private message
Seanwimer



Joined: 29 Jun 2012
Posts: 1110
Location: Western Mass

PostPosted: Mon Mar 17, 2014 3:03 pm    Post subject: Reply with quote

It's a phaez... The amp gets super hot when I use that cab... It also burned up a power tube on me once and a resistor screen plate.
_________________
...
Back to top
View user's profile Send private message
greysun



Joined: 31 Mar 2011
Posts: 421
Location: Chicago

PostPosted: Mon Mar 17, 2014 3:24 pm    Post subject: Reply with quote

they have a Facebook page - check this and contact them with a picture: https://www.facebook.com/PhaezAmplification

I know that when i put my amp together recently, there was conflicting information whether "green" was 8ohm or 16ohm and I had to ask around a bit.

Give them a try. Smile
_________________
this is the beginning...
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    The Guitar Refinishing and Restoration Forum Forum Index -> Pick-ups, Amps and Electronics All times are GMT - 6 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group