# so- ummm- more impedance questions



## fraser (Feb 24, 2007)

ive got an amp head here, had it for years and years. 2 channel, 2 12ax7s and 3 el84- 
my problem is, theres no markings on the OT- nothing anywhere to tell me what speaker impedance the OT is needing.
now, for years this head was used for amplifying vocals, or bass or harp- whatever. never worried about impedance or anything- just plugged it into whatever speakers where available. i always had amps, so never even bothered to try it with guitar.
it hasnt been powered up in about 7 years- 
i took it apart at xmas- cleaned the insides out, then plugged it into a 4ohm speaker- works good, sounds pretty good- not very loud, but the tubes may need replacing, as they are ancient-
in any case- 
i read about applying power to the primaries- then measuring the voltage at the secondaries- to determine the turn ratio- but i have no idea how knowing the turn ratio will let me determine the impedance needed- lol
now, i could just try it through other speakers- this amp was built in the sixties, and since sometime in the early nineties its been plugged into all kinds of speakers and monitors with no regard for its safety, and it still works-
but id still like to know-


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## fraser (Feb 24, 2007)

lol i posted this in the wrong place- sorry guys


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## keeperofthegood (Apr 30, 2008)

http://www.radioremembered.org/outimp.htm

http://wiki.answers.com/Q/How_do_you_use_turns_ratio_to_find_volts_current_impedance


Essentially, you need to know a couple parameters to find others. And yes, ANY transformer in pre-1980 even pre-1990 was VERY poorly marked, if marked at all. Even worse are "coils" used in rf circuits. Lots of times its 16 turns of wire on a 6/32" former with an unmarked ferite slug which was ok when they made it, but a pain in the keaster to replace after accidentally dropping something on it >_>


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## greco (Jul 15, 2007)

Wild Bill helped me get through the same problem. It was actually qute easy to do.

I'll try and find the thread for you.

Cheers

Dave


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## fraser (Feb 24, 2007)

keeperofthegood said:


> http://www.radioremembered.org/outimp.htm
> 
> http://wiki.answers.com/Q/How_do_you_use_turns_ratio_to_find_volts_current_impedance
> 
> ...


thanks keeper- im gonna wait till im sober to attempt to absorb the stuff in those links-


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## keeperofthegood (Apr 30, 2008)

fraser said:


> thanks keeper- im gonna wait till im sober to attempt to absorb the stuff in those links-



:food-smiley-004: good idea, sober + measuring high voltages = a very good idea!


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## greco (Jul 15, 2007)

I looked, both in my old posts and in the archives for the original thread...no luck.

I think I remember most of what I did...too bad the old thread is lost.

Basically, you need to get a *very *small transformer and hook up a 1/4 inch TRS plug to the output side. I found a transformer from a clock radio. The secondary output was about 4 VAC (the primary goes into a wall outlet ...i.e., 115VAC)

You will have to remove the chassis and drain the caps. If you are not comfortable doing this...STOP HERE. 

*With the AMP TURNED OFF * and plug the 1/4 inch jack into the spreaker out and plug the transformer into the wall outlet. You then measure the VAC across two specific pins on the tube socket. *Be careful here also * ...my 4 volts ended up at about 105 VAC !!. 

Now you need Wild Bill (or another amp tech) to tell you how to do the math as you are dealing with a an amp that has 3 x EL84's and I had a single ended 6L6.

If you can't find a small transformer, I'll send you mine with the 1/4 inch plug on it. 

Cheers

Dave


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## fraser (Feb 24, 2007)

thanks dave- im gonna dig around for a transformer-
shamefully i threw away a bunch of broken radios and even a couple crappy amps back in the summer when i moved.
in any case- chassis is out and caps discharged- its been sitting like that for weeks lol, while i pondered.
its low on my list of stuff i want to do right now, but when its turn comes up, id like to be prepared


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## Wild Bill (May 3, 2006)

fraser said:


> thanks dave- im gonna dig around for a transformer-
> shamefully i threw away a bunch of broken radios and even a couple crappy amps back in the summer when i moved.
> in any case- chassis is out and caps discharged- its been sitting like that for weeks lol, while i pondered.
> its low on my list of stuff i want to do right now, but when its turn comes up, id like to be prepared


I picked up a "wall wart" that kicks out about 14 vac from Value Village, Fraser. Only a buck or two! I cut the end off the cord and put a jack on it so it would plug right in to the speaker out on an amp.

I sent Dave my Excel spreadsheet that does the math for unknown OT's today. I would have sent a copy to you but I don't have your email and I can't send attachments through the board.

If you want a copy you can email me at [email protected] and I'll post it to you.


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## greco (Jul 15, 2007)

This is a copy of Wild Bill's email to me:

I forgot to mention to disconnect the speaker as I was doing this on an amp head.

_ "You feed low voltage AC in the speaker jack or clip to the speaker wires 
from the OT. DISCONNECT THE SPEAKER!

Measure the primary AC at the plate pins of the tube sockets, which is 
usually pin 3 for 6V6s, 6L6s or EL34s. You don't have to pull the tubes 
since the amp should NOT be powered up anyway! Still, pulling the tubes 
lets you stick your probes in from the top. " _

Cheers

Dave


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## greco (Jul 15, 2007)

*This is from Wild Bill's post in the original thread:*

_"Zp/Zs = (Np/Ns)2 = (Vp/Vs)2

Looks scary but it's easy when you know the tricks.

First off, Zp and Zs mean Impedance of Primary and Impedance of secondary. We're talking about the output transformer here.

Np and Ns are the Number of turns Primary and Number of turns secondary.

How do we get the number of turns? We don't bother! We can also use Vp and Vs, or Voltage primary and Voltage secondary. Here's the easiest way to do that!

Go to a thrift store like Value Village and paw through their wall-wart plug in power supplies. Look for one with AC output, not DC! Anything from 6-12 volts or so is fine. Current capacity is not important, either.

To make things easier, I put a 1/4" guitar plug on the one I use. That way I can plug it in the wall and feed low voltage AC straight into the amp's speaker jack!

With the amp unplugged and the output tubes removed, you can measure the voltage (stepped up 'backwards' through the output trannie to its primary) either between the two plate pins in a push-pull amp or from the plate to the B+ red wire in a single 6V6 'Champ' style amp.

Now you've got Vp and Vs! This gives you the voltage ratio, which is exactly the same as the turns ratio.

From the formula, this ratio squared will equal the impedance ratio.

At this point we don't know Zs, but we can guess and see if it makes sense! Assume it's 8 ohms and work things out. If you're running a pair of EL34's you should get an answer around 3400 ohms. If it's a pair of 6L6's you should get about 6600 ohms. A pair of EL84's or 6V6's might give 6-8K.

If you get something that seems way off try solving for 4 ohms. This will halve the Zp result. Or 16 ohms, which will double it!

Odds are overwhelming that you will find an answer for a standard speaker load that gives an impedance for the primary that makes sense for the tubes involved. Remember that 4 tubes as in a 100 watt Marshall will require only half the total primary impedance. You will be looking for around 1700 ohms with EL34's. A quad of 6L6's will require around 3300 ohms.

Remember that 'close' is good enough! And it's a good idea to always measure that AC wart at the speaker jack. With the load of the trannies secondary it might be a bit high or low from what it says on the plastic case.

I hate always doing the same math so I set the formula up as an Excel spreadsheet and saved it. That way I can just punch my new measurements in anytime and instantly see the results as I change 4 to 8 or 16.

You can also get a much better explanation here: http://www.aikenamps.com/
Go to 'tech info' and 'Output transformers explained'. I've always used this method. It's cheap! "_


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## JMC Amps (Dec 19, 2009)

Youtube is our friend!

[YOUTUBE]QQNVyYEzcJ8[/YOUTUBE]

Also See: Part 2 and Part 3

Cheers!

-JMc


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## fraser (Feb 24, 2007)

thanks everyone!
armed with all this, ill throw the amp on my bench this weekend i hope-
funny, ive got a wall wart thats been sitting here for ages - output 9vac, with the end cut off. should do the trick then.
ill be back with more updates/ questions!
thanks guys!


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## greco (Jul 15, 2007)

fraser said:


> ive got a wall wart thats been sitting here for ages - output *9vac,* with the end cut off. should do the trick then.


fraser...just remember, that 9VAC is going to be increased significantly when you go from the secondary to the primary...BE VERY CAREFUL when you are taking the measurements.

When I did this, The VAC increased by a multiple of 25 

Hence 9VAC x ~25 multiple = *~225 VAC !! *
Cheers

Dave


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## fraser (Feb 24, 2007)

greco said:


> fraser...just remember, that 9VAC is going to be increased significantly when you go from the secondary to the primary...BE VERY CAREFUL when you are taking the measurements.
> 
> When I did this, The VAC increased by a multiple of 25
> 
> ...


thanks dave- so i should be wearing pants then when i do this?
or do i need to take any special precautions with the meter?
im used to working with lethal voltages at work, but i never poke around in old tube amps at work...........


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## greco (Jul 15, 2007)

Sorry...I had no idea that you were used to working around high voltages.

Also (likely mentioned before) measure the voltage across the tip and sleeve of the 1/4 inch plug from the wall wart....they don't always put out the voltage indicated on the enclosure and you want to have accurate numbers here to use in the formula.

Please keep us updated.

Cheers

Dave


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## fraser (Feb 24, 2007)

thanks dave- ill be sure to check that-


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## fraser (Feb 24, 2007)

ok then!
i think something might be amiss.
i checked and triple checked the readings on the amp-
my wall wart is putting 9.9 volts in-
and im reading 133 volts across the plates.
even figuring for 16 ohms, im only getting a primary impedance of 2887.7- nowhere near the 6-8k wild bill mentioned for the el84s. 
so perhaps something is wrong with this amp- or ive done something wrong lol.
but reading this from wild bills post-


> Odds are overwhelming that you will find an answer for a standard speaker load that gives an impedance for the primary that makes sense for the tubes involved. Remember that 4 tubes as in a 100 watt Marshall will require only half the total primary impedance. You will be looking for around 1700 ohms with EL34's. A quad of 6L6's will require around 3300 ohms.


perhaps thats it? i just dont know how to apply that info however- yikes!


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## greco (Jul 15, 2007)

Hopefully Wild Bill will see your thread again and comment.....if he ever comes out from hiding under his bed..LOL

http://www.guitarscanada.com/showthread.php?31710-Hippie-movement-spawned-by-Conspiracy!-My-stars!

Cheers

Dave


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## fraser (Feb 24, 2007)

greco said:


> Hopefully Wild Bill will see your thread again and comment.....if he ever comes out from hiding under his bed..LOL
> 
> http://www.guitarscanada.com/showthread.php?31710-Hippie-movement-spawned-by-Conspiracy!-My-stars!
> 
> ...


lol! yeah i saw that-
you can hide out down here in the ghetto bill, theyll never think to look for ya here.
theres girls on the corner, and ive got lots of beer.
thats a song- i just wrote it.


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## JMC Amps (Dec 19, 2009)

fraser said:


> ok then!
> i think something might be amiss.
> i checked and triple checked the readings on the amp-
> my wall wart is putting 9.9 volts in-
> ...


Hello fraser,

Just a thought... you say 3 el84s? Perhaps this is a single ended class A amplifier?

Typical operating ratings (or so I've read) of a single el84 class A amplifier has a load impedance of 4500 ohms. Divided by 3 el84s = Ra of 1500 ohms.

Your calculation:
133/9.9 = 13.44 turns ratio
13.44x13.44 = 180.63
180.63 x 8 ohms = 1445.04 (pretty close to 1500)
Perhaps your transformer is a single ended unit rated for 1445.04 (1500 ohms) into and 8 ohm load.

...but, I'm not a tech so...I'm probably missing something in this equation

Just food for thought.

-JMc


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## fraser (Feb 24, 2007)

hi jmc- yup i got 13.4=180.6 pretty much every time i tried it-
your math is exactly like what i got-
each channel is powered by a 12ax7 and an el84- then one extra el84 at the output stage- thats how it appears anyway to me.
2 channels, each with a 12ax7 and an el84- then one other el84 to output. voltage readings are zero if i try the first two plates on the first two el84s-
but either el84, and the 3rd el84, produce the 133 volts-
so it appears this is like any amp with 2 el84s, but with 2 channels of the same- 
i can read a meter, i can poke around without killing myself, but im an infant when it comes to understanding any of it lol
you say-


> Perhaps your transformer is a single ended unit rated for 1445.04 (1500 ohms) into and 8 ohm load.


this is where im lost- if it is correct, and it is an 8 ohm load, im all set, but no understanding of why lol...
ive just been going with the math from wild bills spreadsheet, and double checking with a calculator the same things as in the above videos-
im like the newbiest of the newbs here- i dont even fully understand what a single ended unit is, unless youre talking about the danger that lurks beneath my trousers, and that doesnt make any sense-


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## fraser (Feb 24, 2007)

> Typical operating ratings (or so I've read) of a single el84 class A amplifier has a load impedance of 4500 ohms. Divided by 3 el84s = Ra of 1500 ohms.


its starting to make sense-
but in this case is it really 3 el84s, or is it only 2 per channel- or maybe im thinking too much?


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## Wild Bill (May 3, 2006)

fraser said:


> its starting to make sense-
> but in this case is it really 3 el84s, or is it only 2 per channel- or maybe im thinking too much?


3 EL84's? The extra one must be a reverb driver or something. It would help if we knew what model amp it was and looked at a schematic.

Are you sure you're reading at the plate pins for each of the EL84's connected to the OT? Between the blue and the brown wires? Pin 7 on each tube?

I find it curious that if you had double 133=266 vac across the plates things would make perfect sense! That's why I ask. The formula is right and there is no such thing as a SE amp with 3 tubes in parallel or a push pull amp with an odd number of tubes. There must be a measurement problem.


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## fraser (Feb 24, 2007)

hi bill-
problem is, ive never found a schematic, or another one of these on the net anywhere-
its a pepco made riviera er30.
2 channels, 2 inputs per channel, volume bass and treble for each channel, nothing else-




























as far as the pins- well lol- looking here i see which is pin 7-










however, i didnt know if that is oriented looking down at the socket with the tube pulled, or if it is looking up from the bottom-
i found that i got zero voltage assuming it was pin 7 looking from above, but drew the 133 assuming it was pin 3 from above- which would be pin 7 if from below- now im sounding like an idiot.
there is a white wire going to each of the three sockets at that pin however.

on a fortuitous note- i was out for a walk and found an amplifier someone was throwing out- looks brand new, but has a broken switch- its got a 10 inch fender 30 watt speaker in it- 8ohms- and its good. would make a decent cab for this thing!


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## greco (Jul 15, 2007)

fraser said:


> .......on a fortuitous note- i was out for a walk and found an amplifier someone was throwing out- looks brand new, but has a broken switch- its got a 10 inch fender 30 watt speaker in it- 8ohms- and its good. would make a decent cab for this thing!


WOW...nice find !! What make/model of amp is it? (I am just curious by nature)

I am following this thread, trying to learn from it.

Cheers

Dave


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## fraser (Feb 24, 2007)

greco said:


> WOW...nice find !! What make/model of amp is it? (I am just curious by nature)
> 
> I am following this thread, trying to learn from it.
> 
> ...


hi dave- its a squier sidekick reverb 25- made in taiwan.
looks almost new- has the chrome footswitch for the reverb with it even.
could fix the switch and see if it works, but the cab is just about the right size for this head- and i dont have a cab for it......
in any case, a good bit of luck!


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## JMC Amps (Dec 19, 2009)

What I see in the photo is the first el84 wired as a diode, with AC placed on pins 1 and 7 and your B+ out at pin 3 (internally tied to pin 1). The first el84 is a rectifier. 

When you measure 133 VAC from the first el84 to the other two, you are hinging (read: hinge) on the ceter tap of the transformer. You should see 266 VAC on the last two el84s, or across the OT. Like Wild Bill says, 266VAC makes more sense.

The Math:
266/9.9 = 26.87
26.87x26.87 = 721.99
721.99 x 8ohms = 5775.92
In the 6K primary ballpark now!

-JMc


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## keeperofthegood (Apr 30, 2008)

fraser said:


> however, i didnt know if that is oriented looking down at the socket with the tube pulled, or if it is looking up from the bottom-



When you look at that image, you are looking at the pin side, not the socket side. So, holding the tube in your hand, pins are facing you, gap towards the floor. If you print that off on your printer, and use some fun-tac and stick it underneath beside the socket(s) so you can see the wires going to the socket and look at the printout and see where they are going inside the tube you would be A-OK!


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## fraser (Feb 24, 2007)

keeperofthegood said:


> When you look at that image, you are looking at the pin side, not the socket side. So, holding the tube in your hand, pins are facing you, gap towards the floor. If you print that off on your printer, and use some fun-tac and stick it underneath beside the socket(s) so you can see the wires going to the socket and look at the printout and see where they are going inside the tube you would be A-OK!


hey thanks keeper- so it appears i was testing the right points after all-


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## fraser (Feb 24, 2007)

> What I see in the photo is the first el84 wired as a diode, with AC placed on pins 1 and 7 and your B+ out at pin 3 (internally tied to pin 1). The first el84 is a rectifier.
> 
> When you measure 133 VAC from the first el84 to the other two, you are hinging (read: hinge) on the ceter tap of the transformer. You should see 266 VAC on the last two el84s, or across the OT. Like Wild Bill says, 266VAC makes more sense.
> 
> ...


thanks jmc!

heres what has happened- 
i was wondering why i couldnt get a reading- anything at all, across the last 2 sockets- last night when i got a zero reading i assumed i was testing the wrong points- and thats why i ended up taking the reading using the first socket- which as you point out is the rectifier- id never have thought of it on my own.
now here is where i show off how unobservant i can actually be-
just now i investigated the sockets- and one had a bunch of brownish rusty crap all over the contact point at pin 7, 
i used a welding torch tip cleaner and cleaned it up- 
now im reading 264 v across the two-
where last night, that rusty crap was preventing any kind of reading.
so you were correct, and all is as it should be!
thanks a bunch to all you guys!


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## keeperofthegood (Apr 30, 2008)

fraser said:


> thanks jmc!
> 
> heres what has happened-
> i was wondering why i couldnt get a reading- anything at all, across the last 2 sockets- last night when i got a zero reading i assumed i was testing the wrong points- and thats why i ended up taking the reading using the first socket- which as you point out is the rectifier- id never have thought of it on my own.
> ...



ROSIN lol that is what that is. Rosin from the solder as a residue. You can also use Real Lemon lemon juice to wash it off but you also need to water rinse well too. Thats what pc makers use these days to clean off circuit boards in production.


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## fraser (Feb 24, 2007)

yeah keeper- maybe thats what it was- but i had to actually work the torch tip cleaner like a file to get it off- baked on lol
my impression was its rust- there was some on other pins as well, so ived cleaned them all up, and just vaccumed the guts real good.

now since ive got a jack soldered onto the speaker out wires, im gonna make up a little plate so i can mount it at the back of the head.


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## fraser (Feb 24, 2007)

ok so now ive got a new problem. its way too loud.
ive got stuff rattling off of shelves n stuff now.
i live right on a busy intersection, and there were people yelling up at me, telling me i rock, and stuff.
guess ill have to limit myself to using it in the daytime only lol-
its super quiet- even cranked there is almost no ambient noise coming thru- and its super clean- cranked right up with single coils theres no breakup until i engage the treble booster.
nice fat, chimey sound.
this is a pretty cool amp.

thanks again for the help guys!


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## JMC Amps (Dec 19, 2009)

fraser said:


> ok so now ive got a new problem. its way too loud.
> ive got stuff rattling off of shelves n stuff now.
> i live right on a busy intersection, and there were people yelling up at me, telling me i rock, and stuff.


fraser...thats not a problem! :rockon:


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## JMC Amps (Dec 19, 2009)

Hey fraser,

just curious... Any idea what the plate voltage is (B+). What about the AC voltage on the PT side of the rectifer?

I'd like to know what voltages and limitations are given around an EL84 rectifer. Never seen one before (el84), wired as a diode or used as a rectifier. -or- Perhaps some of the veteran techs out there can eleborate.


-JMc


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## keeperofthegood (Apr 30, 2008)

JMC Amps said:


> Hey fraser,
> 
> just curious... Any idea what the plate voltage is (B+). What about the AC voltage on the PT side of the rectifer?
> 
> ...



I posted about this very idea once before, and got no response on it. I should put it up on the antiques radio forum too, but here are the images again:

http://www.guitarscanada.com/showthread.php?30543-Another-Rectifier-Question&p=266162#post266162




















And yes, it is not common and I would also like to know more about this


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## fraser (Feb 24, 2007)

i can find out- but id need some tutoring on how to measure these things lol-
as you know from all the above im a rank amateur.
to begin- how do i measure the b+ on that tube? from the plate to a ground?
id be happy to help tho-


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