# Amp Building question - wattage



## Lincoln (Jun 2, 2008)

can you guys help me get my head around something?
Not trying to re-invent the wheel, I'm just trying to understand what makes it roll.

If you were to take the power & output transformers from a 65 watt PA amp (2 x EL34) and use them to build a super reverb clone (AB763 circuit 40 watt) and ran the EL34 tubes as hot as they ran in the PA........would you end up with a 65 watt amp? Or would you end with with a very clean 40 watt amp? 

Would the components of the AB763 circuit limit things to the 40 watts it was designed to produce in spite of the bigger iron? Does the output of the phase splitter tube remain basically constant in all amps? 

????


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## dcole (Oct 8, 2008)

You should be able to easily push the EL34's. Your typical long tailed pair phase inverter can have the gain changed as well if lacking. Randall Aiken has a basic method for design on his webpage: http://www.aikenamps.com/LongTailPairDesign.htm

Where is your EL34 based PA coming from?


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## Lincoln (Jun 2, 2008)

Thanks for the info



dcole said:


> Where is your EL34 based PA coming from?


I picked up a Fanon/Masco FMA65 PA a while back. Not sure what to build with it. The iron is huge.


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## dcole (Oct 8, 2008)

Are you selling these on ebay? I seen a few from Edmonton: http://www.ebay.ca/itm/FANON-MASCO-...pt=Vintage_Electronics_R2&hash=item1e7fba4c7e


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## Lincoln (Jun 2, 2008)

dcole said:


> Are you selling these on ebay? I seen a few from Edmonton: http://www.ebay.ca/itm/FANON-MASCO-...pt=Vintage_Electronics_R2&hash=item1e7fba4c7e


That's where I got it from, a tech in Edmonton has a bunch of them for sale. Great guy. Local pickup for me. I'd hate to think what shipping would be on one.


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## dcole (Oct 8, 2008)

That's cool. I don't know what the iron is in their or the supply voltages but they may be close enough to clone a Marshall.


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## Lincoln (Jun 2, 2008)

dcole said:


> That's cool. I don't know what the iron is in their or the supply voltages but they may be close enough to clone a Marshall.


I haven't found a schematic for it yet or measured the voltages, but here's what it looks like without the covers. Should run 4 - EL84's with no problem I would think


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## dcole (Oct 8, 2008)

Cool. So much for matched tubes eh? The right EL34 is noticeably bigger than the left one. Any markings on the transformers you can list? If they are Canadian you'd think they would have used Hammond tx's.


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## nonreverb (Sep 19, 2006)

The right EL34 looks like a Mullard rebranded Sylvania


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## dcole (Oct 8, 2008)

I do concur.


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## Lincoln (Jun 2, 2008)

dcole said:


> Any markings on the transformers you can list? If they are Canadian you'd think they would have used Hammond tx's.


No Hammond part numbers or any brand name markings. PT is marked 251-30 ANC9, OT is marked 252-50 AND-9
I can fire it up and check voltages for you but what you really need is current capacity, right? I have no way to test that.

Maybe give the seller a call. Cool guy and the best tech around this area - Chuck Frank - C4 Sound Works. (780) 478-8160 His shop is always full of Marshalls, he might have a good idea what flavor of Marshall those transformers would be best suited to.


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## nonreverb (Sep 19, 2006)

The output tranny isn't ultralinear or is it?


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## Wild Bill (May 3, 2006)

Lincoln said:


> No Hammond part numbers or any brand name markings. PT is marked 251-30 ANC9, OT is marked 252-50 AND-9
> I can fire it up and check voltages for you but what you really need is current capacity, right? I have no way to test that.


Those numbers are most likely Pepco or Pine in-house inventory numbers. Manufacturers do this because they don't care if the trannie came from Hammond or Stancor or whoever. They would have had a number of companies bid on THEIR specs and then approved those vendors they wanted to supply the part. Once it went into THEIR part box they no longer cared from where it came.

What's more, a manufacturer would not want those specs to be public knowledge. Why let the competitors have it easy? If someone needed a replacement they could call the Pepco factory and be sold one at a ripoff replacement part price.

Of course, when the factory has long ago become a parking lot a replacement just isn't available. However, all is not lost.

Just think like a manufacturer and use some common sense! Transformers are probably the most expensive parts inside a guitar amp. There is no way the designer would call for an output transformer that could handle 4 6L6s in an amp that is only going to use 2. In other words, a 50 watt amp will have 50 watt capable transformers, both the output and the power units.

So knowing the original specs of the amp where the trannies came from is very important. If the output tranny was driven by a single pair of 6L6s then you can make an educated guess that the primary impedance will be somewhere around 6600 ohms.

If you are a REAL tech you should know how to measure this! If you haven't got to that point yet on your journey to the tube amp godhead then a simple google will help you out. Basically, you feed a low AC voltage into the speaker side and measure the voltage across the primary plate leads. I suggest you use clip on "gators" before you apply the juice. The voltage on the primary will be stepped up to perhaps a couple of hundred volts! That can HURT!

Once you know your voltages and secondary impedance ( - did you use a 4 or 8 ohm tap?) then its just a simple bit of math to calculate the unknown primary impedance.

R G Keen's www.geofex.com site will help you out. You'll have to poke around but you won't mind. You will trip over TONS of other great tube and pedal stuff while you're at it!

For the power tranny, you can directly measure the voltages on the various windings. Keep in mind that a winding with no load on it will appear about 10% high. If you understand how the typical full-wave silicon diode or tube rectifier circuits work you can get a good guess as to what plate voltage would be the end result. The technician's Rule of Thumb is to take the rectified voltage and once it is applied to the first filter cap use a multiplier of 1.3 for the plate voltage with some plate current actually happening. You will be close enough for rock and roll! If you are using a tube rectifier, remember to include the rectifier's voltage drop in your total.

Tube data sheets will also give you some good guesses. They usually show typical plate currents with different plate voltages.

If you intend to "play" with the specs, as in using a trannie designed for a pair of EL-34's and instead use TWO pairs of smaller tubes like EL-84s then you will have to take into account that the optimum plate load will be different. With the tubes I've mentioned you will be close enough to not matter. A pair of EL-34s usually get a 3400 ohm OT. A pair of EL-84s are most often given an 8000 ohm plate load so two pairs would need half of that - or 4000 ohms. Close enough to 3400 ohms not to care. Tubes just aren't that anal about those values.

If you need to know how much current the power transformer can supply with its filament windings just count the tubes supplied by that winding! Again, the tube data sheets will tell you that a 12AX7 being fed with 6.3 volts will draw 300 ma of current. Count them and add them up! Then look up the current draw for an output tube. Multiply by the number used in the amp and add that into the sum too!

Once you have done this a few times it becomes second nature.

Remember, the manufacturer wanted to make as much profit as possible! He would never have spec'd in an output tranny that could handle 100 watts when his design only needed a 50 watt unit! Why on earth would he spend the money? So some hobbyist 60 years later could use it in a 100 watt homebrew design? Not on your life! If anything, he would spec trannies that were just strong enough to do the job, hopefully with a bit of a safety margin. That's why some amps burn out trannies more than others. The early Booger Nomads were notorious for burning out power trannies. The later ones were shipped with a fan blowing not on the output tubes like you usually see but on the PT!

Hope this is useful!

Wild Bill/Busen Amps


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## greco (Jul 15, 2007)

Wild Bill said:


> Basically, you feed a low AC voltage into the speaker side and measure the voltage across the primary plate leads. I suggest you use clip on "gators" before you apply the juice. The voltage on the primary will be stepped up to perhaps a couple of hundred volts! *That can HURT!*
> Wild Bill/Busen Amps


With Wild Bill's guidance, I did this something like this a long time ago. Wild Bill was helping me to determine the output impedance of the OT so that I could match the speaker impedance to it.
Wild Bill gave me an Excel file with a formula to do the calculations involved.

I built a small AC voltage supply using a transformer from a clock radio. The AC output voltage from this little clock transformer was around 1.0 VAC. When I hooked it up to the secondary of the OT and measured the primary, it was over 100 VAC ! ....Scared the hell out of me!!

@ lincoln...I think I still have that little 1.0 VAC transformer, PM me if you want it.

Cheers

Dave


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## Lincoln (Jun 2, 2008)

nonreverb said:


> The output tranny isn't ultra-linear or is it?


I don't think so. Not to my understanding of ultra-linear anyway. There are only 4 wire on the "primary" side of the OT. One to big power, one to each EL34, and a mystery wire that runs back to the output jack area. On the secondary side, everything runs to outputs 2, 4, 8, 16, and 24 ohms.

Wouldn't an ultra-linear circuit have 2 wires going to each EL34 tube? Could it even use EL34's?


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## jb welder (Sep 14, 2010)

Not sure if this is of any use, but there is a schematic for FMA-35 here:
http://www.scribd.com/doc/13118360/sed952#
And another thread regarding the 35 model here:
http://www.ax84.com/bbs/dm.php?thread=378885


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## Lincoln (Jun 2, 2008)

Wild Bill said:


> If you need to know how much current the power transformer can supply with its filament windings just count the tubes supplied by that winding! Again, the tube data sheets will tell you that a 12AX7 being fed with 6.3 volts will draw 300 ma of current. Count them and add them up! Then look up the current draw for an output tube. Multiply by the number used in the amp and add that into the sum too!
> 
> 
> Hope this is useful!
> ...


I come up a little short on my super reverb idea. The PA amp is powering 4.7A @ 6.3 volts worth of tubes, I calculate a super reverb with EL34's rather than 6L6's would require 5A @ 6.3V
Hard to believe that a transformer that massive can't support what I'd like to do with it.


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## nonreverb (Sep 19, 2006)

I'll bet it can. 300ma more probably won't do it in. Is it using a 6.3V indicator light as well? If it's using something light a #47 bulb, there's another 150mA to boot!


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## Lincoln (Jun 2, 2008)

nonreverb said:


> I'll bet it can. 300ma more probably won't do it in. Is it using a 6.3V indicator light as well? If it's using something light a #47 bulb, there's another 150mA to boot!


Actually it's got two 6.3V indicator lamps. That gives it back the missing 300mA! I can always use an LED instead. Thank you


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## nonreverb (Sep 19, 2006)

Even if the bulbs are a little lower current draw, it"s close enough for rock 'n roll :smile-new:


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## Wild Bill (May 3, 2006)

Once again, a bit of common sense. Is it likely that the manufacturer spec'd a 4.7 A winding? It would probably cost MORE to make than just something using standard wire gauges that would handle AT LEAST 4.7 A!

Don't worry, be happy!

Wild Bill


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## Lincoln (Jun 2, 2008)

a Thank you to everyone who replied to this thread and furthered my education :smile-new:


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## Lincoln (Jun 2, 2008)

Well, I've had this Fanon/Masco FMA65 PA amp sitting on my bench for over a year now, I've started thinking it would make a better Marshall than it would a Fender. Maybe something like a 2204? (50W lead with MV) I hooked up an A/C cord to see what I had for voltages and to make sure I had a bias supply winding. The numbers I got were confusing. I'm thinking the wire colours must not be standard. There were some pieces missing so I had several "loose" wires.
This what I have for wires on the PT. 

2 - black 18ga
2 - red 18ga
2 - dark green 18ga
2 - dark blue 18ga
2 - light green 20ga
1 - white 18ga (grounded, must be a CT)

I realize now I should have done a ohm test to the windings before I hooked it up to power, but I didn't. I hooked up to the black wires as I'm used to doing on PT's.
On the red wires which should be the HV, I got 280V to ground on one wire, 80V to ground on the other. From red to red I had about 120 volts. Green to green gave me 7 volts, but each green to ground to ground gave me 80 volts. Nothing was what I expected.
Backing up a step & pulling off the A/C cord, I took some ohm readings. Red to red gave me a reading of about 37 ohms. All the other pairs gave me readings of 1 ohm or less.

My primary winding must be the red wires, right??


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## WCGill (Mar 27, 2009)

This might help, unfortunately nothing was or is standard.

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


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## Lincoln (Jun 2, 2008)

WCGill said:


> This might help, unfortunately nothing was or is standard.
> 
> http://www.radioremembered.org/xfmr.htm


Thanks for the link. Looks like I better grab a different meter before I try it again. Something is not right.


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## jb welder (Sep 14, 2010)

How do you mean white is grounded? You will need to disconnect all the wires for testing and start with resistance checks.
If you have already done this, do you mean that white is connected to the xfrmr casing?
Does white show resistance to any other wire?
Check resistance between 2 blacks, between 2 blues, 2 reds, 2 lt.grn., 2 dk.grn.
post results.
I don't think any windings have CT, I'm guessing blues are bias winding, reds HV winding, and 2 sets of different heater windings (greens).


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## Lincoln (Jun 2, 2008)

jb welder said:


> How do you mean white is grounded? You will need to disconnect all the wires for testing and start with resistance checks.
> If you have already done this, do you mean that white is connected to the xfrmr casing?
> Does white show resistance to any other wire?
> Check resistance between 2 blacks, between 2 blues, 2 reds, 2 lt.grn., 2 dk.grn.
> ...


Yes, the white wire was soldered to the chassis when I got the amp.

red pair 23.4 ohms
blue pair 53 ohms
black pair 1.3 ohms
dk. greens .6 ohms
lt. greens .9 ohms

None of the wires have any continuity to the white wire.
Looking at those numbers, I'm surprised I didn't let the smoke out when I hooked the black wires directly to 120 volt A/C


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## epis (Feb 25, 2012)

Lincoln said:


> Yes, the white wire was soldered to the chassis when I got the amp.
> 
> red pair 23.4 ohms - *High voltage*
> blue pair 53 ohms - *Bias supply*
> ...


Hi Lincoln, white wire is for electrostatic shield (one single layer between primary and secondaries).
http://en.wikipedia.org/wiki/Isolat...electric_Barrier_and_Electrostatic_Shield.jpg
These transformers are good for 50W Marshall. Good luck with your project.


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## jb welder (Sep 14, 2010)

Lincoln said:


> None of the wires have any continuity to the white wire.
> Looking at those numbers, I'm surprised I didn't let the smoke out when I hooked the black wires directly to 120 volt A/C


The blacks will be the primary as you suspected. The only number that seems off is the HV winding. You said you measured 120VAC red to red. But the resistance of the HV winding reads lower than the blue winding.
Can you measure the AC voltage from bue to blue?
And double check resistance blue to blue and red to red?
Sounds like the HV winding may be toast.


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## Lincoln (Jun 2, 2008)

jb welder said:


> The blacks will be the primary as you suspected. The only number that seems off is the HV winding. You said you measured 120VAC red to red. But the resistance of the HV winding reads lower than the blue winding.
> Can you measure the AC voltage from bue to blue?
> And double check resistance blue to blue and red to red?
> Sounds like the HV winding may be toast.


More like my meter was toast. I brought home my good meter from work and suddenly everything fell into place. Ohms stayed the same pretty well but voltages came around.
red to red is 425VAC
blue to blue is 66VAC
both sets of greens are 6.9VAC
Just perfect. 
I should have known to question the first meter, not the colour codes on the transformer. Rookie mistake 
Thanks for the help everybody


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