# Transformers without center taps?



## Lincoln (Jun 2, 2008)

Looking through my "treasures" today, I realized that a transformer set I bought years ago, had no HV center tap. That struck me as unusual, so I started playing it just to see what it was all about. 

HV to HV it was showing just over 400 volts AC. It came out of a 65 watt, 2xEL34 PA, so I was expecting a bit more voltage out of it. 

It has 2 sets of heater wires, so I hooked up a quick & dirty rectifier setup with a GZ34. On pin 8 of the GZ34 that would normally be the B+ wire, there is no DC reading at all. And AC is showing about 125V. So obviously this thing isn't going to work for anything but a paper weight. Sort of looked around the interweb, didn't find anything that answered all the questions for me. I did see a picture of sort of virtual HV center tap using a pair of diodes to ground.

I never found a schematic on the PA these transformers came out of, so I have no idea how it was meant to function. Any ideas?


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## Dorian2 (Jun 9, 2015)

Sorry. Can't help you there..


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## Lincoln (Jun 2, 2008)

Now that I've had a chance to think it over, there is no way this power transformer could have been tube rectified without a center tap. I'll rig up a full wave bridge and do some more testing.


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## dtsaudio (Apr 15, 2009)

Yes, it's made for a bridge rectifier. Be careful here if you put a filter cap on it. Unloaded you can expect about 560VDC.


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## Latole (Aug 11, 2020)

I often built amps from scratch and to drop B+ supply, I use zener stack :






Amp Mods


Generic Guitar Tube Amplifier Modifications



robrobinette.com


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## Paul Running (Apr 12, 2020)

You can use your GZ34 in a hybrid configuration as so:


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## Paul Running (Apr 12, 2020)

Lincoln said:


> I never found a schematic on the PA these transformers came out of, so I have no idea how it was meant to function. Any ideas?


Do you know the brand and model number for your PA amp?


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## Lincoln (Jun 2, 2008)

Paul Running said:


> Do you know the brand and model number for your PA amp?


Fanon/Masco FMA65


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## Paul Running (Apr 12, 2020)

I believe that the schematic is not common on the net. I have the FMA-100...a slightly different config. If you could confirm your tube line-up...as I am aware, the FMA-65 has a tube line-up of: 2 × 12AX7, 7199, 2 × EL34 and a 6CY7.
It is a 4-channel, a triode for each channel...that takes care of the 12AX7s.
The 7199, has the pentode for the mid-stage from a tone network, the network may be an integrated module,
PN-92-04.
The triode section of the 7199 may serve as the paraphase PI, driving the EL34s.
The 6CY7 may be used for the screen supply/regulator for the output tubes...it's adjustable on the FMA-100, by a pot.
I say may, because it was common to duplicate circuitry from model to model for many of these manufacturers.
Fannon was the Canadian affiliate for Masco in the 60s.
The tone module:










The schematic for the FMA-100 attached.


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## Lincoln (Jun 2, 2008)

Paul Running said:


> I believe that the schematic is not common on the net. I have the FMA-100...a slightly different config. If you could confirm your tube line-up...as I am aware, the FMA-65 has a tube line-up of: 2 × 12AX7, 7199, 2 × EL34 and a 6CY7.
> It is a 4-channel, a triode for each channel...that takes care of the 12AX7s.
> The 7199, has the pentode for the mid-stage from a tone network, the network may be an integrated module,
> PN-92-04.
> ...


The FMA-100 Looks pretty darn close to the FMA-65. Only I have 2 x EL34's instead of 4 x 6L6's . Helps me understand what's going on, thank you.

btw, what's the purpose of that 70 volt output on the output transformer?


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## Paul Running (Apr 12, 2020)

That's the line distribution, a much higher output impedance; usually used in a multi-speaker configuration as in a building with thousands of square-footage, like commercial buildings...really what they were primarily designed for...anyway, lucky for us, they incorporated the lower impedances for guitar amp applications.


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## Lincoln (Jun 2, 2008)

I've seen some full wave bridges drawn with .002 uF caps in parallel with each of the 4 diodes. Would there be any advantage to doing that in this case?

What's their purpose in life?


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## Paul Running (Apr 12, 2020)

I call those snubber caps, there are other names for them. Snubbers absorb energy and may be used to eliminate voltage spikes caused by switching. Semi-con diodes can have very fast switching-times (fast slew-rates). The object of the snubber is to eliminate the voltage transient that occurs when it stops conducting (it will go into reverse-bias, during one-half of the AC cycle). It provides an alternate path for the current to flow through the circuit. The inductance of the power transformer produces these spikes. This is usually not a problem with a tube rectifier because of it's slower slew-rate...it cannot switch off and on as fast as a semi-co device...a bit long-winded here but there are some good articles on the net concerning snubbers...mainly used in switching circuitry.
You could try an experiment with the snubbers. First without them in circuit, power-up the amp with no input...if your amp is very quiet, turn up the gain to hear the noise in the speaker. Try to remember the level of noise. Then power-down, discharge the filter caps, connect the snubbers and then repeat the test. It is possible that a particular noise will be attenuated...the diode switching noise may be reduced or eliminated. Different values of caps, will snub certain frequencies. I believe that Mr. Carlson has a video about this phenomenon.


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## nonreverb (Sep 19, 2006)

Paul Running said:


> I call those snubber caps, there are other names for them. Snubbers absorb energy and may be used to eliminate voltage spikes caused by switching. Semi-con diodes can have very fast switching-times (fast slew-rates). The object of the snubber is to eliminate the voltage transient that occurs when it stops conducting (it will go into reverse-bias, during one-half of the AC cycle). It provides an alternate path for the current to flow through the circuit. The inductance of the power transformer produces these spikes. This is usually not a problem with a tube rectifier because of it's slower slew-rate...it cannot switch off and on as fast as a semi-co device...a bit long-winded here but there are some good articles on the net concerning snubbers...mainly used in switching circuitry.
> You could try an experiment with the snubbers. First without them in circuit, power-up the amp with no input...if your amp is very quiet, turn up the gain to hear the noise in the speaker. Try to remember the level of noise. Then power-down, discharge the filter caps, connect the snubbers and then repeat the test. It is possible that a particular noise will be attenuated...the diode switching noise may be reduced or eliminated. Different values of caps, will snub certain frequencies. I believe that Mr. Carlson has a video about this phenomenon.


Ultralinear Twins use snubber caps in the rectifier circuit.


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## Lincoln (Jun 2, 2008)

What do you guys think of this style of bridge rectifiers? Any good for amps?


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## nonreverb (Sep 19, 2006)

Lincoln said:


> What do you guys think of this style of bridge rectifiers? Any good for amps?
> View attachment 360524


As long as the voltage rating is adequate....definitely don't have to worry about the current rating.


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## jb welder (Sep 14, 2010)

Lincoln said:


> What do you guys think of this style of bridge rectifiers? Any good for amps?
> View attachment 360524


Sure, they work great. And if you bolt them to the chassis you have a good heatsink.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> As long as the voltage rating is adequate....definitely don't have to worry about the current rating.





jb welder said:


> Sure, they work great. And if you bolt them to the chassis you have a good heatsink.


Yeah, I think they are all rated for 25 amps. A least the ones at work are. Unfortunately only 200V, but I know the same style is available right up to 1KV. It was only about a year ago I realized what they were and that they could be useful on amps. Why they are in 24V DC circuit, I still have no idea. Must be to smooth out ripples or something.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> Yeah, I think they are all rated for 25 amps. A least the ones at work are. Unfortunately only 200V, but I know the same style is available right up to 1KV. It was only about a year ago I realized what they were and that they could be useful on amps. Why they are in 24V DC circuit, I still have no idea. Must be to smooth out ripples or something.


They are usually found in solid state amps. Bass amps in particular where high current demands are present.


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## Lincoln (Jun 2, 2008)

Do you guys ever check your new diodes before you install them? I never have until now. I found a bad one!


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## Paul Running (Apr 12, 2020)

That's a wise practice...pre-inspection of components prior to assembly. It may consume additional time but beneficial, especially when a balanced circuit is important to you; if that diode was installed in the B+, FW-rectifier side and full ACV hit it...imagine the stress on the first electrolytic-cap in the filter section.


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## Latole (Aug 11, 2020)

Diode with short leads, look a used one. I always check these


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## Lincoln (Jun 2, 2008)

Paul Running said:


> That's a wise practice...pre-inspection of components prior to assembly. It may consume additional time but beneficial, especially when a balanced circuit is important to you; if that diode was installed in the B+, FW-rectifier side and full ACV hit it...imagine the stress on the first electrolytic-cap in the filter section.


it never got to the filter caps. But I did waste a few fuses trouble shooting the problem.


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## Lincoln (Jun 2, 2008)

Latole said:


> Diode with short leads, look a used one. I always check these


the diode was new when I installed it. I pulled it out to check it property. Diodes are not expensive, I would never install a used one.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> the diode was new when I installed it. I pulled it out to check it property. Diodes are not expensive, I would never install a used one.


That's strange! I don't recall ever having a brand new defective diode. That said, it's bound to happen as you've found out Lincoln.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> That's strange! I don't recall ever having a brand new defective diode. That said, it's bound to happen as you've found out Lincoln.


my experience is very limited compared to yours, but I've never seen a bad diode either. I've seen them burn right up fail open in mobile equipment, but never seen one pass in both directions. 
My Fluke has a setting for "diode test" but I never used that setting until last night. 😌

Thinking back, these could be Amazon diodes, so probably Chinese of questionable quality. Either that or Newark.


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## Latole (Aug 11, 2020)

Lincoln said:


> it never got to the filter caps. But I did waste a few fuses trouble shooting the problem.



Use bulb current limiter, you will save fuses and trouble shouting will be easier


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## Lincoln (Jun 2, 2008)

My experiment was a wash anyway. With either my home-made rectifier bridge or a commercial made rectifier bridge, it was still making 408VDC open circuit. That's not enough for what I want to do. I find it strange because the PA this PT came out of was a 65 watt beast.

Another strange thing about this PT, there is a winding with a pair of blue wires that checks out at about 59 ohms resistance. The red pair is about half that resistance, and yet the blue pair is only making approx. 70VAC. Not understanding why a substantial winding is not making more voltage. It doesn't help that the rectifier was missing when I bought the amp, and all these wires were floating in the breeze......

not the first $200 I ever wasted, and won't be the last either.

I thought about a voltage doubler bridge, but when you double the voltage, you half the amperage, right? Can't do that either.


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## jb welder (Sep 14, 2010)

Lincoln said:


> Another strange thing about this PT, there is a winding with a pair of blue wires that checks out at about 59 ohms resistance. The red pair is about half that resistance, and yet the blue pair is only making approx. 70VAC. Not understanding why a substantial winding is not making more voltage.


Voltage of a winding has to do with the number of turns. Resistance will be partly due to the current capability of the winding (wire gauge). The low voltage winding is probably very thin wire (comparatively) with low current capability, maybe a bias winding.

You had said at the start that it was putting out just over 400VAC, now you are getting the same approx. 400VDC rectified?


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## Paul Running (Apr 12, 2020)

Lincoln said:


> Another strange thing about this PT, there is a winding with a pair of blue wires that checks out at about 59 ohms resistance.


The c-supply or bias-supply.


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## Lincoln (Jun 2, 2008)

jb welder said:


> Voltage of a winding has to do with the number of turns. Resistance will be partly due to the current capability of the winding (wire gauge). The low voltage winding is probably very thin wire (comparatively) with low current capability, maybe a bias winding.
> 
> You had said at the start that it was putting out just over 400VAC, now you are getting the same approx. 400VDC rectified?


Yeah, it's 408VDC rectified with no load on it. Didn't really change all that much. I was expecting a boost.

About the winding, I'm used to seeing low resistance, .2 ohm or .3 ohm on the low voltage heater windings, 20 to 30ohms on the HV, about 10 ohm on the primary side, etc. 
I wasn't expecting to see a high resistance winding produce a lower voltage. More learning.


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## jb welder (Sep 14, 2010)

Lincoln said:


> it was still making 408VDC open circuit. That's not enough for what I want to do. I find it strange because the PA this PT came out of was a 65 watt beast.


Not sure what you mean 'what I want to do' ?
The transformers are a set, you can get out of them whatever the Fanon could do.
It's not all about voltage, they will be running higher current (lower OT primary impedance) to make the power from lower B+. 

Look at the plate voltage for the 50W Marshall EL34's in this example.


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## Lincoln (Jun 2, 2008)

jb welder said:


> Not sure what you mean 'what I want to do' ?
> The transformers are a set, you can get out of them whatever the Fanon could do.
> It's not all about voltage, they will be running higher current (lower OT primary impedance) to make the power from lower B+.
> 
> ...


I certainly see your point. I've been judging everything by vintage Fender specs with PT's at 600 to 700 open circuit voltage. Time for a re-think.


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## dtsaudio (Apr 15, 2009)

Lincoln said:


> Yeah, it's 408VDC rectified with no load on it. Didn't really change all that much. I was expecting a boost.


It should be higher unloaded. Is there are filter cap?


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## Lincoln (Jun 2, 2008)

dtsaudio said:


> It should be higher unloaded. Is there are filter cap?


no filter cap. I've got the PT set up with a proper cord, and a off/on toggle, just taking voltages and playing with rectification. Red wire to red wire.


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## Latole (Aug 11, 2020)

No filters caps ! Right DC volts reading need filter caps


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## Paul Running (Apr 12, 2020)

Lincoln said:


> no filter cap.


Latole is correct, you are measuring pulsating DC:
This is what you would view on an oscilloscope.


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## dtsaudio (Apr 15, 2009)

Add a cap and watch the voltage go way up.


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## Lincoln (Jun 2, 2008)

dtsaudio said:


> Add a cap and watch the voltage go way up.


Well gentlemen,
I learned something again today. Add one 32 uF electrolytic, and 408 volts becomes 568 volts. It's magic. 
Thank you all.


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## greco (Jul 15, 2007)

Lincoln said:


> Well gentlemen,
> I learned something again today. Add one 32 uF electrolytic, and 408 volts becomes 568 volts. It's magic.
> Thank you all.


Be careful with that stuff. Don't get any on ya !


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## Lincoln (Jun 2, 2008)

greco said:


> Be careful with that stuff. Don't get any on ya !


No worries Dave, I've worked with my hands all my life and I still have all my fingers. This high voltage thing is just more of the same type of awareness. "Don't put your finger where you wouldn't put your dxxk". 😋


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