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Old 4th June 2012, 11:36 AM   #1
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Join Date: May 2010
Default 18 watt toroidal transformer questions

Hi.
Im building a matchless spitfire amp (2X 12ax7, 2X el84, 1X gz34)

Ive bought this powertransformer:Link nr 1:

Tube-Town Online-Store - Ringkern Netztrafo 106VA / Marshall-18Watt-Style tt-t106

It comes with this diagram: Link nr 2

https://www.tube-town.net/info/daten...-Anschluss.pdf

Im using the 3.15 - 0 - 3.15v winding for the heaters
The 0-5v winding for the rectifier heaters
And obviously the 300-0-300 winding for the high voltage

BUT: There are two centertap wires on the 300v winding? (green and white)
Do I solder them both to chassis? Or is one supposed to be left alone?

Im unsure about the meaning of the powertransformer diagram in link nr 2?

The amp layout Im using is this:

http://www.ceriatone.com/images/layo...eCeriatone.jpg

Finally one more question: In my country, the voltage in the wall is 230 volts. If I was to connect the powertransformer, using the 240 v primary, wouldnt the secondary 300v winding drop a bit, maybe even close 2 the desired 290 volts? (Or would it affect the other windings too much?)
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Old 14th June 2012, 03:29 AM   #2
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Join Date: Oct 2004
Location: Adelaide South Oz
bagudan,
Since no-one had replied here is what I think.

When you have 2 windings like this which can be wired in series or in parallel it is important to know the start and end wires of the 2 windings ( the phasing).
We can see that one winding is red to green and the other is white to blue.

If hooking up these in parallel it is ESSENTIAL to get the phasing right. One way (wired in phase) the currents will add and the windings work together. The other way the winding will fight each other and result in the transformer trying to pull large primary currents and blown the mains fuse.

What is less understood is that it is IMPORTANT to get the phasing of the winding right when putting them in series as well. You suggest green and white together as the centre tap. Before connecting these wires to your amp use a little terminal block to connect the red wire to one terminal, the green and white to another termonal and the blue to a 3rd terminal, use a multimeter to check the voltage from red to blue, if you have it correct then you will read 600V if you have it wrong then you will read about 0v.

If it reads 600V then proceed by connecting green and white to the 0V point. If you read 0V then you will need to swap the wires on ONE of the windings and then check again that you have 600V end to end.
By doing this you make sure that the rectifier tube is using alternate anodes on opposite half cycles of the HV AC waveform. If you have it wrong then both anodes of the rectifier will conduct to cathode on one half cycle and none will conduct on the other half cycle, giving you a half wave rectifier (which would not be good).

As far as which primary voltage tap to use start with the 230V. When you have your amp up and running then measure the heater voltage and change the tap to get that as close to 6.3V as possible. Yes the HV will change a littel but thats not really critical.

Hope this helps,
Cheers,
Ian

Last edited by gingertube; 14th June 2012 at 03:39 AM.
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Old 14th June 2012, 04:15 AM   #3
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Hi Gingertube.

Thanks for the reply. I do read 600 volts when tying green and white together, so I should be set to go, I think. Another guy also mentioned the primary voltage wasnt too critical, So ill do what you suggest. The only reason for wanting too keep b+ a bit down, is that The cirquit Im building (Matchless Spitfire) is already runs a tad hot. But Ive bought different values for the cathode resistor, and also a couple of 5w zeners if I should want to lover the high voltage winding a bit instead.

Thanks

_Kasper
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Old 14th June 2012, 04:58 AM   #4
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Join Date: Oct 2004
Location: Adelaide South Oz
Kasper,
You can also lower the B+ a little by putting dropping resistors in each of the HV wires to the two rectifier tube anodes. Try 47 Ohm 2Watt metal film resistors (can go up to 100 Ohms if necessary). This will also protect the rectifier tube itself a little by limiting peak currents through it (and make it last a bit longer). This is the "preferred" method for dropping the B+ a little.
Cheers,
Ian
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Old 16th June 2012, 08:53 PM   #5
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Hooked it up tonight. Having problems with the heater voltage,
Im worried that my gz34 is drawing to much current, because the filament voltage on the powertubes and 12ax7s is only 4,5 volts

It should be alrigt though?

The 0-5 winding should be able to handle 2 amperes if I read it right?
And the 3.15-3.15 should handle 3 amperes

Tube-Town Online-Store - Ringkern Netztrafo 106VA / Marshall-18Watt-Style tt-t106
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Old 17th June 2012, 10:10 AM   #6
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Join Date: May 2010
Well, its not the rectifier causing the voltage drop. I pulled the rect. out, and the filament voltage is only 4.20 volts when the rest of the tubes are in? must look elsewhere for mistakes...

But is it safe to use a gz34 that draws 1.9a, when the o-5v winding puts out 2 amperes?
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Old 18th June 2012, 03:40 AM   #7
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Join Date: Oct 2004
Location: Adelaide South Oz
bagudan,
The 2 Amp rating of the 5V winding is the maximum current it can supply. It will provide the 1.9 Amps drawn by the GZ34 heater without any problems.

There must be something else loading down the transformer outputs or you are connected to the wrong primary tap. Check the unloaded voltages on the transformer secondary. Without a load these should all read about 10% high. That is the 6.3V should read about 7 Volts etc.

Cheers,
Ian

Last edited by gingertube; 18th June 2012 at 03:42 AM.
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Old 18th June 2012, 07:09 AM   #8
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It puts out 7,1 volts when there is no load. I must run through the cirquit once again. I tried with an old transformer, amd the problem was the same, so obviously it must be wiring related
Thanks for the reply!

Best regards
-Kasper
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Old 18th June 2012, 08:29 PM   #9
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The voltage is fine now....rewired all the heaters tonight. Im using an old teisco amp, and I left the original heater wires in. Obviously something must have been wrong with them... because the new wires solved the problem
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Old 18th June 2012, 09:41 PM   #10
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Finished the build tonight. I have sound coming through.... Not great sound yet, but Ill look into that later.

First thing is the b+. As expected, it is high....quite high actually. With all tubes in, I read 384 v dc at the first filter cap. The schematic calls for 352 v dc.

When checking bias, by using the weber online bias calculator, I get 15 watts/41 ma per tube.

Cathode resistor is 150 ohms, cathode voltage is 13 volts, and the plate voltage was approx 368.
I put some russian 7189as in there, so theyre fine and there is no redplating.

But what is the best way to lower b+ that much? Zeners on the high voltage center tap, or dropping resistors in each of the HV wires?

_Kasper
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