# Transformer question



## Jean GODBOUT (Oct 31, 2009)

Hi amp guru

I'm planning the construction of a new amp, I select the AX84 4-4-0 witch is a Trainwreck Express clone, mod by the AX84 guys using a 6SN7GT double triode as power tube in push pull mode for an approximative 2 Watts output. The schematic is calling for a Hammond 125C (8 Watts) Secondaries set to 22K.

My question is, if I buy a larger one, let say a 125E rated for 15 Watts would the performance be the same, worst or better ?

Schematic and layout : http://www.ax84.com/4-4-0/AX84_4-4-0_Studio_100216.pdf

Thank, Jean


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## keeperofthegood (Apr 30, 2008)

Hammond Mfg. - 125 Hook-Up Data

Use the load table, 8 ohms is 8 ohms is 8 ohms, the reflected inductance is what it will be depending on the lug-taps used with the 8 ohm speaker. 22K ohms reflected to the primary with an 8 ohm load is taps 2&4 no matter which [FONT=Arial, Helvetica, sans-serif]*125A, 125B, 125C, 125D & 125E*[/FONT] you use.


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## keeperofthegood (Apr 30, 2008)

Another thing to know about inductors: Self Inductance of a Coil from Example No. 1:



> A hollow air cored inductor coil consists of 500 turns of copper wire which produces a magnetic flux of 10mWb when passing a DC current of 10 amps. Calculate the self-inductance of the coil in milli-Henries.


So, if we drop the current to 1 amp or raise it to 100 amps, the results would (inaccurately because the flux would change too however the principle holds true, hey its a 7 page tutorial @[email protected] lot of reading all at one time for the PERFECT example) be :

500 * 0.01/1 = 5000mH
500 * 0.01/100 = 50mH

Same coil, simply changing the current affects the inductance of that coil. And as you lower the current, you raise the inductance because it is an inverse relationship. Higher inductance has better frequency response than lower inductance. Lowering the current then also broadens the frequency response range of the OT.


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## Jean GODBOUT (Oct 31, 2009)

*Beef up transformer*

Hi keeperofthegood 

Mainly because I'm French and not perfectly fluent in English, I really don't understand your reply.

Is it good or bad to beef up the transformer ?

Thank you, Jean


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## keeperofthegood (Apr 30, 2008)

Jean.

IF (big if this) I have learned this all correctly (AMP BUILDERS PLEASE STEP IN) what you MIGHT get is more frequency response. Otherwise, as far as the tube itself is concerned all five of those transformers would be the same, they would all be a 22K ohms load.


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## dtsaudio (Apr 15, 2009)

As Keeper said you MIGHT get slightly better performance. Current handling is the biggest difference though. From a reliability standpoint the 125E is better, but is overkill.


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## Jean GODBOUT (Oct 31, 2009)

Thank for your input. The main reason for me to use a bigger transformer is, I'm tempted to install two 6SN7GT (instead of one) in case that it's sound thin. If I do that, should the Secondaries set to 11K ?

Jean


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## WCGill (Mar 27, 2009)

Jean, It would be the primary side to match the tubes. I'd think it should be 11k for 2 6SN7's in parallel if one is 22k. The smaller transformer should still be adequate as Hammond underrates.


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