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28th September 2007, 06:30 PM  #1 
diyAudio Member
Join Date: Feb 2005
Location: ann arbor, mi

Hammond Transformer Secondary Connections
Hi Everyone:
I am confused on how strapping the secondaries of a tube output transfomer work, specifically the Hammond 1650T Hammond has the secondaries on the 1650T list as a single 4 ohm tap and a serial 4 and 8 ohm tap, (see picture). Hammond states that if the two 4 ohm taps are connected in parallel, the result is 4 ohms. I thought it should be 2 ohms. lasltly, how does strapping two 4 ohm outputs in series yield 16 ohms? Thanks for your help. Ciao  Minto 
28th September 2007, 06:55 PM  #2 
diyAudio Member
Join Date: Jun 2002
Location: Macedon NY

Paralleling the windings doesn't change the impedance... because the winding ratio is still the same  the output voltage is the same for each winding, or in parallel. You COULD use only one secondary, but the winding loss would be higher, and more importantly, the high end of the frequency response would suffer, due to poorer coupling.
The impedance ratio is the square of the turns ratio (voltage ratio). When you have twice as many secondary turns, the optimum load is twice the voltage, half the current: four times the impedance. 
28th September 2007, 07:36 PM  #3 
diyAudio Member
Join Date: Feb 2005
Location: ann arbor, mi

Oh.
How does the secondary arrangement of an amp such as the Grommes G101a work? If paralleling the secondaries has no effect of allowing a reduced load, are so many 8.4 ohm taps available to run 70 volt lines? Thanks  Minto 
28th September 2007, 08:45 PM  #4 
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Join Date: Jun 2002
Location: Macedon NY

Assuming it's a 100W amp, it would be about 140V output with all six in series  sometimes used for high power distribution. Series Parallel for 70V, parallel all for 25V or 8 Ohms. Other transformers were used too  a friend has a couple of these amps.

1st October 2007, 03:18 PM  #5 
diyAudio Member
Join Date: Feb 2005
Location: ann arbor, mi

I looked at the the rear panel of my Grommes G101a and it says that all six of the 8.4 ohm taps in parallel will serve a 1.4 ohm load. Also all six taps in series will serve a 50 ohm load.
I'm not really sure why parallel/series configuration is so simple for the Grommes but when two 4 ohm taps are connected in parallel or series in a Hammond they behave so differently. This is all very confusing. Could you recommend a source that I could look into that would clear up the confusion? Thanks  Minto 
1st October 2007, 03:50 PM  #6 
diyAudio Moderator

I think the issue is that the Grommes rear panel is just wrong, paralleling or adding windings in series is not simple addition or division  as Tom pointed out in his earlier post. (You can't just add winding impedances together in a transformer.)
Placing a whole bunch of windings in parallel does not change the transformation ratio of the transformer  it is simply the analog of one winding with a much larger conductor size. (Ignoring non ideal transformers.) Note in the series case the new load impedance is simply the square of the change in winding ratio  since all six are now in series the turns ratio has changed by a factor of 6  square that and multiply the 8 ohm individual winding impedance by the result or 36 x 8 or 288 ohms  this is obviously a high voltage tap for distribution. (Possibly 140V) Not obviously stated is that the rules are the same for all output transformers regardless of who made them or when, and in fact for all transformers in general when looking at winding impedances. RDH4 has a very good section on designing audio transformers, and pdfs of this and other worthwhile publications are available on Pete Millets website as well as elsewhere. Google for further information on the subject. I believe even wikipedia has some information on this subject.
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1st October 2007, 04:58 PM  #7 
diyAudio Member
Join Date: Feb 2005
Location: ann arbor, mi

Thanks for the direction to Pete Millett's page. I didn't realize he had so many resources available online.
Ciao and thanks again  Minto 
1st October 2007, 08:13 PM  #8 
diyAudio Member
Join Date: Jun 2002
Location: Macedon NY

If the secondaries are 1.4 Ohms, then six in series will be 1.4 * 6^2 or 50.4 Ohms. And 100W in 50 Ohms is 70V. So that much makes sense. They offered other transformers in these amps  the schematic may show a different one.
Old saying among military strategists... "The map is not the territory". 
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