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Old 31st March 2013, 07:53 AM   #1
Haman47 is offline Haman47  Australia
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Default output impedance

I have been given an amp from an organ built in the 50ís with huge power and o/p transformers and filter choke (twice the size of those on the 100w Marshal amp I also have on the bench). Valves are 2x6SN7 and 2xEL34 in a Williamson-style design. Plate voltage is 900 volts. The o/p transformer has 2 separate speaker windings of unknown impedance. By running a known ac voltage through the HT winding and measuring the voltage on the speaker windings I was able to determine that the winding ratios were 23:1 and 76:1. These seem much lower than I expected. Does anyone have any idea what are the output impedances of the o/p transformer are then? Thanks for any suggestions.
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Old 31st March 2013, 08:53 AM   #2
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Quote:
Originally Posted by Haman47 View Post
I have been given an amp from an organ built in the 50ís with huge power and o/p transformers and filter choke (twice the size of those on the 100w Marshal amp I also have on the bench). Valves are 2x6SN7 and 2xEL34 in a Williamson-style design. Plate voltage is 900 volts. The o/p transformer has 2 separate speaker windings of unknown impedance. By running a known ac voltage through the HT winding and measuring the voltage on the speaker windings I was able to determine that the winding ratios were 23:1 and 76:1. These seem much lower than I expected. Does anyone have any idea what are the output impedances of the o/p transformer are then? Thanks for any suggestions.
The 23:1 would be ~4K:8, and the 76:1 is probably used for feedback? 900V on the EL34?! Wow.

Jaz
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Old 31st March 2013, 02:27 PM   #3
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Just curious .... did you measure 900 volts on the plates, with a load, or is that calculated? Is this using a tube rectifier? (Or likely, a couple - 5R4s?) From your description, the era, and the size of the choke this might have a choke input supply (most of the 'old' stuff I've seen from that era does), in which case, to achieve 900 volts, the power transformer must be greater than 2000 vct (1000-0-1000).
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Old 2nd April 2013, 07:50 AM   #4
Haman47 is offline Haman47  Australia
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Default output impedance

Perhaps I should have given a little more detail. Rectifiers are a GZ34 and a 6X5. Smoothing is by a capacitor-choke-capacitor arrangement. The output from the power transformer is 900-0-900 volts which end up at about 825 volts at the valve sockets. Not surprisingly this makes me a little wary about using the amp, but with so much iron it may give 100 watts with a good bottom-end frequency response.
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Old 2nd April 2013, 12:22 PM   #5
GoatGuy is offline GoatGuy  United States
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Well... considering that the majority of those big old tube-based organs were sitting in churches, funeral homes, synagogues and temples... and needed to output enough acoustic energy to be an adequate accompaniment for hundreds of attendees ... singing ... I'm not at all surprised by the size of the thing.

Unlike amplifiers built to reproduce the exquisite range of detail and subtlety in the high fidelity recorded-audio spectrum, the organ amplifier was much closer to the "P.A. amp" in design, meant to be run continuously between 1/10th and 7/10th full power. Thus, its output would be almost entirely class B, with complete output tube cutoff and little overlap. Watts baby... that was what they were shooting for. Watts.

It will probably disappoint as a high-fidelity amp.

PS: "all that iron" is kind of an interesting observation in historical perspective... the saturation magnetic flux of the hard iron alloys used before the 1980s (and especially before the 1970s) was quite a bit lower in a design sense ... than today. Aligning magnetic grains wasn't well practices, annealing wasn't as refined an art, and hysteresis above surprisingly low flux levels was something of a problem. Now consider that, and that electronic-organ manufacturers were intensely competitive, and would cut just about any and every corner they could to ensure a profit ... and it puts into perspective just how stretched an amplifier design you have sitting on your bench.

GoatGuy
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Old 3rd April 2013, 09:20 AM   #6
Haman47 is offline Haman47  Australia
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Default Output impedance

Thanks for the insight and advice GoatGuy. Perhaps I will just put the amp on the back shelf with all of the other oddities not to be bothered with for the present.
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