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Old 17th April 2012, 11:39 AM   #1861
dhtrob is offline dhtrob  Netherlands
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Andy,

I always prefer(red) low static resistance in choke/IT/OPT designs. This usually results in using thicker magnet-wire and bigger cores, which I like wrt lo bass-response..
Core material is also of big influence on sound (and performance)...
Interesting that you prefer the smaller Hammonds..

Oh, and I often use a low value resistor between plate and choke to "tune" the sound...

Rob
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Old 17th April 2012, 11:57 AM   #1862
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Rob - you may want to try out four of these 156C chokes. I'm damned if I know why they sound as good as they do, but my ears tell me they are quite special in this particular context. Defies reason, but it won't be the first time!

andy
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Old 17th April 2012, 12:30 PM   #1863
dhtrob is offline dhtrob  Netherlands
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Hi Andy,
Already looked them up on Ebay.. Thanks!
Rob
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Old 17th April 2012, 07:14 PM   #1864
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If you are in Europe the cheapest place is probably Bluebell Audio. Philip, the owner, is very into tube audio and is a big fan of the Hammond broadcast transformers. He does good deals if you buy a bunch of stuff, and anyway his prices on most stuff are cheaper than anybody else in Europe by some margin. Recommended! He's been selling the 156Cs in bunches of four. Apparently Hammond has increased production of them 200% in the last year and were surprised that so many were going out the door.

Bluebell Audio

Andy
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Old 18th May 2012, 12:53 PM   #1865
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Does anyone have a 26 datasheet? Can't seem to locate one on google.

Cheers

Charlie
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Old 18th May 2012, 12:58 PM   #1866
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Hi Charlie. How about three of them?
Attached Files
File Type: zip 26_HB3.pdf.zip (242.8 KB, 133 views)
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Old 18th May 2012, 01:03 PM   #1867
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Thanks Rod!

It's slowly coming together - the only time I have for it is over my lunch break. Trying to get through Andy's 4mm Aluminuim is proving to be hard whilst holding a ham sandwich...

Charlie
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Old 21st May 2012, 01:39 AM   #1868
mach1 is offline mach1  Australia
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Andy,

How do you position your double 156Cs ? base to base ala Gary Pimm, or side by side (parallel or at right angles). Do you wire the coils in opposite directions? Do you insulate the chokes from the chassis/earth?

thanks in advance.
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Old 21st May 2012, 10:08 AM   #1869
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I just follow Gary Pimm - he measured the responses with a scope. So bottom to bottom with the wires coming out the same side, pin 1 to pin 1. I assume I've understood this correctly. If I haven't, let me know!

As for whether to ground the chokes - good question!! Can we have some views? The chokes are dipped in some varnish stuff and you have to strip down to the metal to make contact.

I have reduced the choke input power supply in my 27 line stage to a 150Hy Hammond 156C and a Solen 10uf capacitor per channel. Still clean and quiet. I will be adding a table of values to the web site soon. One trick I did to get more bandwidth from them was use 2 in series. This doubles the inductance and halfs the capasatance. Frequency response went from 16-30K with one 156C to 9-80K with 2 in series. The high frequency went up more than double because there was a dip at 50Khz that was -4db with 1 choke and it was only -2db with 2 chokes. With 2 in series the frequency response was -2db at 50K rising to -1db at 60Khz then rolling off to -3db at around 80Khz. The self resonance is very low Q, a gradual hump between 50K and 70K that is only 1-1.5db high. Proabably low Q because the DC resistance is almost 3K. I varied the current from 5ma to 10ma and didn't see any frequency response varaition. Could have been the plate resistance decreased with the increase in current and counteracted the loss of inductance caused by the higher current. Another trick that you can do with 2 chokes is mount them bottom to bottom on standoffs so one choke is "right side up" and the other is "upside down" and wire them out of phase. This makes the whole setup "hum bucking". I measured the hum rejection at 20db as compared to both chokes mounted in the normal way. The test was having my Weller soldering station sitting 1 foot from the line stage. Turning one choke over dropped the induced 60Hz voltage by a factor of 10. Gary Pimm


Nice sounding, especially for the money! Use 2 in series. This has 3 things going for it.
- 1st, the inductance is double so you get 1 more octave on the bottom end of the frequency range.
- 2nd, because the capacitance of the chokes acts as 2 caps in series you will get better high frequency response. In the 26 preamp I built the 3dB down point went from somewhere near 30K to out past 70K. The single choke had a 4dB dip around 40K. With the pair of chokes the dip was only 2dB. There is a broad very low Q resonance around 60K so with 2 chokes in series it drops 2dB down near 30K, then rises back up gently and finally drops below the -3dB point somewhere above 70K. The numbers are vague cause I'm working from memory of a project done several years ago...
- 3rd, you can mount the chokes on standoffs, bottom to bottom and wire the chokes with one winding backwards. This makes the pair of chokes hum buckers. In my test setup the pair of chokes picked up 20dB less hum than a single choke. Gary
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Old 21st May 2012, 11:33 AM   #1870
hihopes is offline hihopes  South Africa
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Default Hum/buzz on 26 preamp

I hope someone can help with this. I have a 26 pre with a loud hum/buzz. The 26’s are resistor loaded and LED biased, with Rod’s boards supplying the heaters. 50K stepped attenuator, star earthing in amp box, PSU connected via 2 umbilicals (1 for B+ and 1 for DC to Rod’s boards).
The PSU has a 100VA EI transformer with screen and HV secondary + 2 x 6.3V 3A secondaries; hybrid bridge with a dual damper diode (6BY5GA). The screen is connected to 0V. Smoothing is fairly heroic - for B+ it is clcrcrc (7H choke, 1st C is polyprop 5uF and others are 330uF) and to Rod’s boards it is crcrcrc (large value caps). By all accounts and according to PSUD, ripple should be almost gone in both supplies.

I got a slight improvement by snubbing the SS diodes in the bridge, so suspected that the problem had to do with the bridge. The heater draw of the 6by5 is 1.6A, which is quite a lot of ac current! Suspected it might be coupling somehow, so I tried supplying the rectifier valve’s heater with DC from a switch mode power supply. The frequencies within the noise were slightly different, but the noise was even louder.
I suspect that the problem has something to do with the PSU (which is quite compact). I hope it is not transformer problem.
Anyone got any bright ideas? All help much appreciated.
Best regards,
William.

Last edited by hihopes; 21st May 2012 at 11:39 AM.
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