NFB network for 16 ohm load

a DIY, modified Mullard 5-20 monoblock design

Postby erichayes » Mon Oct 09, 2006 12:37 am

Hi All,

FWIW, the original HF-89 used a rather convoluted global feedback network consisting of two 3k resistors in series coming off the 16Ω tap, going to the middle of the 1K/100Ω voltage divider on the cathode of the 1st AF amp. There was also a 225 pF cap across the sum of the 3K resistors, as well as another 500 pF cap going from the 4Ω tap to the midpoint of the 3K resistors.

When Eico came out with the second generation HF-89, which used a different OT (the one my 50 watt OT was cloned from), the NFB became more straightforward: 7.5K shunted with 225 pF from the 16Ω tap to the 1K/100Ω junction.

Since, for some reason, I can't pull up the eiclone schematic, I don't know what the resistance values are for the NFB or the 1st audio stage. It's a moot point, however, for the following d'oh reason:

Contrary to popular belief and Gertrude Stein, an OT is not an OT is not an OT. I'm not talking about comparing a transformer that weighs 12 pounds with one of the scrawny OTs used in the K502; they 're obviously in different leagues. What I am trying to emphasize is that the NFB resistance and capacitance values need to be tweaked to get optimum performance from the OT you are using. Most of you don't have the test equipment to verify how much NFB you've applied, so use the eqiuipment Mommy and Daddy gave you: your ears. A 50K pot in place of a fixed resistor will allow you to "dial in" the NFB value that sounds right to you. For safely's sake, put a 1K resistor in series with the pot so you don't accidentally take the output to (essentially) ground.
Eric in the Jefferson State
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Postby Shannon Parks » Mon Oct 09, 2006 6:13 am

Here's the updated NFB tuning for 8 ohms that hasn't been edited into the schematic yet, but is in the doc & mod thread:

Code: Select all
High Frequency Tuning
1) Change R8 to 2K (Mouser pn# 71-RN65D-F-2.0K)
2) Change C7 to 220pF (Mouser pn# 5982-15-500V220)
3) Change C8 to 470pF (Mouser pn# 5982-15-500V470)


The tuning of the Eiclone and the Poseidon Mark III is very similar. Here we have 2K feedback from an 8 ohm load. If we multiply this by the square root of 2, or 1.414, we get 2.8K. On the Poseidon, indeed we have a 2.74K resistor in there from the 16 ohm tap. If we divided the above bypass cap value by 1.414, we get 150pF - close to the 120pF bypass in the Poseidon. These are actual tested values, btw - they just luckily end up pretty close.

So if wanted to tweak it in by ear, maybe use a 2.5K as the safety resistor in your pot jig. You can probably try a handful of bypass caps valued from 100 to 220pF. I use these jigs when testing - sometimes two at a time! I assume Eric does as well. Also, start hoarding all the variable caps from old radio tuners, too. They are handy for this type of transformer tuning.
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