Tweak for Hammond Transformer Users

[Shannon Parks]

I'm kind of suprised no one called me on this. Anyhow, I wasn't too concerned about it, but here's the scoop folks:
The original Dynaco ST-35 used the 16 ohm tap for its negative feedback circuit using a 27k resistor. Normally, one adjusts the NF resistors R30 & R31 and measures J4-3 and J3-3 (without the tap attached) until you get 28.3k resistance to ground. The extra 1.3K is the cathode resistor.
But since the Hammonds usually use the 8 ohm tap for the negative feedback loop, this needs adjusted in order to more closely match the original feedback circuit. If using 8 ohm taps for NF, then adjust the NF resistors in order to read 20.3k from J4-3 and J3-3 to ground, respectively.

Shannon

[EWBrown]

I was wondering about this during initial check out, but it sounded so nice I figured it probably didn't matter too much. I haven't yet tried the square wave and o-scope NFB tweeking, that's in the future...

Still sounds better than the re-vamped SCA-35.

/ed Brown in NH

[Shannon Parks]

I was wondering about this during initial check out, but it sounded so nice I figured it probably didn't matter too much. I haven't yet tried the square wave and o-scope NFB tweeking, that's in the future...

Still sounds better than the re-vamped SCA-35.

/ed Brown in NH

Yeah, while testing on my scope a 1kHz square wave looks superb on the stereo 35 no matter how much I fiddle with the adjustable feedback resistor. Stout little design. I would guess the extra dB's of feedback would give you added stability, but technically not much difference in sound. Those not using a preamp shouldn't probably bother, as you'll lose precious gain in the first stage.
But then again, tweaking kind of goes hand-in-hand with this hobby. ::wink:

Shannon

[nineno]

I ended up using the Hammond 1650E (Impedance: 8,000 CT) on my amp. I was wondering if there were any additional modifications (biasing or component wise) that needed to be made in light of this modification.

I'm using the 8Ohm tap for feed back (but there is also a 4 and 16Ohm tap). I'm running 8Ohm speakers, too. Or should I feedback with the 16Ohm tap?

Any help anyone can give me would be greatly appriciated!

Early returns are great, however. I fired the amp up, and it amplified. Once i get the chassis built I'll send Shannon some pics.

drew*

[Shannon Parks]

Any help anyone can give me would be greatly appreciated!

Drew,
You are fine. I don't think you have a 16 ohm tap (in your configuration) as the Hammonds let you wire it up as either 4/8 ohm or 16 ohm.
We look forward to the pics! BTW, I wonder what the origins of the term "fire up" are? :)

Shannon

[nineno]

I don't think you have a 16 ohm tap (in your configuration) as the Hammonds let you wire it up as either 4/8 ohm or 16 ohm.
We look forward to the pics! BTW, I wonder what the origins of the term "fire up" are? :)
Shannon

Shannon, are you sure that i don't have a 16Ohm tap on my 1650E? From the schematic on the bottom of this page http://www.hammondmfg.com/1608.htm it looks like the green wire is a 16Ohm tap.

For whatever it's worth i have the black (ground) wire on the 1650E going to J2/J5-3 (where the blk/ylw on the 1650F goes). I have the black/green (8ohm tap) going to J2/J4-3 (where the yellow on the 1650F goes). I have the black/red (4ohm) and green (16ohm?) wires un-connected, floating in electron space along with the CT for the filaments....none of them go anyplace.

However, i was thinking that i could use the 16Ohm tap to go to J2/J4-3 location for my feedback so that it would no longer be floating around in no-where land, and then i could hook my 8ohm tap right up to the output jack, rather than having it bounce to pin J2/J4-3, and then to the output socket. Does that make any sense? Frankly, I'm jsut trying to avoid having floating wires and clean up the wiring in the chassis by avoiding "bounced" connections from one place to the next.

As for the "fire-up" part, it was REMARKABLY uneventful. No snap-crackle-pop, and nothing lost it's smoke. A rewarding feeling indeed!

Thanks for the continued support.
drew*

[Shannon Parks]

Shannon, are you sure that i don't have a 16Ohm tap on my 1650E? From the schematic on the bottom of this page http://www.hammondmfg.com/1608.htm it looks like the green wire is a 16Ohm tap.

Drew,
I was too lazy to look and you are indeed correct. :oops:

The 1650E doesn't require the same goofy Hammond wiring scheme as the others. I would concur with your thoughts on using the 16 ohm tap. Cleans it up some, eliminates a jumper cable, etc. Cheers!

Shannon

[EWBrown]

If one were getting really zealously precise, should the feedback cap (27 pF) also be increased for the 8 ohm versus original 16 ohm impedance?
If the resistor was reduced to 19.2K, maybe the cap should be increased
by sqrt 2 to 39 pF to lower its reactance proportionally.

Just an idle thought... :?

/ed B

[Shannon Parks]

If one were getting really zealously precise, should the feedback cap (27 pF) also be increased for the 8 ohm versus original 16 ohm impedance?
If the resistor was reduced to 19.2K, maybe the cap should be increased
by sqrt 2 to 39 pF to lower its reactance proportionally.

Just an idle thought... :?

/ed B

Ed, Thermion and I were just discussing this last weekend. The answer is "yes" - but more correctly, the answer is "I don't know". Once the Eiclone hubbub has died down, I will thoroughly research that cap value, the B+ plate feedback cap value and along with other possible tweaks with in depth stability testing with my o-scope. I'm thinking I might even try a much larger feedback cap.

Shannon

[EWBrown]

I can try some 39 pFs on my in-process rev B board, and see if it makes a difference. I went w/o the 20 pF UL screen tap feedback caps on the first one at first, and didn't hear any difference after putting them in.
I guess Dynaco had a reason for that one, they are in all their amps.
Might have been a way to get around the 10% and 20% component values they were using way back then.

/ed B