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In Reply to: RE: Rookie needs help please: wiring the Hammond 266M2 for CT (2.5V CT) posted by RC Daniel on April 05, 2014 at 22:07:36
Hi,
"To do this, do each of the 1.25V wires connect to a filament pin and both of the 0V wires connect to the top of the cathode circuit"
Yes, the ends of the winding go to the tube socket's filament pins.
To determine wiring, use an Ohm meter on connected wires, to find about half the DCR ( the center tap ) AND, fire up the primary and measure, VAC on the secondary. Should measure above 2.5 VAC END TO END on the entire end to end winding, unloaded. Check lead outs both measurement ways, DCR and VAC and have one method confirm the other.
Once you get it running OK, re-do the four lead outs to double TCSS. This means two TCSS on ends, and either three or four total on Center tap. Keep cathode return to ground leads as short as possible, Use the length of the Rk power R to make up the distance of the ground return. On SE amps, I shoot for two inches or less lead length on the secondary center tap.
Very important to change the primary lead ins to triple TCSS, to maximize dynamics and transient response on a SE DHT amp. The lead outs on BOTH sides, when optimized, will do two things sonically, flesh out the instruments, and add transparency, greater dynamics too, three gains in total.
Have fun. Listen to it step by step and learn.
Jeff Medwin
Follow Ups:
> > "To do this, do each of the 1.25V wires connect to a filament pin and > > both of the 0V wires connect to the top of the cathode circuit"
> >
> > Yes, the ends of the winding go to the tube socket's filament pins.
It would be easier if the 266 series were simply a 1.25V - 0V - 1.25V arrangement, but we have four, not three, ends... unlike the 166 designs intended for the US. We have 2x 1.25V and 2x 0V - so, to be absolutely sure, I should connect the two 0V windings to cathode and each of the 1.25V windings to a filament pin?
I will be trialling all kinds of stuff - some of your recommendations included - during the year. Regarding filament transformers for future builds, I will probably end up having some custom jobs wound...
As for this build, I have laid out the amplifier so that the DHT cathode C (and R) form the return to the PSU filter cap that supplies the stage - the ground circuit is about as short as can be. Makes for a short cathode circuit... though I would like to have been able to arrange the relative positions of the output transformer and socket to reduce the full length of the current loop... but hey, that is tough when the PSU cap that supplies the output is shared between channels, want to keep the cathode circuit short and meet some other objectives.
I am trying not to get too obsessive with this build: it is really about having a reliable yet still nice-sounding benchmark that honours a well-documented design. My next amp will be rather different.
Thanks again for you assistance.
“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
" I should connect the two 0V windings to cathode and each of the 1.25V windings to a filament pin?"No!
If you already are using the two windings in series (and it works) then you already know the proper phase.
Leave it hooked up just the way you have it. Get rid of the resistors/humpot and connect the cathode resistor to the point between the two windings (that's your center tap).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/06/14
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“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
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