Re: Turner... Filaments and OPT question



Engineer wrote:

> "flipper" <flipper@xxxxxxxx> wrote in message
> news:48d8o1dmuhpm03654fal5dg9klvjvg8s0d@xxxxxxxxxx
> > Believe it or not I really am working on that first tube amp but
> > hurricanes and such have slowed me down a bit. Still, I have the
> > entire case done and components mounted with the HV section, -V
> > supply, and filaments CCS now finished.
>
> (snip... filament CC was question fully answered)
>
> > Minor question on the OPT. Is there a color standard on the wires? I
> > know which is primary and secondary but I'm wondering if I can tell
> > phase (which on the secondary to use for feedback) by the wire
> > colors.
> > Primary is green with a red stripe and red with a black stripe.
> > Secondary is yellow and black.
>
> Don't bother. I use a very simple technique for f/b phasing with an
> unknown (to me) OPT (and that's most of them!), viz:
> 1. Power up the amplifier in open loop i.e. no NFB (keep the volume
> control at zero and have no input signal.) This also serves as a
> "smoke test"! I use a variac, check all voltages...but this is OT.
> 2. Apply a very small input signal compared to the amplifier's design
> specs (gain with no NFB will be very high!)
> 3. Crack the volume control until you get a small sound from the
> speaker or a small signal on the 'scope (in this case use a load
> resistance on the OPT secondary.)
> 4. Use a high f/b resistor (e.g. 100K) from one side of the OPT
> secondary (other grounded) to wherever it goes in the design
> (typically to a low value resistor in the first stage cathode
> circuit.)
> 5. Connect one end of the resistor and touch the other to the f/b
> point.
> 6. If the volume goes down (heard or seen), you've got the phasing
> right, so put in the design value (and its paralleled phase advance
> cap) or, if an experimental design, reduce the resistor value in
> stages (and experiment with the capacitor) until you have the desired
> square wave or frequency response - details on this beyond the "scope"
> of this reply (no pun intended!)
> 7. If the volume goes up (or if it squeals, likely it won't with
> 100K), reverse the secondary and continue as in #6.
> That's it. It's almost easier to do than describe.
> Cheers,
> Roger

Your method is right, although there *must* be a load at the output
or the amp may oscillate.

If the phasing of the sec is wrong, and the FB R is a high value,
then the slight postive FB you are adding should merely increase
the gain, but not enough to make any oscillations happen.

Patrick Turner.




.

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