NFB Question

[dartanion]

Hi Folks,

I have seen in the past that having NFB can cause nasty oscillations if the OT Primaries are connected with incorrect polarity. Simply swap OT primary leads and the squeal goes away. IIRC, connected with the wrong polarity causes positive feedback. I saw this on a build where when I engaged the NFB, it got louder and squealed like a pig.

Anyway, my questions are these:

How does the OT polarity cause positive feedback if connect backwards?

Is this effect only observed in PP amps or can it effect SE amps too?

[Allynmey]

Dart, when the signal is injected back into the PI from the speaker jack, being out of phase (neg feedback) it will cancel some of the gain and give broader freq. response. If it is in phase, you have just created an oscillator (Pos feedback). Positive feedback from the output section causes oscillations and howl.

I know this is a very simplified version but, it's quick and simple. I think Randall Aiken's site has more if you would like to read up. It really is a artform to use local NFB loops in the preamp to control gain and broaden some of the Freq. rsponse.

Allynmey

[stoo]

As far as SE amps are concerned..There is no phase inverter to deal with but it depends where the FB is injected. Each triode reverses the phase of it's input signal so you must chose which stage the FB is injected.
Stew

[dartanion]

Yeah, I've read the Aiken article about a dozen times, and understand the principles behind it. Randall's article doesn't really go into details such as what I described. So, it's a crap shoot as to if you get your OT primary polarity correct on the first try then right? Any way to figure out which way is correct before hooking it up?

[Allynmey]

Dart, If you get the same trannys from the same manufacturer all the time, write down the orientation of colored wires. You have a better chance of getting it right the first time. I leave a little extra on each plate wire until fire up and then trim it down.

Allyn

[stoo]

Some tranny companys put a dot on there wiring diagrams marking the like phases . So if you have a + signal on the dot end of the primary you'll get a + going signal on the dot end of the secondary.
I've always considered the top of the primary diagram to be the same phase as the top of the secondary diagram.
Then you work backwards through the power stages inverting the phase as you go from plate to grid ect. The normal place to inject the NFB is on the non inverting ip of the longtailed pair ( marshall styled arrangement)
Stew

[novosibir]

Before you fire up an amp the first time, dime the presence control!

If your OT wires are swapped by accident, w/ presence fully cranked the amp doesn't squeal. Yes, in this case of 'positive feedback' the presence is acting as a dampening control.

A simple way to 'do it right' at the first stroke is:

Measure your OT's primaries from the CT to its ends, the resistances are at least slightly differing. The 'end' w/ the higher resistance to the CT always should be connected to the output tube(s) next to the preamp tubes.

Btw. This 'end' definitely is the end of the primary winding, the other one is the beginning

Larry

[dartanion]

Thanks for all the tips guys.

How about for an SE OT. I can't really figure out which lead is the start or end of the winding. Is this just a trial and error approach?

I have started leaving the OT primary leads long-ish so that I can swap them if needed. I made the mistake of cutting leads down to length on one build a year or so ago, then had to splice in a section of wire to swap. DOH!

Anyway, I just knew there had to be a better method than flipping a coin.

[statorvane]

Not to hijack this thread, but Larry.... those are AWESOME lookin' amps on your website!

[novosibir]

Sorry for being so far off topic, but...

... Thanks for your kind words, statorvane

Larry

[Wild Bill]

Thanks for all the tips guys.

How about for an SE OT. I can't really figure out which lead is the start or end of the winding. Is this just a trial and error approach?

There is an engineering standard for transformer colour codes. Blue (and brown) is for the plate, red for the B+ feedpoint. On the secondary side black is common, green is 4 ohms and I forget the rest! Any old copy of the ARRL ham radio manual or a good book like the Radio Design Handbook (you can google up a download!) will give you these and other conventions.

Anyhow, of course Marshall had to pull some weird colours out of their butt but most (like Fender) seem to stick to the standard.

[dartanion]

Hey Wild Bill,

Yeah, it's true that there are some standard color codes, but I have found that color doesn't always go by the codes or even by what the manufacturer says it is.

[Johnhenry]

50 Watt Power for Marshall =
Primary
* Blue - common
* Black - 120 V.
Orange - 220 V.
Yellow - 240 V.
Secondary :
Orange - 6.3
Black - 6.3
Red - AC out
Red - AC out
Black - CT
Brown - CT
100 watt Primary =
Primary :
*Orange - common
Green - GRD
Red - 120 V
*Blue - 220 V.
Violet - 240 V.
Secondary :
Blue - AC Rectifier
Grey - AC Rectifier
Yellow - AC CT.
White - Bias
Black - 6.3 V.
Black - 6.3 V.
Green - Grd.
Note : A continuity chk. must be made between (*) primary wires to ensure that they are not interchanged with secondary wire's of the same color.

50 watt output : Primary :
White-Plate
Brown - CT
Red - Plate
Secondary :
Orange ( 2 ) - common
Yellow - 4
GRN-8
Grey - 16
100 watt output :
Primary :
White - CT
Brown - Plate
Red - Plate
Secondary :
Brown - ( 2 ) Common
Green - 16
Black - 4
yellow-8
Hope this helps you on the Marshall's,
Johnhenry

[Johnhenry]

I know if you dump an even amount of Stages into the P/I and you wire the output like a B/F Fender amp Blue to the power tube Away from the preamp and brown closest to preamp, Red to B +, Blk to Grd., and green to speaker out your speaker's should play forward and your amp is in phase,
on where to hook your N/Fb in, someone already posted the way to do it,
it does make a diff where you hook though !
I'm not for sure, but on a S/E amp it may make a difference also where you tie it in, i've been having luck tieing it in on the Cathode of the phase inverter.
i'm sure there are people on here that know,
Where's PRR when ya need him ? LOL !
Johnhenry

[novosibir]

I know if you dump an even amount of Stages into the P/I and you wire the output like a B/F Fender amp Blue to the power tube Away from the preamp and brown closest to preamp...

Vice-versa, vice-versa!

Otherwise you'll have a squealing pig and a wife, crying for divorce!

Larry