Global NFB resistors & EMI

[tubeswell]

So how much EMI might a typical NFB resistor from the OT to the PI cathode produce in the wrong location? (I decided this was a theory/design question)

(The reason is I have built an amp and gone and located the NFB resistor between the tail part of the LTP circuit and the coupling/plate resistor- to-P.A. grid part of the circuit, and I am getting a hiss (like the vol controls are dimed - but they're not) in the PA even with the 'tail ends' of the coupling caps to the P.A. disconnected. The output tube grid cables are shielded. Scratching my head and thinking this is possibly the only thing that could be causing it. I know the output tubes are good and the OT is hooked up the right way.)

[Chuck H]

With the tail ends of the PA coupling caps grounded the NFB loop is moot. Even if the NFB resistor is creating a parasitic on the preamp or PI it would never get to the power tubes so I would look elsewhere for the problem. Does the amp hiss with the PI tube pulled?

Chuck

[tubeswell]

Yep it does hiss with the PI pulled. It also hisses with the PI-to-P.A. grid coupling caps taken out, so it is in the power amp. There is occasional static intermingled with the hiss and when the static gets intense there are blue flashes on the plates.

I took the GFB resistor out and the problem was still there, so I am left wondering whether it is something to do with the bias supply.

[Chuck H]

I've heard filter caps cause a hiss (with occasional pops) when they're going bad. As we've discussed here before, just because a filter is new doesn't mean it's good... Take it from me.

The bias supply WOULD be the only other thing in the chain by this time. So it HAS to be in the power supply or the bias circuit. If it's a new build check for caps that have too much voltage for their rating or even (I do know you wouldn't do this out of ignorance) installed in reverse polarity of how they should be. I once built an amp with the grounds on the opposite side of the preamp board from how I usually build. Sure enough, I installed all the preamp bypass caps backwards.

Chuck

[Enzo]

Hiss with the PI pulled????

Pull the powr tubes and try them one at a time. I bet one of them hisses and the others don;t.

And try each socket one by one to see if there is any socket related clues.

[tubeswell]

Hissing problem fixed after I re-did the layout at the bias supply end of the board. (BTW It wasn't in one of the tubes or in one of the sockets, it was in both tubes in either socket. It wasn't the filter caps either, because I swapped in new pre-amp and screen caps and that didn't fix anything. So in the end I mounted the bias circuit on a separate board and then spaced the Pre-amp B+ and screen/RT B+ eyelets further apart from each other. Maybe the type of board I am using does have the 'tweed disease' after all, and you need to space the high voltage eyelets further apart from each other? I haven't had this problem with this board before and I've used it in 3 other amps already without issue. Anyhow now the hiss is gone with the PI pulled. But now there is crackling and random static there with the PI inserted.)

I had earlier pulled the Global NFB out and that didn't fix anything (in terms of the hiss - which I later solved), so it wasn't that, and therefore I am leaving this thread and going back to my thread in the debugging BB.

Thanks to all who replied

:-)

[Chuck H]

Bonus... And good info to have for designing!!! I wouldn't thing there would be any conductivity issues with the board your using. Those problems with the "tweed" amps (and many BF amps IMHE) is caused by the fact that the black pigment in the boards is CARBON. Also known as "lamp black" and to this day is the "go to" pigment for making anything black. The rub is that carbon also makes a nifty resistor. Just add heat and high voltage and those black boards start to do all kinds of interesting things.

Chuck

[Steve Conner]

Hey Tubeswell

I had similar problems in my Crown SXA restoration, crackling and popping in one channel that turned out to be a leaky tagboard. I had to take the offending node off the tagboard and float it in the wiring.

[tubeswell]

Hey Tubeswell

I had similar problems in my Crown SXA restoration, crackling and popping in one channel that turned out to be a leaky tagboard. I had to take the offending node off the tagboard and float it in the wiring.

Heya Steve

Bingo - that's precisely what I was now thinking of doing (I notice the Leo F also kept those high voltage wires floating away from the tagboard) I'll let you know. Cheers M8