NFB problem -- screams

[bluefoxicy]

I dropped a 33k resistor between 16 ohm hot and R9+ (pin 8), and the amp gets louder and more distorted. SPST switch in between to flip it on and off.

 

Ah, oops, didn't raise C3.

 

So, I have a SPST switch as follows now:

 

Common: pin 8 of preamp

1: Cathode cap

2: 33k resistor to 16 ohm output

 

Result: Switching to NFB mode removes C3, connects pin 8 to output, and screams like crazy.

 

If I'm understanding this right, that gain stage inverts; and then the power tube inverts again. That being the case I'm getting positive feedback and I should really be connecting to pin 3, not pin 8. ?_?

[bluefoxicy]

Reversed the OT and it worked.

[wpkg]

Reversed the OT and it worked.

 

yep I had the same problem. a super loud motorboat sound. same fix....

[bluefoxicy]

Any thoughts on using the NFB into the power amp, or using it with parallel preamp (PA in), or into the first preamp stage?

 

I'm using a 10k resistor in NFB (1/4 watt) @_@

[wpkg]

Any thoughts on using the NFB into the power amp' date=' or using it with parallel preamp (PA in), or into the first preamp stage?

 

I'm using a 10k resistor in NFB (1/4 watt) @_@[/quote']

 

not a one! i gotta say most of your posts are way above my head man. I barely know what youre talking about. good luck with that though.

[bluefoxicy]

 

not a one! i gotta say most of your posts are way above my head man. I barely know what youre talking about. good luck with that though.

 

Pick up a copy of RC-30 and glance through the first 50 or so pages.... I tend to be overly technical in everything I do, this is a known problem.

[layboomo]

What exactly are you trying to achieve with NFB and is this just a valve jr we are talking about? Most push pull amps insert NFB at the phase inverter...but obviously thats not an option on an SE amp like a VJR.

[bluefoxicy]

I'm trying to push a button and get clean volume (not just headroom, but real volume). It's working somewhat. A 10k resistor takes gain away and then you crank the volume and it distorts at a lower volume than normal; I threw in a 33k I had on hand, same headroom but almost no volume drop. I'd imagine the 47k value that the modders settled on was picked for good reason-- the lower the resistance, the more gain I lose, and at a point I'm only removing gain rather than any harmonics.

 

Also, I should put a capacitor in the NFB circuit, probably 0.48uF to cut anything lower than 14Hz to reduce muddiness and oscillation... or try to, and see what happens. See Fender.

[layboomo]

I'm trying to push a button and get clean volume (not just headroom' date=' but real volume). It's working somewhat. A 10k resistor takes gain away and then you crank the volume and it distorts at a lower volume than normal; I threw in a 33k I had on hand, same headroom but almost no volume drop. I'd imagine the 47k value that the modders settled on was picked for good reason-- the lower the resistance, the more gain I lose, and at a point I'm only removing gain rather than any harmonics.

 

Also, I should put a capacitor in the NFB circuit, probably 0.48uF to cut anything lower than 14Hz to reduce muddiness and oscillation... or try to, and see what happens. See Fender.[/quote']

 

Well "real volume" as you put it ......there is only so much available in a SE amp like a VJR but the tone shaping with NFB is interesting and most people do it with a tweed style tone control or a Bitmo at the r-6 location. You might find this link interesting lots of technical info. http://home.comcast.net/~stphkeri/NegativeFeedbackTone.pdf

[bluefoxicy]

Holy crap.

 

Is that a tone knob that sends its discarded signal back to the input of the previous inverting triode, rather than to ground? Interesting!

[bluefoxicy]

Oh I see, it sends it back to the input of the tone stack to prevent tone stack distortion. Still interesting!

 

This got me thinking of grounding the Big Muff to the previous grid, for NFB...