Drawbacks of no NFB

[fusionbear]

Hello gents. I built a 50 watt M clone a while ago and I was experimenting with removing the NFB because I liked the tone much better. Any drawbacks? I noticed the presence stops working when you do this. Any thoughts?

[Richie]

make the presence switchable,or rewire it to be a cut control. the amp will be more raw sounding without the feedback.

[Ian444]

It was discussed in this thread if you feel like a read, mostly page 2 I think.

https://tubeamparchive.com/viewtopic.ph ... highlight=

I would not use NFB if it didn't make the amp sound better. Its not a "good practice" requirement.

The prescence control is a bit of a gimmick IMO. When NFB is used, the prescence control prevents the higher freq's being fed back, acting as an artificial top end boost. When the prescence control is set to min treble, then all freq's are being fed back the same. Why bother, but people seem to like it. Especially when you consider that in some amps, considerable effort goes into pulling the "fizz" out of them. The prescence control puts it right back in there...just MO.

[Roe]

In a M the PI becomes even more unbalanced w/o nfb. look for redplating on v4

[fusionbear]

In a M the PI becomes even more unbalanced w/o nfb. look for redplating on v4

Should I then stick to 100k on both instead of the usual 82K/100K ?

Any reference material to read up on to learn?

Thanks!

[Gaz]

I've never seen the PI become so imbalanced that it causes red-plating, just by removing the feedback. That sounds crazy!

Anyway, if that were the case, the yes, you would just adjust the plate resistors to balance it again.

[Alexo]

I've never seen the PI become so imbalanced that it causes red-plating, just by removing the feedback. That sounds crazy!

Anyway, if that were the case, the yes, you would just adjust the plate resistors to balance it again.

Yeah, I'm not buying it. Won't affect idle current and I've never driven a power tube hard enough to cause it to redplate - even in SE amps where you have an essentially 100% unbalanced pi. Maybe if your nfb was the only thing preventing an ultrasonic oscillation, but if that's the case, you've got other problems.

[Roe]

not 100+100, if v4 redplates you may need to decrease the 82k to 68k. redplating is a normal problem on marshalls, esp. 100watters. this not only has to do with a mismatched PI and low NFB (100k at 4ohms) but also with running the el34s way too hot (1k7 Z and voltages as high as 530v)

[philo43]

Red-plating means the tube is biased too hot. If you drive the grids too hard you will hear some really ugly sounds (blocking distortion) long before seeing a red glow on the plates. Check the bias after changing anything in the output section to make sure it's still in line.

As for the steep loads that Marshall used, I agree they were pushing things a bit. They sounded more aggressive than other amps, though, and tubes were less expensive back in the day. I'm not sure jimi was thinking about tube life when he laid into his stack(s).

[Roe]

Red-plating means the tube is biased too hot. ..

no it doesn't necessarily mean it's biased to hot if you mean biased at idle. cf. https://tubeamparchive.com/viewtopic.ph ... sc&start=0

[fusionbear]

I measured my voltages this evening. Let me know if they are too hot. Maybe I need to decrease. I haven't noticed any problems and the amp sounds great. I haven't removed the NFB yet. I would like your advice first.

[IMG768]http://i169.photobucket.com/albums/u223 ... G_3195.jpg[/img]
[IMG768]http://i169.photobucket.com/albums/u223 ... G_3194.jpg[/img]
[IMG768]http://i169.photobucket.com/albums/u223 ... G_3192.jpg[/img]

You guys are great! thanks!

[Alexo]

I would not give a second thought to removing the nfb, I usually prefer to run most of my builds this way, but it's a matter of taste.

...however, running upwards of 480 volts is pretty hot! Not really hot for an old Marshall, but hot for power supply caps that are only rated at 500 volts.

My old apartment had a mains voltage of 128 volts - if you plugged that amp in there, you'd be looking at 512 on the first node of your power supply, theoretically.

[fusionbear]

Its my son's amp, so I will put a switch. NFB on when he is playing his drop tuned metal stuff and no NFB when I borrow his amp...

Still hoping I am not exceeding the ratings of the 12AX7's though with the voltages...

BTW, I use a regulator with all my amps. I have 2 Furman p1800's

[rocketeer]

Since your son is drop-D tuning do what Diezel does to really "tighten" up the output section. 10k FB on 8Ω. Absolutely wonderful IMHO. This goes against what a lot of guys post here but it works well if he's using high gain settings and wants the low stuff to articulate and not mush out. You'll have to give your PI's more input to recover some of the lost gain....... turn it up!

[Roe]

Its my son's amp, so I will put a switch. NFB on when he is playing his drop tuned metal stuff and no NFB when I borrow his amp...

Still hoping I am not exceeding the ratings of the 12AX7's though with the voltages...

BTW, I use a regulator with all my amps. I have 2 Furman p1800's

just use a pot in series with a resistor, 100k and 27k or even higher for the pot