Not enough NFB... OT problem?

[Ralle]

Hi Guys.
Question: Can an ot cause a poor nfb?
I've checked everthing else in the amp...all the voltages looks fine, resolderd all of the joints... checked the bias circut, caps, resistors... all the stages looks fine...
When I look at the scope connected to the output, there's way too much gain ( flatting out ) for a 27k/16 ohm nfb... it overcompresses in a bad way...
It's like there's not enough amplitude from the ot to send back through the nfb... the presence works fine... but it's still too much of everything... You know when you dissconnect the nfb, you get insainly too much gain... it is with in those realms it sounds... imagine that plus having a working presence controll... it cuts off the lows like it should, but it's like the ot don't have the straingth to send it back through the nfb as it should... The sound kinda bends over at the top... getting thin and ugly...

Anyone seen this before?

//Ralle

[STEINBERGER]

Do not tell me that 67 marshall amp of yours that has, (in my opinion the best sound on this forum) is giving you trouble?

[danman]

Have you checked the entire NFB circuit for bad connections, failing resistors or cold solder joints? That would be the only reason that I can think of for the lack of NFB.

[Ralle]

It's not the nfb circut... I've checked it several times, and it's not the bias circut... everything looks fine...
It's when I hook up the scope and look at the sinewave on the outout, it overcompresses, it's way too much gain for a 5k pot/0.1uF/ 27k/16 ohm nfb...
The nfb is a result of the amount of amplitude from the ot... if there's not enough amplitude, there's not enough nfb... I guess one could compare it to a master controll... lowering the master, you get less nfb... It's just like that but whithout the lost of gain... On top of that; it's not like the presence controll don't work... it does, in the way it should but... it's just too much gain...

[Haze13]

I've seen several times that old resistors drift under the load (or when there is a voltage across them). Say the "Tale" resistor in the PI circuit is getting larger than you have less NFB and you have more gain. It's more likely that, than a lower output from a transformer, but in electronics you can't be sure on a 100%.

[darkbluemurder]

but in electronics you can't be sure on a 100%.

Agreed, but what you can be sure of is that if something is not working right there is a good reason for it - it is just a matter of finding it.

Presence control works - that means there is a feedback signal, otherwise it would not work.

I don't think it's the bias circuit, unless your circuit is such that if the bias pot wiper fails it puts maximum negative voltage to the grids - but then the voltages would not be fine as mentioned before. In the most prevalent bias circuits a failure there would hurt or kill the output tubes.

Are you sure that you have taken the NFB from the 16 ohms and not the 4 ohms tap?

The only instance where I can think of an OT putting out not enough signal for the NFB is when it's close to dead, but then you would have a very very weak output from the amp altogether.

I would check the phase inverter connections just to make sure.

Last thing to check would be to disconnect the NFB temporarily just to see what difference it makes. If it's drastic, the NFB circuit probably works OK and the fault lies somewhere else.

Good luck,
Stephan

[VelvetGeorge]

This seems to be down to the phase inverter. Please post voltage readings from all V3 pins at idle and under load. These should identify why the NF is not having the expected affect.

george

[Ralle]

Thanx guys... I went and took the whole amp apart... I mean everything... I stripped it clean... cleand up the chassi, resoldered every point, cleaned all the pots, cleande all the ground points, put in new tube sockets ( power tube sockets ) cleaned the preamp sockets...
I have installed the powersection, heaters, sockets, filtercaps... and I have been VERY carefull about the grounds...
I have yet to install the preamp card, pots and connect the preamp tubes...
If anything is wrong after this, I'm selling the amp... No, just kidding... if there's still somthing wrong after this, it's likely as you said; the PI section...
You might think all this is unnessecary, but it was due for a cleanup anyway...
I'll get back with the voltages on V3...
Talk to you soon...

[Ralle]

Guys... I have just finnished the rebuild of my amp... I haven't tested it yet, but when I installed the main filtercaps ( two 100's in serie ), I got a strange notion; I think I might have installed the one closest to ground backwoards during all of my testing since I don't know when... it could have happen like a month ago... If that's the case ( talk about a simple but yet soooo serious fault ), this symtom I've been having would make sence...
I'll get back tomorrow after I'f firerd it up, and messaurerd everything...
Keep all the fingers crossed...

[neikeel]

Thanx guys... I went and took the whole amp apart... I mean everything... I stripped it clean... cleand up the chassi, resoldered every point, cleaned all the pots, cleande all the ground points, put in new tube sockets ( power tube sockets ) cleaned the preamp sockets...
I have installed the powersection, heaters, sockets, filtercaps... and I have been VERY carefull about the grounds...
I have yet to install the preamp card, pots and connect the preamp tubes...
If anything is wrong after this, I'm selling the amp... No, just kidding... if there's still somthing wrong after this, it's likely as you said; the PI section...
You might think all this is unnessecary, but it was due for a cleanup anyway...
I'll get back with the voltages on V3...
Talk to you soon...

Hi Ralle

Was this a clone or an original ?:palm:

[Ralle]

The PT and OT are clone, 1203-80 and c1998 selflead, both from George's... The rest is original... well, there's been some swapping of some resistors and some filtercaps. All the couplings are original mustards.

[Ralle]

This seems to be down to the phase inverter. Please post voltage readings from all V3 pins at idle and under load. These should identify why the NF is not having the expected affect.

george

Here's the PI ( v3 ) pin readings with standby on ( ready to play ):
pin 1: 219v
2: 21,7 v
3: 35,2 v
4, 5: 3,15 vac
6: 209 v
7: 22,5 v
8: 35,2 v
9: 3,15 vac

What do you make of that?

[VelvetGeorge]

Does the tone change now if you lift the NF?

george

[Ralle]

Yes it does... the gain gets insain...

[Haze13]

Means the the NFB is there where it should be... Have you tried to replace the resistors in the NFB chain? If there will be no change, than resistors are OK and the problem is somewhere else...