Sound city 50 plus negative feedback loop

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Sound city 50 plus negative feedback loop

Postby Cookieand on Tue Mar 15, 2016 8:02 pm

I was looking to get a little more dirt from my sound city 50 plus b50 model. I'm after some advice, would it ok be to fit a switch to isolate the negative feed back loop? I've read on various forums this can be done to certain amplifiers and I wanted to check it would be OK on the 50 plus? Also would it be worth while swapping the phase inverter tube from the ecc81 to a ecc83?
Thanks
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Re: Sound city 50 plus negative feedback loop

Postby pdf64 on Wed Mar 16, 2016 10:41 am

I think some 50+ variants (Lead model I think) run with the power amp open loop, so it shouldn't be a problem.
The schematic as drawn is open loop http://bmamps.com/Schematics/Sound_City ... 50plus.pdf though not sure if that's an error.
And the LTP should run fine with a 12AX7; the gain increase is marginal, but the tone / character will change too.
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Re: Sound city 50 plus negative feedback loop

Postby Cookieand on Mon Mar 21, 2016 10:00 pm

ok that's great, thanks!
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Re: Sound city 50 plus negative feedback loop

Postby Papa Dog on Tue Apr 05, 2016 1:12 pm

Cookieand wrote:I was looking to get a little more dirt from my sound city 50 plus b50 model. I'm after some advice, would it ok be to fit a switch to isolate the negative feed back loop? I've read on various forums this can be done to certain amplifiers and I wanted to check it would be OK on the 50 plus? Also would it be worth while swapping the phase inverter tube from the ecc81 to a ecc83?
Thanks

pdf64 wrote:I think some 50+ variants (Lead model I think) run with the power amp open loop, so it shouldn't be a problem.
The schematic as drawn is open loop http://bmamps.com/Schematics/Sound_City ... 50plus.pdf though not sure if that's an error.


there are other schematics at Sound City Site
A ton of 40+ schematics, various incarnations.
I would recommend this one

yes, the schematics show a negative feedback loop.
Yes, you can bypass that with a switch. Would I recommend it? Nope. But, try it and if you like it please report back.
You might get more than you bargain for, and might find that increasing the value of the resistor would give you less NFB and be just what the doctor ordered.

As to changing from a 12ATto a 12AX7...here is the skinny...lot of guys run a 12AX7 because for a long time you could not get 12AT7s...they were out there, and it may seem funny, but before the internet a lot of things just could not be found by the people who needed/wanted them. The difference is there. The AX has a bit more gain and less "headroom". Remember, these amps were built to run very clean. Changing the circuit and trying to "dirty them up" gives unpleasant and unexpected results. Good luck.
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Re: Sound city 50 plus negative feedback loop

Postby pdf64 on Tue Apr 05, 2016 6:47 pm

Papa Dog wrote:schematics at Sound City Site
A ton of 40+ schematics, various incarnations.
I would recommend this one

yes, the schematics show a negative feedback loop


Just to note that the above schematic doesn't have an operational global negative feedback loop around the power amp, due to the 'other' end of the 10k feedback resistor being connected to 0V.
Whereas this guy does have an operational loop http://soundcitysite.com/sc_lb50_plus_schem.jpg

My theory is that the loop is used with the pure fixed bias variants (B or L/B?), open loop with mixed bias types (ie 68 and 22 ohm cathode resistors on the power tubes, may be L types?), as the inherent local negative feedback that they create may mitigate stability issues.
My L50 MkIV Plus doesn't have a negative feedback loop; the bright channel volume control in boost mode is something of a hair trigger!
But I think that's mainly due to the 4n7F bright cap, as things calm down rather if a more sensible, lower value bright cap is used.
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Re: Sound city 50 plus negative feedback loop

Postby Papa Dog on Tue Apr 05, 2016 8:16 pm

pdf64 wrote:Just to note that the above schematic doesn't have an operational global negative feedback loop around the power amp, due to the 'other' end of the 10k feedback resistor being connected to 0V.
Whereas this guy does have an operational loop http://soundcitysite.com/sc_lb50_plus_schem.jpg

My theory is that the loop is used with the pure fixed bias variants (B or L/B?), open loop with mixed bias types (ie 68 and 22 ohm cathode resistors on the power tubes, may be L types?), as the inherent local negative feedback that they create may mitigate stability issues.
My L50 MkIV Plus doesn't have a negative feedback loop; the bright channel volume control in boost mode is something of a hair trigger!
But I think that's mainly due to the 4n7F bright cap, as things calm down rather if a more sensible, lower value bright cap is used.


The pream of the 50+ is different from other Mark4 (120+ and 200+) amps. Same "bright cap" however, on the "normal channel". Eh....snip, snip, better.
I have seen hundreds of schematics, and with Mark4 do not see a dedicated L or B...they all say L/B.
the "hybrid bias" you are noting in the schematic which you posted is NOT the way it came from the factory....I have nothing to indicate that there was even a tech bulliten making the suggestion for a 50+, however the '73 schematic for the 120+ suggests that mod and says it can be done to the 200+ as well. Some of my schematics show the NFB loop going to ground in the vicinity of the PI cathode circuit, others show it hitting the junction of the .15 mfd cap and the 4k7 resistor. It would seem to me that both would have an effect, but would tend to change the ground on ANYTHING on that rail. I could be worng.
The '73 schematic that i referenced earlier shows a jumper to ground the NFB, but does not indicate to remove the connection to the cap.
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Re: Sound city 50 plus negative feedback loop

Postby Cookieand on Tue Apr 05, 2016 9:24 pm

Ok, I tried swapping the phase inverter tube to a 12ax7 and I wasn't happy with the sound, it seemed quite harsh.
I then tried fitting a switch to interrupt the negative feed back loop and I wasn't happy with the result. I got a lot more back ground noise which I didn't like.
So I'm back to running the amp as it should be and just using a valve overdrive pedal (English muffin) which seems to do the trick.
Thanks for all the helpful information and comments.
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Re: Sound city 50 plus negative feedback loop

Postby Harvey58 on Sat Oct 29, 2016 5:44 pm

Try some easy tricks, you cant do any bad. Check what resistors are on V2 and V3 plates. If you find 100k, change them to 220k. Check the first valve also, there should be 220k. Check the first valve cathode resistor, try a little bigger, from 1k5k go to 2k2 for example. This could put some dirt in the sound.
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