Sound City L120 mk4 Biasing

[eddiep1966]

Hello everyone,
I have a 71 Sound City L120 MK4 that has been on the self for years, Well decided to bring some life back into this thing. Recap the power section and got her ready for new tubes. I was wondering if anyone had any suggestion on were to bias this thing at, well a good starting point.
Thanks for any thoughts or suggestions.

[pdf64]

Does your amp have the 68ohm and 22ohm cathode resistors?
I get the impression they were a later addition, not fitted to the earlier ones.
If not, consider fitting 1ohm cathode resistors, then it’s quick and easy to set a suitable idle current (perhaps in the range 30-35mA) and identify any outlier valves.

Note that without the 68 & 22ohm resistors, the magnitude of the bias supply can be marginal, and a voltage doubler (edit: on the bias supply rectifier) may be necessary to get idle current under control. A simple half wave type is perfectly adequate and is easy to implement with a bit of tagstrip. Be sure to get the ecap orientation correct

[Stevem]

With almost 500 volts on the EL34 outputs I would not idle them at more then .032 amps.

.030 amps is 60% and .035 is 70% and starting to get into the hot side of things.

Set the bias to .028 amp, play the amp hard for 3 minutes and then do your final bias/ current setting.

[eddiep1966]

Thanks for the input guys it is very helpful. No my amp does not have the 68 and 22 ohms resisters. Form what I have read that was implemented in 1973 but who knows a lot of info has been lost on these amps over the years. But I never understood why they were different values for each side unless it was making up for some kind of imbalance in the output transformer. Maybe you guys can shed some light on that subject. So the 1ohm resister does seem like a good idea since these old things do have six output tubes. Notice some socket arcing so I'm gonna replace all of them and see were it gets me. I have read on some of these amps you may have to redo the bias circuit with a voltage doubler to get the voltage up enough

[Stevem]

That’s certainly true of some Highwatts when fitted with strong testing output tubes!

[gldtp99]

I've Rebuilt/Reporposed dozens of SC 120's over the years ---- Hiwatt preamps, Modern Hi Gain preamps, Blackface/Brownface Fender preamps

To help out the sometimes marginal Bias Supply, I've taken to installing 100 ohm/5 watt Cathode resistors on all six Output tubes

Then building a Hiwatt DR103 type Bias Circuit (with adjusting Trimpot)

I started doing this before MHuss posted the Bias Voltage Doubler circuit (seems like a good idea) on his Hiwatt website and kept doing it the way I had been because it works great and is extremely simple

I bias these amps to 60% of Plate Dissipation and the output tubes last for years and years

The only "complaint" I've gotten (from the owner of the last one I did) was "I play bass in several bands and if i turn this amp up above "2.5" the other bandmembers all get mad at me ---- This 120 watt head is louder than a 600 watt solid state Bass head I've tried ---- Is it supposed to be this Loud ???!!!"

SC 120 output sections are solid, massively powerful, and wonderful sounding IMO

I'm not a fan of the SC 120 Mk4 active preamp, to put it mildly

I've still got two SC120's in storage that need rebuilds

I'll get to them one day if i find someone who needs that much raw power

[eddiep1966]

Well Hello gldtp99
I don't know if you remember we had some conversations about this L120 over at the Vintage amp forum many years ago with papadog. Man I sure miss that forum. It's a shame that its a thing of the past now.
Did you mod the bias circuit in your L120 when using the 100 ohm resistors or did you leave it stock? I was thinking of putting the 1ohm resistor to measure the current but since i have a custom diy bias probes that will do six tubes ( inspired by this amp) Its not really necessary. Would rather use the probes instead building a doubler in it. One Thing for sure your right they are Very, Very, Loud. If you have never own one of these amps then kind of hard to understand raw power when comparing to Marshall 100 watt but I think the pig has got it beat.
I have seen pics of your work and it is very nice indeed.
Yeah really I'm not a fan of the active tone circuit ether but that is something us SoundCity guys have to put up with unless we mod. LOL
But when it comes down its all said and done I still use my Marshalls. I just want to get the old girl working like new and if i need her she is there.

[eddiep1966]

Then building a Hiwatt DR103 type Bias Circuit (with adjusting Trimpot)

Reread the post and there it was, I do have some good stupid moments.

[eddiep1966]

Then building a Hiwatt DR103 type Bias Circuit (with adjusting Trimpot)

Reread the post and there it was, I do have some good stupid moments.

[palmatown]

I've Rebuilt/Reporposed dozens of SC 120's over the years ---- Hiwatt preamps, Modern Hi Gain preamps, Blackface/Brownface Fender preamps

To help out the sometimes marginal Bias Supply, I've taken to installing 100 ohm/5 watt Cathode resistors on all six Output tubes

Then building a Hiwatt DR103 type Bias Circuit (with adjusting Trimpot)

I started doing this before MHuss posted the Bias Voltage Doubler circuit (seems like a good idea) on his Hiwatt website and kept doing it the way I had been because it works great and is extremely simple

I bias these amps to 60% of Plate Dissipation and the output tubes last for years and years

The only "complaint" I've gotten (from the owner of the last one I did) was "I play bass in several bands and if i turn this amp up above "2.5" the other bandmembers all get mad at me ---- This 120 watt head is louder than a 600 watt solid state Bass head I've tried ---- Is it supposed to be this Loud ???!!!"

SC 120 output sections are solid, massively powerful, and wonderful sounding IMO

I'm not a fan of the SC 120 Mk4 active preamp, to put it mildly

I've still got two SC120's in storage that need rebuilds

I'll get to them one day if i find someone who needs that much raw power

Hi gldtp99,

I have a handful of Sound City amps I am really digging working on, 120’s and PA’s. I am relatively new to amp building/work. I have some biasing problems, way to hot first of all on the PA’s and mismatched mA reading staying with the sockets (mA readings don’t follow the tubes).

Have you shared the HiWatt bias circuit you mention here in addition to the 100 ohm/5W Cathode bias resistors per output tube? I’d love to give it a try!

Thank you for any help!

[gldtp99]

Here's a 100 ohm/5 Watt Metal Oxide Power Resistor that Mouser currently has in stock --- It has better specs than the other 3 similar resistors they have right now:

https://www.mouser.com/ProductDetail/YA ... ZAZA%3D%3D

Buy 10 of them so you pay less money (you need 6 anyway to do this job)

Install them into the Sound City 120 amp (first removing the existing cathode connection on each output tube socket) and installing these Power Resistors as Cathode Resistors on the tube sockets

That's all you need to do -- they should work with the stock Sound City Bias circuit just as well as the Hiwatt type circuit