Sound City L120 - noise issues

[StingRay85]

All tube amps makes some noice . Most of the time it is in the gain stage . The more gain the more noice .

Thank you for your wisdom

 

[gldtp99]

That sounds tempting. Do you have schematics?

mhuss Hiwatt schematics and layouts:

https://hiwatt.org/tech2.html

 

[StingRay85]

mhuss Hiwatt schematics and layouts:

https://hiwatt.org/tech2.html

Wow that's pretty cool. I need to dig a whole lot deeper into that and see how to convert the active tone stack while preserving as much components as possible. Long therm project for sure. Did you keep the 6 EL34s or is there a way to go to only 4 and still use the original output transformer and impedance selector?

 

[gldtp99]

I've done six of the SC120-to-Hiwatt gut/rebuild jobs and I've found that keeping the original EL34x6 configuration results in the best sounding amps.

I've tried to run some of them on EL34x4 but they never sound right.

The OT is spec'ed to properly load EL34x6 and that's how it sounds best------ they are not Hiwatt transformer sets, they are Sound City 120 transformer sets and the specs are not exactly the same----- use EL34x6.

Some SC 120's have Partridge transformers and some have similar transformers made by EV---- I've seen some amps that had a mix of Partridge and EV iron---- they both sound great to me.

I've done some gut/rebuild jobs to other SC 120's that have nothing to do with Hiwatt circuits and they don't suffer from noise problems.

Here's one I did for a local Grindcore band that's a modern Hi Gain channel switcher----- here's the Hi Gain channel:

 

[StingRay85]

That's awesome man.

Would you go for something like this?

 

[gldtp99]

That's awesome man.

Would you go for something like this?

View attachment 64440

Yes, the Mid 70's 4 Input circuit is a great sounding Classic Hiwatt circuit.

I've built that one several times as well as the Early 70's 4 Input circuit.

Both are excellent examples of Classic Hiwatt tone.

Remember to take into consideration that the Power Supply sections of the Classic Hiwatts and the SC 120 have some differences---- but the SC 120 chassis is easily adapted to the Hiwatt DR103 type Power Supply circuit.

 

[gldtp99]

Here's a SC 120/Hiwatt gut/rebuild I did many years ago played by my friend Harry (doing some blues).

I don't remember if this one is an Early or Mid 70's 4 Input preamp circuit----- but it really doesn't matter all that much---- both are excellent sounding.

The channels are turned up to give a little "edge" to the tone and the stock MV is rolled back because these are extremely loud and powerful amps.

Original SC 120 era Sound City 4x12 with original Eminence square magnet speakers.

An analog delay was used between the guitar and amp in this recording to fatten up the sound a bit:

 

[StingRay85]

This guy converted it to a Mesa Boogie bass head. The transformers were also placed differently. It's on reverb right now, pretty sweet

 

[gldtp99]

Interesting.

But there's no reason to mount the transformers in a non-stock position with these amps.

The noise problems in the stock L/B 120's have nothing to do with transformer coupling hum. Even though I hear this myth from time to time----- "He turned the transformer 90 degrees and the noise went away like magic !!! I stood there and watched him do it"

Then I ask "Which transformer was it ?"---- and they don't ever quite remember----- And they can't explain how they watched someone turn a SC 120 transformer 90 degrees when I know, for a fact, that there isn't enough slack in the transformer leads to allow this unless the leads have been spliced and lengthened.

I've seen pics, like the one above, of SC 120's that have had a transformer with non-stock mountings---- but it's a waste of time and effort.

The noise problems of stock SC 120's are from the poor design/build of the infamous Mk4 active preamp.

 

[StingRay85]

Cool. In that case I wont be bothered to move them around.

I really dig the idea of a hiwatt preamp design. Probably wouldn't hurt to move the phase inverter to a different spot just in case

 

[gldtp99]

Here's Lalo playing one of my DR504 clones at Trees in Dallas a couple of years ago. He's playing rhythm on this song---- the white head sitting on the Orange cab:



He's been using this DR504 clone for more than 12 years touring all over the southern USA, Mexico, and parts of South America.

In this clip the amp still had the original tube set I built the amp with----- they'd been running for 10+ years at that point.

He brought me the amp earlier this year and I checked it out and installed new tubes.

All new production tubes----- JJ E34L's for the outputs.

This amp has the PI in the stock position next to the transformer----- it's always sounded great and has a very low noise floor.

 

[mickeydg5]

View attachment 64349

This is from a 120 PA unit. Here there is a 90° rotation

All you need is one 90 degrees out. They are also on opposite sides of the amplifier.

 

[mickeydg5]

I actually like the active EQ setup. I think it is just implemented in a bad way.

 

[gldtp99]

I actually like the active EQ setup. I think it is just implemented in a bad way.

I don't think an active preamp is a bad idea in itself.

But the SC Mk4 active preamp leaves a lot to be desired, IMO---- some people do like them but I'm not one of them.

When I did my first couple of SC120/Hiwatt rebuilds I had one stock SC120 on hand that I serviced up and it was a relatively low noise, properly working amp--- and it sounded good.

Until I A/B'd it against the first SC120's that I put the Hiwatt preamp into----- the Hiwatt preamp was just better sounding, much easier to dial in, and lower noise.

Many people A/B'd the amps and nobody thought the stock SC120 was the superior amp in any way.

Another later SC120/Hiwatt rebuild I did had such a low noise floor that it was very hard to tell if the amp was operating at all when it was turned up to stage volume--- it had the transformers and PI tube in the stock positions.

One day a guy who was recording in the studio across the hall from my old shop came over and said he wanted to try out a "big amp". He played a Fender Valve Jr. So I set him up with that low noise SC120/Hiwatt thru a 4x12 and I turned around and continued to work at my bench on some amp.

Next thing I heard was a huge blast of Volume and the guy yelled with fright----- He'd turned the amp up halfway and hit the strings with the same force that he used with his Valve Jr (smashed the strings)---- He'd scared the crap out of himself with the huge blast of volume that came out of the amp---- he said he wasn't sure if the amp was even on because there was no background hiss/noise like most amps have.

The amp was definitely "On".

I plugged in a guitar and gave him a quick lesson on picking dynamics when playing a Big Amp but he didn't seem to want to understand as he only knew how to bash away at the strings using a @15 watt 1x12 combo.

He soon walked out muttering, "Why do they make amps like that?"

 

[StingRay85]

Thats awesome I might hit you up with more questions in the future