sound city 50+ problem

[Shane]

Hi Folks
I have a weird problem on a SC50+.
The anode voltage on one of the EL34's is nearly 30V lower than on the other.(470v against 499v).

I've tried different tubes- same thing.
Tried different OT same!
Bias voltages are ok.

Changed grid leaks and PI coupling caps same thing BUT...
When the output tubes are disconnected from the PI the voltages at the anodes are the same!

The one that is low does not appear to be drawing excessive current.
I thought it might be due to some parasitic oscillation but why only one side of the output?
Any Ideas?

Cheers
Shane

[Mars Amp Repair]

First of all, what's the symptom other than the plate voltage being lower? Sounds bad? What?

Have you actually measured the plate/cathode current in each tube separately?

If the tubes are matched, is the negative bias voltage at the control grids about the same?

Do you have a scope? If so driven into a load, are both halves of the sinewave equal?

I know in the 100watt-200watt models, they have different value cathode resistors for each pair of tubes...I believe it might be to even out the operation of the PI....not certain.

It might be completely normal in this amp. Waiting for more info...glen

[Satamax]

If it's a combination bias, i think this is normal Don't you have two resistor on the cathodes of the power tubes?

[Shane]

Hi Folks
The tubes aren't matched they're quite old though changing the individual bias voltages does change the currents it doesnt seem to change the relative voltage differnce that much.

I havent scoped it yet got to get access to my scope its not onsite at the moment

Though the schematics I have do show different cathode resistors, this doesnt have them also the voltages are considerably higher in this one than shown in the schematic.
The PI is a normal LTP with an ECC81.

Soundwise its very hissy even with the volumes on 0 with the PI connected the background hiss is way too high for me! with a guitar into it its not too bad no weird tones really. maybe this voltage thing is normal for this model! never come across this before though.

I'll get the scope on it and let you know what i find.

Cheers
Shane

[Mars Amp Repair]

Well if the hissiness is the only functional problem with the amp, I think you're barking up the wrong tree to be looking in the power stage.

These Sound Cities are renoun for being noisey. Mods have been posted on other sites to cure the noisiness of these Sound Cities. Not certain where I saw it tho....possibly someone else on the forum knows.

I'd be a bit concerned about the bias adjustments not doing much, but I think that's another issue to address.

I'd start pulling 12AX7's from the 1st preamp up to the PI tube. Somewhere along the line & I'd guess the 1st 2 tubes, you'll figure out what stage the noise is coming from. glen

[Satamax]

Ahhh, sound cities and hiss!

The hiss is kind of inherent of the design i think! Been scoping a lot at them, calculating and all that, and i might tell you few things.

First the "active" tone controls are full bandwidth tubes, fed by a ladder type filter (several L or Π filter sections connected one after the other) and with another stage of filter on their respective input volume. There's no snubber cap across pins one and two (plate and grid) to limit the bandwidth of each stage. Hence some of the hiss. Plus the tone controlsare overlaping frequencies, giving weird results at full output Hence the nasty distortion. In this one, there's no presence, but in the L120 there's one, And GLDTP99 over at plexi place forum has completely goten rid of it and of the send and return of the reverb, seemingly reducing the hiss big time. I've posted a mod adding snubber caps on Michael's verry good site, where you can see the Matti Adolfsen mod, of adding snubber caps. plus some of my own mods, around the presence circuit. And Ken Gilbert's sugestions about the cathode follower.
http://www.soundcitysite.com/sc_webpages/maxscmod.jpg

I think someone could remove more of the hiss by carefully calculating the snubber caps values.

And here's a pic of overlaping tone control nasty distortion

[Shane]

Thanks chaps!
Satamax,I'll check out the site and mods and next week get back into it!!
Cheers
Shane

[fjgaston]

Hi,
I have a sound city 50+
I wish I could get rid of the Hiss.
I also wish I can Have a Master Volume.

I plan to turn my sound city 50+ into a HiWatt custom 50
Do you think it is a good idea/solution to these problems?

I'm studying both schematics but the Hiwatt custom 50 and Sound city 50+ doesn't have the same turret board.
The Eq are really different.

[fjgaston]

I finally did it and it works well.
You can hear it in my Led Zeppelin cover here http://hippymetal.free.fr

[drewl]

Satamax is the SC master....anyway, my L120 was actually pretty quiet especially after recapping.
I removed the fx return jack and installed a footswitch jack to control the reverb and boost functions on mine....