Sound City 50 plus Mk4 - active tone controls?

[pdf64]

See schematic
http://www.soundcitysite.com/sc_webp...plus_schem.jpg
These amps are described as having active tone controls, see Q4 http://www.soundcitysite.com/sc_webpages/faqs.htm
However, I can't see that the tone controls are active; for starters there's no pre-amp feedback loop.
Rather, there's a cascading chain of high pass filters, with the tone control signals being taken off at the beginning, middle and end of the chain, the tone controls simply being volume controls of the tapped signals. Each 'frequency' volume control then has its own common cathode open loop triode stage, the outputs of these being passively mixed and amplified by a further common cathode triode.
Ok there's a little more filtering going on, eg the high frequencies are rolled off the 'middle' stage, but nothing that could be described as active.
Have I got this right, or have I missed something?
Thanks - Pete.

[teemuk]

It’s kinda in the “grey area”. If you asked me, I’d say it’s an active circuit. True, the circuit utilises no traditional negative feedback but it does use active elements that perform a vital function in the circuit. See, the circuit does not work solely on passive filtering; the gain at select bandwidths generated by the passive filtering + active filtering (e.g. cathode bypass) in amplifying stages is also an important, in fact inherent, part of the operation. Uses amplifying devices = active.

[pdf64]

OK, I was taking a rather narrow definition of 'active' controls. Thanks for presenting that perspective.
It's tricky to know where to draw the line, eg a traditional tone stack uses an active device to make good insertion losses but are clearly passive tone controls.
In this case it's the frequency dependent cathode bypass that clinches the argument, to my mind, and as you point out, makes them 'active'.

[Enzo]

I don't have the schematic open, but one way to look at it might be this. The typical Fender tone stack more or less stands alone. We feed it a signal from a prior stage, and it has an output. We generally boost that output to compensate for losses, but one could just sent the signal off to somewhere else from that treble pot. So no active elements in it. Now look at the tone circuit in question. Would it still function without the active elements?

[pdf64]

Hmmm - yes it would function (at lower signal levels) pretty much (but not quite) the same without the active elements.
It's clearly a grey area.

[sgelectric]

I have a dead Sound City 50 Plus Mark4 .

In reading up on these amps, I noticed where Ken Fisher went out of his way to trash them in an interview, and Ritchie Fliegler says he didn't stop his car to pick up a free one by the side of the road.
I have heard more than one guitarist refer to them as "Sound Sh***y".

What is it about the sound of these amps that engenders this almost visceral level of animus?......Do they really sound that bad?......I have heard them variously desdribed as noisy, hissy,fizzy.....fill in the blank


I'm curious what others think of these amps and this tone circuit. Obviously not everybody hates them.
Thanks

[pdf64]

I thought that the 50 watters have become fairly well respected, at least getting reasonable prices on ebay ~£350 / $500 here? Partridge transformers and Hiwatt connection.
I'm currently working on one that I forgot I had, and has been moldering away in a shed for a decade.
It sounds pretty good. Tone controls work rather differently that the regular tone stack, but that cascading series of high pass filters seems to pick up hum/buzz, probably due to exposed high impedance circuit.
They seem to have undergone various amendments during the production run; this one (later model) has no global feedback on the power amp. The normal channel has the bright cap (a monsterous 4n7)!
One weird fault it had was due to the waxed fabric slide-on insulation failing; a B+ wire was running alongside a grid wire, insulations were touching and the grid was being pulled up a few volts! Seperate the wires and all was fine.
A major annoyance is that the bias supply isn't quite sufficient for modern tubes and requires a voltage doubler on it.
Also it would benefit from extra B+ filtering for the screens; they are fed from the plate node, but that has 100uF on it, so it's just about acceptable.

[sgelectric]

Thanks for the info.My understanding is that not all Sound City amps had Partridge transformers.I suspect mine doesn't.It lacks the distinctive stickers although it is an open frame "Partridge-Style" transformer.
In some ways this "active" eq reminds me a bit of the Ampeg V series eq in that one gets a wide range of tone but much of it is not aesthetically useful.
There's also some good information here Champ Electronics - Vintage Amp Repairs. Lots of cool stuff about British amps.
Thanks again

[teemuk]

They are mostly remembered for the HyLight/Hiwatt connection but "Sound City" was really just another brand of the Dallas/Arbiter conglomerate so "Sound City" is really a kinda broad definition.

[imaradiostar]

I once had a chance to buy one and didn't. Now I'm really kicking myself about it because I could have easily ripped out the crappy "active" tone circuit and build a CP103 preamp in there for a little "Live at Leeds" sound!

I think they're a great platform for mods, even if the current design isn't universally loved.

jamie

[sgelectric]

Please suggest some of the mods you're speaking of.
Thanks

[imaradiostar]

This is from Mark Huss's website at Hiwatt.org:

http://hiwatt.org/Schematics/CP103Pre.pdf

This is supposedly the preamp in all of Pete Townshend's Hiwatts from back in the day.

As an alternative I'd build a clean Hiwatt style preamp for that "Pink Floyd" tone. If I understand correctly he set them clean and use preamps and distortions in front of them for gain.

They're solid amps- tough to go wrong with excellent iron like that! I bet they'd make a fabulous high gain amp with a modern preamp as well.

jamie

[pdf64]

These amps are selling for good money. There are buyers out there that want these amps for what they are.
Surely it would be more beneficial to fix it and sell it to someone that will appreciate it - no-one's going to make any more of these.
A local guy paid £1000 for a good pair of these a few months ago.
Plus although they have good transformers, the transformers aren't THAT special. These days it's so much easier to find multiple sources for this stuff.
And the chassis itself is not a good platform; it's been designed as a bunch of sub assemblies that screw together. The front panel is an external assembly, attached by long stand-offs to the main chassis; the input sockets are totally exposed to, and only 1.5" from, the power transformer laminations.

[jrfrond]

I've always considered these amps to have active tone controls, because gain is introduced for each band, rather than the use of a tone stack and recovery amp. Having worked on more than a few of these craptastic wonders, I also have determined that the active tone circuit is the source of much of the noise as well as the overall brittle tonality these amps have to my ears.

I'm with Ritchie: I'd leave it by the side of the road too!

[imaradiostar]

I would contend that any amp that is a "craptastic wonder" would be much better off as a donor for something that works. Even a simple Blackface Fender or Plexi preamp into a standard schmidt inverter would make a better amp and could easily be built on a simple eyelet or turret board and would keep the tube count low. There is even something to be said for ditching the entire preamp and replacing it with a single pentode and a bass roll off control like a Matchless DC-30 has.

I think your ears would tell the truth- few would pick the stock Sound City over an amp with a single pentode out front. They just sound good!

jamie