It depends on what will that variable AC feed.

If very little current (a few mA) a rheostat (heavy duty wirewound potentiometer) will do, but for any significant load, a transformer is better.

IMPORTANT: A Variac does NOT isolate you from Mains, so secondary is still very dangerous/deadly (pick one), **even if you set it to low voltage**

Now through a 120 to 24 Vac *isolating* transformer as shown, it becomes safe.

You made no mistake and its working as it should.

Only "problem" is that you are omitting a small fact:

A single transistor stage as shown is an *inverting* gain stage, so the higher the Vbb (or Vbe) the lower the Collector voltage (Vce).

Scope "does not lie" and shows it as it is.

But you can!
It just needs the proper probe/adapter.
Everyday I measure power transistor current for servicing, power supply current under load, say when playing Music, current through a speaker, etc.
In this case,all it takes is a 0.1 ohm resistor in series with load, and clipping scope leads across it.
It turns 10A peak current into very measurable 1V peak, easy to display on any scope screen.

Warning: you need to have scope ground floating OR to have that resistor with one end grounded.

Good!!!
Please post the  GX-120/212 diagram  here se we can also compare them.
Thanks.

*There is* line level signal out of that jack, so it should work.

Please post schematic here so we can check where does that jack pull signal from.

Quote from: Loudthud on May 12, 2025, 10:34:01 PMWhat I find curious about the scope photo in post #9 is that the little peaks are 10mS apart but of alternating polarity. The only thing I can think of is that one or two of the diodes have failed open in the bridge rectifier effectively making it two half wave rectifiers. The plus peak of the 50Hz makes a little positive bump in the ground, then on the next half cycle the negative peak makes a little bump of opposite polarity.

Quick edit: Could be that one side of the transformer winding has failed open.

I may be wrong, but what I see there is:

1) waveforms are measured from one point until it repeats "x" time later, always the same..

So a 50 Hz sinewave has positive peaks separated by 1 second / 50 = 1000 milliseconds / 50=20 milliseconds.

Or alternatively, 20 milliseconds separating negative peaks, same thing.

We find both in the picture so I *guess* they are artifacts or parts-of a 50 Hz wave.

50Hz AC because I see no rectification there, both polarities are present, same amplitude,and in the proper sequence: + ... - ... + ... - ... and so on.

I can also guess *where* are they coming from.
Will try to find a drawing, too sleepy to draw anything from scratch now, 02:30 AM here, to show where that waveform is *guaranteed* 

I have found and solved this problem many times, but it plagued me in my early days ... some 40-50 years ago.

Grounding is walking a minefield.
You ground *here* and you have hum.
Screening is useless because ground itself, screen too, BOTH are Hummy     

Now you ground *there*, one inch away, same chassis, same ground bus bar, whatever, and it doe not hum   

You measure ZERO ohm between both points, of course 

What is the exact problem you are trying to solve?

We´ll all keep searching, but for servicing, the fuzzy schematic is still kind of readable.

Fuzzy values can be guessed by where they are and what they do.
 
Agree that layout and parts list are almost unusable.

As a side note, every designer tends to have his pet ideas so many are repeated in different designs, so as you guessed other schematics may help.

So we might help you troubleshoot your amp, what seems to be the problem?

Sorry Friend, but your amp is working like on its first day, nothing broken or to repair, "you have what you have".

It is a very old and dated amplifier, SS Technology was "just born", it works fine by 1970 standards.

What you see is normal.


The hash you see is just residual white noise. Unavoidable.

Some poor design choices were made, volume pot is straight at the front end, only after a unity gain buffer, so following stages are always at full gain, you see *their* noise.
Again, in 1970 most didn´t know any better.

I also see some supply noise.
Notice the alternating higher and lower valleys and peaks, that´s 50Hz (the larger waveform) mixed with 100Hz (the lower one), both always in perfect sync because they come from the same place, both are derivatives of Mains voltage and current.


This is textbook perfect ground supply hum.

Notice full wave rectifiers do NOT charge supply capacitors during all the 50 Hz cycle but only on narrow peaks:



IF load current is, say, 1A continuous, transformer is actually recharging filter caps only once every 1/100th of a Hertz, in a way larger narrow pulse, say 5A to 10A.
Which averages to 1A continuous, what load demands.

Now that STRONG current peak travels through chassis or some ground wiring.

Chassis is never ever true ZERO ohms, it is physically impossible, a few inches of aluminum or iron sheet (chassis) , a ground return wire, even if thick copper, will have *some* resistance.

Your meter will show zero (you already checked that), but it´s actually, say, 0.01 ohm  , "Nothing"

"Nothing" really?
It depends.  Remember those 10A charging pulses and compare them to 1A *average/DC* current demanded by load.

0.01 ohm times 1 A = 0.01V DC, so under load your supply will drop from 42VDC to 41.99Vdc .... nothing indeed.

Now for signal/noise/audible hum analysis

0.01 ohm times 10A =  100mV hum at 50 or 100 Hz!!!!!!

Straight at **ground**!!!!

Ground, your supposed friend, now is your enemy, because if you ground your preamp there, "everything" will have 50/100 Hz peaks mixed in!!!

That´s why layout, grounding, ground loops are SO important, and best results come from experience.
LONG experience.

Not much experience possible in the early days.

To boot, I see 2 problems:



1) preamp is on a separate chassis, connected to supply thnrough long wires, to boot using connectors.

2) I see main supply caps very far from each other, separated by the power transformer.

Modern practice is to place them side by side and join them by a large copper pad, to minimize resistance.
IF outside caps are used , those mounted to chassis with clamps, they often are joined by a solid copper *bar* , not kidding (or at least an aluminum one)

Bonus points for the 220V Mains switch mounted 1 inch away from 7.5mV sensitive input.

But again, not dissing that beautiful and very well made amplifier, "you have what you have", nothing to "repair" except a full redesign.
After which it would not be a WEM any more.

Hey!!! it was good enough for Pink Floyd!!!!     

Quote from: Tassieviking on May 06, 2025, 11:20:12 AMI would like to add that some heat transfer paste is conductive and will cause shorts if used with heatsinks and mica washers.
The conductive heat transfer paste is often used in computers for the main processor heatsink, it makes no difference in there since there are no electrical terminals close by.

Make sure you only get non conductive heatsink paste if you get some, a small tube of really good expensive paste will last forever and ever so don't get anything cheap and nasty.

Computer builders and modders swear by "Arctic Silver" paste, claimed to contain silver dust.
I much suspect Aluminum dust instead but in any case, any metal present will be conductive.

That 100 ohm potenttiometer i9s to fine adjust output DC offsetr.

It is a primitive version of famous RCA "Brute 70" amplifier, which was a revolution when it came out in 1969-70

I built many back in the day and is the base design for most modern SS amps.

But it uses a single input transistor (later ones used a differential pair there which self balances) so 40406 needs about -0.6V on its emitter (this is shown on schematic) which is "stolen" from driver transistor 40409 emitter.

Try to reduce offset, but 200mV is good enough, not sure your hum comes from there.

It may come from poorly filtered main rails (+/-42V) or grounding issues.

What happens if you short across input 18k resistor with a bit of wire?
Does hum change in any way?

Vbe looks "like a rock" because it is being force fed throughn am high-ish resistor (100K)

As of the wide Hfe disparity:

small signal (preamp) transistors are optimized for high gain, they don´t worry about high current because they typically will be used around 1-2 mA or so.
Low dissipation also, 300-500mW (tops).

Power transistors on the contrary are optimized for POWER     , high dissipation (70 to 120W), high current (think 10-15 Amperes), etc.

*Everything* in this World is a compromise, so the heavy price paid is abysmal Hfe.

You will find than in all power Amps, power transistors are never used by themselves, *always* driven by a medium power driver transistor, in a Darlington or Quasi Darlington configuration, so their Hfe multiply.

For example my rule of thumb for 50-100W power amps I design is a 2N3055 (Hfe 10 @ 10A) driven by a TIP 31/32 (Hfe 40 @ 1A)) for a combined Hfe of 400, go figure.

I am very conservative (which is safe), in practice combined Hfe is quite higher, but that is a guaranteed minimum value.

Later I switched to power Darlingtons, TIP142/147 (which are a sort of Industry Standard, being also used by Fender Marshall Ampeg Laney Peavey Crate, H&Kettner which should mean something.

Guaranteed Hfe @ 10A is 1000, go figure.

Thamks for the update.

Don´t worry, pots are the same and will work fine.

In the old days, component values often followed the "2-5-1" scale, so you would find 10 - 25 - 50 100 k pots and so on. .02 .05 .1uF caps, etc.

Today a more evenly distributed "exponential" scale is used, similar to the one used in resistors, 10 - 22 - 47 - 100 k pots and so on.

Pots actually have wide tolerasnce, think 20-30%, yet amps work fine.

    

If at all possible upload a copy, it may help others.

Congratulations on your experiments.
Nothing beats hands-on.
Most books are old school (transistors were a BIG novelty in the 50s and 60s) so examples are "dated" but all of those have modern equivalents.
For 2N3053 you can use any modern mid power transistor, think cheap and plentiful TIP31C or similar.

Quote from: g1 on April 29, 2025, 03:02:13 PMAlso, the noise you experienced must be due to the dress of the wires.  Where you routed it through the back of the chassis must be a sensitive area.

Usng a metallic uninsulated jack bolted to chassis shorts a great part of NFB , increases gain big time and makes "natural" noise unbearable.


Solution is usin g a properly insulated jack, properly wired.