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Please don't laugh... first attempt at tracing out a schematic

Redboy

Redboy

a few good watts
Here's my first stab at mapping out a schematic, from my little Olson console amp. Can some of you tube-savvy fellas take a look at it, and point out any glaring errors? What's up with the driver cathode being coupled to the output like that (or am I looking at that all wrong)?

Some of the values were printed on the components, some were not. I also used a cool Resistor Value Calculator that I found online.

The idea here is that I'd eventually like to clone this little amp, since it sounds so good. I'll need some help with transformer values when it gets that far.

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I think the 8 ohm and common legs of the OPT are switched, and the driver cathodes are simply earthed through the resistor/cap. Looks pretty dang legible to me, for a first try I'd say very well done. :yes:
 
earthman said:
I think the 8 ohm and common legs of the OPT are switched, and the driver cathodes are simply earthed through the resistor/cap...
Click to expand...
I looked at this again, and I don't think so...

It's 12AX7 pin 3 to wire to paralleled resistor&cap to L out. Green OPT wire attaches to L out as well. The black OPT wires are both connected to the same middle pin, which grounds to the chassis.

Still seems screwy, huh?
 
Not screwy,eh?

Redboy said:
I looked at this again, and I don't think so...

It's 12AX7 pin 3 to wire to paralleled resistor&cap to L out. Green OPT wire attaches to L out as well. The black OPT wires are both connected to the same middle pin, which grounds to the chassis.

Still seems screwy, huh?
Click to expand...

Absolutely not screwy. You're describing the global corrective feedback circuit of this amp. Another common term for it is "negative" feedback. I use the term "corrective" because negative sounds so negative.

The topology you've drawn is really quite common in a general sense. One detail however: In your schematic you've not grounded the "common" leads of the OPT sec. The feedback loop probably won't work properly unless common is grounded which it no-doubt is in the physical circuit.

Now that you know the amp utilizes "negative feedback" you may hear from True Believers who will tell you that the amp will become more wonderful if you remove the feedback loop. Gain will certainly increase but I suspect that overall, it won't become more wonderful. Regardless of my speculation, if you're confident of what you're doing, you may want to experiment with the value of the feedback resistor (>15K= less feedback and vice versa)
 
Steve O said:
Absolutely not screwy. You're describing the global corrective feedback circuit of this amp. Another common term for it is "negative" feedback. I use the term "corrective" because negative sounds so negative.

The topology you've drawn is really quite common in a general sense. One detail however: In your schematic you've not grounded the "common" leads of the OPT sec. The feedback loop probably won't work properly unless common is grounded which it no-doubt is in the physical circuit.
Click to expand...
Yup. Those leads are grounded to the chassis. Thanks for the synopsis.

Regardless of my speculation, if you're confident of what you're doing...)
Click to expand...
Nope! Not me. :no: Not confident, but willing and hoping to learn someday! I'd really like to do my best to put together an outright clone of this thing, then maybe play with it later. Actually, the whole project will have to be later, maybe much later. Too many things on the board these days.
 
mhardy6647 said:
They've really got 60 uF on the first P/S filter cap? That doesn't seem like a good idea, does it?
Click to expand...
I guess I wouldn't know, but that's what's in there... Does that mean the rectifier's burning up?
Looks like 50uF max (if I'm reading data sheet correctly).
Click to expand...
Change the 60uF to a 50uF, then?

Oh man, I'm in over my head already...
 
What really matters is the peak current vs. the rating of the rectifier. A 6CA4 / EZ81 can handle a lot more than 60 uF if there's enough winding resistance in the transformer. This one is maybe 250-300 Ohms end to end?

If you're running the rectifier at it's full ratings, the max C rating DEFINITELY does apply.
 
For a First Schematic, I'd say that was outstanding! (I used to draw schematics professionally)

A couple of suggestions:

Don’t use 4 way connections unless it is a specific part of a component representation, use 3 way connections and be sure to use the "Solder Dots" as we used to call them. This avoids confusion between connected points and crossover traces. It also negates the need for the funny little jumper hoops that haven’t been in common use for at least 35 years. (You did 4 ways a couple of times for the ground symbol; pull it to a corner, or move it to another spot in a “T” arrangement for the lines)

Don’t be afraid to make the 3 way connection “solder dots” bigger (so they are obvious)

Show your transformers with distinct connection points using the small circles you have, and make them evenly spaced from the Xformer

You did a good job avoiding unneeded dog legs, very good!

Don’t get me wrong, I’ve seen so called “professionals” do worse schematics, all in all, I’m impressed!
 
Tom Bavis said:
What really matters is the peak current vs. the rating of the rectifier. A 6CA4 / EZ81 can handle a lot more than 60 uF if there's enough winding resistance in the transformer. This one is maybe 250-300 Ohms end to end?
Click to expand...
How would I know? "Winding resistance" is a new term to me. I've got no specs for the transformers in this amp.
 
I've had a couple RCA SE 6BQ5 amplifiers that had 80 uf hanging off the 5Y3 rectifier. Tubes were cheap in those days I guess. So the rectifier had a short life, go buy another.

Lifting the common on the OPT will disable the feedback compensation circuit.
I noticed this messing with the SE 6BQ5 breadboard circuit. Lifting the common increases the gain. Along with noise. The circuit also seemed ratty and unglued sounding. With the common and feedback connected, gain was less, and things sounded way more relaxed. And way more quiet.

Maybe the no feedback BS is a bad thing when refering to a SET amp. Though the triodes filament/cathode introduces a degenerative feedback. Sumthin' like that.
I tried a triode strapped 6BQ5 circuit once with no feedback, I didn't like it.
I didn't like a pentode connected 6BQ5 with no feedback compensation either.

You can mess around with the feedback compensation circuit, and noodle with the capacitor values. But doing it by ear is a futile exercise from my experiences.
You need to feed the amp square waves, and adjust values for feedback. The square wave may show spikes and ringing. You adjust the feedback to knock down the spikes and ringing, trying to adjust to make the square wave a "square" as possible.

A single-ended 6BQ5 circuit is rather simple. Keep pondering and playing, you'll figure it out.

Mike
 
outlawmws said:
For a First Schematic, I'd say that was outstanding! (I used to draw schematics professionally)

A couple of suggestions...
Click to expand...
Thanks for the suggestions! I was wondering how many basic laws I was violating as I drew this up. Suggestions noted. One of the trickier parts of this for me was figuring out how to map out the little Centralab PC-81's that are in there (thanks to Mr. Bavis for identifying those in the first place!).
 
>>"Winding resistance" is a new term to me. I've got no specs for the transformers in this amp.

I tend to think of that first, since I have obtained a number of unmarked transformers. The resistance of the high voltage winding gives you a clue to the current capacity. And I always measure transformers with an ohmmeter before I plug something in to the (fused) variac.
 
You can measure resistance in-circuit, after power supply voltage has dropped to zero. Resistors will occasionally read low (if there's a shunt path somewhere), but if they measure high, they are bad. Inductor and transformer winding resistance are marked on the Sams schematics, very handy.
 
I've still been unable to find a model number for this amp. Is it even possible to track down a Sam's folder for something if you don't know the model?
 
Tom Bavis said:
What really matters is the peak current vs. the rating of the rectifier. A 6CA4 / EZ81 can handle a lot more than 60 uF if there's enough winding resistance in the transformer. This one is maybe 250-300 Ohms end to end?
Click to expand...
Tom, I put my leads at pins 1 and 7 of the rectifier, and measured 126 Ohms resistance. What do you make of that?
 
Most of the loss in a transformer is the resistive loss - I^2*R. For a small transformer like that one, it could probably dissipate 2 Watts in the wire of the HV winding, so about 125 mA would be the limit. The amp needs about 90 mA.

The Sams index has about 40 listings for Olson, but without a model number...:dunno:
 
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