s-petersen
Scott
If you use a variac and bring it up slowly to form the can cap, and replace any paper caps in it, it should be good to go.
Updates:
EICO 377 restored:
- all the paper caps including the cans replaced;
- fuse added;
The unit turned looks to be a very nice and very clean, completely original low hour set with a full set of EICO branded tubes:
The after:
Square wave on the scope:
100 Hz:
1kHz:
10kHz:
Do these look about right?
EICO 377 restored:
- all the paper caps including the cans replaced;
- fuse added;
The unit turned looks to be a very nice and very clean, completely original low hour set with a full set of EICO branded tubes:
The after:
Square wave on the scope:
100 Hz:
1kHz:
10kHz:
Do these look about right?
The 100 Hz looks slightly skewed, the 1k is spot on, and the 10k, is slightly rounded, as expected. Does that look about right, or do I have to do more troubleshooting on the sig. gen?
They look like a pretty good tube amp, but not what you need for testing, unfortunately. They should be flat throughout the bandwidth of the generator.
The specs for the Eico 377 say: Square Wave Range: 60-30,000c.p.s. (5% tilt at 60c.p.s., 5% rounding at 30 k.c.), so I wonder if it is within specs or if I need to follow some sort of adjustment procedure to make it perform better. The instructions (https://bama.edebris.com/download/eico/377/EICO 377 AF Sig Gen0001.pdf) seem to suggest some adjustments if "whenever tubes or other components are replaced"....
Thanks for the tip, that's what I did along with the cans too, with the "before" and "after" pictures!
Oh, well, maybe they're not far off then. I'm really talking through my hat a bit here, as they say--I have no experience with analogue generators.
s-petersen
Scott
I think the square waves look off a little more than 5% , but I haven't used a tube audio frequency generator for square waves, most of my use is for RF
For audio my generators are solid state. I use a modified Heathkit 5218.and a BK E-310B
What value cap is C15? You can switch the 6K6s to see if it changes things.
I would measure the resistors around the 6SN7 tube as well
For audio my generators are solid state. I use a modified Heathkit 5218.and a BK E-310B
What value cap is C15? You can switch the 6K6s to see if it changes things.
I would measure the resistors around the 6SN7 tube as well
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That's still more experience than me at this point.
I'll fiddle around a bit more with the thing, try to get it more in shape, maybe try to follow some of the tune-up procedures, see if it helps. It already is a vast improvement over my cellphone and at least I can run a quick test at 1khz, that one does look good.BK E-310B looks really neat with that aluminum dial.
C15 is a 20uf at 150vdc, 85*C, presumably +/-20%. (See photo 2, blue cap hiding behind one of the 6K6 in lower left corner) I replaced it with a 22uf 250v (it sees about 67v dc plus up to 12v signal). Will try switching the 6K6's and will measure resistors and report back. Do you think the calibration outlined on page 11, via C1 and R9 could help at all?
Ok, I'm super confused... The calibration helped a little bit, but switching the 6K6 helped a lot... How or why?
The 100Hz wave is still wonky, maybe the cheap caps I had on hand are high ESR or something:
1kHz:
10kHz:
20kHz:
Even the 30k is more or less recognizable:
Much less rounding and looks a ton better.
s-petersen
Scott
Sometimes as tubes wear, and age their characteristics change, bias points, gain etc, and switching points can change, switching tubes changed the operating points for the different functions.
I am not sure if the calibration setup will affect the square wave shape, I was just thinking that the slope in the output looked like there wasn't enough capacitance in the output for the lower frequencies. What kind of resistance is connected across the output terminals?
You could connect the o-scope at the input of C15 to see if it is flat before it.
Are your probes set to DC?, it helps...
I am not sure if the calibration setup will affect the square wave shape, I was just thinking that the slope in the output looked like there wasn't enough capacitance in the output for the lower frequencies. What kind of resistance is connected across the output terminals?
You could connect the o-scope at the input of C15 to see if it is flat before it.
Are your probes set to DC?, it helps...
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Will test that out, thanks!
The internal resistor R26 is a 10k resistor, externally I have a 1k resistor connected as per calibration instructions.
Is that the AC-GND-DC switch on the scope? No I had that set to AC.
How large of a capacitor would you recommend?
s-petersen
Scott
Set it to DC, and check 100HZ again
Side-by side comparison. I want to say it definitely helped, but the slope is still there... Ok if I sub in, say 47uf in C15?:
s-petersen
Scott
I would try it, but if the slant is before the cap, it will not help
Even if you can't get the 100hertz flat, the 10k is the most important for testing amplifiers
Even if you can't get the 100hertz flat, the 10k is the most important for testing amplifiers
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Ok so the upper trace is the input, the lower trace is the output from the amp:
Left channel:
1kHz low volume:
10kHz low: 
20kHz low: 
Right channel:
1Khz low:
10kHz low: 
20kHz low:
Now, what does all of this mean? Is that oscillation, is it acceptable? Sine wave looks absolutely perfect at the same frequencies, no matter the volume.