Comments for page: Eico Cortina 3070

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Posted by Jacob B October 06, 2020 - 06:40 pm
Hello. I need to replace C6. Any input on what to replace this with? Will any bipolar 2000uF 25V cap work?

Posted by Steve L. September 24, 2020 - 06:31 pm
Hi Mike, Congratulations on restoring the 3070! Glad you brought the output bias current down to a good value. Yes, paralleling R307 with fixed values is a perfectly acceptable way to do that.

That is an amazing price for the Kuman tester. I see that it was originally an open source project and from the 450 ratings on Amazon, there have been a large number of satisfied customers.

It's great that your first solid state project went well and you can enjoy that "space-age" transistor sound :) When I used that term in the article in 2011, it was meant tongue-in-cheek, since the space-age had become dim history in the U.S. But now, with the advent of SpaceX and others, the space-age has come back. Who'd have thought that we would ever see a vertical rocket landing, like they used to depict in old movies?

Posted by Mike S. September 24, 2020 - 06:28 pm
Thanks so much for your article!

I just finished restoring my 3070, which I purchased for $15 on eBay. After a complete (electrolytic) recap, resistor check, new power supply diodes and cleaning, I turned it on and it worked great! I checked the bias current at idle. One side was 50mv (across R11/12/13/14), and the other was 110mv(!). I presume that both should be around 25mv. I was going to use trimpots per your article. Instead, I paralleled higher-value resistors across the 220 ohm R307 until the bias reached 25mv (more or less; tested for 15 minutes). Idle bias is now ~25mv both sides. Amp still works great, and the output transistors stay cool. I tightened every screw I could find. I think this improved grounding and improved the heat sinking of the output transistors. I ordered (10) 2N5089s from China (eBay). Cost: $3 including shipping. I also bought a Kuman Mega 328 Graphic Transistor Tester for $15 on Amazon for transistor testing, identification, and matching. Amazing device for so little money! This is my first "solid state" project. I mostly work on tube amps.

Anyway, thanks for your great advice on the E3070. I've learned a lot.

Posted by Steve L. July 19, 2016 - 02:44 pm
[For benefit of others, Google translates msdava's message from Turkish as, "helpful; one page".] Hi msdava, I guess this is a compliment---thanks.

Posted by msdava July 19, 2016 - 12:37 pm
faydalę; bir sayfa

Posted by Steve L. January 18, 2016 - 11:14 am
Hi Les,
0. Something I must be missing. I don't get VBase Q301 as 1.5V. --- In the Cortina 3070 manual voltage chart, Eico gives the reading as 1.6V so that accounts for part of the discrepancy. [Eico was never known for the dependability of their voltage readings.] The other factor is the loading of Q301 base current. The 1966 RCA Transistor Manual shows beta of the 40317 as 40-200 at Ic=10mA. From the 0.9V emitter voltage, we have it running at 3.3mA, which isn't too far away from the spec point. Taking the geometric mean of beta at about 90, then Ib=37uA. I get the Thevenin source resistance driving the base as 4.9K, so the base current drops the base voltage by 0.18V. That reduces your calculation from 1.84 to 1.66V, which is close to the 1.6V given in the manual.

1. Emitter Resistors R12, R14 1Ω; make sense but I am puzzled by the two Bridging Diodes CR2, CR4. --- R12/14 are there to make quiescent current (Iq) in Q2/4 more stable and predictable. If R12 and R14 were each shorted, the voltage drop across R307, CR301/302 would be working against only the sum of Vbe's for Q303/2/3/302. As we know, Ic versus Vbe rises very steeply, which would lead to unpredictable bias current. With R12/14 in place, CR301/302 are supposed to match the Vbe's, leaving the drop across R307 to fall across R12/14. (There should have been three diodes, not two!) CR2/4 don't come into play with just Iq present. The purpose of those is limit the loss across R12/14 when high current is being delivered to the load. At 19Wrms into 4ohms the peak output current is about 3A, so without the diodes, losses would be high. With the diodes total losses are less than 1W or so.

2. I assume no overcurrent protection circuit against shorting the outputs is needed because of C6 - right? --- C6 could protect against DC or VLF currents but not against AC or transient overcurrent. The sad fact (discussed in the article) is that the poor Cortina isn't short-circuit protected (SCP) and will die in a millisecond or so if the output is shorted while output is present (and in other scenarios too). After all, the five-transistor power amp design was all about simplicity and SCP would have increased the number of transistors in the power amp by 40%.

3. R16 1KΩ at the outputs mean the output never drives into an open circuit. Does that matter? No resistor means no ac current. --- R16 is there to pull the DC output to near zero when no load is attached. Without it, when a speaker is attached, you might experience a loud POP. Moreover, if you shorted the output with that DC present, even without an AC signal, one or more output transistors would surely die. Now, cost-cutters at Eico may well have pointed-out that R312 will serve much the same purpose. The problem with that is the time constant is 54-seconds, which is plenty for bad things to happen. (Not to mention, it would still leave ~1.6V there.)

4. Since this circuit is based on late 1950's technology (PNP's none too good then), would a Full Complementary Output Stage be better? --- Yes, that would be more symmetrical as you point out and one would expect better even harmonic distortion. It was a cost-saving measure at the time but even so, Eico was able to achieve 0.25% distortion across the band at half output voltage. (See "Summary of the Test Results".)

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