| Posted by
Steve L. |
October 06, 2020 - 07:18 pm | |
| Hi Jacob, As covered above in the "Low Frequency Stability Problem" section, I found that C6 (and it's mate, C5) need to be increased to 4700uF to avoid a large peak in the low frequency response. I would make those low-ESR and long life types if possible. A quick look at Mouser shows Vishay MAL202190062E3 ($4.57) would work. It has a 5000 hour rating. No special ESR is featured but the typical value is given as 0.079ohms, which seems good. Let us know how it goes if you get a chance. |
| 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 | |
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