| Posted by Max H | March 18, 2012 - 06:34 pm |
| I hope the 2N2219A holds up too. I will eventually do some more modifications to the amplifier, but for now am just happy to have it working again. This little amp really packs a punch - with my Realistic Mach One speakers, it has more than enough power. I've noticed it gets fairly warm after it's been operating for a while. I replaced all of the many electrolytic capacitors in the 3200 tuner today. With a pair of rabbit ears it's picking up all the local stations with great fidelity. AFC works too. The "stereo" indicator light was burned out, and I replaced it with the wrong bulb, so it's too dim now. Also, the tuning meter doesn't work, so I'm not done yet. Thanks for taking the time to put all this info online, it motivated me to refurbish my old tuner and amp, with excellent results. |
| Posted by Steve L. | March 18, 2012 - 12:56 pm |
| Hi Max, Congratulations on restoring your Cortina! It's great that you were able to find suitable replacements for the drivers nearby. There is a lot to be said for using something that's handy. In a cursory glance, those seem like good choices. The only thing I might carp about would be the 40V max rating of the 2N2219A. I have measured the power supply as high as 45V. When the output swings low, the NPN driver can see almost the full supply voltage. However, I would expect production transistors to exceed their specs, so it should work fine in practice. Please keep us posted on the Cortina tuner progress. Good work! |
| Posted by Max H | March 18, 2012 - 03:30 am |
| Cortina is back in service, after many years. I ended up using 2N2905A and 2N2219A matched as drivers on one side, because they were what I could find locally. TO-66 outputs from Surplus Sales date code 1980. Other side all transistors original. Replaced all electrolytic capacitors. New thermal grease, cleaned controls, etc. Amplifier sounds good. Going to take better care of it this time, tuner is next. |
| Posted by Max H | February 25, 2012 - 08:06 pm |
| Hi Steve -I sent you an email. Let me know if it didn't go through. |
| Posted by Steve L. | February 24, 2012 - 02:33 pm |
| Hi Max, Yes, the good ol' 667 should do the job. I used to use that transistor test function before I bought the Peak unit. Will be interested to see the pix. I hope that you will post a link. BTW, my email address is on the About page. May I have yours? |
| Posted by Max H | February 24, 2012 - 12:53 pm |
| An engineer from Jensen did an interesting article about the Sweet Sixteen concept, in Audio magazine a year or two after the original PE article, complete with a lot of measurements. It can't really be fixed by eq, it has awful comb filtering, a large midrange peak, very little low bass, and basically measures like a loud table radio. The frequency response errors could be corrected using DSP or eq, but it would probably just end up killing the efficiency of the system, and over powering the little drivers below their resonant frequency. So you are right, it's more about a project than about creating a hi-fi speaker system. I also think it might sound good with music which was mixed to sound good on cheap table radios and phonos, since it basically will have the same frequency response. (i.e. early rock music) I was trying tro figure out how I will measure the beta of the driver transistors to match them, until it struck me - my EICO 667 tube tester also tests transistors, and will directly measure Beta. Using an EICO to fix the EICO is cool. Of course I will follow your advice and use higher current for the output transistors. This weekend I'll put some high resolution photos of my units on a photo hosting website. I was comparing the multiplex board from memory, so I am not surprised I was a little bit off. It's very smilar though. There are some small differences between my 3070 and yours, for example the woodgrain cover on mine has a lip folded on the back side, which makes it look a bit less flimsy from behind. |