DuKane Model IA385 Restore

[Chris J]

Starting on this little DuKane amp I got last summer. I began with the power cord which was roached. I thought it a good idea to go with a heavy grounded three prong plug. However I read somewhere that’s a bad idea in this case due to the way the microphone Is supposed to work. Not sure I’ll use it as a PA. Was thinking to use it as a mono amp or maybe eventually convert it to a guitar amp. I see that’s a popular project for these.

Super clean under the hood. It works albeit obviously needs new electrolytic caps. The cans are riveted in and I’m not planning to drill them out. I’m just going to put equivalents under the chassis unless that’s somehow a bad idea. I could shield them I guess but eh.

I’ll start with those and see how it’s working. Too many times I replaced parts that in reality probably Would be fine to leave in place.

 

[gadget73]

Lots of room for under-chassis caps, no point in messing with the can. Maybe mount a terminal strip next to the can and move everything there.

those red tubular caps are likely plastic cased paper. They probably will need to be changed too.

 

[cjkinpa]

Terminal strip is a good idea. Then just use radial caps of the same value.

 

[Chris J]

So turns out the grounded plug is a bad idea. It pretty much sounded the same after the recap until I removed the ground wire.

I used my phone as a source and it sounds ok although very low and distorts past 1/2 way. I guess I need a decent preamp to get some sound?

 

[century tek]

Also check for any out of spec resistors.

 

[gadget73]

into the aux jack it should be basically line level. Into the mic jack it might overdrive. Most phones and similar devices have fairly lame output levels but this ought to have enough gain to get to full power with a half volt or so of input. That aux jack was probably meant to work with a ceramic cartridge and many of those were in the 1/2 volt output range.

Even at full power this is only good for maybe a dozen watts so it may not make a lot of noise with really inefficient speakers either. Whipped tubes that don't make full output won't help either. Probably would be worth checking tube socket voltages against the schematic to see how close they are. This has to be done with all tubes in and power applied, so be very careful. Lots of "kill you to death" voltage available in here. Voltages grossly off from spec might indicate ailing tubes, or maybe just way off value resistors. Carbon comp parts are really prone to drift.

 

[Chris J]

Ceramic input is going to sound like crap as aux input. I’ve tested that on other equipment and yeah, doesn’t work. But line level is line level right? I think my Motorola console tt is ceramic. I could pull the back off and test it with that. But that sounds like more work than reward.

I did replace the 12AX7 and 12AU7 with new. I swapped the 5Y3 with a used one I had on hand and it got weaker so I swapped it back. I have a couple used 6V6 but Im pretty sure the ones in it tested ok. I can test the tubes again just to double check.

I’m using klipsch iFi speakers which I assume are fairly efficient. 8ohm in parallel for 4ohm. I can go back to 1 8ohm. It may have been louder like that with just 1 speaker. I know it’s not a loud amp but for a PA system it’s gotta do a little better than what I’m getting. I’ve seen some posts where folks seem happy with the performance as a mono amp. I’m just not there yet. I have a little altec that can’t be much more powerful and it’s performance is way better. (see pic)

I really don’t like doinking around with hot gear so idk. Maybe this is why the guitar amp mod is popular with this thing? It’s not good for anything else?

I’ll do a little more deoxit and test some more before I start ripping out resistors.

The good news is it had matching 1950s balanced tung-sol 12AX7 that sound super good in the Olson receiver I finally finished. So not a total loss.

thanks!

 

[gadget73]

ceramic inputs are line level, just high impedance. That input feeds a 1M pot, which is about right for a ceramic cart.

Even if you convert it to a guitar amp you need to make sure the voltages are right.

 

[Chris J]

Ok fair enough. If I want to modify it to optimize playback with a typical auxiliary input device such as a cd or dac what would need to change? Theoretically. The volume potentiometer?

Practically though I’m going to do some more testing before I try changing anything.

I guess I could look at the altec and see what’s different between all those input settings.

 

[gadget73]

honestly shouldn't need to do anything. You may find that the position of the two mic level controls slightly affects the sound from the aux input but it shouldn't be all that much. If the CD player is too loud, just turn the aux input control down some. They're using the 12ax7 in mu follower config, which makes it act more or less like a pentode. Low input capacitence, so the Miller effect shouldn't be too terrible.

 

[Chris J]

I guess it’s not that bad. New tubes might do justice.

Source is Apple AirPod streaming music on itunes from a server in my office. I have the Volume on iTunes at 80%.

It’s sounding a little better after warming up some more.

 

[gadget73]

guessing those speakers may not be all that efficient either. 10 watts runs out of steam fairly quick with low efficiency. Kinda why I prefer doing things with measurements. Making rated power or not is a very cut and dry test. No speaker variables to consider.

 

[Chris J]

Thanks for the suggestion. I looked up How to measure output and as far as I can tell I’m getting around 11 watts using 4ohm (the 2 8ohm in parallel). I used a 1,000 Hz signal that produced 6.68 volts.

The only specs I can find on the ifi are attached. It says spl 114 but not sure what that means really. The system came with a powered sub so I can’t find a breakout that shows just the left and right speakers. Klipsch is known for efficient speakers so I assumed these were fairly efficient but 114 sounds like a stretch.

They are small speakers but I am now satisfied with the output since I got it set up in the living room. Something may not have been connected well with the speakers or something earlier? Whatever the case this little amp is starting to grow on me. Probably have to buy a pair of Heresy IV to get the most out of it.

 

[gadget73]

typically speaker efficiency is expressed in db at 1 watt at 1 meter. Kinda means what it sounds like, its how loud the speaker is 1 meter away with 1 watt of input power. Sometimes its expressed at some voltage rather than a wattage or at some other distance but without knowing power and distance the number doesn't mean much. I have a set of speakers rated at 0.5 meter. Sometimes you'll get 4 ohm speakers rated at 2.82 volts, which is actually 2 watts into 4 ohms.

If you're getting ~11w of output, its doing what it should. 10-12w from a pair of 6v6 sounds right. Ideally that should be measured on a dummy load resistor of known value just because speaker impedance can be a bit variable but for this purpose I don't think it matters all that much. If there were some gross problem you'd see a power number way below spec.