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50W monoblock "Engineers Amp"
50W monoblock "Engineers Amp"
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Old 30th March 2018, 01:23 PM   #131
kweaver is offline kweaver
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I hope so. Yea I think they are rated for 101db. Sure doesn't seem take a lot of power to get decent levels out of them.

Will definitely share my impressions though I do not claim to be an audiophile by any means but I do enjoy the tube sound.
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Old 30th March 2018, 10:06 PM   #132
vinylkid58 is offline vinylkid58  Canada
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Quote:
Originally Posted by kolakidd View Post
Sorry the pic of the actual enclosure was terrible.
Congrats, you did a nice job.

jeff
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Old 3rd April 2018, 02:35 AM   #133
kweaver is offline kweaver
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Quick question for you experts. How do you feel about using terminal blocks for connections to the pcb? I'm just thinking it would make it easier to to remove the board for repairs or mods in the future but I am concerned that it might degrade the sound a bit.
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Old 8th April 2018, 01:48 AM   #134
kstylianos is offline kstylianos  United States
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Hi all,

Just finished building this pair or monoblocks and powered them on for the 1st time this evening. Have an 8ohm dummy loan on the outputs and shorted the imputs. One amp is stable and ready for biasing and burn in. The dummy load on the other monoblock gets quite hot within 25 seconds of powering on and needs to be turned off. I swapped tubes between amps and same symptoms. With the tubes removed, the dummy load dosent get hot and allows me to check voltages. Voltages seem to be in the ballpark, but a bit high (tubes removed). Any advise as to how to troubleshoot this?
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Old 8th April 2018, 08:20 PM   #135
kstylianos is offline kstylianos  United States
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Did some more digging and double checked and all component values are correct, electrolytic caps in correct orientation, tube socket pins not shorted. With tubes removed, voltages between both amps are identical. Dummy load resistor starts smoking as soon as bias voltage starts rising.
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Old 17th April 2018, 01:26 AM   #136
kweaver is offline kweaver
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Any luck finding the problem. My chassis just shipped so I will be doing final assembly this weekend so any advice would be appreciated.
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Old 17th April 2018, 02:09 AM   #137
mdurian is offline mdurian  United States
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kweaver,
I recently finished a pair and while I didn’t encounter the problems kstylianos saw, I did have a couple issues worth mentioning.

One of the two amps kept blowing a fuse. Not on first power on, but on the second. I’d turn it on and everything worked fine. But the next time I tried to turn it on, the fuse would blow. I addressed this problem by increasing the fuse value to, I think, 3.1A.

The other problem related to hum from the power transformers. This was purely mechanical hum and did not couple into the speakers. It was just the transformer vibrating on the chassis. I fixed this problem by adding some isolation washers I founds on amazon.

I’d also recommend paying close attention to the CAD drawings of the chassis. In particular notice the stand-off height between the PCB and the chassis top plate. If you use stand-offs too large, like I did, the tube sockets for the pre-tubes won’t be high enough and you won’t be able to seat the tubes in the sockets.

Now that I have things sorted, I’m quite happy with the end result - including some nice power switches with LED indicators I added to the front plate.

mike
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Old 17th April 2018, 02:10 AM   #138
anatech is offline anatech  Canada
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50W monoblock "Engineers Amp"
Hi kweaver,
Terminal blocks will be fine as long as they are rated for the voltages they will see. I would normally use "barrier strips" with some fish paper (insulator) underneath so it can't arc. Thick plastic would also work. The sound will not be affected as the terminal block makes a very low resistance connection.

-Chris
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Old 17th April 2018, 02:54 PM   #139
kstylianos is offline kstylianos  United States
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Sorry I did not follow up with my inquiry. The issue was I switched the NFB leads to the board. I used similar colored zip wire for the NFB and thought I had the polarity correct. Very simple mistake that cost me hours.

I burned them in on the bench for about 1.5 hours with 8-Ohm dummy load and inputs shorted. Continued needing to tweak bias current (55mA), but that finally settled down after 45 mins to an hour. Balance voltage pretty much stayed rock solid after 30 minutes. One of the input tubes went bad/microphonic and after replacement/rebiasing all has been well. Will continue to play them hard and check/adjust voltages as they settle in.

I'll second Mike's observations with the humming PT and making sure of correct standoff lengths. I haven't addressed the PT yet and will probably isolate from chassis as well. I cut my own out of standoffs from a spool of plastic tubing from Lowes.

An interesting phenomenon I'm noticing is that with a pair of Klipsch Heresy (8ohm) connected to either the 4 or 8-ohm tap, the bias current raises to ~70mA. This does not happen with my JMR 3ways (4ohm) or with the dummy load. Still need to figure this out.

Here's a pic with LED voltmeters turned on and amps warming up. I have some higher resolution meters on order.
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Old 22nd April 2018, 04:45 PM   #140
pmillett is offline pmillett  United States
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If the current goes up when connecting a particular speaker, with no audio, it's likely that it is oscillating due to some reactive component of the load. Usually if you place a dummy resistor load of between 20 and 100 ohms across the output, it fixes it...

Pete
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