incorrect voltage at initial build and broken led lead

[freejazz]

The initial build of my Crack got as far as the voltage test. The voltages were all within range except 1 and 7. The values were: 1) 155.4, 2) 170.3, 3) ~0, 4) 170.3, 5) 78.3, 6) ~0, 7) 155.1, ~0, 9) 103.3, and 10) ~0. I'm not sure where the error is here, but I suspect 155.1 v for 7) is way too high. I'm thinking 155 for 1) is also too high. Any suggestions?

Also, I decided to resolder the LEDs since I couldn't tell if they were lit or not. (I build in the garage using daylight since that's all I've got.) I broke one of the leads for one of them during the reinstall. Is this the correct part for a reorder: Mouser 630-HLMP-6000 ?
Many thanks,

/d

[Paul Birkeland]

Yes, that's the part to get from Mouser.  Could you post a photo of the solder joint at A5?

-PB

[freejazz]

Photo attached. Thanks,

/d

[Paul Birkeland]

On lower terminals, I would bend the wires up against the face of the terminal strip so they will stay put.  I would definitely do this on 2L.

The remaining HLMP-6000 needs its leads trimmed. 

You can run a wire jumper from A3 to A8, then recheck the amp to see if the voltages at terminals 1 and 5 both settle to where they should be.

[freejazz]

Thanks, Paul.

I will add the jumper after bending the all of the lower terminal strip leads up. As a quick note, in the current picture the LED and the wire from the volume pot to the center pin aren't soldered. I had unsoldered them to try and start fresh with the LED solder joints. I'll reply shortly. Thanks again.

/d

[freejazz]

Paul,

Not gonna happen today. When trying to wire the jumper I broke the other LED.

Unless.....

Any chance I can wire in resistors place of the LEDs to get the amp working until I get replacement LEDs? What would be the appropriate values in this case?

Thanks,

/d

[Paul Birkeland]

A 470 ohm resistor could replace each LED.

[freejazz]

The closest I have on hand is 562 ohm 1/2 w (Dale CMF55). Since I don't know the intent of the part (I'm assuming to draw down the current) I'm not sure if this value makes sense math-wise. If this won't work I'll just wait for the LEDs.

Thanks for all of you help.

/d

[Paul Birkeland]

I mean, it's kinda wrong, but not so far off that it won't work.  Terminals 1 and 5 will show higher voltages that we would like, as will terminals 7 and 9.  I doubt they will be all that far off. 

[freejazz]

Paul,

I got the resistors in as we discussed and the voltages were as follows: 1) 88.0; 2) 174.7; 3) 0; 4) 174.0; 5) 88.4; 6) 0; 7) 111.6; 0; 9) 111.0; 10) 0. This suggests to me that I had previously incorrectly soldered my LEDs. Is this guess correct? After your last post I was expecting the values to still be pretty far off, but maybe the success of this test with the resistors temporarily in place of the LEDs indicates the robustness of the design.

Am I safe to replace the temp 562R resistors with the LEDs when they arrive from Mouser? Am I safe to test the unit with headphones now, or should I wait until I successfully have the LEDs in place?

Thanks for all of your help,

/d

[Paul Birkeland]

Yeah, you can go ahead and use it.  You should see terminals 1, 5, 7, and 9 drop a little bit with the LEDs installed.

[freejazz]

Thanks again, Paul.

About how long after I successfully have the LEDs installed do you recommend before I install the Speedball? Week? Month?

/d

[Paul Birkeland]

I would run the amp for a few weeks, just to be sure everything is working nicely. 

[freejazz]

Paul,

One more question (for now?!) with a long lead-in: The amp appears to be working perfectly. A few minutes after listening to the amp I first noticed the smell of the electronics heating up. When I touched the top plate it was warmer than I expected so I pulled out my IR thermometer. The temperature of the top plate after about 10 minutes of use was 42c. The 6080 tube is glowing quite bright and is very hot (which might be typical for this application) while the 12au7 wasn't near as bright or hot (relative to its size - again, maybe typical of this application). I didn't think the tubes were going to bring the top plate up to 42c alone. I measured the temp of some of the parts and was surprised at the temperature of the 3k 10w resistor - 67c. That's #@$% hot and is likely what is heating the top plate. So now the question: Is this typical of the design? I believe these big resistors are coming out when I put the Speedball in, but I want to make sure things are running correctly before moving forward.

Thanks,

/d

[Paul Birkeland]

The 3K/10W resistors are rated to run all the way up to 155C.  We find at this temperature, lifetime is really short.  Even at 100C, lifetime is impacted, so we aim to stay under 100C.

The temperature of these parts is governed by the voltages in the circuit, so a 10K/3W resistor getting too hot would also have a bad voltage accompanying it. 

It is 100% normal for the top plate and the tubes in the amp to be quite hot.  The 12AU7 does not generate as much heat as a 6080 because it is a smaller device with a lower dissipation rating.  The funny thing is the 6080 could be run with 5x the plate dissipation and still be OK, now that would be hot!