[adn6244]

What i read from the setup page :
"Both DB channels should average below 50mV or thereabouts, but this depends on the JFET variance. I've seen some with minimum voltage of 25mV and some with a minimum voltage of over 40mV. If the bias voltage is much higher than 50mV, turn the amp off until you figure out whether the trimmers are operating correctly. Once you confirm that the bias voltage is safe, then leave the amp on and proceed with setting up the power supply voltage and tube bias settings, first."

I started to ensure DB bias is below 50mV and proceed to adj the PS bias. I got the 27V for PS and now stuck with Tube bias.

I am sure I got the DB bias ref point correct. I have a feeling I may have snap the Bourns as I went past the click warning twice. I'll double check again later.

Thanks for your prompt reply, really appreciate it. Will update you later if I am able to resolve it...thanks again.

 

[tomb]

Quote:

Originally Posted by adn6244 /img/forum/go_quote.gif What i read from the setup page : "Both DB channels should average below 50mV or thereabouts, but this depends on the JFET variance. I've seen some with minimum voltage of 25mV and some with a minimum voltage of over 40mV. If the bias voltage is much higher than 50mV, turn the amp off until you figure out whether the trimmers are operating correctly. Once you confirm that the bias voltage is safe, then leave the amp on and proceed with setting up the power supply voltage and tube bias settings, first." I started to ensure DB bias is below 50mV and proceed to adj the PS bias. I got the 27V for PS and now stuck with Tube bias. I am sure I got the DB bias ref point correct. I have a feeling I may have snap the Bourns as I went past the click warning twice. I'll double check again later. Thanks for your prompt reply, really appreciate it. Will update you later if I am able to resolve it...thanks again.

OK, I didn't understand that you were still going through the initial setup procedure.

Check those trimmers with your DMM - it should be easy to verify if they're OK.

 

[adn6244]

my apologies for not making myself clear...sleepy after staring at the bright blue led for too long

went to test the amp again after a good sleep...guess what...all readings are ok except one channel - most likely snap the trimmer. will check the trimmer later today.

thanks tomb for your prompt advice.

 

[JamesL]

Is anybody else having trouble sourcing isolated 2.1 jacks from mouser?
The one on the BOM is obsolete, and this one is out of stock. I know mouser stocked another one, but I can't find it. iirc, it was only rated for something like 20v anyways.

 

[tomb]

Quote:

Originally Posted by JamesL /img/forum/go_quote.gif Is anybody else having trouble sourcing isolated 2.1 jacks from mouser? The one on the BOM is obsolete, and this one is out of stock. I know mouser stocked another one, but I can't find it. iirc, it was only rated for something like 20v anyways.

It was rated for 0.5A at 12/24V. Unfortunately, you're correct on the rest. Mouser put us in a hard place by running out of these before a ROHS version was ready. The only alternative I've found is an "outie" availalble at Jameco -
Jameco Electronics Interconnects: VARIOUS: DPJ-021A-R

 

[JamesL]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif It was rated for 0.5A at 12/24V. Unfortunately, you're correct on the rest. Mouser put us in a hard place by running out of these before a ROHS version was ready. The only alternative I've found is an "outie" availalble at Jameco - Jameco Electronics Interconnects: VARIOUS: DPJ-021A-R

Oh yeah.. that kind of sucks.
Anyways, I just ordered a temporary plastic jack until they get the new ones in stock.

 

[oneplustwo]

Hey folks... got a setup question.

I got the DB's down to practically zero. The PS voltage is set to 27V. But I can't get the tubes to bias correctly. One is fixed at 27V, the other is fixed at zero. Nothing happens to either regardless of trimmer position. The LED lights up fine and the tubes start to glow orange after a minute or so.

 

[tomb]

Quote:

Originally Posted by oneplustwo /img/forum/go_quote.gif Hey folks... got a setup question. I got the DB's down to practically zero. The PS voltage is set to 27V. But I can't get the tubes to bias correctly. One is fixed at 27V, the other is fixed at zero. Nothing happens to either regardless of trimmer position. The LED lights up fine and the tubes start to glow orange after a minute or so.

I'm sorry you're having trouble with this. Unfortunately, you should not be able to get the DB's down to zero under any circumstance. Same for a tube at zero - it shouldn't happen. I think we need to see some pics and see if we can spot what's wrong.

 

[oneplustwo]

Here you go:

Hope that helps. Sorry for the noisy pictures. I was trying to get the lens stopped down enough for some reasonable DOF.

 

[tomb]

OK, I don't see anything in particular, but then the photos aren't detailed enough for me to see the part #'s. Still, I don't see anything obviously out of place or deleted.

Tell us exactly what measurement you get when you state the "DB's are near zero." Can you adjust both for more than ~60mVDC? You should be able to bias them up to about 70-80mVDC without any issue of over-heating. Go slow and only about a half-turn at a time once you get them above 50-60mV.

Also, for the tubes - have you racked the trimmers through all their turns in both directions? As I told you earlier, it's quite possible that the tube adjustment can be the last 5 turns out of 20+ turns on the trimmers. You've got 27VDC in the PS, so nothing's wrong there. The tubes are not a complicated circuit - if they're lit, there's very little that can be wrong.

One thing to check is to be certain that you used 2N5087's for the CCS's (some people have mistakenly used 2N5088's there). Those are the little pairs of transistors between the WIMA caps on the sides of the tubes. Also, make certain that you have a 1K and 10K resistor there, too. That little circuit - along with the trimmers - determines the bias on the tubes.

P.S. It does appear that you have very little wicking on the top side of the PCB. That kind of implies some possible soldering issues. I don't want to make that call right now, though, but keep in mind that re-flowing some solder everywhere might help if we can't figure anything else out.

P.S.#2 - I've got to get to bed right now, but I'll take a look first thing in the morning ...

 

[rds]

Looks like all 4 of your BJTs are in backwards (ie you need to rotate the heatsinks 180 degrees.
See here: Millett Hybrid MiniMAX BJT Transistors

 

[tomb]

Quote:

Originally Posted by rds /img/forum/go_quote.gif Looks like all 4 of your BJTs are in backwards (ie you need to rotate the heatsinks 180 degrees. See here: Millett Hybrid MiniMAX BJT Transistors

Good catch, Ryan! You're more up on what one of these are supposed to look like right now than anybody else!

I think you're just suggesting to rotate the heat sinks for purposes of illustration, though.

I'd remove the transistors and re-install them on the other side of the sinks. (Trying to get those heat sinks out can be tough, if not impossible.) Make a solder blob big enough to cover all three pins on the bottom of the board for each transistor. While heating each blob, they may just fall right out. If not, you can tie a loop of string or thread around the transistor and yank on it while the solder is melted. Don't worry about a thermal pad/insulator if you don't have replacements - those are plastic-bodied BJT's and should be OK without them (unless those are MJE's). This will get you going pretty quickly in the right direction.

 

[oneplustwo]

Ah! That's right. That's what I get for just looking at the photos and not reading thoroughly enough. You would think this being my second one that things would go more smoothly. Chalk it up to arrogance. I'll swap them tonight and hopefully everything goes smoothly from there. Thanks folks!

 

[tomb]

Quote:

Originally Posted by oneplustwo /img/forum/go_quote.gif Ah! That's right. That's what I get for just looking at the photos and not reading thoroughly enough. You would think this being my second one that things would go more smoothly. Chalk it up to arrogance. I'll swap them tonight and hopefully everything goes smoothly from there. Thanks folks!

No problem! I've almost done it a couple of times. That, combined with my looking constantly at a MOSFET-MAX the last few months that has them on the left side of the heat sinks - it's easy to get them mixed up.

Let us know how it goes!

 

[linuxworks]

Quote:

Originally Posted by rds /img/forum/go_quote.gif Looks like all 4 of your BJTs are in backwards (ie you need to rotate the heatsinks 180 degrees. See here: Millett Hybrid MiniMAX BJT Transistors

I put all the driver trans in backwards, myself, first time thru!

fortunately, I found this before I applied power. but I did have to unsolder them all.

easy mistake to make. don't feel bad