DIYForums.org

I finished building my MiniMAX this weekend and have a problem. I set my power supply voltage to 27vdc but my tube bias won't go beyond 1.1vdc. I turned the trim pots at RA1L and RA1R both ways click to click and I never got a measurement at TA2L to GND or TA2R to GND over 1.1vdc. Also my db bias measurements at TB1L to TA2L and TB1R to TA2R go from a low of approx. 33mv to no more than 60mv. Any help here is appreciated.

RA5 L/R are the trimmers that you adjust for the tube bias. Did you mean those instead of RA1?

It sounds like you may have some resistors out of place. With the trouble you report on the buffer, it's possible that can be affecting the tube bias as well. Can you provide some pics of the board?

Here are some pics. I hope they help.

More pics.

I notice your output transistors may have been reversed at one point - you've got some heat pad glue/material remaining on the other sides of the heat sinks. Is it possible you applied power to the amp when the transistors were reversed? If so, maybe they're actually damaged in some way.

About the tubes - do you get a glow when powered up? You should be able to see an orange glow at the top center of the tube, even if LEDs are lighting things up.

The tubes do glow and I hear very faint music from the headphones. Yes, I did power up the amp with the transistors reversed. Is there any way to test them or should I just order another set?

Before we go the extent of having you order more, can you verify the resistors in the DB section? None of those resistors are in parallel with each other, so you should be able to verify the resistances with a DMM. Confirm that you have the proper resistors in the proper positions on the PCB (do this with your DMM and the power OFF):

RB1 - L/R (1K)
RB2 - L/R (10R)
RB3 - L/R (10R)
RB4 - L/R (100R)
RB5 - L/R (100R)
RB6 - L/R (220R)
RB7 - L/R (220R)
RB8 - L/R - ensure that jumpers are soldered here.
RB9 - L/R - ensure that jumpers are soldered here.
RB10, RB11 - L/R - ensure these are the 2.2 ohm power resistors
RB13 - L/R (100R)

Also look at RA8 and RA9 L/R - try to confirm 1K for RA8 and 10K for RA9 (you probably won't be able to measure these accurately, but if you can confirm the rating printed on the resistor).

Finally, make certain the transistors (the little TO-92's) are in the correct positions. They'll actually get hot to the point of burning if they're out of position, but make a check anyway, if you can.

I checked and all of the TO-92s are in the correct positions. All of the resistors checked out okay but the RB10s and the RB11s seemed a little high. They measured 2.8R to 2.9R. I ordered 2.2R, the package said 2.2R but I can't tell if the bands are red or orange...ugh

That won't really matter - you're probably reading some of the parallel resistance caused by the MOSFETs. I didn't remember that the power resistors might not be able to be reliably measured directly. Still, that's not enough of a variance to cause an entire operational issue.

If we are confident that all the parts are in the right places, I'm afraid the conclusion is that the output transistors need replacing. Please understand that this is just a guess. I hate to say it, but it may be that you order them and things don't change. However, I've at least a couple of customers who blew out their entire output stage and the results were felt across the board and surprisingly (at the time) it prevented biasing the tubes, too.

EDIT: Contact me directly through the Beezar store e-mail or the PM system here or on Head-Fi. I do not normally list the 2SC3422/2SA1359 transistors for sale - they are reserved for MiniMAX kits because of their scarcity. We can work something out, though.

I just got back from traveling for work and realized I had mistakenly order the BD137/138 power transistors with my original order from Mouser. I soldered them in the board and got the same results. I'm still only reading 1V at TA2 to GND and now I only have 25 to 50mV at TB2 with TA2.

Sorry I didn't see this sooner.

The BD137/138 transistors are a good choice - should not be anything wrong with those, as long as they're oriented correctly. Refer to the key plan at the bottom of this page to confirm that they are oriented correctly:
http://www.diyforums.org/MiniMAX/MiniMAXbjt.php

I can't see any issue in your photos, so let's look at these different things:
1. Do the tubes light? Look for an orange glow at top center.
2. The tube trimmers are 25 turns (as are the rest of the trimmers on the PCB), but the tube bias may be adjusted from zero to max within 5 turns of those 25. This is because there are three different tubes - each with a different bias/gain factor - that can be used with the amp. Some tubes will have their bias adjust near the end of the trimmer travel, others at the beginning, and then others somewhere in-between. So, have you racked the trimmers from one end to the other and confirmed that they still will not bias?
3. Problems in the buffer can lead to issues with biasing the tubes. If you can't measure sufficient current in the buffer after adjusting the trimmers, then there's a part somewhere in the wrong position. Luckily, none of the resistors are in parallel in the buffer circuit, so they can be measured with your DMM and the power off. This will tell us if any resistors are out of position. Tell us what you measure from the following list:

RB1L, RB1R
RB2L, RB2R
RB3L, RB3R
RB4L, RB4R
RB5L, RB5R
RB6L, RB6R
RB7L, RB7R
4. Check to see if the TO-92 transistors are in their proper positions. There's an easy key to remember: odd-numbered transistors go in the even-numbered positions and vice-versa. For instance, QB2L (even) is a 2N5087 (odd) and QB3L (odd) is a 2N5088 (even). Bend the transistors slightly sideways to see their designations.

It's still possible that those output transistors were burned up and need replacing. I wouldn't hesitate to order more of the BD137/BD138 and they'll be cheap.