[Oya?]

I'm getting weird readings on my Millet Hybrid, on the bias for both tubes. Both sides read around 23VDC with the tubes in, I'm feeding the amp 30VDC. Tuning the bias does little to change the readings.

The amp was working fine until very recently. It's in a semi-dismantled state now so I haven't tested if sound comes out fine, but I figured I should fix the bias first anyway before trying any listening.

I've got a Diamond buffer in there which worked 100% the last time I put everything together. I've also swapped in some BUF634s and got the same readings.

Any ideas on where to from here?

EDIT: How can I check to see if the buffers I have are working properly?

Also, I've made tweaks to the board and there was a lot of soldering involved. I've been generally careful but is it possible the trimpots might've fizzled out (if this is even possible?)?

 

[Oya?]

Bump.

 

[tomb]

Sorry, Oya - I've been sort of incognito while trying to get the Starving Student PCB kits shipped out. Still working at it, as a matter of fact.

As for your Millett Hybrid -
Inability to seemingly bias the tubes isn't a bad thing by itself. Since the trimmers are sized to bias three widely different tube types, the range of control for a particular tube set may be only 4 or 5 turns in a 20-turn trimmer. So, it can easily seem that you can't bias the tubes when it may only be that you simply haven't turned them enough.

That said, here are some things to check:

1. Can you confirm that the tubes are lit? You should be able to see an orange glow at the tips and sides of the long vertical rod in the very middle of the tube (the cathode). Even if you're using tube LEDs, you should be able to see this orange glow. If you can't, then the tubes aren't even firing.
2. 23VDC seems a little low for an un-biased tube, especially if you're feeding 30VDC to the amp. Have you measured any of this lately? What are you using for the 30VDC?
3. It's a bit of a PITA, but you can verify the trimmer operation by holding DMM probes on the soldered leads on the bottom of the board while adjusting the screw. If it changes resistance, then you know it's working. (Note: two of the pins are connected together and so will read zero resistance - measure the other one, instead.)
4. The DB's are easy - if you can measure a bias on the output resistors, then they're operating. The bias on the old revMH Diamond Buffers depended on which resistors were used - 2.2R, 2.7R, 3.3R, 3.9R, or 4.7R. For instance, I built all of my revMH Diamond Buffers using 3.3R. So, for 30ma bias, I would read ~100mV bias across the resistor. This is detailed on Steinchen's Diamond Buffer website here:
   discrete Diamond Buffer - assembly
5. Finally, if all that fails - you've mentioned that you've done a lot of work on the board. You may want to supply some pics so we can study what might be messed up.

 

[Oya?]

Thanks very much for the advice Tomb. I'm powering it with a TREAD. My TREAD seems to be playing nice and gives a steady 30VDC.

The old trimmers I've had in the Millet weren't measuring properly/at all so I unsoldered them and put in a new pair. Unfortunately I couldn't clear the tiny pin holes no matter how long I kept at it so the new trimmers are sort of mounted floating. The new pair measures fine when I test in on my DMM.

Despite replacing the trimmers I still got 23VDC approx. on each tube and couldn't adjust it.

I swapped out the tubes with a new pair that I bought a while back, just to check, and the bias started to work again - for a while, but then went haywire. It's basically just going up and down randomly, anywhere from 16V to 23V back and forth, and I can't adjust the bias reliably.

As for the buffer itself I can measure its biases though I'll have to double-check if the values I'm getting are right, gotta read through that page you linked first.

And I'll try to get a pic up.

I've seriously had it with this Millet, I should buy a blank board and start from scratch. Or maybe get an SSMH board and call it a day.

 

[tomb]

Quote:

Originally Posted by Oya? /img/forum/go_quote.gif Thanks very much for the advice Tomb. I'm powering it with a TREAD. My TREAD seems to be playing nice and gives a steady 30VDC. The old trimmers I've had in the Millet weren't measuring properly/at all so I unsoldered them and put in a new pair. Unfortunately I couldn't clear the tiny pin holes no matter how long I kept at it so the new trimmers are sort of mounted floating. The new pair measures fine when I test in on my DMM. Despite replacing the trimmers I still got 23VDC approx. on each tube and couldn't adjust it. I swapped out the tubes with a new pair that I bought a while back, just to check, and the bias started to work again - for a while, but then went haywire. It's basically just going up and down randomly, anywhere from 16V to 23V back and forth, and I can't adjust the bias reliably. As for the buffer itself I can measure its biases though I'll have to double-check if the values I'm getting are right, gotta read through that page you linked first. And I'll try to get a pic up. I've seriously had it with this Millet, I should buy a blank board and start from scratch. Or maybe get an SSMH board and call it a day.

Next time, try poking a spent lead through the holes to clear the solder.

Sorry to hear it's so messed up. If you're sure the trimmers are operating OK, I have no explanation at the moment.

As far as a new PCB - the MiniMAX is the one you want for a direct replacement. It's the old Millett Hybrid and Diamond Buffer with all the bells and whistles - on board heat sinks, headphone delay relay, on board linear regulator, etc.

 

[Oya?]

I might actually just wait until the MAX boards are available.

Little point in putting together a MiniMAX since I don't think I can find an appropriate enclosure to make use of its compactness.

(Got a desktop META42 to keep me happy until then

)

Thanks again for the help Tomb.

 

[tomb]

Quote:

Originally Posted by Oya? /img/forum/go_quote.gif I might actually just wait until the MAX boards are available. Little point in putting together a MiniMAX since I don't think I can find an appropriate enclosure to make use of its compactness. (Got a desktop META42 to keep me happy until then ) Thanks again for the help Tomb.

Well, the custom Beezar/Lansing enclosure is certainly available. Or, you could order one from Lansing yourself - in a couple of different styles.

The MAX is still some time off - I'll be ordering some new prototype PCBs soon, but will have to build them up to be sure.