[THE_SOURCE41]

Hey I haven't been on in awhile because of time restraints but I have finally gotten time to work on and totally finish my Millett Minimax and I just have a question about biasing. I have gotten my DBs perfectly biased at 110mV and the power supply is pretty much constant at 27V after it warms up. The question I have is how long till the Tubes will remain constant. I have set them up and they remain constant then change. I then turn the amp off wait for it to cool down and then turn it on again. I wait for it to heat up again but the tubes voltage will have changed and then I bias it and get it constant and repeat cycle. How long should it be before I get a good constant reading?

thanks
Sam

 

[Beefy]

Bias it to 13.5V about an hour after it has turned on. Forget about it for now, and listen to music.

In about a week, bias it to 13.5V about an hour after it has turned on. Forget about it forever, and listen to music.

 

[THE_SOURCE41]

Thanks very much Beefy

 

[tomb]

Beefy is correct. Another method is what I've been doing lately - just bias them to 14 or 14.5V soon after you turn it on. They'll stablize at 13.5V after about an hour.

Or, try biasing them at 15VDC, period. If you're not using some super-high-impedance cans, the reduction in voltage swing won't hurt much and the tubes will sound a lot better.

 

[soloz2]

powered up the minimax I built yesterday and it's a goodin

 

[tomb]

Quote:

Originally Posted by soloz2 /img/forum/go_quote.gif powered up the minimax I built yesterday and it's a goodin

Sounds great, but it doesn't count without pics!

 

[gspence2000]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Beefy is correct. Another method is what I've been doing lately - just bias them to 14 or 14.5V soon after you turn it on. They'll stablize at 13.5V after about an hour. Or, try biasing them at 15VDC, period. If you're not using some super-high-impedance cans, the reduction in voltage swing won't hurt much and the tubes will sound a lot better.

Aw, now you've got me confused again. I thought that voltage swing was where it's at. Are you saying that the tubes are in their "sweet spot" at 15VDC--more linear or something?

Also, won't this shorten the tube life (not that these things are at all expensive as tubes go)?

I'll try biasing to 15VDC today and see if I can hear a difference.

 

[tomb]

Quote:

Originally Posted by gspence2000 /img/forum/go_quote.gif Aw, now you've got me confused again. I thought that voltage swing was where it's at. Are you saying that the tubes are in their "sweet spot" at 15VDC--more linear or something?

Yes, possibly. Quote:

Also, won't this shorten the tube life (not that these things are at all expensive as tubes go)?

No - the plates are rated for 30V, max. Quote:

I'll try biasing to 15VDC today and see if I can hear a difference.

Yep - best way to tell - try it out.

 

[Beefy]

Quote:

Originally Posted by gspence2000 /img/forum/go_quote.gif I thought that voltage swing was where it's at.

Voltage swing seems to be the tech-spec FOTM right now. Sure it can be important for high impedance phones, but a volt or two one way or the other isn't going to make a heck of a lot of difference...... unless you specifically enjoy tinnitus?

 

[soloz2]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Sounds great, but it doesn't count without pics!

Just a couple from before I wired it up

 

[118716]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Sounds great, but it doesn't count without pics!

I'll be sure to post plenty when it gets to me

 

[adn6244]

i need help...my tube bias one channel can be adjusted to 13.5mv but one channel stays at around 21.5mv and refuse to move no matter how many close wise or anti close wise turns on the trimmer.

anything wrong with the trimmer or tubes?

one thing i noticed is that the tube socket for this channel does not have a tight grip to the tube...could this be the reason?

 

[tomb]

Quote:

Originally Posted by adn6244 /img/forum/go_quote.gif i need help...my tube bias one channel can be adjusted to 13.5mv but one channel stays at around 21.5mv and refuse to move no matter how many close wise or anti close wise turns on the trimmer. anything wrong with the trimmer or tubes? one thing i noticed is that the tube socket for this channel does not have a tight grip to the tube...could this be the reason?

Yes, it could very well have something to do with it. If the pins to the heaters aren't making contact, it won't light. If the pin to the plate is not making contact, you won't be able to bias. Gently bend the pins on the tube outward slightly and see if that makes a big difference.

 

[adn6244]

Hi Tomb,
thanks for your reply. I use a tiny screw driver to retension the socket and now the tubes are firmly in and i can see the centre part lighted up in orange. I adjusted the PS voltage to 27V.

interesting findings :
PS Voltage: V+ with GND = 27V

Now both the 2 tube bias: TA2L/R with GND = around 24.1V to 24.2V
Buffers: Left/Right Channel = 24.1V to 24.2V (The last time I checked was 44mv and I did not adjust the trimmer after that)

Did I just fry something? sigh.....

 

[tomb]

Quote:

Originally Posted by adn6244 /img/forum/go_quote.gif Hi Tomb, thanks for your reply. I use a tiny screw driver to retension the socket and now the tubes are firmly in and i can see the centre part lighted up in orange. I adjusted the PS voltage to 27V. interesting findings : PS Voltage: V+ with GND = 27V Now both the 2 tube bias: TA2L/R with GND = around 24.1V to 24.2V Buffers: Left/Right Channel = 24.1V to 24.2V (The last time I checked was 44mv and I did not adjust the trimmer after that) Did I just fry something? sigh.....

Be sure you're referencing the correct points. The Buffers are not referenced to Ground. You must measure TA2L/R to TB2L/R (or TA2L/R to TB1L/R). Also - at 44mV, the output transistors are just barely switched on (if they are at all). You need to get them up to 90-110mV.

As for the tubes, remember that you may have to turn 15 turns or more on a 20 turn trimmer to see an effect, depending on where the bias shakes out. This is because the trimmers are sized to be able to bias three totally different tubes and their gains.

When it comes to the tubes - if you have power, and the cathodes are lit, these are the only explanations:
1. You haven't turned the trimmers enough.
2. The trimmers are bad.
3. You've got bad connections in the socket.

#3 is highly unlikely if the tubes are lit.
#2 can be easily checked - measure the resistance on the trimmer leads under the board. Turn them about 5 turns, then measure again. If you get different values, the trimmers are working.
#1 is the easiest, but be careful if you don't have clutched trimmers such as the Murata's. Bourns will snap if you turn them beyond the click warning. I believe Vishay's have a clutch that will allow them to over-turn, but I'm not sure.

EDIT: I should add that bad tubes are always a possibility, but again - if the cathodes are lit, this is fairly unlikely also. Two tubes being bad are very unlikely, so switching them is an easy way to confirm.