[Beefy]

It doesn't actually matter which way they go. You might just end up turning them in counter-intuitive directions to get the right settings.

In any case, this photo on the MiniMax website is from my build, and AFAIK, everything is oriented correctly, so yours is all good.

[EDIT] Ooooh, but for the machined case, you couldn't adjust the tube trimmers. This would be a problem. But yours is indeed all good.

 

[tomb]

Quote:

Originally Posted by BlingBlingDr /img/forum/go_quote.gif Things seem to be going smoothly with my build. Thanks for the feedback. I am a bit confused about the trim pots. Based on the schematics, ->3, I have placed them backwards. When I look at previous builds, the pots trim pots are placed as I have them in the picture. Should I be concerned or reposition them?

Yes, Beefy is correct. The only critical trim pots are for the tubes, because as he says - the trimmer screws must be accessible from the holes in the case top. However, not to worry - you've installed them just as everyone does.

P.S. Strictly speaking, the orientation on the schematic may not be correct, but all of the trimmers are interchangeable in their orientation. Most important though, the schematic is rarely used as a reference for physical locations. The board silkscreen is the best reference for that and that appears to be what you've followed.

EDIT: Looks like great workmanship so far!

 

[BlingBlingDr]

And it sings

. Thanks a bunch to everyone who posted their pics, trials, and tribulations. Special thanks to TomB and Beefy for their lightning fast responses to my questions.

 

[tomb]

Quote:

Originally Posted by BlingBlingDr /img/forum/go_quote.gif And it sings . Thanks a bunch to everyone who posted their pics, trials, and tribulations. Special thanks to TomB and Beefy for their lightning fast responses to my questions.

That's great news! Congrats! Give us a pic when it's cased up, too.

 

[BlingBlingDr]

Here she is all cased up and ready to go

 

[tomb]

Looks great! A MiniMAX kit lives!

 

[H22]

Thanks to Tomb, just finnished mine ( after a minor error I made mounting the transistors ).
Didn't have much time to listen to it before I gave it away *sigh*.
Spent Christmas morning with it constantly adjusting the tube bias at My brothers house, He and his wife were blown away by the sound (think i may have to build them one now....). Ten I travelled 3 hours to My fathers house and let My step mother listen to it for about an hour before i told here it was her gift.
It was a huge success, everyone in the house would pop in a CD and just sit and listen, every once in a while there eyes would pop open and they would mouth "WOW", or they would stop the music, take off the phones and tell me they heard something they had never heard before.

Wonderfull little amp, great kit.

I did notice that the bass was a bit lean at first, but after 8-10 hours of use it started coming in nicely. the sound stage also seemed to widen quite a bit.

Now I HAVE to build one for myself. Also thinking of using it as a pre-amp for a gainclone to power a set of mid sized/bookshelf speakers.

Joe

 

[tomb]

Quote:

Originally Posted by H22 /img/forum/go_quote.gif Thanks to Tomb, just finnished mine ( after a minor error I made mounting the transistors ). Didn't have much time to listen to it before I gave it away *sigh*. Spent Christmas morning with it constantly adjusting the tube bias at My brothers house, He and his wife were blown away by the sound (think i may have to build them one now....). Ten I travelled 3 hours to My fathers house and let My step mother listen to it for about an hour before i told here it was her gift. It was a huge success, everyone in the house would pop in a CD and just sit and listen, every once in a while there eyes would pop open and they would mouth "WOW", or they would stop the music, take off the phones and tell me they heard something they had never heard before. Wonderfull little amp, great kit. I did notice that the bass was a bit lean at first, but after 8-10 hours of use it started coming in nicely. the sound stage also seemed to widen quite a bit. Now I HAVE to build one for myself. Also thinking of using it as a pre-amp for a gainclone to power a set of mid sized/bookshelf speakers. Joe

Thank you for such nice comments! Glad to hear you got those transistors worked out!

As for the lean bass at start, you ordered the Black Gates upgrade. Unfortunately, that's a fact of life with Black Gates - long break-ins. This subject is argued endlessly around here, but there are a few things that definitely require break-in, some of them quite lengthy. Black Gates take about a week of average running for the best bass. Most other audio caps in the signal path only take a few hours. Other caps' break-in is best described as starting out cloudy, with an unnatural bass warmth. Then as they break-in, the unnatural warmth goes away and the cloudy turns to transparent.

With BG's, they start out a bit "fuzzy" then get more transparent and detailed as time goes on. However, the bass is the last thing to come in. That's about the only time the MiniMAX can be described as being lean on bass.

Thanks again for the very kind comments.

 

[H22]

Quote:

As for the lean bass at start, you ordered the Black Gates upgrade. Unfortunately, that's a fact of life with Black Gates - long break-ins. This subject is argued endlessly around here, but there are a few things that definitely require break-in, some of them quite lengthy. Black Gates take about a week of average running for the best bass. Most other audio caps in the signal path only take a few hours. Other caps' break-in is best described as starting out cloudy, with an unnatural bass warmth. Then as they break-in, the unnatural warmth goes away and the cloudy turns to transparent.

Well thats good to hear. I had just assumed that this was the case ( caps requiring burn in ) but I was a *bit* woried at first when It seemed light in the bass . That makes me wish that I had longer to play with it!

I just have to build one for myself now, but I am thinking I will opt for the mosfet vesion and possibly set it up to also power a small set of desktop sized speakers, just a bit conserned there about heat dissapation. Looking at the case I don't think it would be too hard to mount some small computer fans ( tiny guys, about 2" square and .5" thick ) to bolw through the bottom of the case. Just have to mount taller feet on the case. I am not wanting huge power here, just something to use on a night stand. if this would work the only down side would be possibly damaging a set of phones by not turning the volume down.
But I guess to make it worthwhile I need to find a set of 2-3 inch drivers that are efficient enough to sound good with the amp.

 

[tomb]

Quote:

Originally Posted by H22 /img/forum/go_quote.gif Well thats good to hear. I had just assumed that this was the case ( caps requiring burn in ) but I was a *bit* woried at first when It seemed light in the bass . That makes me wish that I had longer to play with it! I just have to build one for myself now, but I am thinking I will opt for the mosfet vesion and possibly set it up to also power a small set of desktop sized speakers, just a bit conserned there about heat dissapation. Looking at the case I don't think it would be too hard to mount some small computer fans ( tiny guys, about 2" square and .5" thick ) to bolw through the bottom of the case. Just have to mount taller feet on the case. I am not wanting huge power here, just something to use on a night stand. if this would work the only down side would be possibly damaging a set of phones by not turning the volume down. But I guess to make it worthwhile I need to find a set of 2-3 inch drivers that are efficient enough to sound good with the amp.

The MiniMAX can power a couple of flea speakers already. You should give it a try.

As for MOSFETs, we really reserved that for the regular MAX (still working on V1.2a). Even if you provided fans, there are other issues involved with going to MOSFETs in the small MiniMAX case. You need about 80ma minimum bias for proper MOSFET operation, but that's more than 50% higher than the recommended bias limit on the MiniMAX. I'm not sure it's a situation with ventilation and the case slots as much as it is the thermal inertia provided by the heat sinks. That much bias really needs 1-1/2" sinks to keep from falling off the cliff into thermal runaway. Your fans may keep it cool in a steady-state condition, but any little upset - especially when making bias adjustments in the first place - could be enough to fry the MOSFETs.

The other factor is that with the MAX and 1-1/2" sinks, the sound seems to improve noticeably the higher one goes with the bias. 80-90ma is not enough to wring out the best performance with the MOSFETs, IMHO - more like 120-150ma. I've been running the first MAX V1.2 prototype at 125ma for months now and it sounds good. I had it set at 150ma for a short while, but felt it was too much for the center two sinks. I still haven't tried to case it up yet, so who knows if that much bias (125ma) can still be supported? That's one of the things yet to be developed in the MAX V1.2 prototyping.

 

[Beefy]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif That much bias really needs 1-1/2" sinks to keep from falling off the cliff into thermal runaway.

MOSFETs have a negative tempco. Not to say you couldn't still manage to fry them with too high a bias......

 

[tomb]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif MOSFETs have a negative tempco. Not to say you couldn't still manage to fry them with too high a bias......

Technically you're correct about the negative temperature coefficient, but the more I learn about MOSFETs, the more I find that's not worth much. Heat is heat, eventually any substance will reach a breakdown point. More - there are specific references to thermal runaway for MOSFETs in the literature:
Power MOSFET Thermal Runaway
Power MOSFET Thermal Runaway-Severns example
Interactive Power Electronics Seminar (iPES) - Thermal Runaway of a MOSFET

Quote:

"Like many others, I used to list immunity to thermal runaway as one of the advantages of power MOSFETs over bipolar transistors. Then I read Rudy Severns application note and discovered otherwise."

- referenced in the links above.

 

[H22]

Quote:

As for MOSFETs, we really reserved that for the regular MAX (still working on V1.2a). Even if you provided fans, there are other issues involved with going to MOSFETs in the small MiniMAX case. You need about 80ma minimum bias for proper MOSFET operation, but that's more than 50% higher than the recommended bias limit on the MiniMAX. I'm not sure it's a situation with ventilation and the case slots as much as it is the thermal inertia provided by the heat sinks. That much bias really needs 1-1/2" sinks to keep from falling off the cliff into thermal runaway. Your fans may keep it cool in a steady-state condition, but any little upset - especially when making bias adjustments in the first place - could be enough to fry the MOSFETs. The other factor is that with the MAX and 1-1/2" sinks, the sound seems to improve noticeably the higher one goes with the bias. 80-90ma is not enough to wring out the best performance with the MOSFETs, IMHO - more like 120-150ma. I've been running the first MAX V1.2 prototype at 125ma for months now and it sounds good. I had it set at 150ma for a short while, but felt it was too much for the center two sinks. I still haven't tried to case it up yet, so who knows if that much bias (125ma) can still be supported? That's one of the things yet to be developed in the MAX V1.2 prototyping.

well thats very usefull advice. Sounds like the best way to implement it would be either the standard "MAX" or "MINIMAX" in a completelly separate enclosure, As I have read that remote mounting the transistors is a bad idea, possibly mounting them under the board on a large (PCB sized) heatsink would be a good plan, while not ideal for airflow (fins would be pointed down) the huge amount of thermal mass might make up for it?

While I know a gainclone is a far cheeper and simpler route to a small power amp, I like the tubes! and I am not ready just yet to jump into a full bore all tube power amp. As I said, im just looking to power a set of 2"~4" drivers at a moderate level, and I think a modified "MAX" would be a fun project.
If I were to use a large heatsink as the "bottom" of the enclosure, and use the 3amp PS regulator and requisite powersuply, would I be safe to assume that 20W per channel is acheivable?
If the PS circuit is pushed to ~2.5 amps @ 27v, that is ~65 watts, given the tube heater/bias current draw that should stilll be enough power left over for what I want.
Plus it would save all the fuss of using fans.

 

[Oberst Oswald]

Fast question... RB8L/R & RB9L/R do these get jumped when using Black Gates or are the jumpers only for a MOSFET version? I'm building the Beezar kit. The BOM says.... "NOT USED - For MOSFET ONLY - jumper out". Thx

 

[tomb]

Quote:

Originally Posted by Oberst Oswald /img/forum/go_quote.gif Fast question... RB8L/R & RB9L/R do these get jumped when using Black Gates or are the jumpers only for a MOSFET version? I'm building the Beezar kit. The BOM says.... "NOT USED - For MOSFET ONLY - jumper out". Thx

That's correct. The use of Black Gates has nothing to do with it - only whether the diamond buffer uses BJT's or MOSFETs. All MiniMAXes use BJT's, so RB8L/R and RB9L/R are jumpered out.

P.S. It's also summarized here:
http://www.diyforums.org/MiniMAX/MiniMAXbjt.php