Lavane
Lunatic Member
Thanks, that's another idea I like.
A box arrived.
- Thread starter Lavane
- Start date
Legrace
Super Member
Lavane
Lunatic Member
Nice. I found some mahogany today I'm going to use. Nice TT's. I ways liked the "flying saucer" decks. What are you using for the driver tubes in your M-125's? I'll probably get another pair of 12BH7's, I already have two. I'm using JJ ECC802S long plates right now. They're my favorite new production 12AU7. Bob Latino stated in a post on the Klipsch forum that because of the higher voltage swing of the 12BH7's, they push the amps to full output easier. I'm still scratching my head over that post.
Legrace
Super Member
Wood sides are walnut pieces 2.5" tall x 1/2" thick that I got from home depot. Counter sunk holes on the sides so the screw heads are hidden. Adding wood dresses them up nicely. For driver tubes myself have settled on RCA 12au7 clear tops, and prefer KT120 for power tubes.
Pio1980
AK Member
Might mean longer service life driving four 6550s.
Lavane
Lunatic Member
Maybe. I'll try to find it again. The way it was worded it seemed he was saying to use 12BH7's fo drive these amps to full power.
Lavane
Lunatic Member
Pio1980
AK Member
That's what it seems to say, that 12BH7 provides more drive.
Pio1980
AK Member
That's probably where any benefit occurs.
Lavane
Lunatic Member
I've read that a few times myself. I just thought his statement about needing the 12BH7 to drive the amp to full output a bit odd. He doesn't state that on his website and ships 12AU7's when the kits are shipped with tubes. Why originally design the amplifiers to use 12AU7's if they won't drive the amplifer to full output? I will get a second old stock 12BH7 to use as a splitter. I thought I had two, but remembered one half of the second one I have is bad.
those tubes bias slightly differently, so it may be a case of its actually more optimized for one over the other. Its still splitting hairs between them though.
...............................12au7..................12bh7
plate V......................250......................250
grid V.......................-8.5......................-10.5
plate C.....................10.5......................11.5
plate R.....................7700....................5300
transconductance....3100...................3100
amp factor................17.......................16.5
so the 12bh7 is a little lower impedance and has slightly less gain. As a split load inverter the gain comes out slightly less than 1 so that doesn't matter. The slightly lower impedance also isn't that critical, thought it will be slightly more able to handle a lower value grid resistor. Probably not enough to really matter though. Used with the same resistors it will likely end up with the 12bh7 drawing even more extra current but still probably not enough to make a big difference.
unless it was optimized for a 12bh7, then the 12au7 may end up slightly over-biased and not draw enough current to be as linear as it ought to be.
...............................12au7..................12bh7
plate V......................250......................250
grid V.......................-8.5......................-10.5
plate C.....................10.5......................11.5
plate R.....................7700....................5300
transconductance....3100...................3100
amp factor................17.......................16.5
so the 12bh7 is a little lower impedance and has slightly less gain. As a split load inverter the gain comes out slightly less than 1 so that doesn't matter. The slightly lower impedance also isn't that critical, thought it will be slightly more able to handle a lower value grid resistor. Probably not enough to really matter though. Used with the same resistors it will likely end up with the 12bh7 drawing even more extra current but still probably not enough to make a big difference.
unless it was optimized for a 12bh7, then the 12au7 may end up slightly over-biased and not draw enough current to be as linear as it ought to be.
Lavane
Lunatic Member
Thanks. That explains some things. I think it was originally optimized for the 12AU7 if I'm not mistaken, like the ST-70 and ST-120 is.
hm, further thought, are these actually using a long tail pair inverter ? thread is long and I didn't trip over the driver schematic. If thats the case, the inverter gain is more than one, but still 16.5 vs 17 is not significant. If its got a CCS, then the current is whatever that sets it to so the bias voltage gets less critical.
so I'm still thinking that both tubes would give pretty much equal results.
so I'm still thinking that both tubes would give pretty much equal results.
Lavane
Lunatic Member
That's kind of what I thought. These do have CCS. That's why his statement had me a bit puzzled.
Lavane
Lunatic Member
Lavane
Lunatic Member
I just used a thin coat of wipe on semi gloss poly coat. I still haven't decided on what to use for a PCB cover. I have some 1/8" thick wood I could use. My other choices are plexiglass and paint the bottom side, or perforated steel. Getting perforated steel used to be easy to find around here, but the steel yard closed years ago. It was handy whenever you needed any type of steel cut to your needs. I was surprised it closed since this is a heavily agricultural area with tractor dealers and ag machine manufacturers around.
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