Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DummyLoad on September 30, 2014, 02:38:21 am
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stout with a modified tone stack. replace with doug's.
this is what i'll breadboard in a couple of days...
could also use mojo PT p/n MOJO758
use doug's fender choke 022699, turret board, can caps, toobz, etc.
--pete
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There is something I don't understand
the cathodes of EL34 are connected directly to ground (at the 6.3v CT - X connection)
then there is a standard cathode bias connection (via the 100uF cap || 250R resistor) to ground at the HV winding CT (#1 connection)
and the NFB is connected anywhere
Franco
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There is something I don't understand
the cathodes of EL34 are connected directly to ground (at the 6.3v CT - X connection)
yes, free DC - makes for quiet - better than grounding CT.
then there is a standard cathode bias connection (via the 100uF cap || 250R resistor) to ground at the HV winding CT (#1 connection)
?????
and the NFB is connected anywhere
not used.
Franco
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Sorry before I was assuming the X point was connected to ground but it isn't, so
the cathodes of EL34 are connected directly to ground (at the 6.3v CT - X connection)
is totally wrong and the only connection to ground is at point # 1 via the || cap and resistor (for cathode biasing)
the X connection is an elevated heater reference for heaters :BangHead: :BangHead:
Franco
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what does SW2 do?
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what does SW2 do?
Ha, that's a good one! As shown, the C7 connects to the top of the tail resistor. That is the famous Hoffman Stout wiring error. Doug still wont tell us if this is intentional.
Flip the switch and C7 is connected to ground, which puts the second grid of the LTP at AC signal ground, just like 99% of all LTPs that don't use the second grid for feedback or to mix another preamp channel.
I bet Pete put the switch in there more to hear if there really is any magic sound in Doug's 'error' circuit. :wink:
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i did. i actually have used SW2 in other stout builds. helps clean tone. the LPTI wiring error puts a significant imbalance on the output transformer and output tubes. swap the output tubes every once in while.
what i did do in the schematic posted, is completely ignore the fact that the LTPI needs tuning to drive EL34s: probably needs a 15-22K tail and not 47K. i'll tune it on the breadboard and post recommended values.
--pete
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draft. please comment, suggest, or flame...
some eyes to check the layout would be appreciated as well.
most dimensions on the chassis are uncertain until i can get one to take measurements off of.
--pete
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doug has company with his new stout build in the new chassis, so i started this project... it never did get built and tested on the breadboard.
shooting from the hip on this one.
will pete hit or miss?
had to chop up the new punched chassis some... milled the PT cutout about 75-80mils wider. vibrolux PT now fits. mounted doug's TW-35 35Watt mojotone 791 OT and doug's 4H 90mA fender choke.
i had sourced a vibrolux PT from AES: they still carry it and it looks like a mag components based on EIA 989#. hammond 290CX is a drop in (with machining) as is the hammond 273CZ. either will yield 425V B+ or thereabouts; depending on the tubes and bias setting.
other PT sources:
http://www.classictone.net/40-18041.pdf (http://www.classictone.net/40-18041.pdf)
https://taweber.powweb.com/store/022723sch.jpg\ (https://taweber.powweb.com/store/022723sch.jpg\)
link to some build pics:
http://s2.photobucket.com/user/pmitchel/library/Boilermaker (http://s2.photobucket.com/user/pmitchel/library/Boilermaker)
--pete
EDIT: the PT i'm using was sourced from AES- not new sensor.