Hi
I'm looking for modifying a Rogue Audio M120 magnums monblocks from autobias to fixed bias.
Up to now i succeed to have separate bias supply for biasing a quad of EL34 or KT88.
Driver stage remains the same 12AX7 input and two 12AU7 mu-follower.
All voltages are present and stable and no smoke or bang!
I can follow signal amplification from input to 12AX7 both phases outputs to 12AU7 all fine, signal in the range of 20vp-p at 400Hz.
This signal drives output stage in each grid of EL34 or KT88.
Output tubes have 10 ohms cathode resistors without bypass capacitors and 470ohms grid resistors plates are direct to output transformer at B+ 500Vdc.
Problem is from driver stage to input grids.Driver tubes signal is dropped.
Signal with 2ch scope shows coupling caps in at let's say 10vp-p but dies down to 0,5vp-p on the other side of coupling caps??
I have replaced original coupling caps from 2,7uF 400V to 0,47uF/630V to check if this is an impedance problem?
Same, no amplification, with or without power tubes inserted??
Pin 5 grids of all power tubes measure very high with ohmmeter.
Does anyone have M120schematics or M150 ??
I've asked Rogue Audio, of course they don't have it claiming they lost the computer where the designs were..... well if you work on a design, you have it in your head especially this type of amplifier....
BTW this amp works in autobias with 660ohms cathode resistors on each power tubes....... lots of heat wasted there and hi-current in the PC boards makes it problematic,....
Thanks
Season's Greetings
I'm looking for modifying a Rogue Audio M120 magnums monblocks from autobias to fixed bias.
Up to now i succeed to have separate bias supply for biasing a quad of EL34 or KT88.
Driver stage remains the same 12AX7 input and two 12AU7 mu-follower.
All voltages are present and stable and no smoke or bang!
I can follow signal amplification from input to 12AX7 both phases outputs to 12AU7 all fine, signal in the range of 20vp-p at 400Hz.
This signal drives output stage in each grid of EL34 or KT88.
Output tubes have 10 ohms cathode resistors without bypass capacitors and 470ohms grid resistors plates are direct to output transformer at B+ 500Vdc.
Problem is from driver stage to input grids.Driver tubes signal is dropped.
Signal with 2ch scope shows coupling caps in at let's say 10vp-p but dies down to 0,5vp-p on the other side of coupling caps??
I have replaced original coupling caps from 2,7uF 400V to 0,47uF/630V to check if this is an impedance problem?
Same, no amplification, with or without power tubes inserted??
Pin 5 grids of all power tubes measure very high with ohmmeter.
Does anyone have M120schematics or M150 ??
I've asked Rogue Audio, of course they don't have it claiming they lost the computer where the designs were..... well if you work on a design, you have it in your head especially this type of amplifier....
BTW this amp works in autobias with 660ohms cathode resistors on each power tubes....... lots of heat wasted there and hi-current in the PC boards makes it problematic,....
Thanks
Season's Greetings
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Did you end up using a small transformer to get that 70-80V grid bias to each tube?
BTW in stock form did you see a whopping 78-85mA across each tube using actual current measurement?
What were they even thinking???
BTW in stock form did you see a whopping 78-85mA across each tube using actual current measurement?
What were they even thinking???
Found that my bias supply design to loads down side of output stage grid .
I did have a 100Kohms resistors on each leg that goes to each output to fixed bias, but it needed another 100K in series, now signal from driver stages swings
the output tubes to full power.....
Hi,it's been 6 years ago, had to look at my notes.
Yes did use a small power transformer to produce a fixed bias of -80 volts dc per monobloc.
Each output tube had an adjustment pot and test point showing volts across a 10 ohms cathode resistor.
Did bias at 60- 80mA each.
The mod was good and stable.
But this amp design as a auto bias with cathode resistor of 600 ohms and bypass capacitors had a high B+ 575 VDC.
Originally they were a lot of overheated parts on the pcb.
That's reason why I did the redesign
So KT88 tubes needed to endure a such a high B+.
Sonic Frontiers power 2 were like that.
KT88 from JJ's couldn't be used as they arked.
Sold the Rogue's m120 magnum modded to fixed bias, and customer never report back, I guess he was happy with them.
Yes did use a small power transformer to produce a fixed bias of -80 volts dc per monobloc.
Each output tube had an adjustment pot and test point showing volts across a 10 ohms cathode resistor.
Did bias at 60- 80mA each.
The mod was good and stable.
But this amp design as a auto bias with cathode resistor of 600 ohms and bypass capacitors had a high B+ 575 VDC.
Originally they were a lot of overheated parts on the pcb.
That's reason why I did the redesign
So KT88 tubes needed to endure a such a high B+.
Sonic Frontiers power 2 were like that.
KT88 from JJ's couldn't be used as they arked.
Sold the Rogue's m120 magnum modded to fixed bias, and customer never report back, I guess he was happy with them.