Hi guys!
I'm converting a friends MO100A to KT120s. I replaced the PS caps and changed the bias for -34V and 83ma. The power supply is now 675V 55-56watts plate dissipation. I'm using only one pair of KT120s.
Connecting an 8 ohm load to the 6 ohm ct the output looks very good but only about 38Watts. When connected to the 6 ohm tap I get a not so good 74 watts.
Do those who convert use 4 KT120s or a different tap configuration?
Thanks!!
Scott
I'm converting a friends MO100A to KT120s. I replaced the PS caps and changed the bias for -34V and 83ma. The power supply is now 675V 55-56watts plate dissipation. I'm using only one pair of KT120s.
Connecting an 8 ohm load to the 6 ohm ct the output looks very good but only about 38Watts. When connected to the 6 ohm tap I get a not so good 74 watts.
Do those who convert use 4 KT120s or a different tap configuration?
Thanks!!
Scott
The problem is the 12AX7 stage can't drive more than 30Vp-p before it goes to crap.
IT needs a new front end.
IT needs a new front end.
The problem is the 12AX7 stage can't drive more than 30Vp-p before it goes to crap.
IT needs a new front end.
Huh?... Why do you say that..
Here's a loadline with a CCS anode load set at 1mA.. The triode is biased at -1.5V, so you could do the bias with a 1.5K ohm cathode resistor.

2Vp-p in = 200Vp-p out.. This is probably on the optimistic side, but where are you getting 30Vp-p from?
If you did a resistive anode load, you obviously wouldn't get this kind of swing out of it, but it would still be significantly more than 30Vp-p.
Or wait.. Are you saying the 12ax7 as configured in that link that JonSnell provided? Or the 12ax7 in general.
It's the way the circuit is designed that limits the voltage swing. It uses a 1.2K cathode resistor and 450K plate and only 68V on the plate out of a 300V B+. Simply lowering the plate resistor gave me the swing it needs. According to the RCA resistive coupled charts 240K plate resistors and 1.8K cathode resistor with 300V B+ will give about 46V RMS 130V P-P. I may leave the 1.2K cathode resistor at first so not to mess with the NFB.
Thanks for the reply!!
Thanks for the reply!!
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