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Bogen MO100A with KT120 ??


2012-12-30 5:07 pm
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?




2009-08-12 3:23 pm
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.


2012-12-30 5:07 pm
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!!