Rms converter to match HP3400A?

Not quite sure what you're asking here.  Is the 'converter' an add-on to the 3400A that you want to build from scratch and put into your meter?  Or do you just need an analog r.m.s. converter with those specs for something else? Or are you looking for a complete equivalent to the 3400A that does all measurements in the analog domain?  Old National Semiconductor app-note books had designs for r.m.s. measurement, although I'm not sure they went to 10MHz.

I have a 3400 with burned out thermocouple converter. I want to rebuild the guts between the input BNC and the analog meter. Building AC amplifiers with 10MHz <1% tolerance is a no issue. The rms converter with 5% error at 10MHz is. Also the crest factor. And again, the parts should be new, and not from an eBay thermal converter surplus.

Right, I see your problem.  The meter having ranges in (I assume) 10dB steps probably means that the converter has to be really accurate over only a 20dB range or so, it's probably sitting somewhere in the meter circuitry where it's working on voltages in the tens to hundreds of millivolts or more. But you're right, the 10MHz requirement is a tough one.  Most of the r.m.s.-ing circuits I'm familiar with use the nonlinear characteristics of transistors in some feedback circuit, which if built with op-amps that can slew up a storm might be good to 10MHz.  Never built one though.  Best of luck!

Right, I see your problem.  The meter having ranges in (I assume) 10dB steps probably means that the converter has to be really accurate over only a 20dB range or so, it's probably sitting somewhere in the meter circuitry where it's working on voltages in the tens to hundreds of millivolts or more. But you're right, the 10MHz requirement is a tough one.  Most of the r.m.s.-ing circuits I'm familiar with use the nonlinear characteristics of transistors in some feedback circuit, which if built with op-amps that can slew up a storm might be good to 10MHz.  Never built one though.  Best of luck!