impedance match of preamp and main amp.

Al's right Cturnem.

But a simple solution for you if you want to go this way and use the Mc602, is to get a technician to change the coupling cap in the output of the Vk 51se and replace it with one of the same quality but say 4 x larger in uF (microfarads).

This would then take the high 4700ohm impedance from the -3db at 20hz region down to -3db at 5hz, where it should have much less effect on the audible bass frequencies.

Cheers George

Thanks guys for all your responses. This helps out A lot.

Good stuff, as usual.

Its good to know detailed impedance measurements when available, but in lieu of that a good rule of thumb seems to be to use an amp with 60Kohm or higher input impedance to be safe with most tube amps. Slightly higher than the 10:1 minimum (as applied to worst case measurement at particular frequency) usually cited is a good and safe insurance policy.

FWIW, you will find most amps that explicitly mention or support use with tube pre-amps tend to have at least 50-60Kohm input impedance spec, often 100kohm unbalanced and twice that even balanced, which is usually preferred for the best and most consistent results.

Thanks for all your help. I think I will look for an amp that matches my preamp. Maybe one of the BAT amps.

Cturnem, sounds like you want to keep your Bat preamp, and that Mac looks to be a nice amp, if your keen on owning it.

Your Bat preamp is a low 410ohms output impedance at most frequencies and into the Mac it has a good ratio of 1.24, the problem is in the bass, and as I said above this is an easy fix if you get a tech to change the Bat's output coupling capacitors to 4 x larger in uF. Then it will be a perfect 1:24 ratio at all frequencies.
This job to do should be around $100-$200 then your Bat will also be a match for any amp you purchase in the future as well.

Cheers George