I need to re-surrect this thread.

My current tubes combo in Moscode is Mullard 12AU7 1960s NOS (Blackburn) as the outside pair, and the stock 6CG7 as the inside pair.
I just received a pair of Brimar 13D5 (12AU7). I took out the Mullard and roll in the Brimar. I was surprised just how similar the sound between the Brimar and the Mullard, I could hardly tell them apart.
Next I tried taking out the 6CG7 and roll in the Mullard as the inner pair, strange thing happen, I hear noise (zzhhzzhhhzzzhh)!! I then take out both the Mullard and the Brimar, switch their location so now Brimar is inside and Mullard is outside, still I hear the same noise.
So now, I leave the Mullard as the outer pair, took out the Brimar and re-insert the 6CG7, the noise is gone!

. I then swap out the Brimar with the Mullard, I heard no noise again!
So, my qeustion is, is there something wrong with using 2 pairs of 12AU7 in Moscode?

Thanks
barry