N-set wrote:

BTW, how stable 6E5P is? I'm asking as I was contemplating biasing the output by the voltage drop on 6E5P's plate choke (DCR is say around 1k), with only some minimal autobias on the output. The drawback is that any drift in the driver will rebias the output (I'm all the time assuming DC coupling with stacked PS).

the question about the 6E5P/6E6P stability I asked myself as well, what I was constructing with DHT version for MF. If you look MF channel of 6ch-Milq

http://www.RomyTheCat.com/TreeItem.aspx?PostID=10915

… then you will see that DHT tube is direct compiled and failure of driver tube will burn out  the output tube, in my case the rare and expensive 4V DH tubes. Yes, I put 100mA fast blow fuse on B+, just in case, but so far I did not have it blowing because a catastrophic failure of  driver tube. As you might see I use 6E5P/6E6P everywhere and since 2004 I went over I think 2-3 dozens of them. Some of them were very severely abused by a Moron with soldering gun (means me), I made many mistakes while I was experiencing with Melquiades and there were events when  6E5P/6E6P were running with no bias and as the result had anode with the temperature nears of Sun surface. Surprisingly, after cooling the tubes still forking fine, even though I did not put them in amps after that.

Uselessly what I take a new 6E5P/6E6P I test them.

I uselessly go the second posts of this thread

http://www.romythecat.com/Forums/ShowPost.aspx?postID=4805

…where the test data is located.

I usually test them at 4V bias, not the 3.4V as I wrote in the post. The 4V is how they run in Melquiades.  The good new tubes have GM around 19000, the well works tubes, after one year, have around 17000. The tube that approach 16000 are candidates for replacement but I sometimes replace tubes with 17000 GM. I look if the tube is old and if it has too much darkness built up on the glass. It might be not right way to do but I have many of those tubes – so why do I need to collect them? Live of high-end audio person is about who accumulate more tube before then die – I do not want to be a winner in this competition. I will be very comfortable if at the time of my final hart attack I will not have any tubes left….

Now, about the rejection rate. For all time I had one or two 6E5P/6E6P that I trashed right from beginning as they were not useable.  Another one or two tube did not give sufficient current. I generally would like to have 35Ma at 4V bias, if tune do not give it then I use it in A, B, C, D channels of 6ch-Milq. I need a full 35mA for single-stage amps of Melquiades. The 6E5P/6E6P are Russian tubes and as any Russian tubes they have notoriously wide parameters. The 6E6P-DR shall be military tube and shall have much less spin off the parameters but it is not what I see. What is good about those tube is that if you get one that operate as you want then it will most like to maintain this operation mode very stable. At least I did not have any single 6E5P/6E6P that failed on me after it was accepted on duty.

It might be no a lot but this is all data that I have. The tube is not used widely for a long time. The history of it’s use in high-end audio started in 1999 what Dima “discovered” it for audio. There is no known to me motioning of this tube in audio before.  If you plan to use 6E5P/6E6P then a tube tester (I am sure you have)  and it will be sufficient for initial filtering out the bad tubes, then they shall be working fine.

manisandher wrote:

Hi Dima, Do you know if this is similar to the approach Tim de Paravicini took in his direct-coupled amp placed inside a Quad? I'm currently using Quad speakers (I gave up on the horn approach through sheer frustration) and it seems to me that this is a great way to go... Mani.

N-set wrote:

Romy, may I ask you why you've employed an autobias on 6E5Pin Milq's MF rather than your biasing technique (if there is any reason at all)?