self bias circuit removal removal risks and rewards

[DrewTube]

You'll need a more specific voltage reading of the cathode voltage of the 6922 and the plate voltage of the EF86, see if you can grab the first decimal place and report back (you can also listen, it's very audible when things aren't in check).

Thanks, CB.  Here are the more precise measurements:

EF86 Plate (at OB) = 103.5v (Socket A) / 101.5v (Socket B)
6922 Plate (at OA) = 149.2v (A) / 149.5v (B)
6922 Cathode (at breg) = 104.3v (A) / 102.4v (B)


BTW, it sounds fine, but PJ's post on the 6922 voltage gap made me nervous.

Thanks again - Drew

[grufti]

A little higher than 1.54V will get you closer to the optimal voltages, if I remember correctly. Not much higher though, it is a trial and error thing, but worth the effort.

[Paul Birkeland]

Those numbers look acceptable.  What I often see is that the EF86 plate is slightly positive with respect to the 6922 cathode, which is seriously undesireable, or that the gap is overly tight (a few tenths of a volt or so).

[DrewTube]

Grufti wrote:

The setup that I have right now actually uses standard red LED's, because the lower voltage of the HLMP-6000 [about 1.56V] had the EF86 tubes at 107V and the 6922 at about 150V. The standard red LED gives me about 1.65V.

[...]

Grufti,

Can you provide any further detail about the "standard red" LEDs that you successfully used (brand, model/part number, etc.)?  I think I'm in a very similar boat where the 1.54v voltage drop of the HLMP-6000 is just slightly too low for optimal biasing.  I went on Mouser and Digikey to look at red LEDs and was overwhelmed (and confused) by all the options.  Any further guidance to narrow my search is much appreciated.

-Drew

[Doc B.]

Are you saying that there is a problem in that the Eros sounds bad now, like distortion? Or are you just worrying about numbers? As PB said, if the 6922 cathode is positive with respect to the EF86 plate by an adequate amount a little difference in the 6922 plate voltage doesn't matter that much. Those numbers can change a little with every 6922 you try. If it bothers you, put the servo back in. That's what is controls.

[DrewTube]

Are you saying that there is a problem in that the Eros sounds bad now, like distortion? Or are you just worrying about numbers? As PB said, if the 6922 cathode is positive with respect to the EF86 plate by an adequate amount a little difference in the 6922 plate voltage doesn't matter that much. Those numbers can change a little with every 6922 you try. If it bothers you, put the servo back in. That's what is controls.

Thanks, Doc. No, it doesn't sound bad...the low bass may be ever-so-slightly rolled off, but that's subtle enough that I'm probably imagining it.  I'm just looking at numbers, but it isn't bothering me...just trying to have my cake and eat it, too.  I'm actually enjoying the process of trying to dial this in...but I made sure I have two spare sets of parts for the servo before I started.  Plan B will be to replace the servo and stop obsessing.

[Doc B.]

Low bass sounding rolled off is most likely just the difference in the sound of the tube itself vs. the original one. Remember that we design for a certain operating point, but that we end up measuring and reporting exactly what we measured in the prototype whether it hit the design values exactly or not. With tube circuits there is always going to be a little variation. Understanding what numbers can be different than the manual and be OK, vs. which ones are bad if they are different than the manual is really the key issue.