Kreg measurement question.

[Raymond P.]

Hello,


I finished the build and all resistance and voltage checks are within spec, with maybe the exception of Kreg (see below). Do they look okay? The B-side is over the 12V spec, albeit by a small amount, and the two sides differ about ~1.5V.


Kreg (A-side): 10.75V
Kreg (B-side): 12.19


Thanks,
Raymond

[adydula]

Hello Raymond,

Have you set the bias for the tubes yet?

I am not the expert, it could be if one of the bias pots are different in value here could be the culprit...so you have had the amp on to measure have you set the boad yet??

One of the real experts will chime in here.
Alex

[Paul Birkeland]

Those Kreg voltages are what feed the little TL431 voltage regulators, and are part of why we can use a 36V rated part to regulate a 225V supply.  As long as Kreg is well over 2.5V but well under 36V, all is well.

Since all the currents in the Mailine are regulated, what you are seeing is just some variation in the 12AU7 doing regulator duty, and this difference doesn't have any audible artifacts. 

[Raymond P.]

Alex,


Yes, T20 and T30 are biased to 145V. (BTW, I found out biasing should be done after giving the amp 15-20mins to warm up and settle, otherwise those voltages will drift, and adjusting a moving target is frustrating.)


-Raymond

[Raymond P.]

Those Kreg voltages are what feed the little TL431 voltage regulators, and are part of why we can use a 36V rated part to regulate a 225V supply.  As long as Kreg is well over 2.5V but well under 36V, all is well.

Since all the currents in the Mailine are regulated, what you are seeing is just some variation in the 12AU7 doing regulator duty, and this difference doesn't have any audible artifacts.

Thanks, PB. I just wanted to make sure there's nothing to be concerned about.


Edited: Hmmm... thinking about your explanation of what causes the Kreg mismatch, I suppose I can use the Mainline to check for how well balanced (or not) 12AU7 tubes are for my Crack. 


-Raymond

[adydula]

Wow!

Thats a really good thing to know IMO....

I bought an ancient tube tester so I could attempt to match tubes.

So now we have a Mainline 12AU7 tube tester of sorts!!

ha!

Alex

So Raymond how does it sounnd???

 

 

[adydula]

Forgot to ask you this:

What kind/brand 12AU7 are you using?

Mine came with a "CONN" brand label...think this is a CONN branded RCA??

Alex

[adydula]

I took a look at a bunch of 12AU7 tubes and found this pix of "RCA Clear Top 12AU7's with the CONN label"

This is exactly what my 12AU7 looks like in the Mainline.

IF this is indeed accurate, many of the NOS tube crowd claim that RCA clear tops which are hard to find are the best sounding tubes in this class....

So here we have a tube that is being used in the voltage regulation part of the amp!

Wonder if taking this 12AU7 and using it in the Crack as the signal tube!

and using a different 12Au7 in the MAinline if the voltage range Paul stated is ok, a little mis-match doesnt seem to hurt~!

Probably way too much analyzing to do here!!

But hey I am retired and what else is there to do !!

Ha!
Alex

[Raymond P.]

Alex,


Here are my quick, initial impressions of the Mainline as compared to my Speedball-ed Crack, listening with the Sennheiser HD650's. Note this is based on less than an hour of listening on the Un-bal, High Impedance settings.

• Larger soundstage
• Clearer details
• Well defined bass, but less quantity.
• Less warmth.

Your Conn 12AU7 is what came with my Crack. You should definitely stick it into your Crack for a listen. The 12AU7 that came with my Mainline has no markings, but I think it's an RCA with black plates and triple mica.

Edited: Added picture.

-Raymond

[adydula]

Raymond,

What headphones are you using?

I definitely agree on the clarity for sure...with the T1 V2's I am using primarly the bass is IMO better overall.
It more depends on the recordings I have and am listening to. The same can be said with piano sound...

Some of my "live" performance cd's...like Joni Mitchel's Miles of Aisles seem so much clearer and more lifelike...I keep
taking of my cans and asking my wife is she talking to me....and she tells me its the voices in my head!!!

I will try this tube in the crack...I have a dozen 12AU7's and most of them are really good IMO.

Alex

Note: One im particular I like is a JAN 5963, by Sylvania 3-71 date code. I found five of them in a group for sale for $35!