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In Reply to: RE: Well that was quick....... posted by vinnie2 on October 11, 2014 at 13:20:19
"the -39 or -40 volts bias predicted by the plate curves is what I get when I measure it "
Dang, that's funny, Vinnie! So, your setup agrees with the published curves. Someone else's didn't, so now you're wondering if you did something wrong. Hilarious. I'm still lafin'. (Not at you, of course.)
BTW, I take it from one of your other posts in this thread, you've discovered that coupling caps aren't the satanic envoys you've been told. I was going to suggest that earlier (you know how I feel about direct plate-to-grid coupling), but my flame suit's getting a little frayed lately.
Keep up the good work!
--------------------------
Buy Chinese. Bury freedom.
Follow Ups:
It actually sounds as good as dc I think. Plus I get all the fun of cap rolling! So far a 0.2 uf k40-y9 is my favorite.
If what DanL says is correct in the above post, I may get closer yet. The filament voltage is down about a volt. I have to snag another 10v filament tranny to stick on the second channel. It will be interesting to see what happens.
Sure the biasing is easier with cap coupling, but what sounds best?
Keep adding up design compromises and you get a mid-fi result.
I think that is where most of us end up - in the mid-fi class due to the willingness to accept compromises in our audio systems.
Few really use the best parts or circuit implementation to be considered Ultra-Fi builders of SE tube amps.
Vinnie
If you want to step it up ...
Get a pair of grid chokes.
You will hear a difference.
DanL
Chip
Last paragraph of OT ...
Also, DanL came up with a nice little cap coupling schem for between the 6sn7 and the 845, so no more dc coupling and everything is right on for the operating points. I have been trying different flavors of coupling caps and it is really starting to sound good.
DanL
Never used them. How do they hook up and what do they do?
Vinnie
They replace the 100K grid leak resistor.
They provide a low DC impedance so
the 845 has a more stable bias.
They provide a high AC impedance so
the driver tube has lower distortion.
A Win-Win situation.
DanL
What do you recommend for a choke?
Vinnie
MagneQuest has the best as Tre' stated.
(Lowest Capacitance)
I use the BCP-16 Ni (Nickel/M4 Pinstripe).
Better performance than the M4 while
without the V restriction of the HN -
the High Nickel version.
2000+ Henries
3850 DCR
DanL
"replace the 100K grid leak resistor."
I don't mean to nit-pick, but that's not a grid leak resistor. I think "grid bias resistor" is the correct term, even when it connects the grid to ground.
--------------------------
Buy Chinese. Bury freedom.
TK
If it was a fixed bias I would call it that
but since it is only referencing ground - NO.
The bias is cathode supplied, not grid supplied.
But then again many things are totally illogical
in their nomenclature.
DanL
Now this is just my opinion and the way I look at this;I think a grid to ground resistor should be called a "grid resistor" unless the value of the grid resistor, in conjunction with the grid leakage current (not grid current), is being used to bias the grid positively as is sometimes done with very low plate voltage when the only way to get any plate current flowing is to bias the grid a little positive.
I think they did that is some old car radios and in that case the term "grid leak resistor" should be used.
Just my 2 cents.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/12/14
"I think a grid to ground resistor should be called a "grid resistor" "
That works for me. I just don't think we should refer to it as a grid leak resistor unless it actually serves that purpose. Too much chance for misunderstanding, not to mention the confusion it might cause a newcomer.
--------------------------
Buy Chinese. Bury freedom.
"A Win-Win situation."
Yes it is as long as the grid choke has low shunt capacitance and the driver stage can handle it.
Otherwise you will lose highs.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
So what do you recommend for the choke?
MagneQuest BCP-16 GC
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre'
The 6SN7 should drive any grid chokes.
The only posible exception might be
the EBAY TubeLab ones.
But they don't publish their specs.
DanL
When I ask Kevin what the winding capacitance of the LL1670 is he answered 223pF.
This would be added to the Miller of the tube.
On the other hand Mike said the measured capacitance of the BCP-16 GC is only 15pF.
I should buy a pair.
On the other hand my 5687 driver tube is CCS loaded and the 270k ohms grid resistor gives a pretty horizontal load line and I don't have enough gain in my system to ever overload the output tube.
So, as with most things, the particular circumstance dictates what's needed and what's not.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Sounds good. I will give it a try first chance I get.
So far I can not hear any difference that is significant. In some ways though it does sound better than the dc version. But then I am using a small cap of high quality.
When I was doing a lot of ARTA measurements, I found that the biasing, plate loading and tube rolling were a few things that seemed to affect the % of different harmonics at the output of the amp.
Most of the amps I have are also cap coupled SE but I did not get too much into cap rolling and seeing if that affected the harmonic profile of the circuit measurements to any degree.
I used mostly Russian oil caps or NOS film types for cap coupling.
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