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211 optimum bias point...

122.109.104.132
Posted on July 30, 2015 at 07:41:26
2395
Audiophile

Posts: 355
Location: Australia
Joined: June 4, 2003



Thank you...if you are to advise a layman [me]....I am wanting to bias these amps .........

Based upon the experience and knowledge of those here please ......I am seeking what you would consider to be the optimum bias with the reading in Ma at point X
....lowest distortion bias
....probable best with low distortion and good power
....ok distortion but higher power
....or other explained alternatives as you would use ......

i.e ...if these were your amps what would you bias them at......

Transformers are [I am advised] 10K with a 4 ohm load [that I am using]
Plate voltage is 950

circuit included [as above] including also GE 211 [ VT-4-C] load line graph

again ...many thanks

 

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RE: 211 optimum bias point..., posted on July 30, 2015 at 10:26:14
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
 I assume you mean above the 1 ohm resistor.
At the X, it is ground all the time.
Mimimum .04V and max of .06mA for Class A1.
You are able to go class A2 (grid current).
So .07V will give you 68% duty on the 211
which is good for longevity.
.1V is the absolute max for the 211
which will shorten lifespan alot.
The higher the bias voltage, the more power
but will lower the 211 lifespan.
The trade-off.
I would start at .06V and see
if you have enough power.
If you have plenty of power then good.
If not, increase until you do.

DanL



 

RE: 211 optimum bias point..., posted on July 30, 2015 at 12:22:46
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 It's not critical. 70-75mA will put the 211 in the right place.

 

RE: 211 optimum bias point..., posted on July 30, 2015 at 13:01:02
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
 But he asked several questions -
....lowest distortion bias
SO .06V for max class A operation
10 Watts (36 Watts Peak into A2)
....probable best with low distortion and good power
HERE .07V for Class A2 operation and "68% sweet spot"
30 Watts
....ok distortion but higher power
THEN .1V for max power and low 211 life
40 Watts

DanL



 

RE: 211 optimum bias point..., posted on July 30, 2015 at 16:35:12
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Here's some reasoning:

First question, how deep into grid current can you go? Grid current for the 211 at low plate voltage is about 10mA at +20v, and 25mA at +40v.

Now look at the 6SN7 cathode follower. When its output is at zero volta, quiescent current is about 9mA (-200 volts divided by the 22K cathode resistor), and the plate voltage is about 200v. At +20v output, the cathode resistor takes 10mA, and the 211 grid current is 10mA, total 20mA. The plate to cathode voltage on the 6SN7 is then 180v, so the 6SN7 grid bias is -1v. That's probably the limit; driver grid current will start creating limiting distortion at greater drive.

In practice you won't get down to 50v at the 211 plate, so the grid current will be less and you can drive it a little harder - let's say +25v. At +25v bias on the 211, the minimum plate voltage is 250v at 140mA, leaving 700v for swing. The maximum voltage is then 1650 volts, total swing 1400v which is 140mA into a 10K load. That puts the quiescent 211 plate current at 70mA. Increasing that current will reduce the peak negative swing, reducing power. Decreasing that current will increase distortion on the positive swings. So that's probably close to the lowest distortion/maximum power point. It's also close to the maximum dissipation of the older 211s, though some later versions were rated up to 100 watts.

 

RE: 211 optimum bias point..., posted on July 30, 2015 at 16:44:00
2395
Audiophile

Posts: 355
Location: Australia
Joined: June 4, 2003
 amp meter is in circuit [x is the meter]....thanks

 

RE: 211 optimum bias point..., posted on July 30, 2015 at 18:27:03
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 Verify my sim but distortion seems high for 16watts out
Running 211 at 75ma




 

RE: 211 optimum bias point..., posted on July 31, 2015 at 07:00:59
2395
Audiophile

Posts: 355
Location: Australia
Joined: June 4, 2003
 thanks...you simulation is fantastic...please....based upon your sims what would you set the bias at [if your amps] to get the best for general listening........best low distortion against usable power ....

 

I made measurements, posted on July 31, 2015 at 07:25:17
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 It's been a few years, but I remember there was no "sweet spot" with 1KV on the anode when I measured mine. I could vary bias from at least 65mA to 80mA with very little difference. I settled on 70-75mA as the best compromise between dissipation and output power. This is from actual experience, not paper charts.

 

RE: 211 optimum bias point..., posted on July 31, 2015 at 10:12:44
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 I'm not well versed yet with this topology so maybe someone else can chime in and I'll change the sim for you. Seems the second tube may be operating above the ratings of this tube.
Is this your schematic?

 

Yep, posted on July 31, 2015 at 10:17:22
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 You're exactly right. The only practical advantage using the direct coupled 6SN7 is overload recovery. It does not have the current capability to establish realistic A2 operation. Regarding anode dissipation, I run all my NOS 211s (mostly GE) between 70W and 80W. The latter is the limit of my OPTs at 1KV. 80W seems to have no detrimental effect on the tubes (but 100W might). :)

 

thanks, posted on August 1, 2015 at 22:45:06
2395
Audiophile

Posts: 355
Location: Australia
Joined: June 4, 2003
 Yes the schematic is drawn by me [from a bad photo of the original schematic] and from the actual [my] amps, I can email you a better graphic if you would like...the amps are based upon an original Kondo san design.

can I respectfully [and appreciating your time] request a copy of the sims for the following
60ma
64ma
68ma
70ma
again...very much appreciated

 

RE: Here you go, posted on August 2, 2015 at 17:43:41
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
These are the numbers but let your ears do the testing in the end.
You have a pot to quickly dial it in but be careful not to blow your OPT.
You might want to think about protecting your tube and OPT with this bias scheme.



 

your opinion re bias point, posted on August 4, 2015 at 00:01:13
2395
Audiophile

Posts: 355
Location: Australia
Joined: June 4, 2003
 ... very much appreciated….
if these were your amps [I enjoy hi res audio] what would you set the bias at and why………

again…your opinion is very much appreciated….

 

RE: I'd have to agree with TriodeKingdom, posted on August 4, 2015 at 12:56:44
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 It depends on your speakers, volume and general listening habits.
I think 70-75ma would be a good starting point but at the end of the day, let your ears make the decision.

 

RE: your opinion re bias point, posted on August 10, 2015 at 12:36:50
megasat16
Audiophile

Posts: 207
Location: SoCal
Joined: April 15, 2015
 If these are my amps, I would listened to other people's advise and try listening to them at varying bias point.

I would start from 40mA and listened the amp carefully at varying loudness levels. And I would increase 5mA and repeat the listening session till the bias reaches to 65-70mA. I would never bias the 211 hotter than 70mA or colder than 40mA.

Why would I spend time listening to the amp other than just set a bias point that other people said it's the lowest distortion level for them? It's because the amps we used are different, the circuits are different, the tubes are different and most of all, the OPT and the speakers we use are different. It is no joke when people said there is a real good synergy going between the OPT, the OPT tubes and the speakers in the SET amps.

Not only the bias point is important, you should also listened to your 211 amps with the speakers connected to the different output impedance tabs (4 / 8 /16 ohms) if available.
.
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.Thou shall not stand where I type for I carry a bottle of Certified Audiophile Air and a Pure Silver Whip.

 

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