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211 Bias overload.......

101.115.204.126
Posted on July 24, 2024 at 01:57:23
2395
Audiophile

Posts: 359
Location: Australia
Joined: June 4, 2003



Thanks experts...
I am not a technician ...just electronics capable ...

From a visual view nothing seem to be out of place..exploded or overheated....

Based upon the attached circuit.......

Why would all of a sudden the bias to the 211 skyrocket......
The plate is glowing...never seen before and obviously not right...
Turning the bias pot make no difference .......

all valves have been tested and are good [in another amp]
What component or chain is likely the cause....

if not obvious will need to take to tech...

Again ...Many Thanks

 

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RE: 211 Bias overload......., posted on July 24, 2024 at 05:04:00
Triode_Kingdom
Audiophile

Posts: 10401
Location: Central Texas
Joined: September 24, 2006
 This could happen if the 211 socket loses contact with the tube's grid pin. Why isn't the output stage fused? Is this a Chinese amp?

 

RE: 211 Bias overload......., posted on July 24, 2024 at 05:07:11
walge
Audiophile

Posts: 143
Location: roma
Joined: September 25, 2008
 With a great care check the voltage across resistor of 1 ohm and vary the bias voltage.
This is a check point to read the current
The Ia must vary. Then you need to calculate the Pa= Vdc x Ia so you can have an idea of dissipation

Walter

 

RE: 211 Bias overload......., posted on July 24, 2024 at 05:11:07
walge
Audiophile

Posts: 143
Location: roma
Joined: September 25, 2008
 Of course also the voltage on grid of 6SN7 and 211 must vary following the indication of diagram.
Possible a lost if insulation of the coupling cap on driving section

Walter

 

RE: 211 Bias overload......., posted on July 24, 2024 at 07:48:16
Palustris
Audiophile

Posts: 2460
Location: Cape Cod
Joined: September 12, 2008
 It's not obvious where the fault lies and thoroughly checking out the amplifier voltages to troubleshoot means exposing your self to dangerous voltages. Notice the plate voltage on the 211 is 960V! So I would start with the unit unplugged and the power supply caps discharged. Start with the bias supply: measure the resistance of R8, R9, R10 and VR1. Ensure each of these is the correct resistance and that the pot, VR1, has a smooth change of resistance for the entire sweep of the wiper. If you are lucky, then one of these components will be at fault.

Otherwise you will need to check the voltages throughout the amp. Always turn the unit off, apply the DVM leads to the component under test, and turn on the amp. Having multiple DVMs makes the job much more rapid.

 

C3 nt, posted on July 24, 2024 at 19:59:10
Tre'
Industry Professional

Posts: 18056
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: 211 Bias overload......., posted on July 25, 2024 at 06:17:26
dave slagle
Manufacturer

Posts: 5448
Location: NYC
Joined: April 27, 2001
 My guess is the bias supply went south.... check the operating points of the 211 and be aware the voltages inside that box will kill so be careful

dave

 

RE: 211 Bias overload......., posted on July 25, 2024 at 07:47:46
deafbykhorns
Audiophile

Posts: 1083
Location: Florida
Joined: October 17, 2003
 VAL International....
Chinese for sure

 

RE: 211 Bias overload......., posted on July 25, 2024 at 15:53:42
Triode_Kingdom
Audiophile

Posts: 10401
Location: Central Texas
Joined: September 24, 2006
 Got it. All the more reason to suspect the socket. (Or anything else for that matter.)

 

RE: 211 Bias overload......., posted on July 26, 2024 at 02:53:37
2395
Audiophile

Posts: 359
Location: Australia
Joined: June 4, 2003
 Thanks Triode...yes amp is Chinese but has been completely rebuilt [10 years ago] all components except transformers and choke with all new M'cap's and all resistors replaced with Riken and Takman as chosen for best sound. Circuit is basic Ongaku

Am not aware of fused output and have not seen same in either Kondo, Audio Note or AirTight..[sure but not positive so would stand corrected]

What fuse [value] would you place and where...
Also any other suggested changes...

Many Thanks

 

RE: 211 Bias overload......., posted on July 26, 2024 at 02:58:29
2395
Audiophile

Posts: 359
Location: Australia
Joined: June 4, 2003
 Thank you all for Advice ........very much appreciated..

Problem was a soldered wire had lost full contact on a trace.....Trace had slowly lifted and wire was not connecting fully.....
Circuit board had been stripped 10 years ago and some of the traces were compromised. Join had lasted 10 years so not to bad....

Again...Many Thanks All

 

RE: 211 Bias overload......., posted on July 26, 2024 at 03:51:08
dave slagle
Manufacturer

Posts: 5448
Location: NYC
Joined: April 27, 2001
 at what point in the schematic was the problematic point?

dave

 

Output Fuse, posted on July 26, 2024 at 10:30:35
Triode_Kingdom
Audiophile

Posts: 10401
Location: Central Texas
Joined: September 24, 2006
 This was less of a problem 20 years ago before 211 prices shot up. Back then a $100 NOS tube that might be lost due to a chance failure every 10 or 20 years wasn't such a big financial injury. Now the NOS tubes (all I use) are almost prohibitively expensive, due in part to a major collecting effort on the part of Audio Note and others.

Anyway, there are two issues that I'm aware of regarding fusing the output stage. First is of course the high anode current that results from a simple loss of bias (grid goes to zero volts or floats). The latter might occur if the socket loses contact with the grid pin. I've seen this happen if the curved pins of the socket style in your amp become bent or misaligned. After working with these a long time, I have concluded that the type of socket with a flat spring tab under each tube pin is probably more reliable. The sockets themselves are somewhat fragile and can crack if mishandled, but in normal use the tabs can't lose contact unless they're very dirty. Unfortunately, I sold all those off years ago before I realized this (and before I built my amps). The Johnson sockets in my monos are like yours, and I spent a lot of time testing them for reliable contact by twisting the tubes slightly in various directions to be sure contact couldn't be lost during actual operation. Not all the sockets I tested were OK in this regard, and this ultimately provided the incentive to add a 211 cathode fuse to my amps.

The second area of possible damage is the 211 grid. Excess grid current is a real danger in a direct coupled amplifier where loss of the negative supply might move the grid positive. The simplest solution to this that I know of would be a second small value fuse in series with the grid. I haven't done this, but I did install a fuse in series with the driver tube B+ power supply. In my amp, driver tube current is controlled by a negative voltage applied to the driver tube grid, and this voltage is tapped off the main bias supply. So, if the bias supply goes to zero, the excessive driver current will cause its B+ fuse to open, and this will prevent the application of a positive voltage from the driver cathode to the 211 grid.

The schematic below shows how I fused the output stage of my monos. B+ on the output is +1050V. The purpose of the 82V Zener is to prevent the cathode from floating to a destructively high voltage level if the fuse opens:



 

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