Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Oct Sat 25, 2014 4:19 pm


All times are UTC [ DST ]



Post New Topic Post Reply  [ 46 posts ]  Go to page Previous  1, 2, 3  Next
Author Message
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: May Wed 09, 2012 4:34 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 9721
Location: Chesapeake VA
Thanks guys...

Since this mod is so easy and users have been complaining about the issue since Noah saved the first pair of B&K tube testers on the ark, I'm surprised no one came up with this years ago...

Tom


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: May Tue 15, 2012 2:44 pm 
Member

Joined: Apr Sat 30, 2011 2:14 am
Posts: 349
Location: Mississauga, Ontario
Tom,
Thanks for posting this.
What a great idea! I have always been told that the 707 and 747 wasn't good for testing 12AX7's, it's nice to see a solution rather than just pointing out a problem.
Do you know if the B&K 747B is also plagued with this problem? I imagine so since it is pretty much the ss version of the 707?
I have one of those and really like it.
Thanks again,
Mauro


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: May Wed 16, 2012 2:05 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 9721
Location: Chesapeake VA
Mauro, I'm out of town and not that familair with the 747, if someone doesn't post more info, I'll see what I can find later this week...

Tom


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: May Thu 17, 2012 1:43 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 521
Location: Stafford, Texas USA
Mauro

B&K 747 or 747B is NOT plagued with this problem? Plus the747’s are not SS copy of 707’s.

The 12AX7 though tested in the Programmed section socket 29. The Tube is tested for GM,
This being one of the biggest changes in the 747 over the 707.

See manual page 22 switches 13 and 14. The book said set the 12ax7 up for low bias (0.5v)
With a 1.5 VAC signal on Switch 13 position A, GM voltage 190 VDC by set by Switch 14 position A.

By using the Programmed section sockets and some 4,5,6,7 pin adapters one can test most of the old tubes.

Jimmie


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: May Thu 17, 2012 2:18 am 
Member

Joined: Apr Sat 30, 2011 2:14 am
Posts: 349
Location: Mississauga, Ontario
Thanks for posting and clarifying this for me.
Glad to know!
Mauro


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: May Sat 26, 2012 8:02 pm 
New Member

Joined: May Mon 07, 2012 6:13 pm
Posts: 7
Is 22 AWG solid core hookup wire heavy enough for this mod?


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Dec Thu 06, 2012 12:04 pm 
Member

Joined: Dec Thu 06, 2012 11:47 am
Posts: 56
I just followed the procedure and modified socket 24 on 707.

Tested some known 12AX7 that I have, for the original setting, 22 is minimum good, take it as 65%, 33 or 34 should be about 100%.
Test result on this modified socket, reading of the same tube is about 100~105%. It looks pretty compatible.

By the way, I use AWG 22 hookup wire to do this. It seems the wire used in 707 is thicker, maybe #20?

Thank you very much, Tom. I have posted my mod on my facebook. Can I post the link of this thread on it, too?


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Mon 01, 2013 12:05 am 
Member

Joined: Apr Fri 19, 2013 1:25 pm
Posts: 117
Location: Montreal / Canada
I did the modification on my B&K 700, very nice thanks for sharing this!

_________________
http://www.doitfuzz.com


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Sun 28, 2013 4:17 pm 
New Member

Joined: Jul Sun 28, 2013 2:40 pm
Posts: 3
:?: I am seriously considering buying a B&K 707 and learning how to use it to test the power tubes ( EL34, 6550, KT88, 6L6 ), and pre-amp tubes (typically 12AX7's or 12AT7's) in some guitar amps I own. I am a novice and would like to know how to get the very basic understanding of how to test and what the different parameters mean.

Also, I seriously would want to have the mod done because the 12AX7's are in all my amps. Is this difficult for a novice for me to do with basic soldering skills ? When the unit is opened up are the pin numbers readily indefinable ? I appreciate any expertise you would share with me. Thank you in advance !


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Sun 28, 2013 8:49 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 27592
Location: Pocasset, Cape Cod, MA
Looking at the bottom of the socket (or tube) the pins are numbered clockwise starting at the empty space.

The actual soldering shouldn't be difficult, but that assumes you've soldered such things before. Old terminals don't take solder as readily as new ones.

I don't run guitar amplifiers, but would guess you'll wear out ten or twenty output tubes for every 12AX7. So testing 12AX7s may not be a priority. As for testing output tubes, some amps run them harder than others. For those that run them close to or above their ratings, you will want to check that they don't run away as a result of grid emission, and a B&K (or any shop tester) can't do this.

Somebody who does use guitar amps can chime in with actual experience.


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Sun 28, 2013 9:06 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 2113
Location: Florida
Alan Douglas wrote:
For those that run them close to or above their ratings, you will want to check that they don't run away as a result of grid emission, and a B&K (or any shop tester) can't do this.


The B&K isn't going to run them anywhere close to maximum ratings but it does have a very good grid emission test.

RRM


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Mon 29, 2013 2:02 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 27592
Location: Pocasset, Cape Cod, MA
But it won't heat the tube up as much as the amp would.

I've never seen a 6L6 or 6550 run away in a tube tester, but I have watched an 8417 do it in my Cardmatic which monitors grid current continuously on the panel meter. It took a couple of minutes.


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Mon 29, 2013 4:21 am 
New Member

Joined: Jul Sun 28, 2013 2:40 pm
Posts: 3
I appreciate the feedback ! When you say "run away" what exactly do you mean - what happens to the tube, and in which test ? If I am correct, in one of the 50 and 100 watt heads I have, the EL34 power tubes are drawing around 450 - 500 v. Unlike radios, these guys like headroom, but not too much - it's rock n' roll lol ! My main purpose for the B&K 707 is to determine if a power tube is going bad, even though that is often just discernible by ear. I just happen to love vintage analog electronics and want to learn how to test and what all the parameters mean. The unit I am getting has had the mod done to it by the seller (who happens to calibrate B&K 707's) - he just told me this evening, so I am sort of elated that I don't have to do it, although it would have been a good learning experience. The mod is important because each amp has several 12AX7's. Although they rarely need to be replaced, often it is a good idea to swap different brands, current and NOS, to slightly change the tone of a given amp. Therefore the mod is important to test the replacements. Now I just have to learn how and why and what to lookout for.


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Mon 29, 2013 7:01 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 2113
Location: Florida
Alan Douglas wrote:
But it won't heat the tube up as much as the amp would. ............


Agreed, but it will detect very low levels of grid current. If a tube has grid current at lower power levels it will have a lot more at high levels.

On the other hand, the test may be too senstitive for high power tubes and lead to rejection of tubes that will be ok in an amp. The B&K uses a 10 meg grid resistor for the grid emission test and it doesn't take much leakage to make a tube show up as bad. Since the grid resitor in an amp is going to be much lower than 10 megs this is one of those places where the addition of windage to tube tester readings (and/or changing the test sensitivity) comes into play.

RRM


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Jul Mon 29, 2013 7:25 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 2113
Location: Florida
jupiter89 wrote:
.......When you say "run away" what exactly do you mean - what happens to the tube, ......... ? .........


If the tube has grid emission it will start to show up as the tube heats up. Grid emission results in a current that causes a voltage to develop across the grid resistor that is opposite in polarity to the normal bias. This has the effect of lowering the bias, causing the tube to draw more plate current. More plate current means more heat, more heat means more grid current, more grid current means lower bias, lower bias means higher plate current which means more heat and.........etc. This will go on until something gets very unhappy.

RRM


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Sep Wed 18, 2013 1:55 am 
New Member

Joined: Jul Sun 28, 2013 2:40 pm
Posts: 3
Please let me know if this is correct from what I have read. I know nothing about electronics and have limited soldering skills, but just wanted to see if my understanding is correct.
Where would the 2 ground wires from pins 3 and 8 of socket #24 be soldered to ?


Attachments:
File comment: sketch of 12AX7 mod for B&K 707
12AX7 mod.JPG
12AX7 mod.JPG [ 51.7 KiB | Viewed 499 times ]
Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Nov Sun 03, 2013 8:13 pm 
New Member

Joined: Nov Sat 02, 2013 7:26 pm
Posts: 5
Howdy, so I am going to try the 12AX7 mod on my BK-700, when you say ground pins 3 & 8 on socket 24 does that mean just solder them (3&8) to a common ground point? Then connect pins 1&6 on socket 24 to 1&6 on socket 8 then connect pins 2&7 on socket 24 to pin #1 on socket 6 ? Set sensitivity to 91… correct? Thanks,Bubba


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Apr Fri 18, 2014 12:46 am 
New Member

Joined: Apr Fri 18, 2014 12:33 am
Posts: 4
Hi Tom,

Why modify the 707 at all? Socket 8, pins 2 and 7 are all ready connected to the Lo bias. You stated to connect socket 24, pins 2 and 7 to pin 1 of socket 6 which is the Lo bias too . It only is ran through a 1k grid stop resistor between the two sockets which should not affect the test. Put the 12AX7 in socket 8 and dial load setting to 91 and it should test fine without any mods. Am I correct about this?


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Apr Thu 24, 2014 10:49 pm 
New Member

Joined: Apr Thu 25, 2013 9:24 am
Posts: 16
Location: South Australia
The 1k resistor is not a grid stopper but a resistor in a voltage divider
from the high bias to the low bias.
The first resistor in the chain from the
High bias at is -19.5 V
Then R109 2.4K Leaving -7.5V
Then R108 1K Leaving -2.5V
Then R106 470 Leaving -.2V which is the low bias volts
I have not done the mod myself for either of mine B&K 700 but
mod is very sound and worthwhile.
The -2.5V Bias at socket No.8 drive a 12AX7 to near cut off -.2 volts
is a more realistic voltage for the testing the tube this is supplied
at socket No. 24 with the modification in place.
:lol:


Top
 Profile  
 
 Post subject: Re: 12AX7 Test Modification For B&K 707
PostPosted: Apr Thu 24, 2014 11:44 pm 
New Member

Joined: Apr Fri 18, 2014 12:33 am
Posts: 4
crazzzymick wrote:
The 1k resistor is not a grid stopper but a resistor in a voltage divider
from the high bias to the low bias.
The first resistor in the chain from the
High bias at is -19.5 V
Then R109 2.4K Leaving -7.5V
Then R108 1K Leaving -2.5V
Then R106 470 Leaving -.2V which is the low bias volts
I have not done the mod myself for either of mine B&K 700 but
mod is very sound and worthwhile.
The -2.5V Bias at socket No.8 drive a 12AX7 to near cut off -.2 volts
is a more realistic voltage for the testing the tube this is supplied
at socket No. 24 with the modification in place.
:lol:


There's only 1 resistor (R-106) between the low bias -2.5V and socket 8, pins 2&7. Trace the schematic and you will see. The only connection between the hi bias and low bias is via coupling cap C-101. I suppose some voltage drop across R-106 may occur but not enough to effect the test. R-106 is just a 470 ohm. The hi bias is a DC signal where the low bias is a AC signal. There is no way the DC from the hi bias is going to get across C-101 into the low bias. I would think what they are doing there is using the low bias AC to ride on the DC of the high bias for tubes that need the hi bias for the test.


Top
 Profile  
 
Post New Topic Post Reply  [ 46 posts ]  Go to page Previous  1, 2, 3  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 5 guests



Search for:
Jump to:  




















Privacy Policy :: Powered by phpBB