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Post subject: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 7:57 pm
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Today was "Check Amp Bias Day" on both my Weber 5F6A Clone and my Fender BDRI. No particular reason to do so, it has just been a while and I was bored. :lol: Both amps were sounding great.

Using my bias probe, I did the 5F6A first and settled on V4 = 50 mA cathode current (about 48 mA plate current) at 428 VDC plate voltage and V5 =52 mA cathode current (about 49 mA plate current) at 427 VDC plate voltage. Works out to right about 70% max plate dissipation for each tube. Just where I like it. Oh, my line voltage was about 122 VAC.

Next I moved on to the BDRI. I have never been able to get that amp above about 53% max plate dissipation, but it sounds fine at that level. So I hooked up my bias probe and proceeded to take some readings. V4 was about 39 mA cathode current at about 413 VDC plate voltage. V5 was also about 413 VDC plate voltage but showed ZERO mA cathode current! WTF! :shock: I also noticed that V4 warmed up nicely, but V5 was nowhere near as warm to the touch.

So, I first decided to swap the bias probe sockets between tube sockets. No joy. V4 measured normal but V5 still showed ZERO mA cathode current. Plate voltage was normal. Ok. The probe does not appear to be the problem, so I swapped the probe sockets back to where I had them.

Next, I pulled the two JJ 6L6GCs and replaced them with the original GT 6L6Rs. Same thing. V4 measures normal, V5 shows normal plate voltage but ZERO cathode current.

So I pulled out the schematic:

http://support.fender.com/schematics/gu ... _Rev-A.pdf

I first located the two bias resistors, R59 and R60. R60 measured right at 223K Ohms (in circuit) while R59 (the bias resistor for V5) measured 212K Ohms. Then I checked R64, R65, and R66. All good.

So I then started measuring the bias circuit voltages. I measured -53 VDC at TP41. I also measured about -43 VDC at the junction of R59 and R60 and about -41 VDC at the grids (pin 5) of both power tubes (Do these seem low?). BUT I only measured 44 mV at the Fender bias test point where previously I measured 88 mV. So, I pulled out V5 and the Fender bias test point still measured 44 mV. I put V5 back in and pulled V4. There was ZERO voltage measured at the Fender bias test point. So, since I swapped both bias probe test sockets and tubes, I concluded that there is a problem with the V5 tube socket.

I then reflowed the solder on the pins where they attach to the circuit board and measured the resistance between the solder point and the metal pin connectors inside the spocket. All measured zero resistance. I also tried to tighten the connectors inside the socket. These are different from the ones in the sockets of my 5F6A. The BDRI connectors are like little two prong clamps as opposed to the rolled connector in the 5F6A socket. However, even after attempting to tighten the connectors with a pointed metal tool, I still cannot read any voltage at the Fender bias test point with only V5 installed. I have verified the crcuit path between the tube socket and the Bias test point but cannot get a voltage reading. I am 99.999% sure that the problem is the inside connector for pin 5 or pin 8. How do I tighten these? What am I missing? What do I do next?

The really odd thing is, the amp still works. It doesn't sound as clean as it should but it doesn't sound bad. HELP!!!!!

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Last edited by bluesky636 on Thu Aug 23, 2012 9:32 pm, edited 1 time in total.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 8:25 pm
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"Today was "Check Amp Bias Day" on both my Weber 5F6A Clone and my Fender BDRI. No particular reason to do so, it has just been a while and I was bored. Both amps were sounding great."

Good idea!
There should be a National Amp Test Day
and an Annual Fender Stratocaster Setup Day :wink:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 8:33 pm
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:roll:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 8:38 pm
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bluesky636 wrote:
Today was "Check Amp Bias Day" on both my Weber 5F6A Clone and my Fender BDRI.


Oh my gosh, silly me :oops: :oops: :oops:
I was just enhancing the "Day" concept :lol:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 9:17 pm
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Ok. Some more info:

I swapped tubes between V4 and V5. It made no difference. Still no voltage on the cathode pin of V5 or at the Fender bias test point.

I was able to clip some small test leads to pins 5 and 8 of V5 and push it in enough to make contact with the socket. I was able to measure zero resistance between the solder points on the board and the open end of the clip lead. So the pins for the cathode and grid appear to be making contact inside the socket. Why is there no voltage measured at the cathode of V5. The one ohm resistor used for the bias test point is good. V4 measures OK, why not V5?

ARRRGGGHHH!!!

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Last edited by bluesky636 on Thu Aug 23, 2012 9:47 pm, edited 1 time in total.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 9:19 pm
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bluesky636 wrote:
...I first located the two cathode resistors, R59 and R60. R60 measured right at 223K Ohms (in circuit) while R59 (the cathode resistor for V5) measured 212K Ohms.


Those are not Cathode resistors, they are bias resistors connected to the grid resistors (R64, R65). I know that you know the Cathode resistor is connected to the Cathode and is the test point resistor R66. :wink:

bluesky636 wrote:
So I then started measuring the bias circuit voltages. I measured -53 VDC at TP41. I also measured about -43 VDC at the junction of R59 and R60 and about -41 VDC at the grids (pin 5) of both power tubes (Do these seem low?).


Not low, same reading as schematic, this is good.


bluesky636 wrote:
BUT I only measured 44 mV at the Fender bias test point where previously I measured 88 mV. So, I pulled out V5 and the Fender bias test point still measured 44 mV. I put V5 back in and pulled V4. There was ZERO voltage measured at the Fender bias test point. So, since I swapped both bias probe test sockets and tubes, I concluded that there is a problem with the V5 tubes socket....


Do you have the Plate voltage at pin 3 on both sockets, measured directly at the sockets? (remember a problem here before). My guess is that the circuit trace supplying Plate voltage is broken before it gets to the socket (maybe where the blue wire from the OT connects to the board). With output tubes removed, insert positive volt meter lead into pin 3 of each socket to see if Plate voltage is present at both sockets.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 9:30 pm
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shimmilou wrote:
bluesky636 wrote:
...I first located the two cathode resistors, R59 and R60. R60 measured right at 223K Ohms (in circuit) while R59 (the cathode resistor for V5) measured 212K Ohms.


Those are not Cathode resistors, they are bias resistors connected to the grid resistors (R64, R65). I know that you know the Cathode resistor is connected to the Cathode and is the test point resistor R66. :wink:


DOH! You're right. I mispoke. I'll correct that.

shimmilou wrote:
bluesky636 wrote:
So I then started measuring the bias circuit voltages. I measured -53 VDC at TP41. I also measured about -43 VDC at the junction of R59 and R60 and about -41 VDC at the grids (pin 5) of both power tubes (Do these seem low?).


Not low, same reading as schematic, this is good.


Ok.

shimmilou wrote:
bluesky636 wrote:
BUT I only measured 44 mV at the Fender bias test point where previously I measured 88 mV. So, I pulled out V5 and the Fender bias test point still measured 44 mV. I put V5 back in and pulled V4. There was ZERO voltage measured at the Fender bias test point. So, since I swapped both bias probe test sockets and tubes, I concluded that there is a problem with the V5 tubes socket....


Do you have the Plate voltage at pin 3 on both sockets, measured directly at the sockets? (remember a problem here before). My guess is that the circuit trace supplying Plate voltage is broken before it gets to the socket (maybe where the blue wire from the OT connects to the board). With output tubes removed, insert volt meter lead into pin 3 of each socket to see if Plate voltage is present at both sockets.


As noted in my original post with both tubes installed and using the bias probe: "V4 was about 39 mA cathode current at about 413 VDC plate voltage. V5 was also about 413 VDC plate voltage but showed ZERO mA cathode current!"

Just pulled both tubes and reverified using the bias probe, both plate voltages came in at 442 VDC.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 9:36 pm
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Oh, you have the bias probes that measure Plate voltage at the socket?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 9:44 pm
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shimmilou wrote:
Oh, you have the bias probes that measure Plate voltage at the socket?


Yeah. I've been using them since since I accidently shorted the plate terminal to the adjacent pin a few years ago. That is probably what you remember. :lol:

Anyway, I cannot find any reason that I am not measureing cathode current for V5. All circuits look and measure normal, both resistance and voltage.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 9:56 pm
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:? So much for my idea.

It seems that you've checked everything but the screen grid voltage at pin 4. :idea:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 10:08 pm
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shimmilou wrote:
:? So much for my idea.

It seems that you've checked everything but the screen grid voltage at pin 4. :idea:


DING! DING! DING! WE HAVE A WINNER!!!!!!

V4: 423 VDC on power supply side of R62, 422 VDC at pin 4.

V5: 422 VDC on power supply side of R61, ZERO VDC at Pin 4.

Looks like I'll be replacing the 470 Ohm screen grid resistor!!!!

Thank you shimmi!!! :D :D :D :D

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 10:11 pm
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Just measured R61: It is wide open. :(

Don't have any 470 ohm, 1 watt resistors on hand, so I'll have to order some. Gonna replace both just to play it safe. :D

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 10:24 pm
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Cool! SGR really should have been our first thought. :oops:

1K would also work, FP of course.

If you want to experiment, I really like the sound in my HRDlx using 2K, slight compression, nothing dramatic but cool.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 10:28 pm
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I prefer the BDRI clean and use my Fulltone FD-2M for OD. :D

I've been looking at modding the PI/power amp section of the BDRI to be like the 5F6A Bassman. They are only a few resistor values apart. :wink:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 10:36 pm
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Sounds cool, please post about it if you do.

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