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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 12:05 pm
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shimmilou wrote:
bluesky636 wrote:
...
C31 = 49.6 mfd
C33 = No reading
C35 = No reading
C36 = No reading...


You should still get a reading on the caps, even in the circuit. Isn't an unfiltered DC lower in voltage than a filtered DC?

Make sure that you didn't blow a fuse in your cap checker though (not that I've ever done that, ahem, cough, cough). Three completely open caps seems unusual, but if they are open, that could certainly account for your lost voltage, if I'm thinking correctly.


No, it's not a fuse in the DVM. My spare 20 mfd caps and the 47 mfd in the amp all measure correctly.

I am going to try measuring the 22s again later today, even if I have to disconnect them. It would be odd for all three to have failed. It is even odder that Tissan's bias measurements are almost the same as mine, though his are even colder. :?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 1:18 pm
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I will re-bias this WE probably close to 80mV and I will give you exact measurement values.
BtW, with a bias of 120mA (shortly experimented) B+ dropped from 433 to 423V but I don't remember anode voltage.
8)

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 1:32 pm
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Tis-san wrote:
I will re-bias this WE probably close to 80mV and I will give you exact measurement values.
BtW, with a bias of 120mA (shortly experimented) B+ dropped from 433 to 423V but I don't remember anode voltage.
8)


That seems to contradict what you said earlier:

"The plate voltage is about 420V in both cases with 68mV of biasing"

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 9:33 pm
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No time to really work on my amp today.

Checked the caps again with the following results:

C31 = 49.6 mfd
C33 = No reading
C35 = No reading
C36 = No reading

I find it hard to believe that all three 22 mfd caps could be bad.

I have four 20 mfd, 500 volt caps that measure 23.68, 21.49, 21.28, and 20.78 mfd. I will swap the three highest reading ones in tomorrow. Should prove interesting.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 9:47 pm
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I think that you've got it. Your meter is working and the caps should read in the circuit. Those three caps are bad alright.

I bet these caps were duds right from the China factory, and since the amp produced sound no one knew that they were bad. This could be the reason that your output tube failed. :idea:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 9:57 pm
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I'll find out tomorrow. Keep your fingers crossed. Off to bed.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 10:11 pm
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Good night John Boy.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Sep 05, 2012 9:55 am
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May be I made a confusion on the measurement naming : anode (plate) and B+ for the reporting, Bluesky, as you know B+ is DC after rectification diode;Anode (plate) voltage is on PIN 3 of 6L6.

Nevertheless, I will redo all measurements and write on paper to make sure about no confusion. I will add also Vac because I have variation at home.

:)
:)

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Sep 05, 2012 6:36 pm
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Ok. Back at it.

So, I decided to just replace the three 22 mfd caps with three 20 mfd caps I had left over from my 5F6A build. I checked all three of the 22s and once again, got no reading. All three 20s checked in between 21 and 23 mfd. Good.

So I clipped both leads on the first 22 (C33) after the choke. Before trying to peel it off the silicon goop they attached it to the PCB with, I decided to measure it while it was completely out of the circuit. It measured ..... drumroll please .... 24.8 mfd! HUH? So I next soldered the leads back together and measured it .... no reading. WTF?!?!?!?!?

Ok. I clipped the + lead on C35 and measured it .... 24.3 mfd. Clipped the plus lead on C36 and it measured 24.46 mfd.

Soldered them back up and got no meaningful reading.

OK. I am totally baffled.

Time to check power supply voltages without the tubes installed (I decided to pull all the tubes as I have never done that before).

Line in =120.2 VAC

B+ = 446 VDC
Z = 446 VDC
Y= 445 VDC
X = 445 VDC
6L6GC plate = 448 VDC (From bias probe)

Same voltages with V1 installed:

B+ = 442 VDC
Z = 441 VDC
Y= 422 VDC
X = 380 VDC
6L6GC plate = 444 VDC (From bias probe)

Same voltages with V1 & V2 installed:

B+ = 442 VDC
Z = 442 VDC
Y= 417 VDC
X = 365.2 VDC
6L6GC plate = 444 VDC (From bias probe)

Same voltages with V1, V2, & V3 installed:

B+ = 438 VDC
Z = 437 VDC
Y= 400 VDC
X = 356 VDC
6L6GC plate = 441 VDC (From bias probe)

Same voltages with V1, V2, V3, V4, & V5 installed:

B+ = 406 VDC
Z = 403 VDC
Y= 369 VDC
X = 324 VDC
6L6GC plate = 405 VDC (From bias probe)

Bias setting = 43 mA cathode current, measured with bias probe (about 41 mA plate current) = 57% max plate dissipation

Heater voltage = 6.55 VAC with all tubes installed.

So, what is the bottom line? I cannot find anything wrong with the amp beyond the fact that my voltages measure low compared to what they should be at my chosen bias point. I can't find any reason for the voltages to measure low. The amp sounds great. It just won't bias any hotter than a little over 50% max plate dissipation.

Unless someone can come up with something that I have missed, I have to consider that the amp is working as designed and this is characteristic of the BDRI. I sure wish someone who has been able to bias this amp hotter would chime in.

I think I am finished.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Sep 05, 2012 8:38 pm
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bluesky636 wrote:
...Unless someone can come up with something that I have missed...


Something is definitely amiss. There is no reason that those caps should give "no reading" while in the circuit, and then measure OK out of the circuit. I verified that mine read 22 uF in the circuit.

Are you reading directly across each cap in the circuit, or from the chassis to the positive of each cap?

No way that this is normal, your B+ voltage is too low, and there must be a problem with the rectifier circuit or the caps. Maybe something simple such as a ground problem, polarity issue, or loose connection somewhere, but something is definitely wrong.

You are also a fan of math, so you know that a specific amount of AC fed into a bridge rectifier will produce a specific amount of DC, the math does not lie. The readings that you have can not be normal.

At this point, I would have to check this out for myself to see WTF is going on here. :?

Can you scope your B+, with all tubes installed and amp turned on as if you were going to play, and see if you have something like this? 3 volts ripple, 120 Hz. You might have a little more ripple (maybe 6 volts), I have doubled my first stage cap value, but the waveform should be similar, and the frequency should be the same.
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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Sep 05, 2012 9:54 pm
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shimmilou wrote:
Something is definitely amiss. There is no reason that those caps should give "no reading" while in the circuit, and then measure OK out of the circuit. I verified that mine read 22 uF in the circuit.

Are you reading directly across each cap in the circuit, or from the chassis to the positive of each cap?


I have done it both ways: Directly across the cap and DVM positive lead to cap positive with the negative lead of the DVM connected to ground. Made absolutely no difference.

shimmilou wrote:
No way that this is normal, your B+ voltage is too low, and there must be a problem with the rectifier circuit or the caps. Maybe something simple such as a ground problem, polarity issue, or loose connection somewhere, but something is definitely wrong.


As noted in an earlier post, all diodes read the same. All are installed correctly with the proper polarity. I would think that a ground problem or loose connection would manifiest itself with noise, either hum or random crackling. The amp is dead quiet and sounds great.

shimmilou wrote:
You are also a fan of math, so you know that a specific amount of AC fed into a bridge rectifier will produce a specific amount of DC, the math does not lie. The readings that you have can not be normal.


I don't disagree. Looking back, my AC input to the rectifier is lower than yours. So for the voltages to be as low as they are, there would seem to have to be a problem with BOTH the power transformer AND rectifier. I don't know how to test for either of those.

shimmilou wrote:
At this point, I would have to check this out for myself to see WTF is going on here. :?


Sure! Come on over. I'll throw some steaks on the grill and pop open a few tall ones. :lol:

shimmilou wrote:
Can you scope your B+, with all tubes installed and amp turned on as if you were going to play, and see if you have something like this? 3 volts ripple, 120 Hz. You might have a little more ripple (maybe 6 volts), I have doubled my first stage cap value, but the waveform should be similar, and the frequency should be the same.


OK. I hooked up my new scope ($75 from E-Bay). The is the first time I have used it. :lol:

Image

Settings are 2 volts/cm and sweep time of 2 ms/cm. Looks like 4 volts of ripple at 120 hz.

It's 1:00 am in Virginia. Time for bed.

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Last edited by bluesky636 on Sat Nov 03, 2012 5:06 pm, edited 1 time in total.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 1:48 pm
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Here is new measurements (written on paper !) but a couple of mn of warm up.

Vbias (TP26) = 71 mV
VAC = 237VAC
VB+ = 446V --> drift to 438 after 15mn, VAC mesured on B+ = arround 3 VAC (not bad at all, less than 1%)
VZ = 445V
6L6 P4 = 444V
P3 = 444V
P5 - 46V


Vbias =92mV
VAC not measured
VB+ = 425V
VZ = 424V
6L6 P4 = 422V
P3 = 420V
P5 = -41V

I noticed a slow drift in voltage probably due to the fact that the amp was not temperature stabilized.

I think this makes sense with your's, all components are connected, volume at "zero". Still your B+ DC is low, have you measured AC on B+, may be caps are dead. I saw your oscillocope but no time to bring mine righ now.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 2:07 pm
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Tis-san wrote:
Still your B+ DC is low, have you measured AC on B+, may be caps are dead. I saw your oscillocope but no time to bring mine righ now.


The oscillosope photo is the ripple on the B+ measured directly at the cap: 4 volts AC at 120 Hz. Seems reasonable based on shimmilou's measurements.

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Last edited by bluesky636 on Thu Sep 06, 2012 3:54 pm, edited 1 time in total.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 3:17 pm
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bluesky636 wrote:
The oscillosope photo is the ripple on the B+ measured directly at the cap: 4 volts AC at 120 Hz. Seems resonable based on shimmilou's measurements.


Fully agreed !

But Still strange low B+ ???

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 3:52 pm
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Well, just for grins, since I use a bias probe with 1 ohm, 1% tolerance precision resistors to measure both cathode current and plate voltage individually for each power tube instead of Fender's 1 ohm, 5% resistor that measures both tubes at the same time, I decided to bypass the Fender resistor and diode at the test point. I ran a ground wire from the amp chassis to the junction of the two cathodes and test point resistor and diode, shorting out the resistor and diode and tying the cathodes of both tubes to ground like in the original Blues Deluxe.

Did it make a difference? Unfortunately, no. Bias is still sitting at about 53% max plate dissipation (41 mA cathode current, 405 VDC plate current). At the factory setting of 30 mA cathode current, plate voltage measures 410 VDC, about 40% max plate dissipation. With the bias pot cranked all the way, the cathode current measures 42 mA and plate voltage 402 VDC, about 53% max plate dissipation. That's as high as it will go.

I can't find a reason for any of this.

Any other thoughts/suggestions?

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