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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 5:32 pm
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You got me excited there for a second. :(

Yeah, your AC from the tranny to the rectifier is only about 8 volts lower than mine, and only drops about 10 volts with tubes installed, which is consistent with mine only dropping about 10 volts also. The DC is just plain low to begin with, only part of it seems to be due to the lower AC supply. I discounted the tranny as the source of the problem, but maybe it has something to do with the tranny itself, the lower voltage being a clue. I'm out of ideas otherwise.

But I would definitely like to get together the next time that I am in VA. Are you far from Fairfax? I can only imagine that it might be a long while before I get back out there again, but if I do I would certainly like to look you up.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 5:38 pm
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shimmilou wrote:
You got me excited there for a second. :(

Yeah, your AC from the tranny to the rectifier is only about 8 volts lower than mine, and only drops about 10 volts with tubes installed, which is consistent with mine only dropping about 10 volts also. The DC is just plain low to begin with, only part of it seems to be due to the lower AC supply. I discounted the tranny as the source of the problem, but maybe it has something to do with the tranny itself, the lower voltage being a clue. I'm out of ideas otherwise.

But I would definitely like to get together the next time that I am in VA. Are you far from Fairfax? I can only imagine that it might be a long while before I get back out there again, but if I do I would certainly like to look you up.


I'm out of ideas too. I posted a request for help on the vintage forum here, but so far nothing useful. I could change the caps, but since I can't measure the existing caps in circuit but they are fine when out of the circuit, there is no guarantee that new ones won't exhibit the same behavior. I am baffled as to how you can measure yours in circuit but I can't.

I live in Centreville, about 15 minutes west of downtown Fairfax. Please do contact me if you ever come out here.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 5:45 pm
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If anyone else out there has any ideas, we would sure like to hear them.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 5:46 pm
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Tis-san:

Are you able to measure your cap values while they are installed in the amp?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 8:08 pm
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This is like an addiction, I can't quit!

Just another thought, is there much AC voltage drop across the thermistor, P1 to P3, after warm up? It should be very little to none. :idea:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Sep 06, 2012 10:41 pm
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shimmilou wrote:
This is like an addiction, I can't quit!

Just another thought, is there much AC voltage drop across the thermistor, P1 to P3, after warm up? It should be very little to none. :idea:


I have no idea, but I will check it later on Friday. What is the thermistors purpose? I see that it appears in the modern amps and Reissues, but not in the original amps. I guess it is one of those mandated "safety" features like tube cages and rubber booties on the toggle switches. :lol:

Ah. Found this on-line: "NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. "

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Fri Sep 07, 2012 7:52 am
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For reference, I read 12 ohms across thermistor with amp off at room temperature (probably a good idea to unplug from electricity for this reading). After switching on power and allowing about 5 minutes warm up, I read about 1.4 VAC across thermistor.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Fri Sep 07, 2012 8:31 am
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shimmilou wrote:
For reference, I read 12 ohms across thermistor with amp off at room temperature (probably a good idea to unplug from electricity for this reading). After switching on power and allowing about 5 minutes warm up, I read about 1.4 VAC across thermistor.


Thanks. I will check this later today.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Fri Sep 07, 2012 3:02 pm
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My thermister measured 11.4 ohm (cold) to 3.3 ohms (hot).

Voltage drop across it went from 3.4 VAC (cold) to 1.3 VAC hot).

Just for grins, I measured the DC voltage from the negative lead of each cap to ground. It measured between -3 mVDC and -8 mVDC.

I cannot see anything else to measure.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Fri Sep 07, 2012 5:16 pm
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So what is the conclusion here?

Well, it is obvious that the power supply voltages are about 20 VDC lower than what they should be. However, I can find absolutely no reason for them to be that way. Although I cannot measure the power supply caps while in circuit, they measure well within tolerance when pulled from the circuit. There is no evidence of leakage. All components in the main power supply circuit from the power transformer to the tubes are correct and measure within tolerance. There is no evidence of excessive AC ripple from the power supply.

The bias supply of the BDRI is the same as the bias supply of the new Hot Rod Deluxe III. All components match and measure within tolerance. The math calculating the bias voltages matches my measured values. Yet, I cannot bias the amp above about 55% of maximum plate dissipation.

The only thing left to do would be replace the power transformer, rectifier diodes, and power supply caps. However, I can find no reason to justify going to that expense with no guarantee that it will change anything.

The amp sounds great as is. I can therefore only conclude that it is what it is. Unless someone has a brilliant revelation, I can think of nothing else to measure or do except put my tools away and get back to playing the guitar.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Fri Sep 07, 2012 9:10 pm
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Decided to just pull the Fender 1 ohm resistor and diode and bag them. Installed a jumper wire where the resistor was and removed the temporary jumper to ground that I added. Looks neater.

Final bias setting (using my bias probe):

43 mA cathode current (about 41 mA plate current) per tube
410 VDC plate voltage per tube
56% max plate dissipation per tube.
Bias readings are more stable with the resistor and diode removed.

Amp sounds great.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 11, 2012 3:49 pm
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I DID IT!!!!!

My plate voltage is still lower than I think it should be, but I was able to bias the amp at about 67% max plate dissipation with the ability to go higher if desired (previously I was limited to a max of about 57%). It took a few tricks, but here is what I did.

I have been working with the following schematic for the BDRI:

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

During my investigation I discovered that the bias supply in my amp did not match the schematic. I discovered that it matched the schematic of the Hot Rod Deluxe III.

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

The original Blues Deluxe and Hot Rod Deluxe had a half wave rectifier for the bias supply. The BDRI and the HRD III use a full wave rectifier for the bias supply. So apparently Fender made a running change in the BDRI and continued that change into the new HRD III. Interesting.

With that out of the way, I turned my attention to the main power supply. Everything in my BDRI matched the BDRI schematic. I checked all components and found they measured per the schematic. No reason could be found for the power supply voltage to drop the way it was doing and my inabilty to increase the bias beyond about 42 mA cathode current.

I was going to give up when I decided to look at the power amp stages of the BDRI and HRD III. It was there I found my first clue as to what I needed to do.

Look at the phase inverter of the BDRI, particularly V3-A. Notice the plate resistor arrangement. Now look at the phase inverter of the HRD III (also V3-A) and you will see the typical phase inverter plate resistor value used in most tube Fenders , 82K ohm on one tube and 100K ohm on the other. What would happen if I changed the plate resistors on V3-A of my BDRI to match that of the HRD III?

So I did. What happened? Not much. I still was only able to get a bias setting of about 56% max plate dissipation, but it occurred at a slightly lower setting (more counterclockwise) than previously. It then occurred to me that the bias pot was almost at the end of it travel and turning it more clockwise did not increase the cathode current beyond about 44 mA. So I decided to change the 68K ohm resistor (R77) that limits the current to a smaller value. The next smaller value I had was 56k ohm. So I swapped that in.

EUREKA!!!

I was now able to increase the cathode current beyond 60 mA. So I set the cathode current to 52 mA (average of the two 6l6GCs) and plate voltage dropped slightly to 402 VDC (again, average fo the two tubes). BUT, using the Weber bias calculator shows that 52 mA cathode current is about 50 mA plate current. Multiply that value by 402 VDC plate voltage and you get 20 watts plate dissipation at idle which equals 67% of maximum plate dissipation! HOORAY!

I also added the 47 pfd cap shown connecting the PI plates in the HRD III schematic (I would assume this is for noise or oscillation suppression). With that change, the PI and output stage circuitry is nearly identical to that of my 5F6A clone (the 5F6A clone does not have the 1.5K ohm grid stoppers that the BDRI and HRD III have, plus a few minor value changes in PI resistors).

So, what does the amp sound like now?

There is actually a slight but noticiable change in the tone. To me it is an improvement. The lows are a little tighter and less flubby. Mids are solid and the highs don't pierce. I have this amp set up for my PAF type humbucker equipped guitars and the 5F6A clone set up for my Fenders with stacked noiseless type pickups. Bottom line, it sounds better to me than it did before.

This was an interesting exercise. I went from a failed tube/screen grid resistor to being able to bias the amp hotter than I could before. In the process I learned about running changes that Fender made in the amps and how they affect the amp's performance. It was frustrating at times, but most learning exercises are. Thanks to shimmilou, Stratele52, Arjay, and BMW2002ti (in the vintage amp forum) for their suggestions and guidance.

Gonna go play my guitar now. :D

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 11, 2012 3:57 pm
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bluesky636 wrote:
There is actually a slight but noticiable change in the tone. To me it is an improvement. The lows are a little tighter and less flubby. Mids are solid and the highs don't pierce. I have this amp set up for my PAF type humbucker equipped guitars and the 5F6A clone set up for my Fenders with stacked noiseless type pickups. Bottom line, it sounds better to me than it did before.


Not surprising since both jugs are now pumping to spec with a nominally-ranged idle bias.

CONGRATULATIONS, BILL!

We're awarding you the "Toppscore Trophy".

:mrgreen:

RAWK......ON!

8) 8) 8)

Arjay

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 11, 2012 4:08 pm
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Wow! I'm glad that you had some success here, definitely a unique case. Your 56K value for R77 is a little lower than mine (about 75K) but sounds like it worked out just fine. IIRC from speaking to someone at Fender, you are correct about the 47 pF cap (maybe RF).

Good job!

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 11, 2012 4:42 pm
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Thanks, guys.

For comparison, here are the bias settings for my BDRI and 5F6A clone:

BDRI
CATHODE CURRENT 52 mA
PLATE CURRENT 50 mA
PLATE VOLTAGE 402 VDC
% MAX PLATE DISS 67%

5F6A Clone
CATHODE CURRENT 51 mA
PLATE CURRENT 48 mA
PLATE VOLTAGE 427 VDC
% MAX PLATE DISS 70%

Even though the 5F6A clone has a tube rectifier (5AR4), it has a much higher B+ voltage than the BDRI. I forget what it is at the moment, though. I still think the BDRI voltages are lower than they should be, but at least I can now bias it where I want it. :D

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