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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 8:18 am
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shimmilou wrote:
Yeah, I remember now that we never really knew why the Plate voltage was so low to begin with. It really should be very close to 430 VDC at your bias setting. I can't remember what we discussed before....possible open diode (D6, D7, D8, D9) or bad filter cap (C31, C33, C35, C36)? Or, is the AC from the transformer too low? Are you sure that the transformer connections to the circuit board are on the correct terminals? Just throwing stuff out there. :?


Haven't checked those items specifically, but I can say (and will verify later today), that the plate voltage as measured using my bias probe and DVM with the power tubes removed is right about 430 VDC. With the tubes (JJs) installed and bias set to the factory specified 30 mA (cathode current) per tube, plate voltage drops to in the neighborhood of 425 VDC. This works out to about 42% max plate dissipation with my JJs. At 40 mA (cathode current) plate voltage drops to around 407 mA. This works out to about 54% max plate dissipation for the JJs.

Tis-san wrote:
Bluesky and Shimmilou,

I confirm also that I cannot bias properly JJ valves on my BDRI (55mV on Tp, let be 22.5 mA per valve best case, very south pole "cold") since the 5881 are biased at 68 mV a hot condition with still a margin on the pot. I bring back a couple of resistors for adjustment ..., I've also noticed that the bias resistors in the circuit are not conform to my schematic.

:)


I don't use the Fender test point any more to measure bias. The 1-ohm resistor is only a 5% tolerance. I refer to use my bias probe with 1% tolerance resistors.

There is no real difference in bias settings between the JJs and the stock Groove Tube (White label, #2) 5881s in my amp. Both bias about the same although I do not bias the GT 5881s as hot as I do the JJ 6L6GCs.

What resistors did you adjust (change?)? The two 220K ohm bias resistors measured right at 223K ohm for one and about 212K ohm (well within 10% tolerance) for the other.

Are you using the correct schematic? The one I posted earlier is for the BDRI, not the original Blues Deluxe.

shimmilou wrote:
Tis-san,

What is your Plate voltage with JJs?

What is your Plate voltage with 5881s?

I have heard of some BDlx that seem to have a mixture of BDlx and BDvl parts. :?


As I noted, there is no significant plate voltage difference beween tubes in my amp.

The schematic I posted shows both the Blues Deluxe Reissue and Blues Deville Reissue components and measurment values. My amp coresponds with the BDRI as shown.


Gotta go walk my dogs. Be back later. :D

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 8:39 am
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bluesky636 wrote:
...the plate voltage as measured using my bias probe and DVM with the power tubes removed is right about 430 VDC...


With output tubes removed, the Plate voltage should be closer to 460 volts. With output tubes installed and approx 40 mA Cathode per tube, the Plate voltage should be closer to 430 volts. I would really have a look at the bridge rectifier circuit, maybe a bad diode or solder connection?

I'm sure you'll figure it out if you can keep at it.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 11:05 am
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shimmilou wrote:
bluesky636 wrote:
...the plate voltage as measured using my bias probe and DVM with the power tubes removed is right about 430 VDC...


With output tubes removed, the Plate voltage should be closer to 460 volts. With output tubes installed and approx 40 mA Cathode per tube, the Plate voltage should be closer to 430 volts. I would really have a look at the bridge rectifier circuit, maybe a bad diode or solder connection?

I'm sure you'll figure it out if you can keep at it.


Ok. I'll check them out. My new DVM includes a diode test function. Can the diodes be tested in circuit or do they have to be removed?

I would think if it was a diode that the amp would be pretty noisy and perform poorly due to poor rectification. It is actually quieter than my 5F6A clone and sounds great with the new tubes.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 1:33 pm
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You can check them in circuit, make sure amp is unplugged, caps are discharged and standby switch open. Might be difficult to check the solder joints without pulling the board.

You might be right about noise, I just can't figure why the DC would be so low. If diodes are good, can you get an AC voltage reading from the transformer section that feeds the diodes, with switch in standby vs in play?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 3:41 pm
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For reference on my amp, checking AC volts from CP11 to CP12 (P11 to P12 for BDRI), where red wires connect to board.

Standby - 344 VAC
Play - 334 VAC (438 VDC B+, approx 40 mA Cathode current each output tube)

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 4:57 pm
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Schematic used:

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

OK. Here are my measurements.

Rectifier diodes (D6, D7, D8, D9) all measure good (0.577 V forward, open reverse). Since the BDRI PCB component mounting holes are through plated, the solder joints that I can see are all good, so I will assume that the joints on the underside of the board are also good.

Line voltage = 120 VAC

Voltages measured WITHOUT power tubes installed:

The AC input to the rectifier is 336 VAC. I measured between the cathode of D9 and the anode of D6 as shown in the schematic.

B+ (TP37) = 440 VDC (solid)

Plate voltage at each power tube = 440 VDC (solid)

Voltages measured WITH power tubes installed:

The AC input to the rectifier is 326 VAC. I measured between the cathode of D9 and the anode of D6 as shown in the schematic.

B+ (TP37) = 410 VDC (average)

Plate voltage at each power tube = 408 VDC (average)

Cathode current at each power tube = 40 mA (average)

There seems to be a discrepancy with TP 41 as shown on the bias power supply schematic. At the junction of D15 (anode), C43, and R76, the voltage is -101 VDC. At the junction between R76 and the top of the bias pot (R82), the voltage is -52.6 VDC. I also note that D10 does not show in the BDRI bias supply schematic but does show in the Blues Deville Reissue bias supply schematic. However, D10 IS present in my amp. At the junction of D10 (anode) and D15 (cathode), the voltage is -40VDC.

I'm not sure what else to measure that would add any value to this investigation.

Other than those discrepancies noted between the schematic and my amp, I don't see any reason that these measurements are not normal.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 5:38 pm
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You are losing about 35 VDC for some reason from the bridge circuit. Rechecking my math, 336 VAC times 1.414 equals approx 475 VDC for the B+ that you should have with the output tubes removed, and dropping to approx 440 VDC with output tubes installed. Since you only have 440 VDC with the tubes removed, something is wrong in that bridge circuit to cause a loss of approx 35 VDC. It could also be a faulty power supply filter cap dragging the voltage down.

With roughly twice the voltage on the bias supply (-101 VDC), that would seem to indicate a center tap on the part of the transformer for that supply. Do you have connections from the transformer at P15, P16 and P17?

Looks like you have one of those BDlx that has a mix of BDvl parts. :idea:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 5:58 pm
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shimmilou wrote:
You are losing about 35 VDC for some reason from the bridge circuit. Rechecking my math, 336 VAC times 1.414 equals approx 475 VDC for the B+ that you should have with the output tubes removed, and dropping to approx 440 VDC with output tubes installed. Since you only have 440 VDC with the tubes removed, something is wrong in that bridge circuit to cause a loss of approx 35 VDC. It could also be a faulty power supply filter cap dragging the voltage down.


I don't know what to say to this. I can't see anything wrong with the power supply. All measurements are consistant. I guess I could measure the power supply caps. I have some extra 20 mfd, 500 volt caps on hand, but no 47 mfd 500 volt caps.

shimmilou wrote:
With roughly twice the voltage on the bias supply (-101 VDC), that would seem to indicate a center tap on the part of the transformer for that supply. Do you have connections from the transformer at P15, P16 and P17?


I don't know. I have put the amp away for the evening. I'll throw the question back at you: the part numbers are the same in your amp and mine. Do you have a center tap in the bias supply? The Deville appears to, but that is a different power transformer than in our amps. Note though that there is only -52 VDC from the input of the bias pot to ground and not -101 VDC. I think that is correct and that the schematic is wrong as far as TP 41 is concerned. Note that the 59 Bassman LTD also shows -52 VDC across the bias pot to ground for a very similar bias circuit. I am not sure what the purpose of D10 is in my bias circuit. There is no equivalent diode in the Bassman LTD bias circuit either. What do you think the impact of removing D10 would be?

shimmilou wrote:
Looks like you have one of those BDlx that has a mix of BDvl parts. :idea:


Seems like it. :lol:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 6:53 pm
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Since I have put the amp away for the evening, I decided to do a little research and took a look at the Hot Rod Deluxe III schematic, particularly the bias power supply.

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

Note that version III has both D15 AND D10 but no center tap. Also note that the voltage measured at TP 41 is -106 VDC which closely corresponds to my measured -101 VDC at the same point. I would be willing to bet that you would measure about -53 VDC at the top of the bias pot just like in my amp. I have not compared the component values in the bias supply in my amp to the HRD III, so I will do that tomorrow (we're having thunderstorms now and I really don't feel like poking around inside a live amp with lightning around :shock: ). I think based on this discovery, it is safe to say that my bias power supply is working as intended. That leaves only what, the main power supply caps as a potential problem?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 7:02 pm
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Wow, when I hit preview for my post, I just saw your post about the HRDlx III and I didn't realize the bias circuit was different. So, it seems that it's just full wave vs half wave accounting for the voltage difference in the bias supply. Yep, looks like maybe your bias supply is OK and the B+ power supply has an issue. Oh well, I had this whole post typed out, but it isn't relevant now so I've deleted it.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 7:58 pm
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Sorry. :lol: :lol: :lol:

Tomorrow I'll check the main power supply caps and compare the values caps of the BDRI bias circuit components to the HRD III bias circuit components.

It occurred to me that I have four 20 mfd, 500 Volt caps on hand so if the 47 is bad, I can parallel two of the 20s and that should be sufficient. Or replace one of the 22s with a 20. My 5F6A clone uses five 20 mfd, 500 volt caps with two in parallel and three singles and that works great so it should work just fine in a BDRI. Save a few bucks. Just like Leo. :lol: :lol: :lol:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Mon Sep 03, 2012 10:28 pm
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OK. Back at it again.

First I compared the actual bias circuit component values in my BDRI to the schematic values in the BDRI and HRD III:

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

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

HRD III C37 = 47 mfd
BDRI C37 = Not present
My amp C37 = 47 mfd

HRD III D10 = 1N4006
BDRI D10 = Not present
My amp D10 = 1N4006

HRD III D15 = 1N4006
BDRI D15 = 1N4006
My amp D15 = 1N4006

HRD III C43 = 47 mfd
BDRI C43 = 100 mfd
My amp C43 = 47 mfd

HRD III R76 = 82 Kohm
BDRI R76 = 1.5 Kohm
My amp = 79.6 Kohm (measured)

HRD III R82 = 25 Kohm
BDRI R82 = 25 Kohm
My amp R82 = 25 Kohm

HD III R77 = 68 Kohm
BDRI R77 = 100 Kohm
My amp R77 = 66 Kohm (measured)

HRD III C38 = 10 mfd
BDRI C38 = 22 mfd
My amp C38 = 10 mfd

So, the bias circuit in MY BDRI is identical to the bias circuit in the Hot Rod Deluxe III. Makes sense. The Blues Deluxe came out first in the mid/late-90s. Then the Hot Rod Deluxe replaced the Blues Deluxe and incorporated some changes (notice I didn't say "improvements" :lol: ). The Blues Deluxe Reissue came out around 2004. I bought my BDRI on the 4th of July, 2010. Obviously some running changes were made to the amp. The series III Hot Rods came out in what, 2011? So the changes made in the BDRI were incorporated in the HRD III. 8)

Now, on to the main power supply.

The main BDRI power supply schematic and main HRD III power supply schematic are identical in both voltage values and component values. My amp matches both schematics.

R74 = 4.67 Kohm (measured)
R75 = 9.79 Kohms (measured)

I have a minor problem in measuring the power supply caps. In order to get an accurate reading I probably need to take them out of the circuit which I really don't want to do right now (its 1:30 am). At DC the four caps are effectively in parallel through the DC resistance of the inductor (162.8 ohm measured), R74, and R75. Disconnecting the choke from the circuit, I got the following readings:

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

I'm confused.

So I turned the amp on and proceded to take power supply readings (all readings done at my current bias point of 40 mA cathode current per tube):

B+ = 411 VDC
Z = 409 VDC
Y = 374.6 VDC
X = 329.3 VDC

Interestingly, all four voltages are almost exactly 20 volts lower than the schematic says they should be.

I'm really confused. Are all three of the 22 mfd caps bad or do I just have to disconnect them to get good measurements. I can't believe that the amp would work and sound as good as it does if the three 22 mfd caps were bad.

Now what?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Mon Sep 03, 2012 11:40 pm
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Sorry for late answer.
In fact, my amp was not working properly and I worked on sunday all day long.
One output valve was not filament supplied properly (very low low low red compare to the other that is "incandescent" and the same with JJ on the same side) and the other was working mainly in class A. I found a bad soldering on the back side of the power PCB, the PCB can be remove easily by removing 4 valves and unscrew all screws of turret on the outside of the steel box. Now I can bias properly 5881 and JJ without added resistor in // to R77 with more max current for the 5881 (valve characteristic dependancy).

The plate voltage is about 420V in both cases with 68mV of biasing (sorry I use the TP, I don't have a valvemeter, I think I will survive with 5% error).

:)
:)
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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 7:38 am
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Tis-san wrote:
The plate voltage is about 420V in both cases with 68mV of biasing (sorry I use the TP, I don't have a valvemeter, I think I will survive with 5% error).


That is still colder than my amp.

420 x .034 = 14.28 watts

14.28/30 = 0.476 = 47.6% max plate dissipation for a 30 watt JJ

14.28/25 = 0.57 = 57% max plate dissipation for a 25 watt 5881

Both values will actually be slightly less since you are measuring cathode current instead of plate current.

My current settings are:

40 mA cathode current at 407 VDC plate voltage.

407 x 0.04 = 16.28 watts

16.28/30 = 0.54 = 54% max plate dissipation for a 30 watt JJ

16.28/25 = 65.12 = 65% max plate dissipation for a 25 watt 5881

Slightly less if using plate current values.

Your BDRI is basically behaving the same as mine. I bet if you set the bias a little hotter you would get similar numbers to mine.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Tue Sep 04, 2012 10:03 am
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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.

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