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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 10:40 pm
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Grab several of those 470-ohm resistors, Bill. They're common to most tube Fenders and you're sure to need more eventually if you do any commission repair work.

And kudos to shimmilou for a theoretical troubleshooting job well done!

Rawk on!

8)

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Thu Aug 23, 2012 10:49 pm
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Just ordered twenty, 470 ohm 5% 2-watt 500 volt metal film resistors from Newark Electronics for $1.66. Priority mail will probably be twice that. :lol:

Going to bed now, its 2:00 am.

Thanks again, shimmi.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Fri Aug 24, 2012 6:06 am
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My total bill for 20 resistors from Newark:

Your Merchandise Total $1.66
Your Freight Total $5.15
Your Tax Total $0.08
Your Order Total $6.89

Guess I should have bought 100 to make the shipping worthwhile. :lol:

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sat Aug 25, 2012 5:50 am
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and an Annual Fender Stratocaster Setup Day :wink:


More like bi-annual!

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Aug 29, 2012 3:37 pm
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Well, the new 470 ohm screen grid resistors arrived today so I promptly installed 2 of them in the amp. Fortunately, they are located underneath near the front of the tube PCB. That, along with the fact that the holes are through plated allowed me to unsolder the old resistors, pull them off, and install the new resistors on the top side of the board without removing the board itself.

Image

I plugged in the old GT 6L6Rs that came with the amp, fired her up and watched the cathode current and plate voltage of BOTH tubes climb to their desired settings. Cheered on by that, I swapped the GTs for my JJ 6L6GCs and fired the amp up. As soon as I threw the standby switch there was a loud crackling noise from the speaker and bright, visible arcing inside V5. I promptly switched the amp off and checked things out. Everything looked OK, so I pulled the JJs and checked the open circuit voltages. They were good. I put the JJs back in and same thing occured. I then put the GTs back in and everything is fine.

So, it appears that something failed in V5 (the screen grid maybe) that took out the 470 ohm screen grid resistor. I replaced the old 1 watt 470s with 2 watt resistors and both seem fine. The amp has been sitting turned on for about 20 minutes now. Just finished playing through it and it sounds great. I have a pair of new JJ 6L6GCs on order from Eurotubes.

BTW, the ugly looking solder joint on V5 is where I accidently shorted the plate voltage pin to the pin behind it when I was trying to measure plate voltage some time ago, pre-bias probe. Shows what can happen when you aren't careful. :shock:

Oh, I decided not to mod the PI and output stage to match the 59 Bassman as I found that those parts are all the way to the rear on the underside of the tube PCB and cannot be accessed without removing the board, something I am loathe to do. :cry:

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Last edited by bluesky636 on Sat Nov 03, 2012 5:05 pm, edited 1 time in total.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Aug 29, 2012 3:59 pm
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Does this fall into the "no complaints about JJ" category?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Aug 29, 2012 4:21 pm
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shimmilou wrote:
Does this fall into the "no complaints about JJ" category?


I guess no one has ever had a tube fail under mysterious circumstances?

And for the record, I have had one Groove Tubes 12AX7, one cheap Chinese 12AX7, and one NOS JAN Phillips 5751 fail on me.

Whether the tube failed and took out the resistor or vice versa, I have no idea. In any case, the amp is back up and running and I am happy.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Aug 29, 2012 5:02 pm
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Not mysterious at all, just probable.

The tube failed and took out the resistor. Impossible to be vice-versa.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Wed Aug 29, 2012 5:17 pm
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shimmilou wrote:
Not mysterious at all, just probable.

The tube failed and took out the resistor. Impossible to be vice-versa.


Ok. So what? I still like JJs and you still prefer Groove Tubes. I'll still recommend them to anyone that asks. Like I said, the only thing that matters is that the amp is fixed.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sat Sep 01, 2012 8:14 pm
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The new JJ 6L6GCs arrived today. I popped them in, biased them, and teh amp sounds great. Noticeably louder than before. Puzzles me how the amp managed to work and still sound decent with one bad tube.

The only other odd thing is that I have never been able to bias the amp above 53% max plate dissipation. It is currently set at 40 mA cathode current (about 38 mA plate current) at a plate voltage of 407 VDC. Right at 52%. Any thoughts about why this may be the case? The bias supply does not seem all that high. Its -52 VDC at the output of the diode and about -45 VDC with the bias pot set to max. I've though about modifying the bas circuit, but I'm not sure how. My 5F6A clone runs right at 70%.

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

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sat Sep 01, 2012 9:31 pm
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I have tried several types of output tubes in my HRDlx and some tubes, such as the stock tubes, will only draw a max of about 35 mA Cathode current for each tube, while others would draw much more. Your thought about modifying the bias circuit is probably the best option.

One thing that I tried was to parallel a 330k ohm resistor with R77 (R77 is 100K), which made R77 effectively approx 75K. You could just change R77 to 75k, but paralleling another resistor was easier. R77 is the resistor in series with the bias adjust pot (same on the BDRI I believe), and this allows the negative bias voltage to go lower, giving more adjustment range of current for various output tubes. It works great, and now the same tubes that had the lower current draw can go much higher, and I can still get the higher draw tubes low enough to be in a good range.

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Forgot to show you, I did the same as you and mounted my SGRs on the other side of the circuit board as it was easier (bottom right in picture). Changed mine to 2K though.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sat Sep 01, 2012 10:08 pm
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Actually, I have considered replacing R77 with either a 56K or 82K ohm resistor (that's what I have on hand right now). However, it is not really plate current I am concerned about, but the plate voltage. Right now I am sitting at 40 mA cathode (about 38 mA plate) and 407 VDC plate voltage (line input is 122 VAC). I can get almost to 45 mA cathode current, but plate voltage drops to about 400 VDC. The BDRI and your HRDlx have the same power supply and transformer and the same output stage and output transformer, so I am puzzled as to why my plate voltage drops so much. Don't get me wrong. The amp sounds great where it is currently biased. It is more of an academic interest as to why I can't bias the output tubes any higher than 52%.

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sat Sep 01, 2012 10:25 pm
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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. :?

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 12:53 am
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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.

:)

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Post subject: Re: Strange Problem With My Fender BDRI
Posted: Sun Sep 02, 2012 7:05 am
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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. :?

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