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Post subject: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after mods
Posted: Mon May 28, 2018 1:45 pm
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So, 30yrs as in the closet musician, new to form, not new to tube amps or fender gear. Technical background.

Picked up BJ-III, looked into BillM mods, but he passed away couple of years ago and his son isn't really doing the business (at all!) Got Fromel mods, https://www.fromelelectronics.com/products/bjr-mods, Supreme for MIM. Standby switch went fine. Then installed/replaced tone stack's C5 (250pf), C6 (.1uF) & C7 (.015uF) (not sure why C5 was included). Fromel's kit does not include signal coupling caps (will do later). Cooler bias resistor R52 (33k). "Twin Stack Mod" bridge/short Mid pot's pin 1 & 2 w/bare wire short. PwrSupply main filter electrolytic cap (100uF/450v - doubled in value), did not do the rest of the electrolytic filter caps, they looked fine and kit. used the same values.

Turned it on to standby, all good still, went for full on and seconds later R47 was smoking. Oh, wait, I transposed the R# and put the 33k R52 resistor into R25 - Doh! Ah, fixed that and all the original resistor values measured the same so the slight warming of R47 did not change it's value, nor did it really get very hot and burn the PCB. Nevertheless, after straightening all that out, turning the amp back on, smoke still began streaming from R47.

Now on BillM great web site, a note in a Q&A follow on to Bad Electrolytic Caps page, he points out "The R47 burn is usually caused by a failed screen grid on V4 or V5, so need new output tubes. The 100 ohm screen resistors (R35/36) are supposed to go first.... but real world they don't and shorted screen takes our R47."

So, being Memorial Day I can't find a store open that has a set of new EL84s.


If all resistors are back in place and still correct values, even the slightly burnt R47, all I need is new pwr tubes?
Thanks for focused responses.


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Tue Jun 05, 2018 9:49 pm
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Well, first thing is to pull the power tubes and see if the problem went away.
First question is if the power tubes are bad, why didn't it smoke the 2.2k resistor before?
Always, you have to swallow your pride and look over your work first. If that checks out, then the standard remedies are applied one by one.
Pull one power tube at a time and see if one smokes and the other doesn't. If either smokes the resistor, check bias voltage. Or measure the bias current directly at the red wire of the output transformer. Two tubes should idle at 35 to 54 ma there.
A lll model can't be old enough to have bad caps. Power tubes are most likely to be the cause


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 06, 2018 1:09 am
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"Swallow your pride and look over your work first."

Definitely, what I'd do first. Esp if amp was fine before the mods. Check the polarity of those electrolytic cap(s) you redid.

Have a question. Is R47 in your amp the same as the one in the schematic I found on the Net? Never mind the blue highlighted portions. It's from a site with other mods.

Image

Because this R47 has nothing to do with the output section of your amp.

Here's the Blues Jr (not sure about III) schemo phase inverter and part of output section.

Image

R20 & 23 are the grid stopper resistors. R24 & 25 are the screen resistors. R 21 & 22 are the signal grid-to-ground resistors. All part of the EL84 output section.

My bet, is your "R47" is part of the PSU. Seen as R32, in the following schemo. An indication that there is a short in the PSU. Polarity issues with that 100mfd/450VDC cap. Or a short, due to soldering/wiring issues.

Image

Good luck! Keep us informed. :)


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 06, 2018 5:05 am
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SawBuck wrote:
If all resistors are back in place and still correct values, even the slightly burnt R47, all I need is new pwr tubes?
Thanks for focused responses.


If the smoke has been let out, even a little bit, I'd replace that resistor.


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 06, 2018 6:03 am
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Ya, smokes with both tubes out.

R47 is a 2.2K 2W resistor - schematic is Rev D for a BJ III. There are now four versions of BJ's.

https://www.thetubestore.com/lib/thetub ... -Rev-D.pdf

Just got new power tubes, have various preamp tubes laying around and new resistors just came in the mail, per BillM notes I upped R47 to a 3w.

Bought this, and did this super simple mod before I realized how fragile these BJ's were. Wish I still had my SFBMR.

Fortunately I still have my BFPR so I'm not out an amp.


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 06, 2018 6:27 am
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Forgot to say, does not smoke with all tubes out but I'm not sure what that tells me since not really working w/all tubes out.


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 06, 2018 6:50 am
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That is the first resistor in your PSU power supply rail. You have a short somewhere. You need to find it. Use a DVM set to resistance-DC. Unplug amp. Drain main caps.

I do NOT agree that increasing the wattage of that resistor to 3-watts is the answer. The amp should not be "smoking" the 2-watt OEM resistor.

Now measure the resistance from B+ side of PSU power rail to chassis (ground). You may get the meter charging, as the caps charge up. But, you should not get near zero resistance. If you get near zero resistance, find the short-to-ground, before trying to turn the amp on.

I have a feeling that 100mfd/450VDC electrolytic cap is miss-wired or shorted (or defective) and you are getting a short to ground, at that point. If so, pull one lead of the cap and try measuring B+ to ground static resistance, again. If it goes to high resistance, you have found the culprit. Need a new cap.

Once you fix the short, get a bunch of proper fuses, prior to firing up, again. Pull ALL tubes. Do you have access to a Variac? It would be better to measure the B+, as you ramp-up the outlet voltage.

The voltages on the rail should be (+/-) 20% VDC of values in the boxes, along the PSU rail. If not, you may still have an issue with the circuit.

Tell us how it goes! Good luck! :)


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 06, 2018 7:32 am
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OK, if it doesn't smoke with the preamp tubes out, that's significant.
Pull all the tubes. Now put in one tube into v3 and turn it on. Next, do v2, one tube only. Then v1, one tube only. My bet is on v2. That's where you did the tone stack work. If you shorted a cap or did the mod wrong, it will pull down the plate voltage of v2 and overheat the resistor.
Rather than waiting for smoke to appear, a more technical approach is to clip a dc voltmeter onto each leg of the resistor. With no tubes installed, it will read just a few volts. When the power tubes are inserted, the voltage drop across the resistor will go up. Each preamp tube will add a little more. The one causing the smoking will register a large increase in the meter reading as the voltage drop across the resistor goes up.


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Thu Jun 07, 2018 9:51 pm
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Yes, & thank you TimsAudio,

Measuring B+ to gnd (across the main filter 100uF electrolytic cap) upon amp on, starts out around 2K-4K and quickly climbs to whatever you want to wait for with the volt meter charging the cap. I am assuming that is enough away from near zero to imply the electrolytic is not incorrect or otherwise screwed up.

V3 alone nothing, V2 alone we are smokin' R47 (2.2k 2W), (where the tone stack mods are). Never bothered trying v1.

Unfortunately visual examination everything looks solid. Will dig deeper and re-solder things this weekend. I have so little time, I thought this would be simpler than taking a sliverface bandmaster reverb to blackface specs/tone and changing the OT to David Allen 3 tap.

Note to self - never buy anything but point-to-point tube amps.

More horrid details:
Now I did, for a very brief moment mind you, when attempting to change the bias resistor R52 from stock 22k to 33k (I know some use 27K but Fromel's kit had 33k - so it doesn't run the pwr tube so hot) manage to transpose the R52 into R25 (a stock 7.5k) and thus I placed a 33k there instead and R52 is still 22k - Opps! This is where/when the smoke started.

Argh! Thinking this was going to be so simple, I was in a hurry and maybe a little dislikeablesic.

After the original smoke event and amp off, amazingly these resistors still showed there correct values (while still in circuit) even R47 (albeit some what smoked now - plan to change R47 once I get a better understanding of what whet wrong.)

Fromel's kit changes from stock:
C5 250pf silver mica cap
C6 .1uF WIMA box cap
C7 .015uF WIMA box cap
R52 (bias) to 33k
C25 100uF/450v elcetrolytic
R14 (mid pot) shorts pins 2 & 3 on pot (on trace/solder side, short wire applied to "pot pins/hole closest to master vol and center hole")
Funny - claims Bj III has the "Sparkle Mod", and says C9 won't be there, however my version MIM in 2009 has cap in C9, says "122J", perhaps someone already installed a .0012uF cap to return sparkle mod back to BJ II version.....?????)

Did not do the Input jack mod.

Thanks for your feedback


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Fri Jun 08, 2018 6:58 am
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Good on ya, got it fixed.
The sparkle mod at c9 is very much according to taste. A 1200pf will cut a lot of treble. No cap is too bright to my ears.
I use 680 to 820 pf for my mods.
I still don't get the mid mod. I have a different approach.
I'll be posting my Blues jr mods soon. They actually make it sound better,, not just different.


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Fri Jun 08, 2018 12:42 pm
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TimsAudio wrote:
Good on ya, got it fixed.
The sparkle mod at c9 is very much according to taste. A 1200pf will cut a lot of treble. No cap is too bright to my ears.
I use 680 to 820 pf for my mods.
I still don't get the mid mod. I have a different approach.
I'll be posting my Blues jr mods soon. They actually make it sound better,, not just different.


I don't get the mid mod either. Just crank the pot all the way down (or up).

I think he's saying it's still not fixed.

Pictures would probably help.


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Fri Jun 08, 2018 1:42 pm
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Measure the voltages around V2 and V3. Anode and cathode. Something is drawing too much current. Like said before, those burnt resistors may measure okay, cold... but go all over the place when the circuit is hot.

Photos would definitely help here! Good luck, finding the culprit(s).

:)


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Mon Jun 18, 2018 8:32 pm
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[quote="SawBuck”] ....The 100 ohm screen resistors (R35/36) are supposed to go first.... but real world they don't and shorted screen takes our R47....[/quote]

Common misconception. The SGRs are not now, nor were they ever, designed for any kind of protection. Fuses, and/or “fusible” resistors, are for protection.

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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Mon Jun 18, 2018 10:36 pm
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shimmilou wrote:
Common misconception. The SGRs are not now, nor were they ever, designed for any kind of protection. Fuses, and/or “fusible” resistors, are for protection.


Most correct! :D
As we said many times before, you really don't want screen voltage to go to zero (what it would do if SCR popped). This voltage is to help stabilize direction of electron flow, within the tube. Lowering parasitic oscillations. Esp under extreme overload conditions, like clipping.

I've actually used 5-watt, wire-wound resistors are SCR, to help with their stability under these extreme conditions.

https://en.wikipedia.org/wiki/Tetrode#Screen_grid_valve

Length of both screen resistors and grid stoppers are critical. That is why they are soldered directly onto the socket (from an unused pin to proper signal or screen grid pin).

Image


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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 27, 2018 8:25 am
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While Leo knew about SGR failure and could have factory installed 5 watters, he didn't. There were no fusible resistors at the time and Leo seems to have stayed away from extra fuses. I think he wanted to see a smoking amp still playing to the end of the set rather than wimp out with a fuse. The 1 watt value does just that. Its not really protection, just toughness.
The SGR only pops when a power tube shorts out. Normally, they run cool and don't fail unless the tube takes them down So a 1watt will blow and disconnect the screen supply from the shorted tube. This relieves the short and allows the other power tubes to operate normally.
A burnt resistor also indicates which tube had a problem. A shorted tube can be difficult to spot unless you have a good tube tester.
A 5watt resistor won't blow, so the shorted tube continues to overload the grid supply and draws the B+ down to bad sound in the preamp stages.

The original design shows Leo's advanced engineering skills. Any graduate can design a unit to work. Designing a unit to fail in a controlled way takes a deeper understanding and much experience.
Failure mode design is critical to any successful product. This is the reason switches and relays still exist. Sensors and mosfets are much cheaper to design with. The difference is that relays and switches generally fail in an open circuit state. Solid state devices fail in a shorted state. Usually, in a critical application, open is better than shorted. Sometimes a thermal fuse is attached to a solid state device to provide for an open failure after it overheats.


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