Fender Blues Jr. BIAS

[erfmedeiros]

Hey guys,

I bought this used 2008 Blues Jr. and swapped all tubes with brand new JJ's.

I'm measuring -11.1 V between the top of R31 and ground. Is that too hot for this amp? I can't find that information from a reliable neutral source. I mean, there's a lot of personal opinion out there, but not a lot of real info.

Thanks!

-E

 

[clintj]

-10.7V is the factory bias supply voltage spec, measured at Test Point 30 (junction of R21 and R22). More negative will make the amp idle cooler, all other things being equal. Ideally, Fender picked a number that will safely handle the vast majority of EL84s without issue. So, one option is to check for red plating, listen to the amp to see how it sounds, and just play it.

Just FYI, knowing the bias supply voltage doesn't tell you what the tubes are actually doing. It just tells you that the bias supply is healthy. If you want to find out for sure, you'd need either a bias probe or you'd need to measure a few things and do some math. That involves probing high voltage and taking the appropriate safety precautions while doing it.

 

[clintj]

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

This page has the relevant safety warnings and directions for checking bias, and is my preferred method. However, once you have that info there's not much you can do with it. The Blues Jr is a fixed, non-adjustable bias amp (like Mesa). This means that the bias supply was set at the factory, and changing it requires removing and replacing a resistor on the PCB to adjust it (or modifying the amp to make it adjustable).

 

[Wally]

I just retubed a Blues Jr. with a set of tubes that the vendor sold as being chosen for the Blues Jr. I suppose they chose the tube based on how Fender biased the amp. Those new tubes were dissipating 100/102% of max plate dissipation with a bias voltage of -10.56vdc. That is running to the hot side of things in fixed bias. If I had one of these amps and wanted to keep it, I would buy tubes that had cooler operational parameters than what that vendor provided. Ymmv....

 

[erfmedeiros]

-10.7V is the factory bias supply voltage spec, measured at Test Point 30 (junction of R21 and R22). More negative will make the amp idle cooler, all other things being equal. Ideally, Fender picked a number that will safely handle the vast majority of EL84s without issue. So, one option is to check for red plating, listen to the amp to see how it sounds, and just play it.

Just FYI, knowing the bias supply voltage doesn't tell you what the tubes are actually doing. It just tells you that the bias supply is healthy. If you want to find out for sure, you'd need either a bias probe or you'd need to measure a few things and do some math. That involves probing high voltage and taking the appropriate safety precautions while doing it.

Hey friend, yeah, just adding to that, the test points are in R21 and R22 on the old green board Blues Jrs. On the cream board (from 2001 on, I believe), they changed to R31 and R32.

My intention actually IS to check the bias circuit. I bought the amp with crappy chinese power tubes and V5 started red plating. First thing I did was swap V5 with V4 and the red plating followed the tube. I tested a few suspicious points on the circuit and everything tested OK. So I was pretty confident that the problem was only a falty tube.

Indeed, stock bias voltage for this model is -10.8V but this corresponds to 40% dissipation (or something like that, never did the math), way too hotter and the amp is generally considered a power tube baker. I was shooting for something cooler than that, but was hoping to avoid having to insert a resistor to increase the voltage drop.

Yeah, my point is exactly this, I'm trying to make sure the bias is in the ballpark or on the cooler side, without having to probe the main filter cap.

 

[erfmedeiros]

I just retubed a Blues Jr. with a set of tubes that the vendor sold as being chosen for the Blues Jr. I suppose they chose the tube based on how Fender biased the amp. Those new tubes were dissipating 100/102% of max plate dissipation with a bias voltage of -10.56vdc. That is running to the hot side of things in fixed bias. If I had one of these amps and wanted to keep it, I would buy tubes that had cooler operational parameters than what that vendor provided. Ymmv....

So, I guess JJ's are indeed interesting for this application, if one doesn't intend to mod the amp.

 

[clintj]

Gotcha. I should have asked what rev you were on.

If it followed the tube, it's usually a bad tube. Sounds like you have good bias voltage. The other places to check are the screen grid resistors for exidence of burning and resistance value, and a quick run through of the power supply dropping resistors. Sounds like you caught it before it shorted out the tube, though.

 

[clintj]

I just retubed a Blues Jr. with a set of tubes that the vendor sold as being chosen for the Blues Jr. I suppose they chose the tube based on how Fender biased the amp. Those new tubes were dissipating 100/102% of max plate dissipation with a bias voltage of -10.56vdc. That is running to the hot side of things in fixed bias. If I had one of these amps and wanted to keep it, I would buy tubes that had cooler operational parameters than what that vendor provided. Ymmv....

That's really hot for fixed bias. Wow.

 

[erfmedeiros]

Gotcha. I should have asked what rev you were on.

If it followed the tube, it's usually a bad tube. Sounds like you have good bias voltage. The other places to check are the screen grid resistors for exidence of burning and resistance value, and a quick run through of the power supply dropping resistors. Sounds like you caught it before it shorted out the tube, though.

Amazing news! LOL

Thanks a lot for your help, and indeed I will run a few more tests as prescribed, just to be safe.

 

[PCollen]

Hey friend, yeah, just adding to that, the test points are in R21 and R22 on the old green board Blues Jrs. On the cream board (from 2001 on, I believe), they changed to R31 and R32.

My intention actually IS to check the bias circuit. I bought the amp with crappy chinese power tubes and V5 started red plating. First thing I did was swap V5 with V4 and the red plating followed the tube. I tested a few suspicious points on the circuit and everything tested OK. So I was pretty confident that the problem was only a falty tube.

Indeed, stock bias voltage for this model is -10.8V but this corresponds to 40% dissipation (or something like that, never did the math), way too hotter and the amp is generally considered a power tube baker. I was shooting for something cooler than that, but was hoping to avoid having to insert a resistor to increase the voltage drop.

Yeah, my point is exactly this, I'm trying to make sure the bias is in the ballpark or on the cooler side, without having to probe the main filter cap.

65%-70% dissipation at quiescence is about the norm for most Class AB grid biased power amp tubes. 40% is very cold.

 

[Wally]

ClintJ, that is what I thought. But...as I understand it, Blues Jr amps run hot from the factory.
BillM(RIP) talked about it and modded the amp to allow adjustment. I told the young fellow that it was running hot....but not redplating...even in the dark. I told the young fellow to play the amp in the dark for a while and note the condition of the plates. I have not heard back from him so I suppose he has seen no redplating, either. Side note: I have never measured a redplating tube, so I don't know what it takes in fixed bias to get that situation to manifest itself. But...I have noted more than one fixed biased amp running 100% and everything working along....very lively sonics at that plate dissipation. I suppose I should sit and run a set of tubes in fixed bias right up to thepint of redplating then back it off a little bitty bit and measure the performance just when the redplating ceases to happen.
I suggested to the owner that he buy ''cooler'' tubes next time...to keep the boxes with the specs so he would know...or a vendor would know....what to put in the amp next time. IIRC, the tubes were Apex matched at a current of 57p and a transconductance of something over 11K.

PCollen, I agree that many ascribe to that 70% 'limit', but others push amps beyond that on purpose. I am not in that group because I don't hear the benefits outweighing the wear and tear on the tubes. That is, the sonics don't improve markedly above 70% for me.
IMho, 40%..while being cool...is not cold. Very cold, imho, is way on down there....10-20%??? I prefer something above 40%, but again others choose what they like. Harsh distortion would be occurring at a plate dissipation quite a bit lower than 40%. I have observed a Mesa Boogie MK amp with Boogie tubes running at 32% of MPD....as designed with tubes chosen by Boogie???

 

[erfmedeiros]

Hello again citizens...

BillM recommends changing the screen resistors, from 100 ohms to 1K.

R35 and R36 look a little distressed and seem to have been replaced in a very half-assed manner by the previous owner.

But I can confirm they are both 100 ohms still, just cannot confirm THEY are the screen resistors for sure. Anyone could verify that? And do you agree with the 1 Kohms? Some people recommend upgrading the power rating to 3 W, and some recommend keeping 1/2 W to let the resistor act as a fuse in the case of screen grid short. What do you guys think of that?

Also plan to put the new resistors a little higher above the board, seems like they can get a little too hot.

 

[clintj]

1k 3W is what I prefer; the screen is the Achilles heel of those tubes. Check continuity from the resistor to each power tube's pin 9 to confirm those are the screen grid resistors; the second page in the Fender schematic says they are.

Just my 2 cents, but I wouldn't want something intentionally acting as a fuse on a PCB. When those go, they can go in a blaze of glory and cleaning/repairing a burnt PCB is not my idea of a good time. You can already see what happens with just a little too much heat on a board. If the sockets weren't board mounted (grr!) I'd move them to the sockets and attach them there.

 

[erfmedeiros]

1k 3W is what I prefer; the screen is the Achilles heel of those tubes. Check continuity from the resistor to each power tube's pin 9 to confirm those are the screen grid resistors; the second page in the Fender schematic says they are.

Just my 2 cents, but I wouldn't want something intentionally acting as a fuse on a PCB. When those go, they can go in a blaze of glory and cleaning/repairing a burnt PCB is not my idea of a good time. You can already see what happens with just a little too much heat on a board. If the sockets weren't board mounted (grr!) I'd move them to the sockets and attach them there.

LOL! Yeah, I'm avoiding having to remove this board like the friggin plague. Gotta love these flat cables...

 

[dougstrum]

LOL! Yeah, I'm avoiding having to remove this board like the friggin plague. Gotta love these flat cables...

Yeah~I've had mine apart more than a few times~pita.
Mine is a '93 so maybe different than newer ones. First mod I did
was to bias; changed R29 from 15k to 20k. Didn't notice a sonic difference but it runs the power tubes a bit cooler.

 

[Wally]

Yeah~I've had mine apart more than a few times~pita.
Mine is a '93 so maybe different than newer ones. First mod I did
was to bias; changed R29 from 15k to 20k. Didn't notice a sonic difference but it runs the power tubes a bit cooler.

In the provided schematic, R29 is the 100K power supply resistor for V4A, which is the reverb return triode. R28 is the 1.5k cathode bypass resistor for that same triode. Neither of those resistors would have any effect on how the power tubes were biased, ime. Is your circuit different from the schematiclinked above...which is revision A and the only schematic I find for the Blues JR.?

 

[dougstrum]

DUH!---sorry I was thinking Pro Jr...

 

[Wally]

Doug, you made your mistake and I made mine. I had a VVRI on my m ind and typed that instead of Blues Jr.

 

[MMARSH]

The Fromel kit comes with a replacement resistor that brings the bias at idle down to about (an advertised) 8.5w and it keeps the tubes much cooler. I did many other mods to the board that I can't say what effect re-biasing had on tone, though.

 

[Wally]

The Fromel kit comes with a replacement resistor that brings the bias at idle down to about (an advertised) 8.5w and it keeps the tubes much cooler. I did many other mods to the board that I can't say what effect re-biasing had on tone, though.

8.5 watts of plate dissipation with what tube??? That resistor cooled the biased down, undoubtedly; but one would have to have measurements of a particular set of tubes to know what plate dissipation those tubes were working at. One could have some 'cold' tubes that would not draw that much current, and one could have 'hot' tubes that might draw more current than the 8.5watts of plate dissipation....or some tubes that were in the middle of those two extremes. Not liking to mess with these inexpensively PCBs, I still say that I would order a cold set of tubes and see what they did in the circuit before I would bother with pulling those boards. others' mileage obviously varies..... No matter what I did, I would have to know what the plate voltage and current draw were so I could know where the plate dissipation was.