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FDP Forum / Amp Mods, Repairs, and Projects / Help! Problem with Fender Blues Jr.

Diverted

USA

Ted
Oct 28th, 2016 09:55 AM   Edit   Profile   Print Topic   Search Topic

Hi,

I am having an issue with my Fender Blues Junior and don't know what to do.
The issue is that one of the EL84 output tubes is showing clear heat damage (not both, only one). The amp has about 325v on B+.
Here are the weird things: When I start the amp up from cold, the voltage on the plates come up normally on both tubes. However, after a few minutes my multimeter starts going haywire when I try to measure voltage on the plate of hte second tube. THis problem does not happen on the plate of the first tube. What is hte issue here? Do I have some kind of poor solder joint internally that goes haywire once the amp heats up? Or is something else going on? I can give you any voltages or other info you need; I just could really use some help addressing this issue! Thank you.

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 28th, 2016 10:11 AM   Edit   Profile   Print Topic   Search Topic

Make sure the problem isn't one with the meter...I know this sounds redundant, but you'd be surprised how often that happens. I would be interested to know what you read at the control grids on both tubes (pin 2). Paint burning is not unusual on the EL-84 as they tend to run really, really hot. That said, if you indeed have a tube running hot it usually due to a bias issue. A bad tube can also cause that. We used to see a huge percentage of bad imported 6Bq5/EL-84 tubes exhibiting all sorts of bizarre behavior.

You could indeed have a dodgy solder joint on one of the tube element socket pins. Keep probing and thinking like a tube (that's what I used to tell my technicians).

w

Diverted

USA

Ted
Oct 28th, 2016 10:40 AM   Edit   Profile   Print Topic   Search Topic

Thanks Willie, I was thinking it might be the meter but the problem is 100 percent repeatable (it happens every time) and only on the second output tube.
I'm getting -10VDC on each control grid.

And again just for clarity. Looking at the pcb that the tube sockets are soldered to. The problem is on V4, not V5. (both EL84 tubes).

Thank you so much!

Ted

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 28th, 2016 12:01 PM   Edit   Profile   Print Topic   Search Topic

Okay...

I must presume (not assume) you swapped EL-84s from one socket to the other to see if the issue stayed with V4..right? (since I can't have the amp in front of me, and amp diagnosis is a very methodical process, you have to ask the obvious and never assume anything).

I would expect your screen voltage (pin 9) to be just a bit over 300 volts. If it is I can only conclude the issue is a tube issue unless we can find some reason the for the plate voltage to be modulating...or the tube is oscillating which would be due to some type of positive feedback condition, or a faulty tube.

w

Diverted

USA

Ted
Oct 28th, 2016 12:19 PM   Edit   Profile   Print Topic   Search Topic

You are right about the screen voltage. I checked it earlier (back at work now) and it was hovering right at about 302VDC.

As for the tubes, yep I swapped them out and it happens in just V4 regardless of the tube. I also swapped out the tubes for a fresh (but old stock) set of RCAs. Same thing.

Thank you!

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Oct 28th, 2016 12:49 PM   Edit   Profile   Print Topic   Search Topic

Okay...for now we will presume nothing amiss from outside the immediate circuit of V4. Since you are measuring proper voltages on all the tube pins of V4 and the tube is a known good one, we might want to suspect the tube socket itself. Have you tried cleaning the tube socket tangs with a non lubricating electronic contact cleaner...preferable anhydrous isopropyl alcohol (that's not household type drug store alcohol). The idea being to remove any debris or carbon traces. Then you need to ensure that the tangs are tight on the tube pins by using a small jeweler's slot head screwdriver or other small tool to bend the tangs a bit.

If all that doesn't effect a fix...then not sure where to go from there. The original complaint being that V4 looks like it is overheating and you get spurious DC plate voltage readings on pin 7 of V4. How does the amp sound? Have you taken an AC voltage reading on pin 7 of V4 vs V5?

w

Diverted

USA

Ted
Oct 28th, 2016 12:55 PM   Edit   Profile   Print Topic   Search Topic

Thanks Will.

The tube sockets are super tight, and they're making good contact (I think?)
My only thought is that maybe it's a cold solder joint that somehow goes bad when the tubes heat up?
I'm going to pull the circuit board tonight, desolder the socket and re-solder. I'll let you know how it goes. ... crossing my fingers. Thank you for your assistance, I'll get back to the board soon as I can.

As for the DC reading on each tube: Yes I measured the plate voltage on each. It's about the same on each UNTIL the amp's been on a minute or so. As the plate voltage nears its steady level (about 325 measured on V5, which doesn't have this problem) it starts to get wonky on V4. That is, once voltage hits about 300 on V4 the craziness starts appearing and I get a squeal (very quiet) when I touch the plate lead (pin 7).
I've been running the amp on a variac and the problem does not happen at all when I run the amp at about 100VAC. That is, the plate voltage on each output tube tops out at just under 300VDC steady. Above that, and V4 starts going south.

Ted

(This message was last edited by Diverted at 02:59 PM, Oct 28th, 2016)

Diverted

USA

Ted
Oct 28th, 2016 01:03 PM   Edit   Profile   Print Topic   Search Topic

Just an addendum:

Actually, the amp works OK despite the wonkiness on pin 7. It sounds fine, if a bit tinny, but I don't really know the amp that well so I'm not sure if that's just its natural sound, or an issue.
I'm mainly worried about it because with a wonky, all over the map voltage on one of the plates, it can't be good for the tubes as is evidenced by the charred paper around the v4 tube. Thank you!

E.Carlson

Rome Wisconsin

Too many hobbies
Nov 1st, 2016 08:10 PM   Edit   Profile   Print Topic   Search Topic

It's been a couple of years since I've posted here. Life is busy. I had a Blues Junior with a very similar problem. One of the power tubes was running away. As I recall the phase inverter was oscillating. I remember bypassing a plate resistor with a ceramic cap. I defer to willie and his wisdom. I wouldn't want to send you down a rabbit hole.

pdf64

UK

Nov 2nd, 2016 02:25 AM   Edit   Profile   Print Topic   Search Topic

Just in itself, hanging a meter off a power tube plate can bring on oscillation, as the probe/leads can act to increase coupling back to earlier parts of the circuit.
Try again with the phase splitter V3 removed; that will act to break most positive feedback loops.

Perhaps the tube has got hot due to it having a higher dissipation than the other?
Have you checked the plate or cathode currents, using a bias probe, or volt drop across the known resistance of the OT primaries (see link for method)?
-10V bias with plate and g2 >300V seems ridiculously hot!

how to bias

(This message was last edited by pdf64 at 11:00 AM, Nov 2nd, 2016)

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Nov 2nd, 2016 10:32 AM   Edit   Profile   Print Topic   Search Topic

These amps run hot by design...note the spec voltages are 329 VDC B+, 307 VDC Screens and 10.7 VDC bias voltage. It would be good to know the individual current draws for each tube as that might tell us the tale of why one tube heats more than the other. Amazing how many "matched" sets of tubes are anything but...

Reducing the current draw on the output tubes should cure the overheating but might well reduce the "tone" many/most expect out of these little amps.

w

w

FDP Forum / Amp Mods, Repairs, and Projects / Help! Problem with Fender Blues Jr.




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