1961 B-15N, new problem with my power tubes and sparking...

F

Frederick34

May 12, 2017
96
65
4,586
Texas
#1
So this issue is totally unrelated to my B-18N problem with seemingly unbalanced power tubes, this is a new issue I found today on my early B-15N. I'd better give the full story, in case there are important clues:

Got this B-15N a few months ago and it functioned just fine, to a point... not perfect, but well enough. Some things I had noticed were that there was not much headroom on channel one (bass was breaking up at about 12 o'clock, halfway), a bit more headroom on channel two (bass would break up about 3 o'clock, seemed about right). 6L6GC were a visually matching set of Philips/ECG, not original to the amp, but not new either. Previously, a guitar player had been using this amp (that may be important). I opened it up and the big caps had been changed in 1995 (same with the speaker, and probably the PT). The big caps weren't quite correct (32uf/500v, in both spots where they should've been 20uf and 40uf multis), but apparently close enough. I ordered all the correct caps from Bruce at Fliptops. This amp still has the 2-prong plug and death-cap, so that'll be changed soon, but I'm being quite cautious meanwhile.

The correct caps have now been installed, including the 50uf/50v cap (between the power tubes) that appeared to have leaked it's black oil all over my original schematic, at some point in its history (it was dried up). The end had popped off that cap... why they didn't replace that, I don't know. Some other caps around it had been replaced with orange drops (about 4), but that cap that runs between the power tubes (the 50uf/50v) was shot. However, the amp was functioning even with the dried up leaky cap. I'll assume that's because they put in a bunch of new caps, maybe that was the weak link and it opened up afterward?

So that brings us to today. I noticed that the rectifier tube was actually not correct for this amp, the chassis (and schematic) is marked for a 5U4 but there was a 5R4GYA installed in its place. To me, that probably explains the headroom issues (which I still had, even with the new caps installed). If I understand correctly, the 5R4 uses less filament (2a) than the 5U4 (3a) AND the B+ will run a bit lower (about 25-35v, I think). Two possibilities, in my mind, the guitarist wanted a bit earlier breakup so a tech told him the 5R4 might give him what he wanted OR when they installed the new PT, a tech decided it might be less taxing on the new PT to use the 5R4 (or the other possibility is that the old rectifier just went bad and that's all they had in a pinch, but I doubt that).

I happened to have a good old GE 5U4G recitifier, so figuring I have my new filter caps in there, I'm probably safe to return this thing to stock. I swap the 5R4 for the correct 5U4 and flip on power. Seemed fine (although I might have heard a little pop, could have just as easily been my imagination), so then I flipped off standby and still good, tubes began to get the usual light blue haze, played a few notes on my bass and then I quickly (within 5 seconds) get a bit of sparking inside the 6L6GC nearest the rectifier, right up at the top near the double ring getter, lightning show. I heard the popping also, through the speaker. So I quickly turned the amp off. The 6L6's seemed to heat up awfully fast, but that too could be my imagination... lots more 'cooling off' sounds than usual, after I shut it off.

My hope is, that one of you guys will read this and know precisely what's up, or maybe had this experience before? My theories (which are admittedly not worth much) are:

1) The power tubes are old and marginal. They were fine with the 5R4 rectifier, but the 5U4 revealed the fact that my power tubes are on their last legs. (that's the easy fix, I have a very nice NOS set of Sylvania 6L6GC, but don't want to risk them til I fix this).

2) Someone changed something in there that means I can't really run a 5U4 in there anymore? This old B-15 is cathode bias, so I don't think it's a bias issue. All the caps in there now are either the original little guys, or they are correct value big ones... and all the resistors appear original. The replacement PT appears correct, just has the heavier wrinkle finish on it than the originals.

3) Maybe someone put some super funky PT inside an Ampeg PT can and potted it? This is highly unlikely, in my opinion. But I do suspect a tech was inside this thing in 1995, replaced some caps, PT, new speaker and possibly put those 6L6GC and the 5R4 in there at that time (btw, the preamp and PI tubes appear original and test good).

As a note, I have checked for continuity between pin 2 & 3 on the 6L6GCs (no short there); I've also cleaned my power tube receptacles with Deoxit.
 
Last edited:
#2
Swap the two power tubes and see if the issue moves with the tube. If yes, try the new power tubes.

When you get the power supply working to spec, it can stress other circuits. An amp this old needs to be gone over carefull with a fine toothed comb. Everything needs to be examined. Guitar players do like to modify these amps to make them distort sooner. You never know what may have been changed, including the output transformer.

Check that the power tube cathode capacitor is installed with the + end connected to the cathode. Check the resistor parallel to that cap. Is it the correct one? They played with the value of that resistor and settled on 250 ohms 10W.

You can use a 5AR4 rectifier in this amp. A 5U4G lowers the high voltages more, people call it a more spongy sound. But I would resolve the power tube issue first.
 
  • Like
Reactions: BassmanPaul
#4
beans-on-toast said:
Swap the two power tubes and see if the issue moves with the tube. If yes, try the new power tubes.

... You never know what may have been changed, including the output transformer.

Check that the power tube cathode capacitor is installed with the + end connected to the cathode. Check the resistor parallel to that cap. Is it the correct one? They played with the value of that resistor and settled on 250 ohms 10W.
Click to expand...

Very helpful stuff here, thanks!

I'm a bit concerned about the swap and the sparking, but I can see the point. I don't mind if I fry the old tubes or blow a fuse, just don't want to blow my PT or new caps up. If I'm quick about it, is the risk of damage to those items fairly low?

Also, I'm about 95% certain the OT transformer is original (looks 100% correct and the nuts/washers inside are oxidized and untouched, unlike the PT)

If by cathode cap, you mean that 50uf/50v cap I replaced, it's in the right way (I'll check once more though). The two big resistors (including the parallel) are the originals and have correct values printed on them. No signs of burns or damage.

coreyfyfe said:
The 50uf/50V cap is your cathode bypass cap on the power tubes. The amp will work without this cap (as you noticed) but will sound darker and maybe a little spongy.
Click to expand...

Would the cathode bypass cap being bad for an extended time cause the power tubes to fail quicker? I wondered about that.
 
#5
Just perform a short test with the power tube swap. It will tell you if it is the tube or the circuit at the tube socket. Keep the volume level low.

It isn't unusual that the power transformers are changed. If it is, the question is, are the voltages correct with the replacement.
 
Last edited:
  • Like
Reactions: BassmanPaul and Crowd crusher
#7
agedhorse said:
Have you checked any critical voltages yet???
Click to expand...

Guess that will be my next step, will probably start with power tubes unplugged.

Just tried to check which 6L6 was sparking and of course, it won't do it again! Not sure if I should be happy about that, or even more nervous. With the same 5U4 rectifier in there, same 6L6 pair in there, it sounds fine now. Turned it slowly up til about halfway, sweating bullets, and it won't frikkin spark again. Doh!

Still don't want to put my good 6L6 pair in there til I figure this out. I'm now wondering if the process of me cleaning the 6L6 receptacles with Deoxit (before the sparks) somehow caused just enough of an intermittent pin connection to that 6L6 that it caused the light show. Grabbing at straws now.

I guess the smart thing to do now is properly install the 3-prong plug, discomnect the death-cap, and then check voltages at every check point I can find... in addition to looking over every component again, very carefully, to be sure I didn't miss something. Time to trace the whole circuit. I'll also retension those octal receptacles for the rectifier and power tubes, just to be sure my connections are good.

On a side note (related to my other post on the B-18 power tube temps), this B-15 amps power tubes always remain within 10 degrees F of eachother (more like 5 F, actually). They are almost like mirrors of eachother. Neither got higher than 315 F at the hottest spot. But they ramped up together, and cooled down together, like twins.
 
#8
If you sprayed deoxit, you could have had liquid betwwen socket terminals. This could change the conductivity between terminals. If it is all over use a Q-tip to clean it up. Some alcohol can help. When applying deoxit to tube sockets, put some on a brush (inter-dental) and use that to scrub the contacts. You don't want it anywhere but on the metal.

Deoxit itself is not conductive but you never know what was applied to the socket before you bought the amp. The socket degrades with time, you never know what os embedded in there.

Are DeoxIT® Products Conductive?
 
Last edited:
  • Like
Reactions: bobyoung53
#10
Yeah, I didn't load it up with Deoxit. I used the tip of a pipe cleaner and sprayed it right on the pipe cleaner, not into the socket. Didn't have the dental brush, but I know exactly what you mean. I did drain the power caps prior, btw.
 
#11
Frederick34 said:
Yeah, I didn't load it up with Deoxit. I used the tip of a pipe cleaner and sprayed it right on the pipe cleaner, not into the socket. Didn't have the dental brush, but I know exactly what you mean. I did drain the power caps prior, btw.
Click to expand...

Pipe cleaner is fine. Sounds like you did it properly.

Do the tube pins as well if they are coated with oxidization.

Try pinging the tube and see if it brings on the flashing. Sometimes these issues are also temperature related. Try pinging when the tube is hot.
 
  • Like
Reactions: bobyoung53
#16
nbsipics said:
2 amps with odd problems? Any possibility you have a mains power issue?
Click to expand...

A good thought, that occurred to me too. Our house was completely rewired/modernized by an electrician I trust and have known for years, about 2 years ago. Fully permitted, inspected, tested, etc.

I did check my voltage readings at various sockets and they were fairly smooth (between 118v and 121v at different times and locations). No other electronics in the house are acting up. I'm fairly confident in the power supply, at this point.

On the B-18 with power tube temp issues, I'm fairly certain that it's one of two things:

1) Bias has drifted and/or resistor is out of spec or was changed by someone over the years.

2) All my tubes are mismatched.

My plan on that one (and the B-15) is to trace the circuit this weekend to be sure the appropriate resistors and caps are within spec, measure plate/grid/screen voltages, put nice matching tubes in there, and set bias properly. And I thought I had nothing to do this weekend! That'll keep me out of trouble.
 
  • Like
Reactions: nbsipics
#17
I wouldn't put any new tubes in there until you find what' s going on. Bad power is unlikely to cause arcing in the tubes.

You can measure all the pertinent voltages right on the tube socket; pin 3 for plate voltage, pin 5 for the grid, and pin 8 for the cathode.

Use clip leads when measuring the cathode, and tap on the cathode resistor with a plastic or fiberglass stick; see if the voltage bounces around while tapping.

How do you intend to set the bias if this is a cathode biased amp?
 
Last edited:
#18
byacey said:
You can measure all the pertinent voltages right on the tube socket; pin 3 for plate voltage, pin 5 for the grid, and pin 8 for the cathode.
Click to expand...

May be a dumb question, but I've always done measurements before with power tubes installed. Heard of people measuring voltage at the socket, but with tubes out. Measure with tubes in or out? Or both?
 
#20
I would suspect the cathode resistor as a problem. If it went open or intermittent, it would explain why the electrolytic was destroyed.

Edit: Make sure you have a dummy load ie. 4 ohm resistor connected in place of the speaker while troubleshooting.
 
Last edited:
You must log in or register to reply here.
Top Bottom
Back