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Delta Blues issue. *Update*

Dr. Jimmy

Member
Messages
4,108
Hi gang, had my Delta Blues go down at a gig last night and wanted to post here to pick brains and get ideas etc... Here's some background:

Bought the amp new in '94 and have used it a fair amount, retubed it several times, new speaker a few years back, it's generally in good shape. I have an electronics background, including repair of tube gear (worked in repair dept. of Sam Ash for several years fixing this kinda stuff) so I have a decent clue about this stuff lol.

Last night first tune of the gig, semi full house and the amp quits, no sound. Pilot light on, all tubes appear to be lit just not as bright as normal. Switch from creative brain to technical brain and begin troubleshooting:
-Plug straight into amp, bypassing pedalboard/wireless unit. Nothing.
-Plug into effects return to bypass preamp section, nothing.
-Backup guitar, nothing.
-Spare cable, nothing.

Start wiggling tubes around to see if any weren't fully seated and noticed they were lit up more than before and not only that, I had sound! Thinking something wasn't seated, I plugged back in and away I went.
First song, halfway thru the sound fades and dies out. Look in back, tubes barely lit again. At this point I recall two things:
-These amps have the tube heaters wired in series.
-I have old spares in my gig bag.

I quickly replace the power tubes, everything lights up and we have sound!!! Play the rest of the 3 hour gig with no issues except that the old tubes sound worn but not horrible. Ordered new tubes as soon as I get home from gig at 3am.

My questions to those out there that have experience with this platform of amp:
-Since the amp worked fine after replacing the power tubes, could there be anything else to look at that caused it? Power tubes are in my opinion the weakest link of the chain and do go wonky, so maybe it was just a bad tube.
-I will open the amp up to check for any other issues such as burned resistors but since again, the amp was fine after the swap, I shouldn't expect to see anything, or should I?
-Since the amp is going to be opened up, good time to swap out filter caps? It is close to 25 years old and all original.
-Any tips for extracting the pcb assy. out of this amp? I hear they can be tricky.

Thanks in advance, I appreciate it.

On a side note: It was a weird experience having to switch from the earlier mentioned "creative brain" to the "technical brain" and it definitely took a while to switch back after the amp was up and running. Led to a very stressed, "off" gig for me, as though my head was not really where it needed to be. Very odd.....
 

pdf64

Member
Messages
6,981
The dreaded slow fade out of a cooling heater!
I think it's unusual for tube heater failure to be the root cause of such problems.
Hence the heater circuit may be suspect, rather than the tubes, eg bad solder connection to socket terminal, socket requires retensioning / replacing, loose fuse.
I suggest giving the ecaps a visual inspection, maybe check ripple voltage at idle.
Good luck!
 

Dr. Jimmy

Member
Messages
4,108
Thanks for the reply.

I ordered all new ecaps for the circuit and I think I'll just swap them out anyway since I'll have the board out, I figure they're old enough and I don't want to have to open the amp up again in a year or so to replace them.

I agree about the tube heater failing, in the years I've been around tube amps, I don't recall ever seeing one fail because of that, however there is a first for everything! Another factor was that it was a bit cold out last night and I wonder if some thermal cycling had an effect on a suspect tube and/or solder joint? I'll be sure to go over all the sockets and solder while I have it open. I also ordered screen resistors as well as the two big 'uns that are part of the heater circuit, I'll plan to replace those as well as a preventive measure.

As mentioned, I've had this amp for almost 25 years, got a lot of use out of it initially then it sat for a good 10-12 years then it's been seeing a LOT of use over the last year or two. When it's running it's a GREAT sounding amp.....
 

BOBSMITH

Member
Messages
87
Keep in mind the 4 EL84 heaters are in series. Could be a problem with one of the tubes or tubesocket solder connection.
 

Dr. Jimmy

Member
Messages
4,108
I opened it up a little while ago and inspected it as best I could without pulling the boards out and I could see no evidence of overheating or burning, resistors looked fine and measured to spec, even the filter caps looked ok with no signs of ooze. I cleaned the sockets and retensioned them, threw in the old pulls that got me thru the gig and proceeded to beat on it at gig volume for a good hour or so with zero issues.

I think I may still replace the filters when I get the parts, I want to check the solder side of the boards for anything fishy. I guess at this stage I'll chalk it up to a tube that went bad....
 

pdf64

Member
Messages
6,981
Have you rechecked the set of tubes that were in the amp when it failed on the gig?
My hunch is that the issue was a bad connection (now resolved) rather than a bad tube; perhaps just reseating them a time or 2 may have done the trick.
 

Dr. Jimmy

Member
Messages
4,108
The only time I did that was at the gig when I pulled them out individually and reseated them, it had the same problem which led me to replace them with the pulls I had on hand. Unfortunately there was a room full of people waiting for us so I couldn't be as thorough as I would have liked and did one tube at a time, swap tube locations etc.... I kinda just shotgunned it and replaced them all.
 

TweeDLX

Member
Messages
3,756
Most Peaveys come biased pretty cold, so you can swap in most tubes without a rebias. If it was me, I'd start sticking one tube at a time back in to see if any have an issue. Why waste good tubes? :)
 

Dr. Jimmy

Member
Messages
4,108
Most Peaveys come biased pretty cold, so you can swap in most tubes without a rebias. If it was me, I'd start sticking one tube at a time back in to see if any have an issue. Why waste good tubes? :)
Under normal troubleshooting conditions that's how I would have done it, leave them in and swap one at a time until I found the problem tube, then I would stick that one in a different spot to see if the issue was the tube or the socket etc.... I needed to get up and running asap so I swapped them all. I was about due for a retube anyway.
 

Dr. Jimmy

Member
Messages
4,108
Ok, an update!!!

Had the day off so I dug into this guy and got the pcb's out, not as hard as I thought, and was able to examine the solder side. Believe it or not, the jumpers looked ok, with maybe 2-3 showing very faint cracks in the solder joints. As a precaution, I resoldered all the jumper connections, plus a few others that looked borderline. On the tube pcb, I completely resoldered all the pins on all the tube sockets, I desoldered, cleaned the flux off and resoldered all of them, I noticed a few of the pins looked less than ideal so I just redid them all.

Upon further examination I noticed that the board under R71 and R72 (3ohm 5W in the heater circuit) looked discolored, not burnt, but showing signs of overheating, so I replaced both resistors and left the new ones about 1/16" off of the board to enable some airflow. Screen resistors R43/R53 looked ok, but I replaced them anyway since it seemed like a bad tube is what caused all of this.

More examination showed something really odd. R66 (15ohm 5W in the bias circuit) had what appeared to be a greenish oxidation on both leads, so I took it out and replaced it. After taking it out I rang it out and it was reading intermittently very high/open. I then replaced the caps (C45/C46) that also make up that part of the circuit.

Since I had it opened up, and had the forethought to order the parts, I replaced all the filter caps, I purchased the amp new in '94 and have used it quite a bit so I figured it was due, or would be due soon, so I swapped them out.

Cleaned all the pots, cleaned and retensioned the tube sockets, threw in new set of tubes and buttoned it all back up. Played it at gig levels for about an hour and it all seems well, a lot more clarity and high end, probably due to the new tubes. Either way, I'm going to burn it in over the next 24 to shake it down a bit and see how it goes.....
 




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