Ok I have a question what blew first? My tube or resistor? I have a 1969 (by pot codes) Bassman AB-165 that appears untouched original everything yet no feedback circuit. I don't see anything saying it was ever there. I've been using it quiet at home for a few years now and have never had a problem. I know according to what I read that I should be changing everything inside it if I ever want it to sound good. Yet to me it sounded like the voice of the gods until last week. The day after messing around with a booster pedal I was greeted with very low output. Swapped out the Phase inverter with an EH 12ax7 and bingo back to normal. When I was in there I decided to retube it. EH in the Pre Svetlana for the power. Clipped the .002 caps off pin 5 on the Power tubes and put a switch to the resistor off the bass pot to give me some OD. Even though it is still a bias balance I biased the tubes and got them dead on. Played it for awhile left it on for hours, no problems. A week later(today) I power it on and am greeted by the most incredible distortion with volume on 2. Moments later the light turns out and the Magic Smoke comes out. The tube has red plated and on inspection the Resistor across pin 4 and 6 is BBQ'D. Has this amp just reached it's time? Can I assume I bought a bad tube or just assume it is time to rip it apart. I'm supposed to be starting with a new band and can't have something popping every week. As much as I love the tube tone a Solid State to get me by is looking good right now. 300 bucks worth of parts to get it "new" again could be spent elsewhere. Please Some thoughts from those who have been there.
Ad Widget
Collapse
Announcement
Collapse
No announcement yet.
AB 165 Chicken or the Egg!
Collapse
X
-
Probably just a bad tube (first) that fried the screen resistor (second).
Or the tube lost bias (bad bias balance pot?) and fried. Check the bias voltage on both sockets with the power tubes pulled.
If you want it to be reliable, change ALL the electrolytics if you haven't done so yet (those under the doghouse, the bias cap and the preamp cathode caps).
-
The three prong cord and cap was the first thing I did. I have -42 with the tubes out. Now that I'm a tad bit calmer I'm putting together a list of stuff to order. Looks like I'll be doing all the caps and a few resistors and trying another set of tubes or just putting the old Sylvania's back in. Hoffman seems to have the best pricing and selection as far as that goes.
Comment
-
Originally posted by vdubbus77 View PostThe three prong cord and cap was the first thing I did.
Originally posted by vdubbus77 View PostI have -42 with the tubes out. Now that I'm a tad bit calmer I'm putting together a list of stuff to order. Looks like I'll be doing all the caps and a few resistors and trying another set of tubes or just putting the old Sylvania's back in. Hoffman seems to have the best pricing and selection as far as that goes.
Other things that must be done to make the amp reliable include cleaning all pots, jacks, switches and tube sockets and re-tention tube sockets and jacks. Test the resistor values for drift. Old carbon comp resistors are known to drift (usually up) in value.
That, good tubes and new filters and your amp should be just as reliable as anything you can buy new.
Since the amp was mostly original parts I would guess that the failure happened because of 1) a bad connection between the tube pin and the tube at the grid. This would cause the tube to lose bias and fail. 2) bias is intermittant for some reason. If it was the bias pot then cleaning as above should fix it. It's common to blow a screen resistor when a tube fails. Replace them both."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
Comment
-
Yep -42 on both sockets. I'm going to run to radio shack tomorrow to get a couple resistors just to temp out the screens basically to recreate the failure on the bench if I can. I think the new power tubes I put in one shorted. I'll put it all back together and recheck everything, jiggle and jangle it all around and see what happens.
Comment
-
Whats the point in re creating the problem? Thats impossible anyway now that you have one blown tube. (or is it blown?) Still, I'll bet it was either a bad tube from the start or an oxidized grid connection on the side that failed. Trying to recreate the problem may not work because the failed grid connection might contact better this time only to fail again soon. Better to just clean and retention the tube sockets, replace the blown resistor and the other identical one on the other tube socket, replace the tubes and see if the problem is fixed. I'll betcha it is.
Perform the other fixes above too and the amp will be as good as new.
Chuck"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
Comment
-
I think wanting to recreate the problem is from being an auto mechanic for 20 years. If you can't recreate the problem in front of you then it will usually show up when it is least convienent or most expensive. Tube is toast no doubt. Resistor is broken in half. Old tubes are in and balanced out at about 17 watts a piece seems low but they are even and not glowing.
Comment
-
Originally posted by vdubbus77 View PostOld tubes are in and balanced out at about 17 watts a piece seems low but they are even and not glowing."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
Comment
-
I see no mystery, a tube failed and took the screen resistor out with it. Happens every day.
Recreate the problem? Nah. Determine what was the problem, but no point in recreating it and perhaps damaging more in the amp. If a piston ring breaks and chews up a cylinder wall, do you stick another broken ring in there and watch it do it all over?Education is what you're left with after you have forgotten what you have learned.
Comment
-
I've been burned too many times on Auto Electric to not want to verify problems. It's an old habit. I see what you are saying though. Now I have another problem/quirk maybe. If I put a shield on the PI I get VERY low output. Take the shield off it revs back up. I've pulled and pushed and wiggled and can't duplicate this. I checked for voltage around the screws and shield and nada.
Comment
-
The push and twist little cans that fit over the tube? Put one on and the sound dies? Pull it off, take the spring out of it and put it back on. Does it now work OK? You may not be able to push on the tube and get the same thing, but it sure sounds like the socket is losing contact with the shield on, and the only real way for that to happen is the spring pushing down just so on the socket. Tighten all the socket pins on that socket.
Resistors can't burn up on their own, the only thing that burns them is excess current flowing through them. And since the only path for such current is through the power tube in the case of the screen resistor, a burnt screen resistor is caused by a failed tube.Education is what you're left with after you have forgotten what you have learned.
Comment
Comment