B-52 AT-100 - What does this hint to?

[trip]

Hey guys, just trying to learn further, i know a generic "what is this?" doesn't give any answers without testing but still curious if there's a ballpark thing I should be looking for when testing.

I own two of these guys, the first one is stuck on class A though, the prev owner glued the knob so it couldn't be changed, nbd, no problems, doesn't get too hot, works as it should, now #2 i picked up for about 60 bucks, something is goin on

initially it had a burnt out power fuse (so i'm hinting to think the tubes are going soon, they're sovtek wxt 's and they seem a little dim to me) so I replaced the fuse and bumped it on. Initially it sounded like poo, power tubes 1 & 3 were glowing blue, so I just swapped em around so that v1 & 2 had the glowy ones, sound sounds great. Played it for a few hours total and have had no issues, heat seems fine, albeit the tubes a little dim, but as I mentioned now 1 & 2 have the extra blue glow around them (from which i understand is a pretty normal thing, some amps all 4 do it, some barely at all and some not all).

So I felt like trying the remove the two inner and half the ohms to drop the power since I don't want to run 100 watts into a 2x12.. but when I did this, I immediately noticed v4 wasn't glowing too much, and 1 started to get really hot after a minute, then started to begin redplating so I caught it and turned it off. I don't have the thing open to take any readings but it looked like all the v on the plates went straight to 1, and not 4 (also why i mentioned 1 and 2 had the glow, not sure if it's hinting significance). Put the two middle tubes back in, set the ohms correctly, and then continues to function well for as long as I play, no redplating.

I'm thinking my first step should be new tubes (otherwise how would the power fuse have burned out initially) and checking the plate volts but I don't want to load up new tubes if they're going to burn out due to a fault in the amp. Obviously it's not "100% working" otherwise it'd work fine by halfing the ohms and removing two tubes, but I'm just curious if any one has any hints to what would cause that or what I might be looking for? How could an amp seem to work relatively normally with 4 power tubes but start plating on v1 with just two? Trying to wrap my head around what could cause the issue w voltage because visually v1 just started getting whacked when I tried to half the power.

B52 AT100.pdf

[Enzo]

No, the first thing to do is pull all the power tubes, and check the voltages at their empty sockets. Do you see good B+ - oh, I have no idea, let us say 480-500 volts or thereabouts - on BOTH pins 3 and 4 of each socket? And do you also see good bias voltage on pin 5 of each? Something like -45 to -55 volts. Obviously the amp must be set for fixed bias for the bias voltage to be there. But if you put it over to "class A" there better not be ANY positive voltage on pin 5.

Bad power tubes is probably the most common reasons for fuses to blow, but MANY MANY other things can blow the fuse.

I always put a fresh set of power tubes into any used amp I get. Your old tubes here could be faulty or just one could be funny, who knows until we test them separately.

[nosaj]

No, the first thing to do is pull all the power tubes, and check the voltages at their empty sockets. Do you see good B+ - oh, I have no idea, let us say 480-500 volts or thereabouts - on BOTH pins 3 and 4 of each socket? And do you also see good bias voltage on pin 5 of each? Something like -45 to -55 volts. Obviously the amp must be set for fixed bias for the bias voltage to be there. But if you put it over to "class A" there better not be ANY positive voltage on pin 5.

Bad power tubes is probably the most common reasons for fuses to blow, but MANY MANY other things can blow the fuse.

I always put a fresh set of power tubes into any used amp I get. Your old tubes here could be faulty or just one could be funny, who knows until we test them separately.

I'd also suggest when you have the chassis out to use your eyes and ears to look for any burnt components and to visually inspect the output sockets.

Around here you're not going to find ballpark ideas. Your going to find people who want you to take measurements, because that is what drives you to the problem. If you help yourself by following along you'll either end up with a working amp on one that is cost prohibitive to fix. But one thing is for sure, you will most likely learn something and that is not something you can put a price on. I am thankful every day for the wisdom that is imparted onto me from the many great minds here and each month or so I find myself asking less question and I try to give back when I can.

nosaj

[trip]

Hey Guys,

I do always appreciate the responses and everything I learn from this forum, more than you guys know.

I know there's no ballpark ideas, but I'm trying to understand the general theory, what would make the bias of the first be OD compared to the rest, I know it takes experience to hear a problem and immediately know "yeah thats the op amp chip over here 100%" but I fugred the problem at least hints to some likely cause.

I haven't had time to open it up yet but I will report back when I do. Confirming this type of amp can be run without power tubes installed. I've herd mixed opinions but no professional backing.

On another note, I removed v1(the plating one) and v4 and ran it with v2 and v3, runs cool, no plating. So it runs well with all 4, or just 2&3, but not 1&4. Initially when I got this thing I did take it apart before getting a fuse and not a single thing looked burnt or out of the ordinary, but I didn't remove the pcb completely.

Will report when I take the voltages without power tubes, might be some time, but thanks all, excited to figure this probo out.