Bustificated tube amp...anybody help a brutha out?

[Hammer]

I posted this over on TGP, but thus far have only received an oh so helpful response advising me to not buy cheap Chinese crap. *yawn* :roll: Anyway, here's the deal:

I have a B-52 At-212 I picked up a short while ago for a bangin price and it kicked much arse...until a few weeks ago. I was playing and...it just fizzled out. The sound tapered off and the power light slowly went dim. Now it won't power on. I don't play loud (bedroom guitar hero) and there was not a power surge (everything else plugged into that outlet stayed on...not even a blink). The fuse looks good. Now, I'm handy with a soldering iron, but this is only my second tube amp and the first one to die on me like this, so I am not sure where to even begin. Any tips on where to start poking around or even if I should be poking around vs taking it to a qualified tech?

Even if you can give me just generic tube amp step by steps that could apply to any tube amp, it would be helpful.

 

[capnjuan]

Hi Hammer; have a meter? If not, have some spare known-good preamp tubes? If yes, try subbing them in one at a time. Don't have a clue how your amp is wired but the power circuit that makes the tubes glow ... the heater circuit ... usually includes the pilot light. You wouldn't think the pilot light crapping out would open the heater circuit but ... who knows. Anyway, try a few spares ... see what happens. A few chassis pics for orientation would be helpful.

 

[beinhard]

Hi, Hammer

Do the tubes glow?
If they are all dark you have no heater supply. Power lamp is on the heater supply as well, so this will probably be the case.

You probably know this, but: Poking around inside a tube amp can kill you. There are dangerously high voltages in there, even a while after turning power switch off.

If this was my amp, I would first check components on the primary (mains) side of the power transformer:
main fuse, surge protecting thermistor, power switch, wiring.
Sometimes a fuse can be blown but still look okay. (This amp also has some more fuses inside, but they would not take out the power lamp and tube heaters)
If there is voltage across the power switch, and the wiring is okay, then power transformer is next suspect.

beinhard

 

[Hammer]

Thanks for the help guys.

I do have a meter...somewhere. I haven't used it in so long, it's buried away somewhere. As far as tubes, I don't have any spares, but I do have a Hot Rod Deville...I'll have to double check and see if I can steal any from there to try. The only fuse I looked at was the external one...haven't even taken off the back or dropped the chassis yet. Not sure if the tubes are glowing either. I'll pull the back off and the chassis out this week and snap some pics. Thanx again...much more helpful than TGP. :lol:

 

[capnjuan]

Hi Hammer; dig the meter out ... if it needs it, put a battery in it. Asking you if you have 6.3V AC on the heaters ... without the meter, all you'll be able to answer is 'Maybe' :wink:

You might also have a bad power switch; if you turn it on and get nothing and didn't blow the primary fuse and didn't smoke anything ... The best way to test the switch is to put a meter on it ... or a continuity tester. Anyway, happy to help.

 

[capnjuan]

Just in case you are looking for this stuff, here is the manual, the schematic and the tube layout. Good luck!

Hi Dave and thank you ... the H-Man and I owe you a cup of LTG coffee :wink:

H-Man; this is a fragment from your amp's schematic ... B-52? I guess so ... 100 watts of high-altitude, glass-driven power ... :shock: :shock: If this is your bedroom amp, what are you using on the road? From bottom to top; the lowest red circle is the primary fuse that, as far as you know, didn't blow. The other two secondary fuses are ahead of the diode rectifiers. Even if one of them blew which I don't think happened, you wouldn't have any high voltage when the SS rectifier is switched in but the pilot light (at the top in the black circle) which is fed from the 6.3VAC heater winding on the transformer would still be on.

Besides the fuse, you have a thermistor (green circle / pictured below) and a double pole / single throw power switch (magenta box / pictured below) ahead of the power transformer.

The thermistor acts to snub the in-rush of current and protect against voltage spikes in the domestic power. If the thermistor failed, then there'd be no power in the amp. These are rugged little gadgets so the chance of failure is pretty small but the switch is another matter. Every time the switch is thrown on and just before the contacts 'make', the power arcs across the gap. Over time, the arcing causes one of two things; it either burns up the contacts to the point that, when closed - switch in the 'on' position - there's an air gap between them ... or ... they become welded together ... not all that uncommon ... where the amp is 'always on' regardless of switch position and turning it off means unplugging it from the wall.


Thermistors:

Double Pole / Single Throw switch Antique Electric Supply Part Number P-H520

So ... ... get your meter runnin' ... head out on the highway ... Of course it could be several things but the primary inbound power seems like as good a place as any to start. :wink:

I looked over on TGP; didn't find your post. :? J

 

[Hammer]

Just in case you are looking for this stuff, here is the manual, the schematic and the tube layout. Good luck!

Hi Dave and thank you ... the H-Man and I owe you a cup of LTG coffee :wink:

H-Man; this is a fragment from your amp's schematic ... B-52? I guess so ... 100 watts of high-altitude, glass-driven power ... :shock: :shock: If this is your bedroom amp, what are you using on the road? From bottom to top; the lowest red circle is the primary fuse that, as far as you know, didn't blow. The other two secondary fuses are ahead of the diode rectifiers. Even if one of them blew which I don't think happened, you wouldn't have any high voltage when the SS rectifier is switched in but the pilot light (at the top in the black circle) which is fed from the 6.3VAC heater winding on the transformer would still be on.

Besides the fuse, you have a thermistor (green circle / pictured below) and a double pole / single throw power switch (magenta box / pictured below) ahead of the power transformer.

The thermistor acts to snub the in-rush of current and protect against voltage spikes in the domestic power. If the thermistor failed, then there'd be no power in the amp. These are rugged little gadgets so the chance of failure is pretty small but the switch is another matter. Every time the switch is thrown on and just before the contacts 'make', the power arcs across the gap. Over time, the arcing causes one of two things; it either burns up the contacts to the point that, when closed - switch in the 'on' position - there's an air gap between them ... or ... they become welded together ... not all that uncommon ... where the amp is 'always on' regardless of switch position and turning it off means unplugging it from the wall.


Thermistors:

Double Pole / Single Throw switch Antique Electric Supply Part Number P-H520

So ... ... get your meter runnin' ... head out on the highway ... Of course it could be several things but the primary inbound power seems like as good a place as any to start. :wink:

I looked over on TGP; didn't find your post. :? J

Thanx to you both! I finally got it dropped today. Yea, this is my bedroom amp...along with a Fender HRDv. :lol: No road for me! From just eyeballing, fuses all look good. Here are shots of the guts:

And here's the ever so helpful TGP thread :roll: :

http://www.thegearpage.net/board/showthread.php?t=783927

One thing I did notice is that the rectifier tube seems to be a little...cooked. Not well done or anything, but a lightly sauteed with some dark discoloration to the tube around the base. Is that normal or could this potentially be the problem?

 

[capnjuan]

Looking at the pics now ... even if the rectifier were bad ... I think you'd still have a pilot light. It and the tube heaters are wired like xmas tree lights - even if one is bad, the rest still turn on.

Eh ... how we making out with that meter? I saw where the boyz on TGP were leaning in so far they almost lost their balance ...

More later.

 

[Hammer]

Eh ... how we making out with that meter?

I have to run and get a new one...at some point the leads were yanked from one of the probes...I probably sat something on it wrong in my tool chest. Meh, it was a cheapo from Radio Shack anywho.

 

[capnjuan]

Okay; please confirm that that's the primary power switch on the left and the fuse post to the right - if not tell me which pic to look at to find the power switch:

Never mind; that was the backside of IEC hub on the back ...

 

[Hammer]

Okay; please confirm that that's the primary power switch on the left and the fuse post to the right - if not tell me which pic to look at to find the power switch:

That is the primary fuse on the right, but on the left is the inlet for the power cord. The power switch and standby switch are directly north of those.

 

[capnjuan]

Thanks H. Still looking at it.

 

[capnjuan]

Okay; same pic marked up to show the thermistor and those two fuses ahead of the rectifier diodes. With the amp unplugged and after you make a drawing of which if those in-bound leads to the back of the switch go where, you can pull them off and check if the switch correctly makes a connection between in-bound and out-bound wires. You can also check the thermistor to see if it has burned itself 'open' ... so that no current passes through it.

Past these items, tracking down cooties by post and reply can be pretty awkward and frustrating ... as the poet said: 'working on mysteries without any clues'. Get a meter, we'll check the switch and thermistor. If the switch it bad, you're going to want to take it out and ID the make and model ... because of the variables ... size of the chassis opening ... type of connection lugs ... power-handling ... and so on ... it can be easier finding the exact part than it is finding something else that will work.

 

[Hammer]

Well...now I feel like a dufus. I checked those two secondary fuses and the one on the back of the amp and they all looked good. Then I'm looking at the schematic and I'm all...what the hell is this 5A fuse...? There's a fuse slot inside of the power inlet with, guess what...a 5A fuse! And what does it look like? The inside of that fuse looks like someone splattered the inside with runny scrambled eggs and mustard. Lucky for me, there is also a spare hidden in that same spot, so...wish me luck! :mrgreen:

 

[Hammer]

Looks like I had multiple issues, actually. I swapped out that 5A fuse for the spare, reassembled (which is no fun whatsoever) and plugged it in. Nothing. Crap. So I try another outlet...magic! It looks like there may have been a small surge and it took out 4 outlets on my power strip as well as the main fuse on the amp. So, for now, all is well again and I am back to rockin at unacceptable bedroom levels! Thanx guys!

 

[capnjuan]

:lol: :lol: :lol: :lol: :lol:

Well ... I was pretty sure there was no power in the amp ... :wink:

 

[Hammer]

Well, hrm...new info. So I put everything back in place, pick up a new surge protector and flip the amp to standby to warm up. As soon as I flip it off of standby...guess what...same ol' fizzle and it's dead again. Unless my brain is not working properly, that sounds like a short somewhere to me.

 

[capnjuan]

If you don't want to screw with this, it's fine with me ... If it's the switch, it's a $5-$10 item and about 1/2 hour of bench time at your local tech or maybe $50 or so to get it fixed. If it isn't the switch, you're dragging it down there anyway. Good luck with your amp!

 

[Hammer]

If you don't want to screw with this, it's fine with me ... If it's the switch, it's a $5-$10 item and about 1/2 hour of bench time at your local tech or maybe $50 or so to get it fixed. If it isn't the switch, you're dragging it down there anyway. Good luck with your amp!

Yea, I'm gonna try to find a replacement switch...maybe just replace both since it should be easy enough with the blade connectors. Then it's time to find a tech...and lug this thing there...which is no fun. It is one heavy sonofabitch!

 

[capnjuan]

If you don't want to screw with this, it's fine with me ... If it's the switch, it's a $5-$10 item and about 1/2 hour of bench time at your local tech or maybe $50 or so to get it fixed. If it isn't the switch, you're dragging it down there anyway. Good luck with your amp!

Yea, I'm gonna try to find a replacement switch...maybe just replace both since it should be easy enough with the blade connectors. Then it's time to find a tech...and lug this thing there...which is no fun. It is one heavy sonofabitch!

Okay ... one of the reasons we geezers like dirty old fart-box 6V6 amps ... they're easy on the back ... :wink: