5F6A woes

[asatbluesboy]

A few years ago, I've had my tech build me a 5F6A with Alpha pots, Sprague caps, the works, down to spec, except for a solid-state rectifier and a somewhat cleaner output transformer with 4 and 8 ohm outputs. As he's usually very busy, and in the meantime I got quite handy with a soldering iron and a multimeter, I thought I'd give it a shot and try to solve these problems myself - with the help of your knowledge, that is. Here's the deal:

1) There's a popping noise whenever I stomp on a true-bypass pedal, and it doesn't happen with the same rig through a different amp (tube or solid-state). I've measured the input impedance, and it's missing 1M by a hair (997K), so this shouldn't be the problem. Any ideas?

2) I've rehoused the circuit in a nice pine open-back combo and, despite the popping, am loving it to death. But when I took it to its first gig rehoused this Saturday, something pretty weird happened. Halfway through a solo, the volume dropped, becoming barely audible, even with my ear glued to the speakers. All tubes were glowing, everything seemed to be right, just the volume was ridiculously low even with both channels jumpered and/or cranked. I messed with the controls a little, checked whether the tubes were securely placed, and suddenly it came back to life. 2 minutes later, it died again, so I grabbed a friend's HRD and made my way through the gig. I'm using two 30W Fender 10" AlNiCo blues, and although speakers lose some power on an open cab, I don't think that is the problem - I never had any issues before the rehousing.
Yesterday I took it to rehearsal, plugged it, let it warm up, played one song, and in the middle of the solo of the second one the same thing happened. I stopped playing, turned the controls up and down, got it to work again, but it stopped again 2 minutes later.
I'm pretty sure it might have something to do with either the vibration (as it is a combo now and wouldn't have any issues on a head/cab configuration) or the heat (the tubes were placed horizontally in the head, now they are vertically placed, upside-down). There is also the possibility of something coming loose during the rehousing. The tubes are tight in their places, so it must be something else. What should I be looking for?

Thanks a lot you guys. Please help a brother out. It's pretty frustrating to have a gorgeous-sounding amp for 3 minutes and then nothing.

-Gustavo

[tweed5259]

Hey to me it sounds like something's loose, if you do a "chopstick" test that reveals a lot of issues. just make sure not to get shocked! check if you're OT's connections are good. good luck.

[Enzo]

I'd bet the other amps have blocking caps at their inputs. Popping usually means there is some small DC voltage present.

I don't think that is the problem - I never had any issues before

Famous last words. Everything works until it doesn't. NEVER think up reasons not to check something. Plug the amp chassis into different speakers and see. Play a different amp through these speakers and see.

I'd also be looking for an iffy solder connection, especially on a tube socket.

Whack the chassis with a rubber mallet or your fist. If the chassis reacts to the hit, there is a loose connection.

A tube could be failing. Please don't tell me they aren't old enough to fail. Probably not one of the output tubes, more likely a small tube, considering your description.

[MWJB]

Does it happen on both channels (failing volume pot)? What's your plate voltage/current (overheating)?

Any pics of the chassis?

All 5F6As were combos, had open back cabs and the tubes upside down, they still worked.

[asatbluesboy]

Hey to me it sounds like something's loose, if you do a "chopstick" test that reveals a lot of issues. just make sure not to get shocked! check if you're OT's connections are good. good luck.

I'll be double sure I don't get shocked. I'll try and get a power resistor to drain those mothers dry.

I'd bet the other amps have blocking caps at their inputs. Popping usually means there is some small DC voltage present.

What would be the rating of a blocking cap for this circuit?

Famous last words. Everything works until it doesn't. NEVER think up reasons not to check something. Plug the amp chassis into different speakers and see. Play a different amp through these speakers and see.

I'll double check that. Thanks.

I'd also be looking for an iffy solder connection, especially on a tube socket.

I'll also double check those.

Whack the chassis with a rubber mallet or your fist. If the chassis reacts to the hit, there is a loose connection.

Will do.

A tube could be failing. Please don't tell me they aren't old enough to fail. Probably not one of the output tubes, more likely a small tube, considering your description.

Yeah. I've got NOS RCA 6L6s once. They sounded awesome, but crapped on me a few monts later. Nowadays I don't even trust new tubes anymore. Damn moody things, but oh-so-lovely. Just like women.

Does it happen on both channels (failing volume pot)? What's your plate voltage/current (overheating)?

Both channels, yeah. I'll measure the plate voltage and check it for overheating.

Any pics of the chassis?

I could try that with a bad, bad cellphone camera.

All 5F6As were combos, had open back cabs and the tubes upside down, they still worked.

This is exactly what I thought. The only difference is that mine has only got 2 speakers, a solid-state rectifier and a different output transformer.

Thanks a lot for all the responses. As soon as I open it I'll check those things.

[asatbluesboy]

I hit the damn thing through and through with the back of a screwdriver, with my fist (nasty shock! ) and nothing happened. So I went to practice with it anyways, and decided to try the stupidest workaround possible: turning the amp upside-down. And, to my surprise, the thing worked for 3 hours straight, flawlessly. So I'll try to tighten up those tube sockets and see how that works out.

Anything else I should take a look at? Also, what is the capacitor value I should use between the input jack and the circuit?

[Borg54]

New house, double check the solder joints on the speakers. I had a cold solder once on the speakers and it did the same thing. Also all of the tube amps that I work on I always clip off the push on connectors, and put a good trustworthy solder joint on it. This is the last place you want a short circuit, as the Output Transformer will fry in less than a min. on tube amps.

Drain POWER caps before tighten up sockets, use a dental pick, and consider tube retainers if hanging them upside down in the new house. Also the old disturbance test, from power section to preamp. It's less than a one min. test! No pop! There is the trouble spot. Lift the signal side of caps to test, replace the resistor while your on that V# section, they are only a dollar a dozen. If it ain?t buzzing, don?t chopstick it, only poke at the joints . If the problem still persits you may have a hairline fracture in one of your sockets, in which case it will be time for some pink noise.

How old are the Power Caps?

[asatbluesboy]

New house, double check the solder joints on the speakers.

Will do.

Drain POWER caps before tighten up sockets, use a dental pick, and consider tube retainers if hanging them upside down in the new house.

Yeah, I was thinking about tube retainers as well, just in case. I've actually already got them in the preamp tubes, I just need two for the 6L6s.

Also the old disturbance test, from power section to preamp. It's less than a one min. test! No pop! There is the trouble spot. Lift the signal side of caps to test, replace the resistor while your on that V# section, they are only a dollar a dozen.

Disturbance test?

If it ain?t buzzing, don?t chopstick it, only poke at the joints . If the problem still persits you may have a hairline fracture in one of your sockets, in which case it will be time for some pink noise.

No noise at all. Solder joints are fine.

How old are the Power Caps?

Around 2 years old.

[asatbluesboy]

Well, it was definitely the tube sockets. Any idea on what capacitor (capacitance/voltage) I should use at its input to avoid the popping?

[asatbluesboy]

Thanks a lot to everyone. Ends up it was the speakers, as Enzo said it could be. One day it stopped working altogether (actually, the volume was very, very low), I took it to my tech and he detected one of the speakers was shorted. So it was probably wanting to short already and whenever I'd crank it it would do it.

Thanks again.
Cheers!
-G.

[asatbluesboy]

But wait, there's more! I would hate to be the thankless douche that comes to a forum just to ask a question and disappears when the problem is solved. That solution was only temporary and in fact I ended up needing a new OT. The other one was handmade by a guy here in Brazil who assumed I'd be using it with a 2-ohm load (not 4, as I actually was), which meant he used tiny little frail leads... The bastard. So every time I needed more juice it would be unable to give it. The new OT has been working flawlessly for many a year now.

Now I'm gonna mess with the feedback resistor and screw the whole thing up again... lol

Thanks again for everything. You guys rock.

[Enzo]

And as you discovered, because most of Brazil is south of the equator, you do have to run the amp upside down for it to work right.

[asatbluesboy]

And Leslies need to have their rotors reversed.