How many impact-sensitive components are on a tube amp?

[vigilante397]

It's not a riddle, legitimately curious. I have a Bedrock 621 combo that has recently been replaced as my main amp, so I figured I would take it to work and show it off, maybe find someone interested in it. Unfortunately I forgot it was in my trunk and took a corner too fast, causing it to fall unpleasantly on its face. I got to work and checked it out, everything looks normal inside and out, but I'm not getting output from either channel.

Thoughts on where to look? Thanks!

[Granny Gremlin]

Tubes and speakers; that's it.  (back in the beginning, Traynor's idea of quality control was removing the aforementioned bits, tossing the amp off the roof, plugging everything back in and if it worked it passed)

... though if there was a cold joint in there, or something that was already mechanically stressed, that could break from such a shock.  The cold joint thing happenned to a bud of mine - his 20+ year old ss Peavey 1x15 bass combo started working only intermittantly after it took a bit of a bang up; the problem was one end of a resistor where the cold joint had given but was held mostly in place by gravity. The vibrations from certain low bass notes rattled the combo and made the resistor jump out of it's hole killing the signal.  Took me a while to figure that one out.

[vigilante397]

Tubes and speakers; that's it.

That's what I thought, but for some reason these aren't the things we talk about in class

back in the beginning, Traynor's idea of quality control was removing the aforementioned bits, tossing the amp off the roof, plugging everything back in and if it worked it passed

And that is awesome.

I've taken a couple looks at the circuit board and I don't see anything blatantly obvious that has come undone, but I haven't really taken the time to look at every single joint to make sure it's intact and not a cold joint. I halfway tested the tubes, meaning I swapped out all the preamp tubes and still nothing but I don't have another pair of EL84's so I have to borrow a pair from another amp to test that, which I won't be able to do until Monday.

I've been thinking about testing the speaker, partially because it's the easiest thing to test, but for some reason haven't done it yet. More testing for Monday I suppose. If I have as much free time as I expect on Monday I'll probably go through and re-flow every solder joint just for fun.

Thanks for your help

[PRR]

Speaker IS a prime suspect.

Are you safe to stick your fingers in there?

I would start with basic DC checks. Do you find +400V on the main filter cap? On the '84 plates? Is there a dozen volts on the '84 cathode resistor (if any)? Are the other tube plates mostly "halfway" between 400V and zero? (If a small-tube plate is very high, look to see if the cathode is mid-high; that's perhaps normal cathode follower.)

PCB fans say they are more robust. IMHO this is often over-stated, specially on made-to-price gear.

[vigilante397]

Speaker IS a prime suspect.

Are you safe to stick your fingers in there?

I would start with basic DC checks. Do you find +400V on the main filter cap? On the '84 plates? Is there a dozen volts on the '84 cathode resistor (if any)? Are the other tube plates mostly "halfway" between 400V and zero? (If a small-tube plate is very high, look to see if the cathode is mid-high; that's perhaps normal cathode follower.)

PCB fans say they are more robust. IMHO this is often over-stated, specially on made-to-price gear.

The speaker would be the most expensive fix, but I was never in love with the stock speaker so it would finally force me to throw a greenback in, if that is the problem

I will never feel safe sticking my fingers in a powered-on amp, but I do feel safe sticking some DMM probes in it I'll try the speaker and tubes first, but if neither of them are the problem I'll definitely pull out the multimeter and check voltages. I don't prefer PCB amps (I actually just finished wiring an eyelet board replacement for my new amp's PCB, probably make a whole thread for the mods I'm doing to that thing ), but this one is designed reasonably enough that I can poke around as necessary. I left the amp at work (my music room is too cluttered for extra amps, off to for sale thread...), but I now know what my plan A, plan B, plan C, and even plan D are

[Gus]

Did you check the potentiometers? An impact can damage them.

[Granny Gremlin]

It would have to be a hard and very accurate knock because amp cabinets have a bit of a lip that would protect them in most situations, but yeah, that's also possible. Just not very likely in a situation where it just fell flat on it's face.

[Gus]

It would have to be a hard and very accurate knock because amp cabinets have a bit of a lip that would protect them in most situations, but yeah, that's also possible. Just not very likely in a situation where it just fell flat on it's face.

There could have been something in back of the trunk.  A broken wafer could break the signal path.

[Granny Gremlin]

All true; I did say it was possible (but it would have to a very 'accurate' knock - you really have to bang the knob in just the right way).  The part that breaks is usually not the wafer though (safely inside the chassis in the pot casing), but the shaft.  This usually results in the pot being frozen in a particular position (see this all the time) but in some cases (I have seen it happen, but only once) being completely open (the wiper is knocked off the track completely).  In addition to the above, this is why it's not very likely, but yes, possible.

[R.G.]

I'm not getting output from either channel.

1. Is there hiss in the speaker when you place your ear very near it? There is always some small hiss from an active amplifier output. If it's not there, the power amp or the speaker is not working. If it is there, the circuits before the power amp are not working.
2. If there is no hiss, temporarily disconnect the speaker and momentarily touch and remove a battery across the speaker terminals. Should be a loud, evil thump. If not, the speaker is toast. If yes, the speaker is not being driven.
3. If speaker thumps, check the fuse(s) ***with an ohmmeter***. Fuses are mechanical devices too, with a delicate mechanical filament that can be broken by shock. It's not a big chance, but it's possible.
4. If hiss, does it change even minutely when you mess with the controls?
5. If no change in controls, methodically swap out each and every tube one at a time with a known-good tube.
6. If you got to here and no joy, put down the amplifier, sit down and think for a moment. Do you already know that you can successfully probe around inside a live tube amp with hundreds of volts of AC and DC running around inside without dying, or worse yet, breaking the amplifier worse than it already is? Be truthful; you're the only one listening to the deliberations. If the answer is no, take it to a tech. Music is good, but it's not worth dying for for most of us.

[vigilante397]

There could have been something in back of the trunk.  A broken wafer could break the signal path.

There was nothing else in the trunk, the pots look fine. The board itself is also fine, no cracks, broken traces, or anything like that.

Do you already know that you can successfully probe around inside a live tube amp with hundreds of volts of AC and DC running around inside without dying, or worse yet, breaking the amplifier worse than it already is? Be truthful; you're the only one listening to the deliberations. If the answer is no, take it to a tech. Music is good, but it's not worth dying for for most of us.

Hang on, so breaking the amplifier further is worse than me dying? But frankly I am comfortable poking around high voltage circuits. Unfortunately "taking it to a tech" isn't really an option because frankly the most qualified and experienced tech in town (for about 150 miles) is me.

But if I can't find the problem and fix it easily there still is a bright side, because I bought this from a friend, and his amp tech that used to service this amp has expressed interest in it, so I may just give up on fixing it and sell it to him at a discount

[R.G.]

Hang on, so breaking the amplifier further is worse than me dying?

Just ask an amp collector... 

But frankly I am comfortable poking around high voltage circuits.

OK.

I didn't *really* think breaking the amp more was worse. 

That makes this easy then.

Find the schemo for the amp, open it up and start measuring voltages: first filter cap, screen grid supply, output tube plates, and plate/grid/cathode for all the rest of the tubes. If the filament is not glowing on any tube, that's a Big Hint.

Unfortunately "taking it to a tech" isn't really an option because frankly the most qualified and experienced tech in town (for about 150 miles) is me.

If you're the local capable tech, you have an oscilloscope, no? That will about ice down the rest after power voltage measurements tell you where to look.

But if I can't find the problem and fix it easily there still is a bright side, because I bought this from a friend, and his amp tech that used to service this amp has expressed interest in it, so I may just give up on fixing it and sell it to him at a discount

After all, what are friends for?   

[alanp]

Tubes and speakers; that's it.  (back in the beginning, Traynor's idea of quality control was removing the aforementioned bits, tossing the amp off the roof, plugging everything back in and if it worked it passed)

The Vox doco (How Dartford Powered something or other) showed them kicking an AC30 down a few flights of concrete stairs, then plugging it in and seeing how well it worked.

[vigilante397]

Anti-climactic update: Well the amp works now. Turns out it was some combination of tubes, reverb circuit (looks like the pot is going bad) and the effects loop switch. With the effects loop turned on when there is nothing plugged into the loop it gives the same sound as when a preamp tube is dead, and it looks like the switch was starting to go.

So replaced the switch, replaced the tubes, and whacked it a couple times until the reverb stopped freaking out I'll change the pot today and should be good to go

Thanks everyone for the help, as always, especially since it also applies to another amp I'm debugging

[bluebunny]

. . . and whacked it a couple times until the reverb stopped freaking out

Ah, "Impact Engineering".  You should cover that next semester...   

[vigilante397]

Ah, "Impact Engineering".  You should cover that next semester...   

Actually learned that one from the other "reputable" amp tech in town. But I changed the pot and sure enough it's working fine. Further good news, a guy came it looking at amps and I was able to sell it about 2 hours after the pot was changed

[Gus]

Almost ever part is impact sensitive
The transformers can get ripped from the mounts and possible damage to the lams from something hitting them.
The tubes
The tube sockets
Parts breaking away from solder joints
As you found switches
lamps
chassis tee nuts etc.

If I was to have looked at the amp I would have opened it and checked for broken parts
Check connectivity from point to point
Looked at all of the solder joints
Assumed a tube(s) werre bad and test them individually
etc.