Bad Design, or Bad Tubes?

[Lowspeid]

I picked up a Vox Ac10c1 yesterday off CL for a little less than they are selling for on Reverb. It came with the nice cover, which was a bonus. When I played it at the guys house before I handed over the cash I heard what sounded like a ring/rattle when the gain was pushed past 11:00. I figured microphonic tubes, so I wasn’t too concerned because I’ve heard Vox amps use cheapo tubes that go bad pretty quick. I had a new set of EHX EL84s and JJ ecc83s in my stash, so I’d planned swap out the tubes anyway.

I swapped the tubes today, and saw that the power tubes sit about an inch from the speaker magnet. I’m thinking that can’t be a good design choice for tube longevity. I played for a bit, and it sounded really good. But as soon as I pushed the gain past 11:00 I got the same ring/rattle sound coming from the amp. I opened it up and did the “tap” test with a pencil eraser. They both have that “ring/rattle” to them when tapped, but not excessively. Just to make sure, I swapped the old tubes back in and did the tap test on them, and they make the same ring/rattle as the new tubes. So, either that’s the way EL84s sound when tapped and the issue is something else, or the new tubes are bad out of the gate.

Anyone with an AC10c1 have this same issue and it NOT be tube related? Is the proximity of the power tubes to the speaker causing them to go microphonic? I’m pretty bummed out, and I’m pretty pissed at myself for buying another amp that’s supposed to be “a great deal” but needs money thrown at it to make it reliable/“right”. Any help/ideas/commiseration is appreciated.

As an aside, I bought 2 sets of EHX 6v6s, 6 12ax7, 2 7025s, and 2 12at7s at the same time I ordered the EL84s, and one of the 6v6s was bad out of the box. The company I ordered from sent me a replacement set (which was crazy because tube prices had almost doubled at that point), so that was cool but it was also about 6 weeks after I placed my order. Now it’s been almost 10 months.

 

[Jon Snell]

The speaker magnet has no effect on valves with the exception that if they are touching, vibration may break the glass.
Look for a microphonic capacitor, if the EL84s are definately OK.

 

[Lowspeid]

I had no idea capacitors could be microphonic. Where/how would I look for a microphonic capacitor? Which one would be most likely/common?

 

[VintageSG]

...as soon as I pushed the gain past 11:00 I got the same ring/rattle sound coming from the amp.

'Gain' points more toward pre-amp than power stage. 'More', but not exclusively so. High volume is more normally a power valve thing, but moderate volume with lots of clipping gives square edged waves, and these are ones that give 'metallic' rattles. The parentheses are my weak attempt at disguising poor descriptors.
The EL84 is a tightly packed valve that seems more prone to issues than, say, a 6V6. Can you remove the amp section, extend the speaker wires and try that? The separation would remove mechanical coupling from the question. An external cab would be easier as if I remember correctly, plugging a cab in disables the internal speaker.
The speaker itself rarely causes issues. Induced chassis vibration can be a bear to identify.
In terms of output valves that won't give issue, look toward Soviet 6p14p-EV or -K suffixed.

If you loosen the chassis mount screws a fraction, does the issue reduce?

 

[Jon Snell]

I had no idea capacitors could be microphonic. Where/how would I look for a microphonic capacitor? Which one would be most likely/common?

You must remove the amplifier from the cabinet to "chopstick tap" the internal components.
If you are not trained and competent, get a person who it to do it for you, whilst watching. That way you will gain experience and may be competent to do it yourself next time.
Very gently tap around umtil you find the issue and replace the component, like for like.

 

[NTC]

I know nothing about Vox amp construction, but - is the speaker loose? Are there screws holding the baffle in place that are loose? Are the chassis bolts loose? It is worth making sure these are appropriately tightened before spending lots of time on the electronics. It may be worth checking out if the pots, jacks, and tube sockets are also tight. Any of these things can vibrate.

 

[Lowspeid]

'Gain' points more toward pre-amp than power stage. 'More', but not exclusively so. High volume is more normally a power valve thing, but moderate volume with lots of clipping gives square edged waves, and these are ones that give 'metallic' rattles. The parentheses are my weak attempt at disguising poor descriptors.
The EL84 is a tightly packed valve that seems more prone to issues than, say, a 6V6. Can you remove the amp section, extend the speaker wires and try that? The separation would remove mechanical coupling from the question. An external cab would be easier as if I remember correctly, plugging a cab in disables the internal speaker.
The speaker itself rarely causes issues. Induced chassis vibration can be a bear to identify.
In terms of output valves that won't give issue, look toward Soviet 6p14p-EV or -K suffixed.

If you loosen the chassis mount screws a fraction, does the issue reduce?

Last night I made a speaker extension cable and separated the amp from the cab. I wasn’t able to play with the same volume/gain as I had been when the ring/rattle started due to the lids going to bed, but I will try again today and see if separating them makes a difference.

You must remove the amplifier from the cabinet to "chopstick tap" the internal components.
If you are not trained and competent, get a person who it to do it for you, whilst watching. That way you will gain experience and may be competent to do it yourself next time.
Very gently tap around umtil you find the issue and replace the component, like for like.

I have the knowledge, and can chopstick with the best of them. When I check the tubes today I will “chop” around and see if any caps are making noise.

I know nothing about Vox amp construction, but - is the speaker loose? Are there screws holding the baffle in place that are loose? Are the chassis bolts loose? It is worth making sure these are appropriately tightened before spending lots of time on the electronics. It may be worth checking out if the pots, jacks, and tube sockets are also tight. Any of these things can vibrate.

everything appears right and tight, but I’m give everything another once over to make sure.

 

[schmee]

I think you have another issue. Something is making the amp microphonic and it shows up when you tap the power tubes.. It can often be a pre tube but you changed those.
Does the amp have ceramic power tube sockets? I have heard they can be loose in the metal ring that mounts them causing noises. I think that is a long shot though.

My money is on a bad component somewhere.

I have built some amps with power tubes really close to the speaker magnet. Like 1/2" or less. I did not find issue with them.

It could very well be in the Reverb circuit. Try disconnecting the reverb cable if you can and see if it stops. If it does, clean all the cables jacks to the reverb and try it.

 

[Lowspeid]

I checked everything I could think of and found a microphonic cap and microphonic diode. As soon as I plugged in my guitar both were almost inaudible when tapped. I smacked on the tubes a good bit and couldn’t get them to ring/rattle. I swapped them in and out and couldn’t get them to really ring/rattle. A couple of the preamp tubes made a louder “thump” than others, but none were making the ring/rattle that I was hearing when playing.

I put the set of tubes in there that I thought were the best (EHX el84s, an old RCA in V2, and the original 12ax7 in v3), tightened everthing down that I could find, and buttoned it up. Played it a bit and… Still making that ring/rattle when playing low E/bass heavy open chords. I’m thinking there’s something inside the amp that’s vibrating causing the ring/rattle. Maybe the wire tube holders that Vox uses?

I’ve included photos of the microphonic cap and diode I found, and a recording of the ring/rattle. Ideas?

 

[colnago]

Sounds mechanical to me. Something is physically rattling somewhere.
I hate these situations. Can you play and chopstick around inside at the same time and see if you can tap the suspect rattle?

 

[Lowspeid]

Can you play and chopstick around inside at the same time and see if you can tap the suspect rattle?

Unfortunately not with this amp. It’s pretty much a closed cab once the amp is mounted to the cab.

 

[Dacious]

Sounds to me like something is loose inside. The tube retainers on these are just twisted bits of metal. If you stretch them and they only fit loosely they can rattle against the tube. It's unlikely two sets of tubes would both have loose elements.

 

[Peegoo]

That sounds more like a speaker buzz or cabinet hardware rattle than something wonky in the chassis.

Try this: leave the speaker in the cab, but disconnect it from the chassis connection and power it from another amp. See if the buzz is still present.

 

[Burning Fingers]

I had to find the cause of a buzz noise in a newish amp recently...drove me mad until I connected the amp to an extension speaker and the noise was gone.
So connected the speakers to another amp....noise was there.
Thought at least one of the speakers was not mounted tightly enough or was crook.
Tightened every screw in the cabinet and the speaker screws...no difference so pulled the speakers out and had a look with a flashlight.
There was the problem...2 speaker screws stuck inside one of the speakers and held in position by the magnet.
God knows how they got there or how long they had been there.

 

[Jon Snell]

There you are ... found it.
Two possible issues here;
C59 microphonic and a dry joint. See arrow.
C59 is in a very sensitive position!
Also you will notice R58 and R95 must be at least 1mm off the pcb. If they are not, replacle them both with 220k and 100k 1/2Watt MO or MF type resistors and mount them 2mm off the board to allow for cooling and added reliability.

 

[goldguitarguy]

Sounds like tube rattle in the video. Common issue with the AC10. Also make sure the wires are not contacting the tubes in anyway when the amp is closed up.

 

[Lowspeid]

Sounds like tube rattle in the video. Common issue with the AC10. Also make sure the wires are not contacting the tubes in anyway when the amp is closed up.

What wires? The ones that slip over the tube and “lock” the tubes into the socket? I was thinking about removing them and seeing if that fixes the issue (older AC15/30s never had them, as far as I know).

 

[Lowspeid]

So I removed the wires, new tubes, made sure nothing was touching inside the chassis, removed the speaker and put it back in making sure it was tightened correctly… and still getting the “ring/rattle”. After listening more closely I think it’s coming from the speaker. I’m not sure if it’s the voice coil, or something else in the speaker, but I think it’s something mechanical in the speaker.

 

[King Fan]

Good, you're on the road; keep isolating each variable. Do you have another cab where you can mount that speaker? If not, you can still test the speaker sitting on the bench. If the rattle goes along, you can search threads about speaker noise (debris in voice coil gap, whisker-wire vibration, voice coil rub, subtle tears, etc.) Heh, only a dozen possible causes.

While on the topic, there's a risk of over-tightening the speaker bolts when hunting buzzes. Doing that can *cause* voice coil rub.

If the speaker is quiet in another cab, though, process of isolation/elimination gets even more important -- we have to look at the other 988 causes of buzz/rattle. Only half-joking; I often quote this list sent to me by an amp-smart buddy. He'd had most of these -- and I've actually had a couple others.

• loose board or component mounting screws or nuts -- transformer, tube socket, jacks, pots, etc.
• unused solder lugs on tube sockets (especially the rectifier)
• lugs in use but with enough play and metal-on-metal action at the bottom and a long enough wire to vibrate
• ceramic sockets with little broken chips of ceramic or loose lugs
• leads on the board that look fine, have tight joints, etc. but vibrate like a snare drum snare on the right note
• components that look fine but do the same thing on the board or chassis
• space between the back covers and the furring strips they screw into
• loose aluminum or screen shielding on the underside of the cab top where the chassis snugs up to it
• long flying leads that rattle against a component on the board
• the lamp holder bayonet against the chassis
• the lamp bulb and / or the spring electric contact that pushes the bulb's two little lugs up against the holes in the lamp holder (sometimes filing the solder blob on the bottom of the bulb smoother helps keep the spring in contact with it)
• the lamp jewel and / or the bezel that screws down on it to clamp it to the outside of the chassis
• those bearclaw or spring-type tube clamps
• those noval aluminum tube shields
• plywood voids with dried glue or debris inside
• bubbles in the tolex or tweed that allow the shellaced or hardened glue on the inside of the bubble to rattle against the cab
• loose faceplates or back panels
• small washers, wire fragments, solder scraps etc. in the chassis
• loose or stray metal scraps or washers between the speaker surround and frame
• vibrating speaker baffle screws, amp badge screws, handle screws or t-nuts, chassis bolts, speaker wires, reverb wires, tank mounts...