18Watt.com

Hi All,

I just got that AC30 I purchased from ebay up and running. It sounds good except for the sound of frying bacon with occasional loud pops. This sound happens even when the volume pots are all off. I know that in old Fender amps the fiber board can retain moisture and cause the board to act as a capacitance and will cause this type of problem. Has anyone had any experience with this type of problem in an AC30?

I would have posted this at Plexi Palace but I am not approved by an Admin yet so I thought I would ask here first

Thanks

Greg

what AC30 is this one..new or old..most likely cause..or easiest for you,a preamp tube cab be causeing it. Pull one tube at a time and see if it stops.
A plate resistor can also make the same noise,,or a cracked cathode resistor can also do it. So pulling the tube may help find 2 things. One,if its the tube,the one you pull and it stops making the noise. or if its the plate or cathode resistor,it will also stop the noise for the tube you pulled,so it may narrow it,or where to look for it.

Now if that don't cure it..things like a dirty socket,or loose conection.
Or look for a resistor in that may look like its been hot,they can crack,still work,[sorta] but pop crack and frying noises..\
As always,try the tubes first..even if new,you can have a bad one out of the box.. And since you said,up and running after you got it,what problems did it have.
I've found on the new RI amps,that the cathode resistors come loose,and other loose connections on the PC board. good luck.

Richie.

Hi Rich,

Well i believe it to be a late 60's model . It has mustard caps so I think it is. When I got it there were some components missing. I installed suitable replacements and removed a bunch of stuff I felt were not original such as a separate filament transformer for the pre-amp tubes. I couldn't use it anyway because the primary was for 240VAC. Also there was a LARGE CAP CAN (about the size of a 12oz soda can) that had to be removed. There were also some small caps and resistors attached to the input jacks after the 68k resistors that were just a butcher job that I removed. Anyway I cleaned all the pots and tube sockets with De-Oxit put new filter caps in and put it back as close to stock as I could not having much experience inside an AC30. The amp now sounds pretty good except for the pops and crackle occaisionally. I was told by the guy who sold it to me that it was abandoned because of a noisy tube socket that they were too lazy to fix. I assume now that this snap, crackle, pop I have is what he was reffering to and I don't believe it to be a tube or a socket but I will try pulling the tubes one at a time.

Here is what I can tell you, The problem happens at a nominal volume and is not affected by volume control but is affected by the tone controls (tone wise not volume wise). When the loudest pops occur none of the tubes show any fireworks internally ( I watched it in the dark for 10 min). Otherwise it sounds like an AC30.

Hi HeeBGB,

Wow, you did get that one. Heh, my chassis and boards for the AC30 still lying there on the floor.

Turns out the chassis is a 71 or so. It was rubber stamped inside the chassis. Dont know about the boards, they came off a different amp.

About the problem (other than what Richie said):

What type of resistors are there on the board? If they are the porcelain types, they could be prime suspects. Here (very humid), this is almost always the case with AC30s.

Also, is there a specific pot which makes the loudest pops when adjusted?

Thanks!

nik

Hi Nik,

I think all the resistors are CC's. I am going to post pics later today of the internals and you guys can take a look see. As for the pots...no other than the tone pots changing the tone none of the pots really afect this problem. I really need to put more time into it. I haven't spent that much time on the amp as of yet.

Nik...do you have the parts to make up a vintage style AC30 board?!!

Greg

Hi Greg,

if it´s still the same with the volumes turned down, there isn´t that much left but the PI and output stage. Might be one of the PI plate resistors or one of the power supply resistors (i.e. the 22k that feeds the PI).

It could be a coupling cap as well, but more likely a CC resistor since those are notorious for being unreliable and getting terribly noisy.

You can short the PI plates to ground, one at a time, to see if it´s one of the plate resistors.

My AC30 had a noisy trem channel that turned out to come from one of those crappy CC´s...

Regards,
Adrian

Agreed, almost 100% it´s the plate resistors of the PI. While you are at it, swap all electrolytic cathode caps and all cathode resistors, too.

I have had a few JMIs on the bench that behaved lke this.

Please don´t forget to swap the screen resistors of the EL84:s - they *will* be off spec. Put 1K flameproof stuff in their places.

Cheers /Richard

HeeBGB,

Greg?

Ok, the porcelain resistors I meant are the white colored ones. They look like the carbon film resistors that are in Marshalls. But these have some sort if end caps (red, green) at the ends.

Sorry. I think they are carbon (not sure if film or composite). We know em as porcelain here..hehe..

Anyways, these are notorious to be off spec.

Also, the frying of bacon you hear - is it more like, when you turn the pot, the sound/pop is like something "escaping". Dont know how to describe this, but if you sort of think so, it could very well be grounding adding to the problem.

Try holding the chassis (metal part) while you turn the pot. Does this reduce the loudness of the sound?

I had one amp here wired for 120V connection using step down transformer. I forgot the step down only had 2 prong connection (no ground). Problem (crackle on pot) gone when trans changed to one with ground.

This is in addition to the suggestions which would be likely as well - arcing, resistors, loose connection, etc.

AC30 parts? Which ones do you need?

Thanks!

Nik

Hi Nik,

It's definately not a pot. They are scratchy even though I've cleaned them but it happens with the volumes all in the off position. I haven't had time to replace the PI plate resistors yet but I will get to it.

As far as parts are concerned right now I need the lamp assembly and a 16uf x 16uf 475V cap can with a 1.5" diam. Also I need the Trem/Vibrato rotary switch. I have an old one salvaged form another piece of equipment but it's not the right one, so if I can find a better replacement, I will use it. I am also looking for Vox style component boards if I can find them.

Thanks

Greg

Power tubes or recto tube and preamp tubes can cause the noise too,You might swap those if you have some good ones. Caps can also make the noise. So you see,many things can cause that type of noise.Always try the easy things first,like the tubes.

Richie

Timesaver tip:
If you have access to a ´scope you can see the pop/crackle and instantly verify which stage creates them. Since it pops with all channels at zero volume, you already know it has to be the PI or later. Hook up the scope on the "far" side ("cold" side) of the coupling caps that go from the PI to the power section, set the scope to 100mV/cm or so and a little slower than flicker-free on the timebase, watch the screen. Pops in trace? OK, now move the probe to the input grid of the PI and wait. Pop on screen and in speaker? No pop on screen, but pop in speaker? Now you know where to look for bad resistors.

With a dual channel scope you can watch both sides of the PI at once.

Cheers /Richard

Thanks guys....Problem solved....I think. I replaced the 100k ohm resistor on the TB 12ax7 tube socket between pins 1 & 6 and that seems to have solved it. It is late so I was only able to play it for 30 min or so but prior to changing the resistor it would have shown up in a min or two.
I will do a better test tomorrow.

I did try hooking up my scope but I don't quite have the hang of it yet. I'll have to get my brother to help me out with that.

Anyway..Thanks again

Greg

Hi all,

Sorry, been busy trying to do..er..all these things.

Greg: I am preparing to offer these boards. Email me.

http://www.ceriatone.com/ac30.htm for preview.

I modded the board slightly, esp not having components flying.

Now, need help guys:

1) What would be more desirable - a 4 input non TB with EF86?
OR 6 input with built in TB?
OR 6 input, with TB, but with EF86 in front as well?

2) If the 4 input non TB EF86 is more desirable, is it the case the TB thingie available from Vox to fit in? If not, anyone can help with dimensions of the L bracket unit?

3) I have not seen any added on TB AC30 in my life. Do you have to cut the cabinet to install it?

Thanks!

Nik

Hey Nik,

I would say the TB would be most popular. In vintage amp circles they fetch the most money and are more desireable. That is the one Korg reissued. I would guess that most DIY amp builders would want the TB first but that's just a guess, I really have no clue.

Greg