#1
Hi there,
I've been reading the UG reviews and forums for years, but now have a technical problem of my own. I've read the post guidelines & have searched for an answer, but found nothing similar to this. Seems like this is the correct place to post equipment troubleshooting.

I have had my Ashton tube amp, a VP50 1x12 combo, for about 7 years.
Most of that time used only about once a week, never past half volume.
It is designed in Australia, made in China I think.
It’s a 50w amp, using 3x12AX7’s, 1x12AT7 and 2xEL34’s.

What’s gone wrong:
The amp blew a fuse about a year ago, I replaced it, and it was fine.
Then recently the amp started making crackling noises and popping sounds, and soon after blew the fuse.
It was on it’s original tubes, so I replaced the whole set and biased the power tubes.

The amp is now working properly, apart from when it's in “crunch” mode which causes crackling/popping, or electric buzzing sounds.
On any other mode, the amp is fine.
I don’t use the crunch mode often, so I’m not sure if it was doing this before the tubes were changed.
Will letting the amp make these sounds while investigating damage the new tubes or other components? I've switched channels quickly when it started just in case.

Explanation of channels/modes:
The amp has two channels, clean and dirty, each with separate EQ settings and volume, it also has reverb and master volume.
The clean channel has a “crunch” mode, which adds a slightly overdriven, harsher sound.
The dirty channel has a “filth” mode, which increases the overdrive gain.

Any insight would be greatly appreciated. Why would one mode/channel cause problems, but none of the others?

My next thoughts would are to get into the circuitry and check that the screen
resistors are giving their correct resistance value
(after draining caps of course).

Apart from visual inspection, how can you test if the capacitors are in good condition?

There’s only one amp tech in my area (Auckland, New Zealand), and he wasn't very friendly on the phone, so I decided to replace and bias the tubes myself...

Cheers,
Andy
#2
Have you replaced the 12AX7s? There is a possibility that one of the preamp tube that is used to jack up the preamp gain is starting to go, thus its starting to make strange noises.

If you have replaced the preamp tubes, put the older preamp tubes back in and see if the problem persists.

If you've been using the amp fairly regularly, the caps should not be giving any issues.
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#3
Yea, I replaced the preamp tubes as well.

One tube (a 12AX7 closest to the input jack) has a metal cover, would this be a 'rectifier tube'? I'm not sure if the amp has a tube rectifier or not.

Thanks for the reply, I'll try the old preamp tubes again.

I plan to get into the amp this weekend, even if just for a look and check resistors, as soon as I can make a cap drain tool.
#4
that metal cover is a cover/retainer, and it adds shielding from interference for the tube as well.

12AX7s won't be used as a rectifier (usually. it can tho.) as other tubes can perform the job better. that tube is probably the first tube that the signal sees, which is the one most likely to be blamed for microphonics.

if it's a quality amp, the crunch channel probably gets a section of components independent from the other channels. something might have happened in that part. also, it could be your tube socket that's bad, where you have half a 12AX7 dedicated just for the crunch channel (very likely) and that socket is not making a good connection.

somebody would probably recommend those de-oxit stuff to clean metal contacts.

when you mention "screen resistors" those are on tetrodes and pentodes. 12AX7s don't have screens... your power tubes are fine, since they're used universally with all the modes, not just crunch mode.

if a cap is bad, you'll probably see some goo coming out of one end. or it's completely broken. but if they're bad, you'd smell it by now.

what you can do to check if your caps are discharged, is get a multimeter in DC Volts mode, set 400v or more for now, check voltage across the capacitor. black goes to - of the capacitor, red goes to the + of the capacitor. if you can read anything, theres voltage. lower the setting until you can get a clear reading of the voltage across it. do this to every cap. if they all say less than 10v, i'd say go for it. 10v means it's pretty much one of those 9v batteries. they don't hurt when u put ur finger on both the + and -.
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#5
Hey again,

I made a cap discharge tool and got into the amp today.

I pulled all the tubes, took the amp out of the combo box, and tried to discharge the caps and test for any residual voltage as best I could.

After that I checked all the large 5W resistors were giving their correct ohm values, and cleaned up the circuit board a little (had some flux type material build-up in some areas). I also found one connector that was slightly out of it's socket. Couldn't see anything burnt-out or obviously broken. Quite a few of the caps had goo at the base, it was on so many of them I assume it's a glue.

I put it all back together, and started testing.

On the crunch channel, it seemed to make some crackles at first, but then stopped and played with no trouble for a good half-hour.

Don't know exactly what's changed, but this is the longest it's played on that channel trouble-free in a long time, so I'm happy and will see how it goes.

Information on my amp seems pretty rare, so I'm uploading a schematic I found (seems correct) and a pic of the amp insides. Contact me for better uncompressed versions.

Cheers,
Andy

Ashton VP50 Insides:
http://i68.photobucket.com/albums/i23/dasnoboarder/Untitled_Panorama1small.jpg

Ashton VP50 Schematic:
http://i68.photobucket.com/albums/i23/dasnoboarder/VP50Schematic.jpg
Last edited by dasnoboarder at Jun 6, 2011,
#6
Here's a link to a few specs. Answers the question about the tubes at least.

As for the crackle, I'd bet it's possibly a dirty pot, switch, or tube contacts. I lean toward the switch or pot.