Avt tube problem???

[marcofry]

My avt-20's preamp tube is making a shimmering sound that sounds like the filament of a light bulb vibrating. the sound seems to be amplified and worsens as the amp gets warm. it is also worse along a certain freq range.

Upon pulling the amp I found that the sound stopped when I held the handle end of a screw driver against the tube..while I had it open I also noticed that r53 was burnt beyond recognition any thoughts.....Is the tube in need of replacing??? the amp is approx 5 years old..what about the resistor I don't have the scematic handy....AARGH

 

[CaughtLikeFire]

Re: Avt tube problem???

My avt-20's preamp tube is making a shimmering sound that sounds like the filament of a light bulb vibrating. the sound seems to be amplified and worsens as the amp gets warm. it is also worse along a certain freq range.

Upon pulling the amp I found that the sound stopped when I held the handle end of a screw driver against the tube..while I had it open I also noticed that r53 was burnt beyond recognition any thoughts.....Is the tube in need of replacing??? the amp is approx 5 years old..what about the resistor I don't have the scematic handy....AARGH

not sure about the resistor but I'd get it checked out. it wouldn't hurt to go with a nice new 12ax7 like a tungsol reissue since they're less than $15 but I wouldn't install a new tube until you have a tech check out the circuit.

 

[marcofry]

Re: Avt tube problem???

I'm not too familiar with tubes I'm assuming that the tube is going south...probably...
The amp is playable it is just annoying. the suggestion of the tungsol brings up another question when replacing the tube...certain brands are associated with a certain sound??
electo-harmonics-groove-mullard=???sound. Also what part # would be the orig marshall, it is marked vlve 0065 wk 7/5??

 

[CaughtLikeFire]

Re: Avt tube problem???

I'm not too familiar with tubes I'm assuming that the tube is going south...probably...
The amp is playable it is just annoying. the suggestion of the tungsol brings up another question when replacing the tube...certain brands are associated with a certain sound??
electo-harmonics-groove-mullard=???sound. Also what part # would be the orig marshall, it is marked vlve 0065 wk 7/5??

different preamp tubes sound different. the original is probably an old Sovtek - Marshall used a lot of those but I can't be sure. the number you mentioned sounds like a stamp from the factory - not a model number. If you asked for a Marshall part, they'd send you a 12ax7 from whatever manufacturer(s) they pulled their current stock from. they may not even know the original factory or brand. good news is that it's not like they put anything special in there. if you want the sound to remain the same, I'd try a Sovtek 12ax7wb. A selected Chinese 12ax7 like a Groove Tubes 12ax7C or Ruby would work well too. The Tungsol reissue is a good all-around tube with high output and gain which is why I recommended it. a tube with low gain or current could make your preamp sound not so great. any 12ax7 or Ecc83 (same tube, different name) will work though.

www.dougstubes.com is a great tube vendor that could make a recommendation for you if you're really not sure. personally, I'd buy 2 or 3 different tubes and see which tone sounded the best to my ears (JJ, select chinese, and tungsol would be my picks)

 

[marcofry]

Re: Avt tube problem???

Thanks, for the suggestions...I'll probably buy three or four and try them it looks like they are under 20$ for the most part..
Still wondering about R53...all my tools are at my brothers...my owners manual for the amp too..I wonder if there is a schematic in the manual?????

 

[357mag]

Re: Avt tube problem???

When I had to replace tubes in my DSL 50 head I tried out several until I found a combo that really makes my amp sound like it did new. I put Svetlana Winged C for the power tubes and Sovtek 12AX7WA for the preamp tubes.

I also have a new Marshall AVT20X. I like it but it seems to have kind of an annoying twang in there. I think it's the way the stock speaker is voiced. I opened up the back and it's got a Celestion G10-30 AVT in there. That's a speaker designed and voiced for this amp. It probably didn't cost much and I'm betting it's the cause of that sound I'm hearing.

I'm going to put in a regular Celestion G10 Vintage.

 

[marcofry]

Re: Avt tube problem???

AARGH! The manual doesn't have any schematics in it. I have e-mailed marshall for one..although I have been told I am wasting my time....also the manual speaks of a five year warranty..again wasting my time???

does anyone have said schematic or know what R53 is value wise and use....I found a schematic for an avt 50 and it looked like it was part of the divider for the reverb...but the labels for some of the other pots didnt match my avt 20 so I doubt that it is the same.

 

[marcofry]

Re: Avt tube problem???

hey 357mag I guess I was writing a post while you were....Yes MY avt 20 does have an annoying country twangish sound on some settings...and I have been told that an Eminence red coat will fix this...but I'm not spending any money on speakers untill I get this thing fixed!!

 

[marcofry]

Re: Avt tube problem???

can anyone point me to a schematic and component list for my amp??

 

[StillLearnin']

Re: Avt tube problem???

I don't have anything for the 20 but I do have the AVT50... Preamp section may be very similar - Look at the component designators. I've never worked on a AVT 20 but had a 50 on my bench not that long ago...

Anyway - maybe that'll help

 

[CaughtLikeFire]

Re: Avt tube problem???

can you see even part of each of the colored bands on the resistor? or is it burnt completely thru? you can Google the color coding and find out the value. then just make a trip to the local electronic parts warehouse or Radio Shack

 

[marcofry]

Re: Avt tube problem???

Hey StillLearnin...
The schematic looks to be highly similar to the avt-20 except for a lot of extra goodies It may be close enough.

and CaughtLikeFire, sadly the resistor looks it looks like it has burnt/caughtlikefire:laughing:

 

[CaughtLikeFire]

Re: Avt tube problem???

and CaughtLikeFire, sadly the resistor looks it looks like it has burnt/caughtlikefire:laughing:

:lmao:

man, I don't laugh all that often from forum posts . . . that was just stupid enough to make me laugh.

you may need to get Korg on the phone and pay for a schematic. emails may not be enough. hope it works out

 

[marcofry]

Re: Avt tube problem???

Man I called korg yesterday, the guy I talked to said they don't give out schematics??? I took another look at the avt-50 schem and alot of the resistor values are different from the avt-20...not just in a few places..alot of the cap values are similar though.

 

[CaughtLikeFire]

Re: Avt tube problem???

Man I called korg yesterday, the guy I talked to said they don't give out schematics??? I took another look at the avt-50 schem and alot of the resistor values are different from the avt-20...not just in a few places..alot of the cap values are similar though.

sounding more and more like you may have to take it to a tech . . .

you can try here: http://www.drtube.com/marshall.htm

see if the 1st gen Valvestate 20W or 2nd gen Valvestate 30W have much in common with your amp. that's the last free solution I can think of

 

[marcofry]

Re: Avt tube problem???

Thanks for the help...the avt 50 is the closest...but not close enough.
I really appreciate the links though.

I'm giving up on finding a schematic for free. Marshall said that parts are still covered....parts cost is the least of my worries...its labor cost per hour and the fact that the closest marshal auth serv is an hour+ away...

also, I'm picky about people woking on my stuff...I work with some technicians who can barely solder...although I guess its not really their cup of tea/part of the job.

Maybe when I take it over there I can get a schematic for future use :bigthumb: