Vox AC50 head wrecking problem!!:-(

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leitrim_lad

Well-known member
Joined
Aug 23, 2010
Messages
217
Location
Leitrim, Ireland
Hey everyone,

Im sorry to be posting this here, i know its a bit off track but im wondering if ye could help me with a problem im having with my vox ac50 i cant get my head around..

I have been given an very old ac50 which isnt working.. When i plug in a guitar all i get through the cab is a harsh distortion noise when i play hard. If i play light nothing comes through. When i turn up the volume however when playing light i seem to get the same noise as when i play hard. It happens on both channels (normal and bril). Iv tried different cabs but to no avail.

Would anyone have any ideas what might be wrong? Iv checked all the soldering and everything seems to be ok but id be a fairly new at the DIY game!!!;-)

I followed the schematic and see that the R29 which seems to be a 750r 3w resistor seems to be bloted out. Im thinking it might be blown. Would this cause the sound im getting?

Thanks a million

I can post sound samples and pics if need? Any help or advice ye could give me would be great..

Cheers

Schematic:
http://backfromthesixties.co.uk/images/AC50%20Circuit%20Diagram.jpg
 
Have you checked the voltages?
In particular on the screen grids (pin 4) of the EL34's and R30/R31. Very likely the 750 ohm resistor is open-circuit so the screen grids are all over the place. This is a quite common fault. Under such conditions, the EL34's operate as triodes in class C, so only the high part of the signal gets amplified.
 
I know this is a stupid question but how do you go about checking the voltage of the screen grids and what should they measure? I replaced the r29 (750r) resister and when i turn on the amp it just burns out straight away.. Would you have any ideas? Cheers for the reply, appreciate it..
 
Have you re-tubed it? WIth old amps this is a good place to start.
New power tubes need to be biased with RV4 & RV5.
 
Cheers for the reply, yep i retubed it twice.. with the original and the 2 new set of tubes its the same result. I didnt bias it though, would this cause r29 to keep burning out? How exactly do i go about biasing it with a multimeter?? I read its meant to read between 30-35ma and both tubes are meant to be the same? Not sure if this is correct?

Thanks a million
 
There's one thing to be carefull of is the high voltage present in the amp even after it's turned off and deplugged from mains, maybe you know but it's always better to know again :).
The high voltage will be stored in the Big caps, and it's possible to discharge with a simple tool that you can make yourself or even use the components inside the amp to do so, as Mad ax pointed that out to me when i had a guitar amp problem, and this has to be done each time you'll want to check the circuit when off, one hand in your pocket when you want to check with multimeter and mesuring devices clipped when checkin voltages when power on.
Simple but important safety rules, here is the thread of the guitar amp repair.
http://www.groupdiy.com/index.php?topic=40105.0
 
a friend had a similar problem with his AC30, the first big PSU filter cap was dead (some electrolyte had dripped from it) i changed it and it fixed the problem, but then my friend said it didn't sound like it used to...
tell us when you find the problem
cheers
 
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