Need a little help with AC30 VR sound problem.

Vox and Hiwatt Discussion

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chikov
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Need a little help with AC30 VR sound problem.

Post by chikov » Mon Jan 08, 2018 2:46 am

Hello fellow amp fans and builders. A friend of mine asked me to help to fix his amp... The problem is that it is only a partially tube amp (just one 12ax7 in preamp), the rest is solid state... Every time you turn it on it has a high pitch sound (I checked it with my sound spectrum analyzer and saw it pic at 6.5 kH, 13.5 and 19KH the same time). Nothing affects this high pitch noise except when I rotate the volume knob (barely shifting the pitch - see the video)
https://www.youtube.com/watch?v=aBAbQW1_8pQ
Nothing is plugged in and I removed the 12AX7, still the noise is there. I pocked around with the bamboo stick even checked the waveform with my o-scopeon on the output ( see attached photo).
Does any of you had an experience with the same type of problem and know what is causing it? Does anyone has a link to the schematics of the particular hybrid amp?
thank you.
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pompeiisneaks
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Re: Need a little help with AC30 VR sound problem.

Post by pompeiisneaks » Mon Jan 08, 2018 2:49 am

I don't know loads about solid state stuff. If the tube's out and you've still got the noise, it's likey a problem with the signal path in the SS section. I'm not sure how many people we have here that are great with SS, but I know the Hoffman Forums el34world.com have a good group that is decent with SS. On the other hand, that's a current production amp I think or very recently made at any rate, and Vox like many manufacturers tends to keep it hidden. I would make a guess that some capacitor is gone that would otherwise filter out something like this, but not sure. Have you looked around for burned looking components? Gotten the horrid noise going and probed components with a chopstick? It could even maybe be a solder connection that needs reflowing etc. Just a few suggestions.

~Phil
tUber Nerd!

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xtian
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Re: Need a little help with AC30 VR sound problem.

Post by xtian » Mon Jan 08, 2018 4:33 am

Here is a great forum for tube AND SS electronics:

http://music-electronics-forum.com/f22/
I build and repair tube amps. http://amps.monkeymatic.com

lovetone
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Re: Need a little help with AC30 VR sound problem.

Post by lovetone » Fri Jan 12, 2018 8:52 am

Which Vox amp is this, do you have the schematic?

lovetone
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Re: Need a little help with AC30 VR sound problem.

Post by lovetone » Fri Jan 12, 2018 8:53 am

Sorry has the amp just started doing this?

chikov
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Re: Need a little help with AC30 VR sound problem.

Post by chikov » Sat Jan 13, 2018 5:38 am

lovetone wrote:
Fri Jan 12, 2018 8:52 am
Which Vox amp is this, do you have the schematic?
that is the problem. I do not have and I could not find any schematics for this amp... So I thought, may be someone already have had a similar problem and who can tell me, "yep, it is your power transistors going out" or something like that. Tomorrow I will mess little more with it and try to give you a small update on it.

chikov
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Re: Need a little help with AC30 VR sound problem.

Post by chikov » Sat Jan 13, 2018 5:41 am

lovetone wrote:
Fri Jan 12, 2018 8:53 am
Sorry has the amp just started doing this?
I do not know, plus, does it really matter when It started doing it? i mean, it is doing it now...
Honestly, because the master volume does not attenuate this noise at all, I think the problem lies past it... So, I am leaning towards power amp transistor chip... I just do not have the proof of that suspicion yet

lovetone
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Re: Need a little help with AC30 VR sound problem.

Post by lovetone » Sat Jan 13, 2018 11:33 am

Now I know which amp it is, these are quite a strange circuit.

The valve works with an anode voltage of 15V and simulates a phase inverter in the power stage. The output amplifier is an LM3875 I.C.

The master volume is not as you would expect, it seems to work in a feedback loop from the output stage so does not cut the signal like a regular master volume.

The fault might well be in the preamp stage, maybe the reverb.

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