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#1 2007-06-28 19:00:24

ntek
New member
Registered: 2007-06-28
Posts: 6

amp starts cutting out after about 1 hour

i've had my v4 serviced before and the problem still occurs, power tubes were replaced but after about an hour the amp starts cutting out and or volume drops 75-90%.   anyone have any ideas?  there are so many tubes in this thing, maybe its one of the smaller preamp tubes or maybe capacitors? thanks

Last edited by ntek (2007-07-02 18:24:17)

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#2 2007-07-02 16:18:09

Dr.Tom
Member
Registered: 2007-08-02
Posts: 44

Re: amp starts cutting out after about 1 hour

I've been having the same problem. For me it starts at about 30-45 minutes after having the amp on. I tried blowing a fan on the back of the amp and that seemed to work for a while. My amp tech couldn't find anything wrong with it.

The amp is now making a very loud noise that happenes when I just turn it on. It's off to amp tech again and I'm sure he can figure it out now. I'll let you know how I make out.

Regards,

Tom

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#3 2007-07-02 18:23:41

ntek
New member
Registered: 2007-06-28
Posts: 6

Re: amp starts cutting out after about 1 hour

yeah, my amp also makes that noise.... its like a loud pop when you switch it from standby to the on position.   let me know what you find out!  thanks

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#4 2007-07-03 11:38:33

etb67
New member
Registered: 2007-07-03
Posts: 1

Re: amp starts cutting out after about 1 hour

I just scored a '74 V4-B on ebay that looks to be in good shape, and sounds great, but I'm having the same problem. After about an hour, the amp starts to cut out. So I'll be interested to see what the fix is.

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#5 2008-05-23 16:12:18

Ryan
New member
Registered: 2008-05-23
Posts: 1

Re: amp starts cutting out after about 1 hour

Hello. Any news on the cause of this? Fixes?

I'm having the same problem with mine and would love to hear an update on this.

-Ryan

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#6 2008-05-26 12:51:29

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: amp starts cutting out after about 1 hour

This may not be helpful, but if it starts to cut out after an hour, it sounds like some part is getting too hot and breaking down. You have isolated the problem to the preamp, so this eliminates a lot of detective work. If I were trying to fix it, I'd play the amp for a few hours to get it good and hot and start measuring voltages. Your amp tech should be able to do this.

Here's a great link for problem solving amps:

http://www.geofex.com/ampdbug/ampdebug.htm

Last edited by Diablo (2008-05-26 13:41:13)

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#7 2008-05-28 03:47:02

Sid
Member
Registered: 2008-05-28
Posts: 14

Re: amp starts cutting out after about 1 hour

95% of the time that I experience this type of problem with an amp, it is due to a coupling capacitor that has become leaky.  You can often find the cap that is causing the problem before the problem occurs.  You have to look at the DC voltage at both sides of all the coupling caps in your amp.  If one of the DC voltages is not what you would expect from the circuit, then that is probably the cap that is causing the problem.

This does require a schematic and knowledge of how the circuits work to determine what the proper DC voltages should be.  So the repair person should be quite knowledgeable of tube amps.  If not, then you have to wait for the problem to occur and then measure the DC voltage on all the coupling caps.  By this time, the voltage difference should be pretty gross.

Note that sometimes touching the problem cap with a probe will drain the leaked charge off the cap.  If you do this while the problem is present, the problem will go away and you have found the bad cap.  If you are probing before the problem occurs, you must use a very high impedance scope or DVM or you will never find it.

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