Mark IVa Tech Question. Amp loses all volume after 30min

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mazzith

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So a friend of mine recently picked up a Mark IV Rev A amp. It sounds great but after playing for around 30-40 minutes at low/medium volume his amp loses all volume. It is like someone just was turning the volume back as it is a gradual decline to zero. Tubes are still on and at lights are one. After turning to standbye or off for a 30 seconds the amp volume will return.

We thought this could be a tube so we switched the power tubes from my Mark IV B to his and it still does it.
After it lose the volume we wiggled the preamp tubes to see if it was a unseated tube but was not it.

Everything sounds great until it does this.


Thanks
Aaron
 
Has the amp been hit or dropped? I wonder if there could be a bad solder joint somewhere that may be cracked and begins to lose contact once it heats up. I know that sounds backwards, but it's the only thing that pops into my head right now.
 
Did you replace any of the preamp tubes? It may help to swap a new/known good 12AX7 into each position, one position at a time to locate any failing preamp tube. Also, be sure to clean the fx send/return jacks with non-lubricant contact cleaner to remove any oxidation on the jack contacts - which is a common source of signal loss. Thanks!
 
Forget what I said. I misread your original post. I read it that you turned it off for 30 minutes, not 30 seconds. Sorry.
 
Thanks for the input. We replaced all the the tubes we could. I have a Mark IVb so we were able to take all of my pre amp and power tubes, minus the v4 reverb 12at7 tube. It still had the same problem.

I talked to a tech and they suggested we plug into another amp via the recording/line out when it cuts. This can tell you if its in the pre amp section. But we decided to just take it to a Mesa authorized shop and let them work on it. Being he just bought it not a bad idea to have a tech look it over. The good news is he only spent $800 buying it (no foot pedal though)


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Please let us know what the tech finds. Wondering if it could be the low voltage regulator.
 
They said it was a transistor that had an incorrect voltage.

A $3 dollar part and a $190 bill. They quoted my guitarist for $100 and $35 for rush service. Then they billed him $190 and tried to tell him a power tube was arcing and offered to replace all the tubes and bias it for him for $320 total. We laughed at them trying to sell a bias job on a Mesa with a fixed bias. They are an authorized Mesa repair shop, so we filed a complaint on them and Mesa customer service said they were pissed they tried to sell a bias job and going to try to force them to refund some if not all of the money. Mesa also said if it is not correct they would repair it for free and refund him the $190 themselves.

One thing I'll say about Mesa, they are very expensive but they care about their customer base even if it you were a 2nd or 3rd owner to the amp.
 
mazzith said:
They said it was a transistor that had an incorrect voltage.

A $3 dollar part and a $190 bill. They quoted my guitarist for $100 and $35 for rush service. Then they billed him $190 and tried to tell him a power tube was arcing and offered to replace all the tubes and bias it for him for $320 total. We laughed at them trying to sell a bias job on a Mesa with a fixed bias. They are an authorized Mesa repair shop, so we filed a complaint on them and Mesa customer service said they were pissed they tried to sell a bias job and going to try to force them to refund some if not all of the money. Mesa also said if it is not correct they would repair it for free and refund him the $190 themselves.

One thing I'll say about Mesa, they are very expensive but they care about their customer base even if it you were a 2nd or 3rd owner to the amp.
Click to expand...

Impressive customer service from Mesa!
 

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