Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JET on November 15, 2020, 12:45:41 pm
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First time poster here on this forum. Hopefully on the right section also. Im a relatively long time guitar player but new learner about the tube amps and all other technical world of guitar signal chain.
I am the first owner of Hiwatt Hi-Gain 50 amp head (Made in China edition...) which I play trought Hiwatt NOS HG412 guitar cab. During this year of owning the amp I've had couple of issues with it which made me want to learn more about service of guitar amps. First the reverb tank failed and had to be changed, second time one preamp tube went microphonic and got changed, third time FX loop had bad contact which silenced whole amp signal chain and lastly one of the two power amp tubes failed. I've read on the various internet sources that these China Hiwatts aren't really the most reliable or well-made amps but I like its sound and will try to get it function fine.
Newest problem occurred when I turned stand by switch on after about 5-10 minutes of preheating. After I switch stand by mode off and try to start playing amp makes rustling and crackling sound which isnt really loud but audible. When I turned stand by swtich back ON there was semi-loud pop sound. I waited about minute and re-tried and same thing happened again. After second waiting of a maybe 5 minutes amp started to work as normal and popping sound didnt occur when I was finished and turned stand by ON and power switch off. So it seems like amp needs pretty long preheat time before it functions. Same phenomenom occurred earlier before the power amp valve failure, where I had to preheat amp for 10-15 minutes before it seemed to function and not produce rustling sound. I've been told that normally couple of minutes should be just enough for preheating and I've even read articles about how stand by is more harm than use for tubeamps.
Question is that what could cause the rustling and crackling and is there some components or other things that should be right away checked? Is there high possibility that valve will fail again soon? Friend of mine checked the both power valves (EL34) bias couple of weeks back and set it little bit "colder" since the bias seemed to get little bit too hot after 15-30 minutes warm up time. Bias was measured after a 60 minutes run time about: V1= 33,9 mA, V2= 32,1 mA. Amp gets played about 8-10 hours / month so I believe there should be plenty of service time left in the valves.
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Not much help but just wanted to say that many failures in one year spells LEMON.
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Return back on warranty if you can.
All about how to fix Standby switch Pop;
https://el34world.com/Forum/index.php?topic=26658.0
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Dunno, maybe, you are by far in the best position to answer your question here.
It’s beholden on a valve amp user to have a ‘known good’ spare of each valve type in the amp.
Then anytime there’s a concern, swap in the spare to the most appropriate socket. If no improvement to the issue, replace the original valve and move on to the next most appropriate and repeat, etc.
For power valves, if the amp uses a pair or more, then it’s best for their characteristics to be reasonably close to each other, and to treat them together, eg if there’s a pair, swap in a ‘known good’ matched pair. If the amp is fixed bias, the amp’s bias setting may need checking.
After you’ve followed this process but without any improvement, then the issue very probably isn’t due to a bad valve, so you may then wish to consider a tech’s assistance.