Elgen Bass amp fault diagnosis

[miain]

Hi folks,
I'm a new boy so be patient with me please.

I don't know if anyone out there can help with this but I thought it was worth a try.
I have a very rare Elgen bass valve amp that has just started to go bonkers. After switching on and leaving it on standby as usual, flicking the standby off results and a very loud low hum. It's not a high pitched squeal or earth hum, but a very noisy low frequency hum
I can hear my bass playing quietly in the background but the noise is horrendous!

The amp consists of 4 power amp valves and three pre amp valves

I realise it's a long shot that the problem can be correctly diagnosed like this but I'm just curious if it's possible to tell if it sounds like a valve problem or a blown cap

Thanks

 

[kevinkr]

Moved to Instruments and Amps. All musical instrument amp posts whether tube or solid state should be posted to the I&A forum. Please see the directives under each forum header. Thank you.

 

[kevinkr]

Sounds a lot like it might possibly be a power supply problem. (Possibly bad supply cap) In any event unless you can read schematics, understand basic troubleshooting technique, and know how to work safely with the voltages involved this is best left to a qualified service tech. (Note that if you are willing to make an investment in knowledge and the required test equipment we can probably help..)

Do not operate the amplifier in this condition - serious damage could result.

 

[miain]

Okydoke
Thanks Kevinkr, I am very interested in learning how to service this beasie myself. I do have basic schematic reading skills but I'm not clued up on how to protect myself from the large voltages involved. I think it would be prudent in the long run to be able to keep the thing trucking myself as I intend to use it a lot
Any help in where to start would be very gratefully recieved

 

[kevinkr]

Howdy,
First step is to carefully read this entire thread on high voltage safety: http://www.diyaudio.com/forums/tubes-valves/30172-safety-practices-general-ultra-high-voltage.html

Next read this: http://www.diyaudio.com/forums/tubes-valves/38278-line-tube-learning-newbies.html

Then you'll need some basic tools including appropriate screw drivers, needlenose pliers, cutters, wire strippers, a decent solder iron and a digital multimeter or two..

 

[miain]

Excellent!
I shall do just that...
Just need a decent multimeter

 

[JMFahey]

Try to get the schematic.
There are many Tube sites with links to obscure amps.
Happy surfing.

 

[Enzo]

And even for a layman, trying a different batch of tubes in it is a good first step.

 

[miain]

Thanks for the suggestions

Just an update if anyone is interested. I had to move the amp to do a spot of cleaning and now it works
Do you think it could just be a dry solder or a loose connection?
Can a faulty cap jolt back into life for a short time from just being jiggled?
No wonder Tesla was bonkers

 

[miain]

The amp was a custom job BTW so schematics won't exist. It was based on a Marshall amp though.
It was made for Nick Simper (former Deep Purple) who used two Marshall amps. One for top end and one for low end. The amp was built with two input channels.

 

[ArcticBreaze]

certainly sounds like a dry joint, probably on a power supply filter cap, and no as a rule caps that hav died don't get resurrected (even at Easter lol).

 

[miain]

Excellent!!!
I shall keep reading the safety thread and then crack on with some re-soldering

 

[funk1980]

Concerning the standby before power-up, it's a myth. Copied from Fender without thinking by other manufacturers. it actually puts more strain on the PSU, because tubes will be warm and fully conducting when the HT is eventually applied, while cold tubes will 'ease in' their conduntance. Terms like cathode stripping etc. have no scientific basis. Leaving the heater on for long periods of time without HT actually IS bad for the tube.