Leak Stereo 20-Delivery mileage.....

[Wilbur]

Hi folks
Having a major,major rethink in terms of kit and general life. Ive had this a couple of years.
Believed to be unused since leaving the factory,still with all the lovely Mullard valves,and generally unmolested:eyebrows:
Did the usual "bring up" via the light bulb method to prevent scaring the crap out of it.
It sounds great on one channel...the other sounds like some cheesy 50's sci fi interpretation of what a ray gun would sound like!
So,back in the box it went,and has stayed there.

Id really like to get this going,so if anyone has any pointers,or anyone local with a valve tester...start with the bleedin' obvious,although they havent been used,they may have had a fright.

Will post pics and more info when I get it out....the amp that is.....

[Firebottle]

Hi Paul,

When I looked at Gordons amp I found that one of the phase splitter load resistors had gone high resistance.
Also even though it hasn't had much use I'd check, or if that's not possible, replace the electrolytic caps on the output valve cathodes resistors.

If you are OK with using a multimeter the circuit is readily available on the web, and it's an easy circuit to check out.
If you are not confident, particularly as high voltages are involved, see if you can find a local techie, or a radio amateur who may well have experience with valve equipment.

Good luck with it, it's a great valve amp.

:cool: Alan

[mik_rik]

Hi there simple first thing swap the valves to see if the fault changes channels ,if it does then 1 of them has gone t_ _ s up,if it stays on the same channel then it could be something as simple as a dirty valve socket,try gently removing and refitting each valve each in turn a few times After that it could be more complicated but try the easiest/ more obvious first .If needs be I've got the valves to substitute for you.Don't worry, apart from me, there's nowt made that can't be fixed. p.s I'm . only a few miles up the road

[Wilbur]

Thanks folks!
Im ok with a meter etc,so ill check what i can.Am i right thinking ill have to unsolder 1 end to test the resistors?
Its only 1 channel,so a bit swapsie might help valve wise to track it down.
Give me something to do..:)

[Wilbur]

where ya from Mick?

[mik_rik]

M_ _ ton

[Wilbur]

That IS close!

[mik_rik]

Yep by the way I was born in Peterlee , went to the grammar school there,but was too young to see the likes of Led Zeppelin ,Jethro Tull et al at the Argus Butterfly in the late sixties [yes,really].We moved from there when I was three but I like the place,it was my patch when I was a service engineer many years ago

[Wilbur]

Dont forget Ten Years After! Propper music,I well and truly missed it,im 38!

[Naughty Nigel]

I would be happy to have a look at it for you Wilbur.

My guess is that one of the electrolytic de-coupling capacitors has either broken down, or more likely has gone open circuit (dried out), leading to instability.

Electrolytic capacitors don't usually have a very long life (15 years is typical), but more importantly, their rated working voltage must be maintained by regular charging to that voltage. High voltage electrolytics which have not been used for many years can be re-formed by slowly building up voltage via a high value resistor, (100 KΩ is usually a good start), but this is best done on the bench, as other methods don't really provide enough control of voltage or current. A series bulb in the mains circuit or a Variac will prevent major disasters, but if the cathode in the rectifier valve is hot enough for anode current to flow, the HT voltage will still be 150 to 200 volts or so, and significant current will be available.

Indeed, it is worth noting that the safe working voltage of an aged electrolytic capacitor is only about as high as its normal working voltage. So, if an electrolytic capacitor with a working voltage of (say) 450 volts has only ever been charged to 300 volts, increasing the HT voltage to 450 volts will probably destroy it.

The same advice applies to electronic flashguns; all of which contain large, high voltage electrolytic capacitors. These need to be switched on and charged, and ideally left on for a few hours every three months or so to maintain the capacitor. Otherwise it may just go pop when you need it!

Nigel.

[Naughty Nigel]

I should add that valve amps must never be run without a loudspeaker or appropriate load connected, as there is a very real danger that the open circuit voltage across the output transformer windings will destroy it.

[Wilbur]

Hi Nigel!
Havent had a chance to look at it myself yet,but if you fancy a peek at it that would be great!
I did put a speaker across it when powering it up via a bulb method,just in case!
Thanks
Paul

[Naughty Nigel]

Hi Paul, I am out all day today, but I will send you a PM in a day or two to arrange something.

Nigel.

[Wilbur]

Thanks Nigel!