Durhhh. Bias-ing valves what the?

SMEagol

SMEagol

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Any idea how to bias a push pull 50wpc integrated valve amp. The bloke that had this before me has swapped the tubes from stock 6550's to Russian Genalex Golden Lion KT88's, but I suspect he's not re-biased it.
There's a hole for a voltmeter - I think* - and a screw next to each valve. I have a volt meter, what do I set on the voltmeter (as it looks hard) what thing do I poke down there, and what reading should I be looking for on each valve. Presumably switched on with no music going through.

Hopefully the valves will be glowing blue and not me, I'm not very electrical! Its a Chi-Fi jobby so there's no manual!

(*Chris's legendary electrical prowess coming to the fore.)
 
ok.....let the amp warm up

generically you stick the red probe in the hole provided ..the black to a suitable chassis point or speaker negative ...

set the multimeter to milliamps then slowly turn the screw to give the current desired ...

Ive no idea for the kt88's ......you could start by seeing what the current range is and then set them all to three quaters and then drop it back

more current reduces the life of the valves ....making a fuller richer sound ...less current [to a point] extends the life but will sound thinner ...

as to specifics for your amp that's hard to say as I'm not familiar ...but there should be a biasing current that gives the best comprmise for the kt88 someone will know ...

points to be aware of ........

valves get hot !! if you touch one you will know about it ...

high voltages are involved ....so don't poke fingures or other appendages into holes !

generally take yor time and be careful ......


don't pull valves out if the amp is on a shelf ....yes I smashed on on the underside of the top shelf when it popped out.....

get yourself a spare set of valves too ......always useful in fault diagnosis

hope someone has more detail for you ...

what was the amp ? .
 
Do use a (well) insulated screw driver!!

I have two of the riht size bought specifically for that purpose.
 
And don't forget to keep the speakers connected,or at even better a suitable value resistor as a dummy load instead.
 
Thanks guys, sorry not been about. I am the original "dummy load" we'll see what happens. Could do with the Milli amps for a KT88 though. Does the milliamps icon look like an A with next to it a squiggle and a dotted line under it on the voltmeter, its area of demarkation is surrounded by a red keyline, would I set it to 200m or 200u?


...How good am I at this? :D - Don't answer.
 
SMEagol said:
Thanks guys, sorry not been about. I am the original "dummy load" we'll see what happens. Could do with the Milli amps for a KT88 though. Does the milliamps icon look like an A with next to it a squiggle and a dotted line under it on the voltmeter, its area of demarkation is surrounded by a red keyline, would I set it to 200m or 200u?


...How good am I at this? :D - Don't answer.
Click to expand...


About as good as me, mate.

I must check mine, too.


:o
 
should read mA and the range should be about 150-400mA ...at least it wa for my el34 based amp ...the same on as davidf has.

you can pick up a decent digital mm off ebay for less than a tenner if thats any help
 

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