Gotek

My first valve amplifier - Adjust Bias Current. What's that all about then?

8 posts in this topic

At work all day so time to ask a question:

Got my first valve amp for Christmas. :)

Instructions say:

Step one: install valves. Done.

Step two: Adjust bias current and shows two small screw adjusters on either side and remarks:

Adjust the adjustable resistance to the max, DC voltage is 0.2

Cut in power tube, adjust adjustable resistance, the DC voltage as follows: Power Tube Kt66 : 0.38v & Power Tube EI34 : 0.32v

What the hell does it mean and do I need to do anything? That's all the manual says, no mention of how or what the 'adjustable resitance' screws do on the sides.

Thanks

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I will be suprised if it isn't already set to the right bias for you from the factory. Plug em in and enjoy.:)

Is it a ChiFi amp....google it and check to see what others have done.

Thanks I'll google

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I'm no technie but in some valve amps the voltage (and therefore current) is auto-biased. In others you have to do this your self. There should be a meter on the amp. If not you'll have to use a simple voltmeter. The instructions should tell you what value to set it to.

JJ

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Bias is the amount of current (which needs to be balanced between output tubes) across the elements. Auto Bias is usually cathode biasing which regulates power between tubes, but in some circuits (like mine) you still need to ensure that you're not running individual elements too hot, as this can end up with serious consequences. Usually the valves are installed and connected to a load (ie speakers connected) with a meter across the element pins, and the element current is set to the recommended setting until are are operating at the same level. Remember never to switch a valve amp on without load being present (ie never turn on whilst speakers are disconnected) and remember also that you're dealing with very high voltages in the HT end of the circuit...anything up to 550v.

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Which output tubes are you using? Your amp appears to work with KT66s and EL34s, which have different specs. Your amp has the flexibility of changing the output valve type, but with that comes the onus of manual bias.

What it means is that you first turn the bias resistor to the max resistance, i.e. lowest voltage, to prevent too high a voltage getting to your valves. Then you put the valves in and turn it on, and change the bias resistor so that it is at the right level for your valves - i.e. the voltage is as stated for the output valve type. It may change very slightly as they warm up. Manual bias amps often have a voltmeter to allow you to easily see what you are doing (mine do).

You do want to get this right - running the valves at the wrong bias will significantly shorten their lives and may cause stability problems with the amp in extreme cases.

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Okay. The tubes are: 4 x EL34-B power tubes and 1 x 12AX7 and 2 x 5670

The amp is a Yarland FV-34B and there is no meter.

I have emailed the supplier but do not expect and answer until after the New Year.

So my question relating to the advice given above is:

How do I measure the voltage? I guess I use a voltmeter but where do I put the prods?

I guess I am looking for a measurement of 0.32v? :dunno:

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Gotek, do you have contacts to measure the bias from? For instance, on my Yaqin MC10 there were 4 sets of contacts for each of the OP valves, with a common earth for each pair. You stuck a multimeter into the connectors, then measured resistance, and using an allen key set bias for each valve in turn. It was fiddly, and you felt like you needed two hands, but do-able. My current amp has one connection for each valve, so I need to figure out where the -ive lead from the multimeter goes...

If there isn't anything obvious, you'll need to measure from the internals. Please proceed with caution, and only if you really know what you're doing!! Voltages inside valve amps are often kill-you lethal, so you don't want to be poking round in there. Take it to someone who knows what they're doing, watch and learn for the future...

Jason

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