Valve amp bias question

Davidcat

Davidcat

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Oct 11, 2011
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Hi guys,
After many years of using class A solid state power amps, I have finally taken the plunge and got myself a valve amp. I have used valve pre amps on and off over the years but this is my first biggie. It's a JJ Electronics jj322 300b model. I bought it online from a store as an ex-display item, it looks to have had quite some use , not a problem as it was at a knock down price.
My question is this, how do I check the bias? I can't see any measuring points or trim pots. I don't think it's auto bias, although I might be wrong. Any ideas?
It does sound pretty good but I'm worried because it's used and I had to replace one 300b that was broken in transit (the seller sent a new one). Thanks in advance for any info.
 
Would you approach a car mechanic in the same fashion? "It's running OK now, but eventually something will go wrong. So what do you suggest?"

OK - let me be a little more constructive. Pins 1 and 4 are the cathode, which in your amp, being single-ended, probably connect to ground via some resistor (hopefully with some trim). Remove power, (wait for killer voltages to dissipate), unplug the tubes and measure the resistance. Then put the tubes back in, power up, and without killing yourself or your power supply, measure the DC voltage on pins 1 and 4 (they may differ by a few volts due to the filament current). Bias current = voltage/resistance. Read the tube specs: JJ recommends 60mA for its 300B. I personally like to run them a little hotter for a more German than British sound, but that's just me.
 
The above reply is very strange...The chances are your amp is auto bias, so your best way forward is to check this with the vendor /manufacturer. If it is manual bias you should normally see some bias adjustment points marked on the chassis
 

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