Laney cub 12r is ir fixed bias?

midlifecrisis

had my faulty laney fixed and am about to collect it later but the guy texted me and said he couldnt get the bias steady so he's replaced the el84.    i thought this amp was fixed bias so am i being stitched up like a kipper?

jpfamps

The bias could be unstable due to a faulty valve, so this may be needed.

"Fixed bias" does NOT mean that the bias if "fixed", but that the valves are biased by putting a negative voltage on the control grid (which can often be varied!). Grid current from a power valve will disturb this; furthermore grid current often heat dependent so the bias will drift as the amp warms up, and hence will not be stable.

Amps with EL84s usually run their valves very hard, and so they often need replacing. Looking at our accounts we got through more EL84s (over 150) last year than any other valve including the ubiquitous ECC83 / 12AX7.

You should be pleased that the tech took the trouble to check the bias rather than worry about whether you are being chiselled. You can always ask for your old EL84s back.

ICBM

Yes, this would actually be a symptom of a valve with bias thermal runaway, which is more of a problem in a fixed-bias amp - in a cathode-biased one, it might more or less stabilise, even though then running too hot.

If you do get the valves back, don't use them as spares unless the faulty one is identified - throw that one out. A valve that's running away can then short, which risks blowing something like a screen resistor as well.

midlifecrisis

thanks for explaining, i was a bit surprised , my fault for misunderstanding fixed bias. He seems a good egg so I do trust him

midlifecrisis

ICBM said:

Yes, this would actually be a symptom of a valve with bias thermal runaway, which is more of a problem in a fixed-bias amp - in a cathode-biased one, it might more or less stabilise, even though then running too hot.

If you do get the valves back, don't use them as spares unless the faulty one is identified - throw that one out. A valve that's running away can then short, which risks blowing something like a screen resistor as well.

yes, will do. although my original fault, happened even when i swapped valves with another amp.