Tube Amp bias

[nc42acc]

Anyone with tube power amps try different bias settings? Do you always use the manufacturers specs? For example the ARC REF75 recommends 65ma on the KT120 but I think 45ma sounds much better.

[microstrip]

I have tried several times running amplifiers at lower currents, mainly in summer trying to reduce heat, but always found it degraded sound quality. My experience was with ARC,cj and VTL's power amplifiers. However if your speakers are very efficient, probably you can reduce the bias setting without leaving the class A zone most of the time. Or most probably you just prefer another type of sound than me!

I have read that with some cheap amplifiers that have poorly quality or/and under-dimensioned power supply transformers reducing the bias can improve sound quality - surely not the case of the ARC REF75.

[nc42acc]

Maybe I have a bad REF75 or bad tubes.

[microstrip]

Maybe I have a bad REF75 or bad tubes.

Not forcefully. Not all systems are similar. But on these symptoms I would check the quality of the mains.

[MylesBAstor]

Marty, how are the new amplifiers sounding?

[nc42acc]

Myles it has been frustrating to say the least. Dead KT120 tube, brand new KT150 that wouldn't bias and one died. Makes me want to go all SS.

[mep]

Myles it has been frustrating to say the least. Dead KT120 tube, brand new KT150 that wouldn't bias and one died. Makes me want to go all SS.

You are still up and running with the KT-120 tubes from your Rogue amp?

[nc42acc]

Yes the REF75 is working fine with the KT120. I ordered a new set of KT120 for the Rogue ST100 and one tube is super noisey. UGHH!

[microstrip]

Myles it has been frustrating to say the least. Dead KT120 tube, brand new KT150 that wouldn't bias and one died. Makes me want to go all SS.

If a solid state tube breaks, usually repairs take much longer, take a lot of logistics and cost a lot more. In thirty years of tube equipment, all I had to replace was tubes , a few inexpensive cathode resistors immediately diagnosed by their aspect that were replaced in place and an FET in the power supply of a cj preamplifier.

Golden rules - use good quality tubes from a reliable source, assure they come properly packed and remember that they need to be replaced after their service life, not after they go in short circuit! If possible burn-in tubes without signal for a few tens of hours before applying full power signals - during the first times the small micro- imperfections of the electrodes get "polished", reducing the probability of early failures.

[MylesBAstor]

If a solid state tube breaks, usually repairs take much longer, take a lot of logistics and cost a lot more. In thirty years of tube equipment, all I had to replace was tubes , a few inexpensive cathode resistors immediately diagnosed by their aspect that were replaced in place and an FET in the power supply of a cj preamplifier.

One problem with ss now too is that parts go out of production quickly too.

Golden rules - use good quality tubes from a reliable source, assure they come properly packed and remember that they need to be replaced after their service life, not after they go in short circuit! If possible burn-in tubes without signal for a few tens of hours before applying full power signals - during the first times the small micro- imperfections of the electrodes get "polished", reducing the probability of early failures.

Maybe the most important thing to do for your tubes. Not only does "tempered" burn-in prevent early failures but it extends tube longevity. That is what you are paying for in part when you buy your tubes from say ARC or cj or reputable sources like Kevin Deal. Highly recommended!

[nc42acc]

All tubes were purchased from reputable sources and I paid for a 24 hour burn in. I think going direct to the OEM is the best bet for tubes, albeit at a higher cost.

[MylesBAstor]

All tubes were purchased from reputable sources and I paid for a 24 hour burn in. I think going direct to the OEM is the best bet for tubes, albeit at a higher cost.