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HpW 22nd February 2012 07:45 AM

Cary SLM100 question (older brand)
Cary SLM100 question:

I have two older SLM100 may be from the early 1990 (mono power amplifier, each with 4 KT88). The idle current is set here as "self bias" over a 150E 25Watt power resistor. Each tube has a 10E resistor before connected to the common 150E, then it is connected to a 0.5A fuse and then final connected to ground.

What me puzzles is the following:

a) I had to replace the trani who went hot and noisy, may the cause of the high idle current :mad:

b) the total bias current measured here is about 320mA !!!!! or for each tube it is about 80mA * 450V (Anode/Kathode) = 36W :confused:

c) the latest SLM100 circuits are showing a manual bias (only a single trim-pot for all 4 * KT88 tubes) and recommend to set them to 175mA

in other words, something is here completely wrong or more then different :xeye:

any comments on this are welcome


kevinkr 22nd February 2012 04:22 PM

I believe the new amps are fixed bias, whereas the old design is not. Given the use of cathode bias with a quad of KT88 the idle current is not unreasonable for a 100W amplifier. It is run hot in order to keep it from sliding too far into class B at the high end of the output power range.

It's a design choice, not one I would have made, but probably perfectly reasonable given the design goals of the original designer at the time.

You do need to choose good quality, well matched KT88 for use in this amp's output stage, and in fact for the later model as well since there are no provisions for balancing the output stage.

Are you sure the 150 ohm resistor is just rated at 25W as it might be a bit marginal if that is the case, I figure it is dissipating at least 17W... I'd measure it, make sure it has not drifted over time, and probably replace it with a resistor rated at 50W.

There should be no problem with 320mA idle current with a properly designed power transformer - is this a 120V/60Hz model you are running on an auto-transformer or a bonafide 230V unit? (Not all transformers designed for 60Hz operation work well on 50Hz.)

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