How often does the bias need to be adjusted on BHK amps?

Had to have my 250 serviced. For some reason the left channel XLR input wouldn’t easily release the interconnect cable, so had to have it replaced. It functioned fine, and I doubt it had an affect on the sound quality, which is why I was pleasantly surprised when I hooked the amp back up again and it sounded better than it did before I brought it in. I emailed Mark in service and asked what else was done to my amp, because it sounds better now. The only other thing they did was adjust the bias, which I suspect is why it now sounds better.
I’m now wondering how often this needs to be done, because it makes a notable difference in the sound quality?

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If I were you I’d email Stereophile and ask Michael Fremer ):-

Interesting topic. I note at least one manufacturer has been offering routine checkup and rebias service for amplifiers free of charge for years: Odyssey Audio.

I’m surprised this topic hasn’t gotten more attention. Wondering if anyone else has had their BHK amp rebiased and noticed an improvement in sound quality? My BHK 250 is 3.5 years old, and was obviously in need of being rebiased, based on what I’m hearing now. So now I’m wondering how often this tuneup should be done? If you have a BHK amp, and it hasn’t been checked in a while, it may not be operating at its full potential.

Is it something that users can do themselves? If not, then you shouldn’t be surprised (whether it is actually needed or not)

No, it can not be adjusted by the user, has to be sent in. Not surprised that it’s needed, just wondering how often this tune up needs to be done? It should be mentioned in the owners manual though, so that people are aware that regularly scheduled tuneups are needed.

Even if it can’t be adjusted by the owner (I wouldn’t want to try that myself), it sure would be useful to have a means of monitoring it, with info provided by PSA for what the factory specified range is. Being able to monitor it obviates the need for a canonical ‘how often’ checkup schedule and instead provides a quantitative basis for knowing when a checkup is needed for your particular unit.

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You are spot on and I hope there will be an unbiased (pun for fun) factory response to keeping their gear in the best spec. Companies that honestly address component degradation or drift are tops in my book.

Does anyone know what gets biased on the BHK amps? Is it the input tubes or something in the solid-state output stage? When I owned tube amps in the past, each output tube would get biased, so what gets biased on a solid-state output amp?

I suspect there can be very large differences between different amps (I have two BHK250s).

I feel that one of them takes longer (by, say, 30 seconds) to come up to full output. I say this because I connected two power meters to the input power cables.

From cold one amp drew lots of power and within, say, one minute stabilised. The other amp drew far less current and took much longer.

When I pause the music the amps gradually draw less power by very different rates. If I restart the music I get the feeling that it takes a few seconds for the slower amp to reach full output.

Support were provided with tables of the power draw of each amp - they concluded that the amps were probably at different ends of a spectrum but that it was probably best left alone (after a few seconds I can’t hear any difference between the amps).

I’m not worried about it but the amps really are very different.

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After spending more time listening to my re-biased BHK 250, I believe having it properly, biased makes more of difference than using NOS tubes. i was using some NOS Amprex tubes before I sent it in for repair. Before dropping it off in Boulder I swapped out the Amprex for the stock PSA tubes. I haven’t once felt the need to reinstall the Amprex tubes, because the 250 is sounding do good. Too bad there’s no way to tell if your amp is properly biased or not, because the difference, IMO, is more dramatic than any upgraded tube you can swap in.

And no answer to your original question about company stance on biasing.
I would hazard a guess (based on the lack of response) that the company position would be, “our amps don’t need to be re-biased, which is why we don’t talk about it,” but that’s just a guess on my part. Your experience certainly is contrary to that position, if that is the position. If the position is something different, they should respond.

So if you plan to keep the amp long-term, and based on this experience, will you send it in to be re-biased at some point in the future?

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Tube bias is for output tubes. The tubes in BHK are in the input stage and you just have use a matching pair of tubes.

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“Different tubes require different bias currents, but the BHK will automatically adjust the current to
exactly what is needed.”
I cut this out of the owners manual for the BHK power amps.

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Yes, if i decide to keep the amp long term, i would definitely have it re-biased again. Luckily I only live 40 minutes from PSA, so not a big deal to drop it off.

Interesting. Mark in service told me they re-biased my 250, when I asked him what else was done besides replacing an XLR input. The reason I asked, was because the amp sounded way better after picking it up from PSA.

The amplifiers are Class A/B. They run a small amount in Class A then transfer the majority of amplification to Class B for the higher watts. Although afaik PS Audio don’t publish the Class A/Class B switching point it is likely to be quite low, <10w.
The bias check could be that cross-over point. Occasionally (but not often) it might need recalibration.
In A/B amplifiers I have had restored this is one area the techs have been very “picky” about getting correct. Temperature needs to be stabilised to reference etc.

PS Audio claims the the 250 and 300 auto bias. And should not require manual intervention.

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Are we talking about the input tubes bias, the output stage bias, or both being auto bias?

Actually I thought the class A bias was 1 watt.

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