Biasing an ARC I/50

[milestogo]

I bought a new ARC I/50 this weekend and am planning on replacing the 6550s with Golden Lion KT88s. I measured the stock 6550 bias as 45 mV in the left channel and 48.6 in the right which struck me as both low and inconsistent for a new product. I measured the plate voltage as 462V across chassis ground and pin 3. I can't be certain I'm doing the measurements correctly since I cannot find service documentation so I measured bias from the labeled measurement points to chassis ground.

Does anything seem odd about these measurements? Can I safely raise the Golden Lions to 55 mV? Has anybody found the service manual containing biasing instructions for the ARC I50?

Thanks and apologies if this information is available but I exhausted my search abilities trying to get answers.

 

[Legolas]

I would ask your dealer, or message AR HQ.
But isn't the I/50 auto bias?

 

[milestogo]

The ARC I/50 is not auto bias.

I'm a little hesitant to ask the dealer for the service manual since I don't think they're allowed but I'll probably do it anyways if I can't find the answers.

 

[Legolas]

https://audioresearch.com/new_website/wp-content/uploads/2024/11/I50-manual.pdf

 

[milestogo]

That is the link to the manual which only recommends sending it to a qualified electrician which I essentially am. I spoke to the dealer who is looking into it but as I guessed, even they do not have the service manuals as they send them off for biasing and repair.

Thanks

 

[milestogo]

Interestingly enough, back in 2022 a user on the Audiogon forum contacted Audio Research and received the following reply:

"Dear James

Summary: The I50 is not auto bias. The tubes should rarely need to be biased on this product but if it needs to be checked, the unit needs to be disconnected, turned upside down ,and the bottom cover removed, then there will be points to connect a voltage meter and read the settings. There is a small pot to turn with a plastic screwdriver that will adjust the bias. We are working on written instructions for this as well as a short video. Those will be available in the near future."

Unfortunately no "written Instructions" were ever forthcoming probably due to liability concerns.

 

[milestogo]

Final Update (for those coming across this thread)

The manufacturer provided the biasing instructions to me and will do so to owners on request. You should be experienced as the procedure is dangerous.

I originally did everything correct though they do recommend measuring across the resistors with grabber probes and not using the labeled measurement test points. They recommend 42 mV across the two test resistors. The resistors are hard to locate (two ~.5 ohm) so get the instructions.

I hope this helps anyone trying to do this in the future (be careful!).

 

[cyyin]

Final Update (for those coming across this thread)

The manufacturer provided the biasing instructions to me and will do so to owners on request. You should be experienced as the procedure is dangerous.

I originally did everything correct though they do recommend measuring across the resistors with grabber probes and not using the labeled measurement test points. They recommend 42 mV across the two test resistors. The resistors are hard to locate (two ~.5 ohm) so get the instructions.

I hope this helps anyone trying to do this in the future (be careful!).

Hello

Could you provide the biasing instructions as I also plan to replace the 6550s with Golden Lion KT88s? Thanks!