• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Luxman A3550 with KT88s

D

ddavis1818

New Member
Hi all. New to tube amps and working with a Luxman A3550. Not much in the way of documentation available for this online, so hoping for some advice here.

I'm installing new Mullard KT88 tubes. Upon checking the bias, they're reading from 72 to 77 mA. The lowest I can adjust them down is ~72, which from I understand is not an ideal reading.

I don't have the manual for the Luxman, so a bit in the dark on the ideal settings and recommendations for adjusting (any tips on where to find this would also be appreciated)

Any suggestions on how to proceed with this? What might be causing the high readings and how might I go about fixing it?

Any tips greatly appreciated!
 
Register to hide this ad
can't find a 3550 schematic, most of what turns up is the A-3500 which uses EL34 tubes. Could be a fault with the bias supply, could be leaky coupling caps. If all 4 tubes are in the same realm I'm guessing its not a coupling cap. Could be something fishy in the bias supply causing wrong voltage to all 4 grids though. Hard to say for sure without a schematic to refer to though.

This isn't by chance a 100 volt model being run on 120 volts is it?
 
Have there been multiple versions of this amplifier? The schematic I found online doesn't include a bias adjustment.

Luxman A3550.gif

Jack
 
Not sure thats the right schematic. The pics of the A-3550 I've found online show four 6CG7 tubes, not a 6AN8.
 
Many thanks everyone. Yes, my A3550 was produced later (mid eighties I beleive) and originally used 6550A power tubes. This is the one: https://audio-database.com/LUXMAN/kit/a-3550-e.html

It's strange, the link you sent through is of a different early model A3550 that I didn't know existed.

Mine is indeed a 100V Japanese version. I have it hooked up to a step down transformer to take it to 100V, so, in theory, that shouldn't be a factor (unless I'm missing something in that equation)

Does anyone out there have a recommendation for a good qualified tech in the Northeast (NYC or upstate NY) that might be able to work on this (and willing to work with a non-US model)?

Would love to get it up and running and unfortunately I don't have the time, know-how, and skill to fiddle with it myself.
 
So far I have found this......

Sharing in case somebody can make use of them.

3867089_m.jpg


Would appreciate if someone can post a complete assembly manual.
 
frend2001 said:
So far I have found this......

Sharing in case somebody can make use of them.

Would appreciate if someone can post a complete assembly manual.
Click to expand...

Can you place a copy of that in the AK archives, or maybe upload to HIFI Engine? This forum down-samples large images, and many of the values in your schematic aren't legible here.

Jack
 
Sure looks like a Williamson design to me. Direct coupled split load inverter up front, a second voltage gain stage in the middle, output tubes that appear to be switchable from triode to pentode mode.
 
You must log in or register to reply here.
Back
Top Bottom