I recently purchased a Luxman SQ38u amp used. Seller stated that it's a 120v amp. I found out, while it was being shipped to me, that Luxman does not make a 120v version. The SQ38 transformer can be switched from 100v to 115v by Luxman for either market. This has led me to believe that the seller buys 100v versions and switches them himself misstating the voltage for the market he sells to.
To make a long story short, I can't tell if he actually did it and I am afraid the amp is working too hard at 120v. I can hear some buzzing from the power transformer, but also, a hum through the headphone jack that is very noticeable. Does anyone know how to check a transformer for it's operating voltage and the reason for the loud hum through the headphones?
Thanks!!
To make a long story short, I can't tell if he actually did it and I am afraid the amp is working too hard at 120v. I can hear some buzzing from the power transformer, but also, a hum through the headphone jack that is very noticeable. Does anyone know how to check a transformer for it's operating voltage and the reason for the loud hum through the headphones?
Thanks!!
This is rather dangerous.
Suggest you first check the bias per the manual.
According to the schematic, there are dual primaries (which will be paralleled), each for either 100V or 115V.
So you should make sure the 115V taps are being used, and not the 100V taps.
The 100V/115V jumpers are located at the bottom of the power transformer, nearest the back panel.
Once the taps are correct, measure the DC voltage across each 10R resistor in the output tube cathodes.
Access via the bottom panel. With power off, measure each resistor for 10 ohms. Replace any that are off.
Then with power on, measure the DC voltage drop on each. Should be around 0.50VDC each.
Use the pcb pots near each tube to set the DC bias current for each tube after 20-30 minutes warmup.
Try to get them all the same.
Suggest you first check the bias per the manual.
According to the schematic, there are dual primaries (which will be paralleled), each for either 100V or 115V.
So you should make sure the 115V taps are being used, and not the 100V taps.
The 100V/115V jumpers are located at the bottom of the power transformer, nearest the back panel.
Once the taps are correct, measure the DC voltage across each 10R resistor in the output tube cathodes.
Access via the bottom panel. With power off, measure each resistor for 10 ohms. Replace any that are off.
Then with power on, measure the DC voltage drop on each. Should be around 0.50VDC each.
Use the pcb pots near each tube to set the DC bias current for each tube after 20-30 minutes warmup.
Try to get them all the same.
Another way to check for proper operating voltages is the filament voltages. Tubes can usually tolerate some extra plate voltage and current, but if the filaments are too high that will definitely shorten the life of the tubes. But a noisy transformer is also a problem. I would check you line voltage. Most older tube amps set for 115VAC will tolerate 120VAC but if it's much higher than that they won't be happy.
One solution might be a bucking transformer. You can buy a nice pre-made one on eBay. Generally a handy thing to have for vintage or imported amps:
https://www.ebay.com/itm/1969780110...HvX_mTni-WOfWsOG9k7bOqfC4CecUJ_waAq5AEALw_wcB
One solution might be a bucking transformer. You can buy a nice pre-made one on eBay. Generally a handy thing to have for vintage or imported amps:
https://www.ebay.com/itm/1969780110...HvX_mTni-WOfWsOG9k7bOqfC4CecUJ_waAq5AEALw_wcB
Thank you both for the detailed explanations. I think I will take it to a tech, as I don't want to damage the amp (or myself) trying to diagnose it.
Does anyone know why I get a hum through the headphone jack? I thought it correlated to the transformer hum.
Also, this amp has no inlet with a third earth ground, would that be the cause? I can hear a slight hum when I put my ear up to the woofer of my speaker.
Does anyone know why I get a hum through the headphone jack? I thought it correlated to the transformer hum.
Also, this amp has no inlet with a third earth ground, would that be the cause? I can hear a slight hum when I put my ear up to the woofer of my speaker.
Well, I still get the hum through the headphone jack but the buzzing went down from the transformer when I had the realization that the small transformers powering the aerator for the pond just outside my living room window could be contributing to it. They buzz loudly, so I turned them off and that helped.