I'm restoring an old tube amp, and need some help. The amp is a Maas-Rowe Model 500 power amp. These were originally used with church carillons, and has 50W output power, and are one channel only. It uses four 6L6 tubes on the output. It has a number of different inputs, so there are at least two different input tubes. It uses two 5V4 rectifiers. Unfortunately, I don't have a schematic.
I've replace the electrolytic and paper caps. All the tubes were tested with a Hickok TV2.
The unit passes signal but there is a pretty strong hum on the output - .2Vpp at 120Hz. I didn't match the output tubes - could that be causing the large hum?
I've attached a couple of pictures - they show one tube missing. Its been replaced.
Any suggestions would be apprecited. Oh, and I've built/repaired tube gear in the past. I'm a novice but not an idiot.
I've replace the electrolytic and paper caps. All the tubes were tested with a Hickok TV2.
The unit passes signal but there is a pretty strong hum on the output - .2Vpp at 120Hz. I didn't match the output tubes - could that be causing the large hum?
I've attached a couple of pictures - they show one tube missing. Its been replaced.
Any suggestions would be apprecited. Oh, and I've built/repaired tube gear in the past. I'm a novice but not an idiot.
Attachments
Those are very nice looking amplifiers and I am sure they can be made to sound excellent. I have corresponded with other owners of these amps and the schematic seems to be hard to find. You may have to draw out your own.
The hum problem could be a grounding issue. It might be useful to review the entire grounding scheme to ensure that chassis ground is only at one place. It is a good idea to separate the power supply and signal grounds except for the one place where they ground to the chassis. The input RCA jacks should be isolate from the chassis and grounded to the signal ground.
These amps show up on ePay occasionally so you might find a second for stereo.
The hum problem could be a grounding issue. It might be useful to review the entire grounding scheme to ensure that chassis ground is only at one place. It is a good idea to separate the power supply and signal grounds except for the one place where they ground to the chassis. The input RCA jacks should be isolate from the chassis and grounded to the signal ground.
These amps show up on ePay occasionally so you might find a second for stereo.
Thanks for your reply. I'll look at the grounding. The thing is I'm pretty sure the amp has never been modified, and I just don't think it was designed with .2Vpp hum on the output, so I shouldn't have to modify its original wiring.
I suspect there is a something off in the power supply, or perhaps an input stage is picking up noise for no good reason.
I do actually have a second one of these, and it is doing the exact same thing...
I suspect there is a something off in the power supply, or perhaps an input stage is picking up noise for no good reason.
I do actually have a second one of these, and it is doing the exact same thing...
Don't forget that the speakers could have been 100 feet or more up in a tower; hum may not have been an issue. It is particularly suspicious if they are both the same. If these were mine I would ensure the input RCA was not grounded and that proper grounding techniques were employed throughout the amps. In particular, I would ensure that the chassis was not used as ground.
Good luck, those are nice amps.
Can I suggest shorting the signal input(s) and rechecking, as it may be designed to be part of a connected system. A schematic would help identify if any of the pots can be used to isolate the origin of the hum.
Yes, you should attempt to check the matching of the output stage valves. For maintenance, a small value (1 to 10 ohm) sense resistor should be inserted in the cathode leg of each 6L6, as this is a very practical means to check the valves are within an acceptable balance.
Have you made the mains side safe to modern standards?
Ciao, Tim
Yes, you should attempt to check the matching of the output stage valves. For maintenance, a small value (1 to 10 ohm) sense resistor should be inserted in the cathode leg of each 6L6, as this is a very practical means to check the valves are within an acceptable balance.
Have you made the mains side safe to modern standards?
Ciao, Tim
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