Help restoring old Maas-Rowe amp - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 11th July 2012, 11:27 PM   #1
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
Default Help restoring old Maas-Rowe amp

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.
Attached Images
File Type: jpg IMG_1759.JPG (151.6 KB, 122 views)
File Type: jpg IMG_1766.JPG (167.1 KB, 113 views)
  Reply With Quote
Old 11th July 2012, 11:28 PM   #2
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
Oh, also, these amps were licensed from Western Electric, so I assume they are probably very similar in design.
  Reply With Quote
Old 12th July 2012, 03:44 PM   #3
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
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.
  Reply With Quote
Old 12th July 2012, 05:56 PM   #4
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
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...
  Reply With Quote
Old 12th July 2012, 07:41 PM   #5
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
Quote:
Originally Posted by cuibono View Post
I just don't think it was designed with .2Vpp hum on the output,
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.
  Reply With Quote
Old 13th July 2012, 01:45 AM   #6
diyAudio Member
 
trobbins's Avatar
 
Join Date: Feb 2009
Location: Melbourne, Oz
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Rowe EC-20W 20 watt vacuum tube amp w/ NOS RCA 6BQ5 tubes installed sid6581 Swap Meet 0 22nd December 2011 01:46 PM
Rowe AMI jukebox amp richfoub Tubes / Valves 10 3rd January 2011 02:32 AM
Restoring a B&O RX cygnus x1 Analogue Source 1 10th November 2010 09:18 PM
Rowe Ami 125W Amp Board Inputs coit Solid State 0 11th February 2010 05:18 AM
help with restoring amp skyraider Tubes / Valves 10 8th May 2004 03:35 AM


New To Site? Need Help?

All times are GMT. The time now is 10:16 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2