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Old 10th March 2011, 02:49 PM   #1
maurycy is offline maurycy  United States
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Default Is this amp worth bringing back to life?

I have recently acquired an old guitar tube amplifier. Looks like it was built sometime in 60s so it plugs directly to the AC outlet (no power or isolation transformer). All parts and tubes are there but given the age, I am afraid to even plug it into the wall to check if it is working.

I know that first, I would have to add isolation transformer and convert the cable to three prong so it is properly grounded. My skills are very basic but I think with some help I could pull that off. My question is, looking at the schematic, can you tell me if it is even worth to work on this amplifier? Isolation trafo, capacitors, diodes, etc would probably cost more than the amp is worth. The only reason for me to work on this amp would be to move it to a new case to reduce the size and have a tube guitar amp so I can compare the sound with my solid state amplifier.

Here is the schematic: picture

If I decide to work on the amp, I would remove the tremolo circuit on the right (12AV6 tube) as I do not need it.

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Old 10th March 2011, 03:45 PM   #2
kevinkr is offline kevinkr  United States
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Yes an isolation transformer is absolutely required, and at a minimum you would need to replace the electrolytic caps, rectifier (1N4007 will do fine) and probably the coupling cap to the 50C5.

I wouldn't do anything beyond this, and since a series filament string is used the simplest thing to do is just ignore the tremolo circuit, removing it would require a new power resistor in the filament string.

Listen to it, and decide if it is worth any more trouble than this. It might make interesting tone or not...
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 10th March 2011, 04:59 PM   #3
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About one Watt is what this circuit will give you.
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Old 10th March 2011, 06:40 PM   #4
maurycy is offline maurycy  United States
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Thanks for replies. The outside label says 110-120V , 60 cycle, 55 Watts. What size trafo do I need for this amp? Any sources for cheap isolation transformers?
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Old 10th March 2011, 07:16 PM   #5
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Yeah, that's pretty primitive. Half wave silicon rectifier, you can't remove the tremolo without rejigging the heater circuit, it needs an isolation transformer. >=55VA

Unless someone recognises it as being of historical interest, I don't think it's worth the effort and money.

Every guitarist should have a tube amp sometime. Build a Fender Champ.

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Old 10th March 2011, 10:54 PM   #6
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Schematic looks horrible, therefore this may be the coolest sounding guitar amp you ever had!
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Old 11th March 2011, 01:46 AM   #7
jjman is offline jjman  United States
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I'd like to see a pic. These old amps are worthy of respect in my book. Just need to do the safety thing. My friend has an old Masco that looks like this one. Not sure if it's the power transformerless type.

Click the image to open in full size.
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Old 11th March 2011, 02:03 AM   #8
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Two words come to mind..... boat, anchor
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Old 11th March 2011, 02:24 AM   #9
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I built a isolation transformer out of two transformers salvaged from a pair of 500va computer uninterruptable power supplies wired back to back.
- Nic
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Old 11th March 2011, 02:27 AM   #10
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As mentioned above, you ought to be able to cobble together your own isolation transformer for next to nothing. After that, it's just the cost of replacement caps. If you have even the slightest use for a guitar amp, you may as well give it a whirl. What do you have to lose?
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