Rick Davis
1074 posts
Dec 28, 2012
7:54 PM
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I dropped by Bruce Collin’s shop this afternoon for a little work on the newly acquired Marvel vintage tube amp. This was the first time I’d had the chassis all the way out of the amp. The circuit is sparse and primitive, reminding me of a 1947 Gibson amp I once owned. It has a half wave rectifier, just a tiny diode. Weird.
Mostly I wanted Bruce to install a grounded power cord and test the caps, because I really like the tone of the ratty little amp as it is. Here is what we ended up doing:
-Install grounded power cord
-Remove the “Death Cap”
-Install cathode bypass cap on the power tube. This gave the amp a bit more punch.
-Test all tone caps for leakage. They were all good.
-Test all circuit voltages.
-De-oxidize the tube sockets
-Install speaker jack
We then tried five different preamp tubes:
-The vintage RCA gray plate 12AX7 I had in the amp
-a 5751 from JJ Tesla
-a 12AY7 from EH
-an NOS Westinghouse 12AZ7
-an NOS RCA 5965
Of those tubes I liked the 12AZ7 best, so I have it in the amp for now. We did not change the power tube. The original 7591 tube is a gem; a Sylvania rebranded as a Multivox.
We found some date codes on the chassis and filter cap. It looks like it was made late in 1963.
The amp tested at 7 watts just as it began to clip. It still has the same old-school raspy sound but slightly browner. Sounds good! It’s a keeper.
I'll add a video of the amp tomorrow.
The circuit is about as simple as it gets.
Bruce Collins mapping out the circuit before getting started.
Testing the amp's power.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Last Edited by on Dec 30, 2012 8:48 AM
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SmokeJS
20 posts
Dec 29, 2012
4:24 AM
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You're very fortunate to have ready access to someone such as Bruce. Could you estimate how much time he spent working on the Marvel?
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Rick Davis
1075 posts
Dec 29, 2012
6:46 AM
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SmokeJS, it was a little over an hour. The charge was his basic bench rate.
BTW, I played a Mission 32-20 amp with two 10-inch speakers while I was there. Great tone and articulation. It was ready to be shipped to some lucky harp player.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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Rick Davis
1078 posts
Dec 29, 2012
1:09 PM
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Here is the preamp tube I have in the amp now.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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JD Hoskins
368 posts
Dec 29, 2012
1:23 PM
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12AZ7, same gain factor as a 12AT7, but generally a better audio tube.
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"Technical, schmechnical---put some heart in it, dammit!" ~ Paul deLay
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Rick Davis
1079 posts
Dec 29, 2012
2:01 PM
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JD, exactly right. You know your stuff. Is the AZ7 still made, by the way? I do not know if it is.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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Rick Davis
1080 posts
Dec 29, 2012
8:15 PM
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"even coming from you..."
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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Joe_L
2276 posts
Dec 29, 2012
9:19 PM
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This sounds Marvel-ous...
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The Blues Photo Gallery
Last Edited by on Dec 29, 2012 9:41 PM
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