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Old 9th April 2009, 08:29 PM   #1
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Default Bass amp from spare parts

Well first off, I'm really very new to the whole DIY amp thing, and I've been told I have enough knowledge to be dangerous.

Recently, my dad was cleaning out his basment, and he found an old Webcor reel-to-reel, a HAM radio reciever, and a CB set that he was going to get rid of. Being unemployed at the moment, I have a bunch of time on my hands. So I snagged those and started doing some research.

From the components I pulled out, I came up with a 12AX7A, a 7025, and a 6UE7 (which according to my searches at nj7p.org all have the same characteristics). I also got a 6X5GT rectifier, 2 6V6GTs and a 6AQ5A. And of course the power and output transformers from each, which I am going to have to get a multimeter to test as none of them have manufacturers listed and it's not that easy to find information based on the numbers on the sides of them.

So, looking at various amp schematics to find something with a similar tube configuration, I came across the Fender Bassman 20. It has one 7025 in the preamp and another one feeding 2 6V6s in push-pull.

Now, being that this is my first attempt at anything like this...it would make a lot of sense to just try and recreate the Bassman 20 circuit and deal with the 18w it's supposed to produce.

Unfortunately, I've always had a bit of a mad scientist streak. (I blame my dad's family). So what I'm looking to do is add in another 7025/12AX7 gain stage, use my tube rectifier instead of the SS one in the schematic, and convert the power stage from 2 6V6s in PP to the 6AQ5A in SE class A into the 6V6s in PP class B. Plus I'd like to add an effects loop so I can use this 31band EQ i've had sitting around since my church upgraded their PA.

So any thoughts, comments, suggestions are appreciated, but you don't have to tell me I'm crazy...I'm already well aware of that. From my research I know that what I'm looking to do is reasonably possible. I just need an assessment of the feasability. Whether modifying this schematic will work or if there's something better, or if, the worst option, starting completely from scratch would be the easiest.

Thanks in advance for any help.

PS, the schematic for the Bassman 20 that I've been working from.
http://www.schematicheaven.com/fende..._bassman20.pdf
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Old 9th April 2009, 08:37 PM   #2
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First of all, what kind of transformers did you salvage?
Can you list models of the gear you took apart?
Transformers are more expensive parts than tubes.
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Old 9th April 2009, 08:42 PM   #3
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Default Re: Bass amp from spare parts

Quote:
Originally posted by Chaos5522
and convert the power stage from 2 6V6s in PP to the 6AQ5A in SE class A into the 6V6s in PP class B.
Hope you've got lots of duct tape!
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Old 9th April 2009, 09:51 PM   #4
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Default Re: Re: Bass amp from spare parts

Quote:
Originally posted by Wavebourn
First of all, what kind of transformers did you salvage?
Can you list models of the gear you took apart?
Transformers are more expensive parts than tubes.
The reel-to-reel is a Webcor EP2712-1
It had 6EU7 and the 7025, the 6X5GT and the two 6V6GTs
there is no manufacturer information on the power transformer but the numbers on it read "68P028-5 596603"

The CB is an Eico Sentinel Pro, the numbers on the transformer read "30079 10976645"

The HAM reciever is a Knight Star Roamer

Quote:
Originally posted by leadbelly


Hope you've got lots of duct tape!
Of course, duct tape fixes everything
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Old 9th April 2009, 09:51 PM   #5
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Actually i stand corrected...the Webcor had a 5635 and the 7025
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Old 9th April 2009, 09:58 PM   #6
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Do you remember when you took transformers which wires were soldered to where in that reel-to-reel?
It may help. Looks like you have everything for the amp.
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Old 9th April 2009, 11:28 PM   #7
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Actually i stand corrected again. The power amp I'm looking to do is Class A SE/Class AB PP based on the schematic on pg 6 of the tubecad article on mixed class/topology amps
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Old 9th April 2009, 11:28 PM   #8
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This isn't the greatest pic ever...it was too washed out with the flash and just too dark without it, but all of the secondary lines coming off the transformer went right to the socket for the rectifier.

The pic is too big to attach and the resolutions a little high to just include it in the post, but it's at http://www.resilienceguild.net/files...econdaries.jpg
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Old 10th April 2009, 02:22 AM   #9
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Quote:
Originally posted by Wavebourn
Looks like you have everything for the amp.
Yeah in theory I should have everything i need with the exception of capacitors I'll need to replace, but it's a matter of reconfiguring things into a bass amp as opposed to the hifi that the reel-to-reel was.
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Old 10th April 2009, 02:37 AM   #10
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Bass amp is a hi-fi. I would start from making the original one to work.
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