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Old 17th March 2014, 05:17 PM   #1
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Default New Project: BIG bass amp 4x6l6!

Hey guys,

Just scored on an old Allen Organ amp chassis that I will be using for bass guitar. It uses HUGE power/output transformers, 4x6l6, 2x5u4, 2x6sn7, and a 6au6. I haven't traced out the circuit yet but I'm planning on leaving everything except maybe the 6au6 input tube since my goal is to build in a B15 preamap and run it into the power section. Should be pretty doable, correct? I like the idea of the 4x6l6's as I could pull two of them and run it at half power most of the time.

I cannot find ANY information on this amp/schematic. I've wanted to build a bass amp and knew you needed a large output transformer but this is ridiculous! The pictures below are of one on ebay now, it's identical to mine and shows what I have to work with. I plan on replacing the electrolytics just in case they're on the their way out. I'm pretty damn excited to get going on this thing. I think running it through a couple 15's would be nice...
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Old 17th March 2014, 05:22 PM   #2
kevinkr is offline kevinkr  United States
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Moving to the instrument & amps forum per forum policy. You'll get a lot more input for your design questions on I & A.
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Old 17th March 2014, 05:25 PM   #3
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Ah, gotcha! Thanks a lot
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Old 17th March 2014, 05:46 PM   #4
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Congrats on your score! Based on your description, I think you have the guts from an Allen C-3 organ, the amp was rated for 60 Watts. I think it is safe to re-cap it to start with, then you probably want to get rid of the OC3 regulator and run the screen at a higher voltage to get the power up, then adjust the gains of the various stages, and play with the feedback a bit... Looks like a fun project.

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Old 17th March 2014, 05:57 PM   #5
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jazbo8: C-3, you nailed it! I've looked for such a schematic but turned up nothing! Thanks for this, it really gives me a starting point to understanding what's going on inside the amp. I've built a couple of guitar amps and repaired some as well but I'm still learning as I go. Think I could remove the first two tubes (6au6 and 6sn7) and slap in a B15 preamp section before the PI 6sn7? I'm really going for the old school soul tone so I think that Ampeg circuit is a decent goal.

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Old 17th March 2014, 06:23 PM   #6
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The B15 preamp is quite different from the C-3, so if you go that route, it is basically a complete gut job, perhaps a mini-SVT? the circuit topology is a bit closer.
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Old 17th March 2014, 06:36 PM   #7
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Ah, I see. I just figured I could run any preamp into the power section of the c-3. I know it's got two more 6l6's but I'm ok with that I'm not entirely set on any specific amp. I just want to get a decent preamp section in and get this up and running!
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Old 17th March 2014, 07:33 PM   #8
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May be I mis-understood you, I thought you want to build the B15 preamp from scratch... if you already have a B15, then it is possible to plug its output into the power section of the C-3 without much trouble.
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Old 17th March 2014, 07:39 PM   #9
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No you read right, I wasn't clear. I don't have one but i wouldn't mind building a single tube b15 preamp and inserting into the chassis at a logical point before the power section. You're right, it won't really be a b15 but I think any decent preamp/tone stack with this amp would be just fine. I've never seen transformers so big! I think it'll stay in my practice space most of the time...
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Old 17th March 2014, 08:12 PM   #10
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Yes, with those large transformers, I think you can easily get more than 100W, since the original use was for an organ, the OPT's LF response should be quite good as well.
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