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 Post subject: It goes to 11!
PostPosted: Sun Mar 22, 2020 9:19 pm 
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I've been working on this amp for awhile now and i just finally noticed all the knobs go to 11! That made my day. I guess this will be my build thread. I think I'll post the pictures when it's all done, which shouldn't be too far off.


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 Post subject: Re: It goes to 11!
PostPosted: Wed Mar 25, 2020 2:04 am 
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I took my time building this amp. As it was my first amp build. I used this as a model for studying amp design and operation. I had some preliminary study from Dave Hunter's "Guitar Amp Handbook". Plus years of internet and youtube research. As I assembled the Tramp I studied everything i was working on at the time, and if i didn't feel i fully understood it, i'd stop and hop on the web to figure it out. I decided to watch quite a few of Uncle Doug's youtube series on tube amps and that really helped clarify some of the mysteries of the vacuum tube for me.
I do have one question on the Tramp design i haven't figured out. Does the negative feedback in "tweed" mode only affect the preamp gain structure? Or does it clean up the power section as well? I'm trying to find the best setting for the least preamp distortion while heavily overdriving the power tube. I figure i'll get good results with the PAB as it is boosting post 12ax7. However I'm wondering if having the gain lower in tude mode vs gain higher in tweed would provide clean front end, dirty back end. Hence the question of where the negative feedback is functioning.

Anyways, The amp works! It also has virtually no noise floor. I did use a lot of shielded cable for the signal where possible (as i had a roll of it lying around), but with no comparison, i don't know how much noise i prevented.


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 Post subject: Re: It goes to 11!
PostPosted: Wed Mar 25, 2020 2:10 am 
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As you might notice. I decided to add just a little mojo that is probably near inaudible but it felt good to customize it a little changing the first 2 input coupling caps to Sozo nextgen mustard (same values). I also used their somewhat new Silver Mica cap for the the treble tone cap. Its pretty darn big rated at 1000v but it fit.


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 Post subject: Re: It goes to 11!
PostPosted: Wed Mar 25, 2020 2:26 am 
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I also went back and changed the bias resistor configuration to what "lighthouse" had designed. I haven't tried the larger octal tubes yet but it will be interesting to see if i can hear the difference between the two "high" bias settings. I debated tweaking the tone stack but realized that would be silly having never heard the amp. I think it's good as it is. I haven't tested the PAB i installed but i decided to go with a 220k resistor after studying the boost in the tone stack calculator. It looks like it will give it a good boost but without boosting the bottom end too much. The 1meg showed a big jump in the bass above the treble. With the 220k the EQ structure changes much less while still giving a significant boost. I can always change it later.

I'm running it into a closed back Avatar cab with a well broken in 16ohm UK made Celestion M Creamback. It sounds good to me.


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 Post subject: Re: It goes to 11!
PostPosted: Fri Mar 27, 2020 11:13 am 
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GonzoBananas wrote:
I also went back and changed the bias resistor configuration to what "lighthouse" had designed. I haven't tried the larger octal tubes yet but it will be interesting to see if i can hear the difference between the two "high" bias settings. I debated tweaking the tone stack but realized that would be silly having never heard the amp. I think it's good as it is. I haven't tested the PAB i installed but i decided to go with a 220k resistor after studying the boost in the tone stack calculator. It looks like it will give it a good boost but without boosting the bottom end too much. The 1meg showed a big jump in the bass above the treble. With the 220k the EQ structure changes much less while still giving a significant boost. I can always change it later.

I'm running it into a closed back Avatar cab with a well broken in 16ohm UK made Celestion M Creamback. It sounds good to me.




I played with the PAB, using a CTS 500K push/pull for switching. Didn't notice much difference in the boost, so I ended up getting a 1meg CTS pot and swapping out the swiper pad, so I had a 1meg push/pull.
I really like that. Here is my post on the PAB mod I installed. viewtopic.php?f=16&t=6267&start=20

Was running the WGS Blackhawk Alnico, now I picked up a 12" Creamback Neo. I like it a lot for rock & Blues. Gives the amp a little more low end, which I think it needs. Running the KT88 Big Can in it.

Scott


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 Post subject: Re: It goes to 11!
PostPosted: Fri Mar 27, 2020 1:49 pm 
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Hey, that's a cool mod. Too bad the boost is subtle at anything less than 1meg. It's strange the Tone stack calculator shows over a 10db boost across the board with just adding the 220k resistor, but i measured it with a DB meter on single notes and it was showing 3db differerence. Enough to notice something changed but not really feeling louder. I'm thinking of putting the 1meg in, as it's showing 3db gain in bass, 2db mid 1db treble from the 220k. So if over 10db translated to 3db in the room, i'm thinking it will only add 1db going to 1meg. I was thinking about the larger tubes as i have a pair of 7581a that drifted away from matching so they are a bit noisy in the push/pull amp. However they'd be running cold with the 6L6 bias resistor. I'm supposing your kt88 is just clean for days even with the PAB boost? Assuming you aren't pushing the preamp into distortion.


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