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 Post subject: 5e3 Octal Build
PostPosted: Mon Feb 04, 2019 7:26 pm 
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The folks at Trinity were nice enough to punch out a chassis for me to build an octal preamp.

Image

Image

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Image

I started with a stock build but made a few tweaks along the way.
The biggest change was a pre pi master volume that I snuck in to the bright low input. It may or may not stay, but for right now it’s been useful. I also wanted a little more “oomph” out of the 6SL7s so I goosed the plate and cathode resistors a bit.
More clips to come and more details on what tweaks I’ve made as I decide on which I like best but here are a few quick clips I made last night before the Super Bowl started

I’m still working on the cab for the amp so this clip is going through a V30 2x12

https://youtu.be/SrbhaXToMGI

https://youtu.be/fLlLOEtmApE


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 Post subject: Re: 5e3 Octal Build
PostPosted: Wed Feb 06, 2019 8:24 pm 
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Sweet like the tone :)


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 Post subject: Re: 5e3 Octal Build
PostPosted: Sun Feb 10, 2019 8:52 pm 
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Found a guy locally selling an unfinished cab. Tweaked the preamp a little bit and gave it some volume and it really came alive

https://youtu.be/rZW-mq7XS5M


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 Post subject: Re: 5e3 Octal Build
PostPosted: Mon Feb 11, 2019 11:27 pm 
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Awesosme stuff. Love the cranked one. Must be loud!
Let me know all the tweaks and I'll create a schematic and layour for it.

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 Post subject: Re: 5e3 Octal Build
PostPosted: Tue Feb 12, 2019 5:32 pm 
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it is quite loud! but just the right amount though. loud enough to feel it but not enough to knock stuff off the walls. It feels very "big" for being a "low wattage" amp (I'm a 50 watt marshall guy).


I did very little changes on the board, but did change some values around a bit. The ones I settled on for that clip were

Load Resistors: (similar to earlier octal models) to give the 6sl7's some more swing and overdive
R7, R8 and R14 bumped to 220k
R13 to 100k

Cathode bypass caps lowered these a little bit to tighten things up early on.
C3 3.9uf
C7 22uf
C10 22uf

C2 coupling cap on normal channel down to .047uf (may try this on bright channel instead) I'd like to have one channel better suited for my Les paul and another for my strat with a higher cap/less filtering

I also bumped the R19 250R resistor to 10 watts, but only because I like the look of the big wire wound resistors

another change I made was replacing the 1MEG grid leak resistor with a 1MEG pot, as a make shift pre PI master volume/farting out control, but I found with the tweaks I made some of the other parts of the amp, I preferred the pot to be totally open, so it is acting as if the resistor was in anyways. to make room for the pot, I removed the low bright input and corresponding 68k grid stopper.

in the clip you can see me reach for a control a couple times. each time is me turning up that 1 meg pot a little bit more


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 Post subject: Re: 5e3 Octal Build
PostPosted: Tue Feb 12, 2019 9:28 pm 
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With the 6SL7s, and the other changes, could you do a voltage chart?
On the TripTop, with 6SL7s, we use 220K and 470K plate resistors.
What brand tubes did you use?
I will start a schematic.

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 Post subject: Re: 5e3 Octal Build
PostPosted: Tue Feb 12, 2019 9:53 pm 
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I'd be glad to do a Voltage chart.

I ended up using new JJ 6sl7s. I tried some Kenrad tubes I found in an old tube radio as well. They sounded very nice at low volumes but were very microphonic at higher levels. The JJ's hold together very well


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 Post subject: Re: 5e3 Octal Build
PostPosted: Wed Feb 13, 2019 10:55 am 
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First schematic for review. put coupling cap in v1 to bright channel.
need to fill in all DC voltages in boxes

Attachment:
Tweed Octal_v1.jpg
Tweed Octal_v1.jpg [ 854.54 KiB | Viewed 1934 times ]

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 Post subject: Re: 5e3 Octal Build
PostPosted: Wed Feb 13, 2019 12:01 pm 
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Aynirar27 wrote:
I tried some Kenrad tubes I found in an old tube radio as well. They sounded very nice at low volumes but were very microphonic at higher levels. The JJ's hold together very well


You can try silicon rubber rings around them to see if that helps.

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