Simplified 5B5 Pro Build.

[ThermionicScott]

Hey folks. My first amp build was a 5E7 Bandmaster that I enjoy a lot, but I've had the urge to build something with a 15" speaker and an earlier, cruder circuit for a while now. So I'm going to re-use the speaker cab and build some new guts for it! I'll use the TV front Pro (5B5 circuit) as a starting point: grid-leak octal preamp into a single-knob tone control, into a paraphase PI, into a pair of cathode-biased 6L6's with no negative feedback. Here's the schematic:



I'm going to simplify it down to one input per channel. That lets me eliminate one of the 6SC7s. I don't plan to have more than two people plugged into this amp at a time, so it's no big deal. I used a 5C3-style eyelet board:



Behind it, you can see the 1961 Jensen C15P I bought for this project. I won't be building a 100% authentic 5B5 Pro, but it'll be in the spirit... here's a pic of it with the new baffle. I'll be staining the baffle dark brown, like other TV front and wide-panel tweed amps (narrow-panel baffles used black paint). I used regular 3/8" plywood, so if it's too buzzy, I'll make one from birch ply.



I decided to try my hand at forming a chassis from scratch. It's 0.063" H14 aluminum, a little "whippy" but it should be stiff enough not to cause problems:



I'll continue this in the next post.

- Scott

[ThermionicScott]

Here's the other side of the chassis:



Dad got me this neat NOS octal-sized chassis punch for Christmas last year. Thanks Dad!



Now, onto the transformers. Going by the 5B4 Super layout, the early 2x6L6 Fenders used a power transformer with a 700V/140mA secondary. The OT had a 6.6K primary and a wattage in the low 20's. Luckily, I don't need to go boutique to replicate these specs -- Weber's Deluxe PT has a 680V/150mA secondary, close enough! And the Deluxe Reverb OT has a 22-watt, 6.6K primary.



The interesting thing is that this is the iron that ships with a lot of tweed Deluxe kits, so I'll essentially be able to switch between tweed Pro and tweed Deluxe "mode" by swapping the rectifier and output tubes!

More to come!

- Scott

[eugenedunn]

Really ambitious and cool Scott! I love the idea of an octal preamp with big bottle tubes. Have you ever thought about KT66's? Even options between Tube and a Copper Cap rectifier? Anyway.... NICE PROJECT!

[JohnnyCrash]

You might consider two inputs if only for jumpering channels... I'd be interested in hearing what a parallel'ed 5B5 Pro sounds like

Cool choice! Very basic/primitive. I can't wait to see the finished product!!!!

[Dr.Ow!]

Should be a great sounding array and with a stand alone 'verb...mmm.
Greenlee is nice too isn't it. Hope our good 'ol USA can get back to producing real and useful stuff, like tools.
Look forward to seeing the all octal line up.

[ThermionicScott]

Thanks for all the kind words!

Geno, KT66's would be cool -- I've never played around with those. I also have a pair of good-testing 6BG6G's (an early 6L6 derivative with the sexy ST-16 bottle and a top-cap for the plate). Think I'll try 6L6's first though!

JC, the inputs on the original 5B5 couldn't be parallelled -- each input had its own triode. That said, I've got a lot more space for jacks if the desire strikes!

Dr, I hope it sounds good. I've only heard a couple of YouTube clips, but I'm sure I've heard it on a bunch of old blues/R&R records records. Boy, and this punch is the best. So easy and smooth, I didn't want to stop punching holes!

Tomorrow I hope to drill a couple more holes for routing transformer leads and mount all the tube sockets.

- Scott

[winny pooh]

Cool project Scott, would love to hear how this will sound with an archtop doing jump blues yeah!

[sjhusting]

You might think about using a 6sl7 instead of a 6sc7, if you didn't want the shared cathode. I suppose it doesn't matter here. You could also use a switch to jump the channels internally.

other than that nice amp; I've been looking at octal pres and grid leak for the next one myself.

jm2cw

steven

[FenderLover]

I did a 5C5 myself. And I ONLY use octals. Went there once, never went back to 9-pins. The paraphase is cool too. Hell, Tweeds are cool, period.
Looks good!

[ThermionicScott]

Steven, I may resort to 6SL7s if the three or four 6SC7s I have are impossibly microphonic. I certainly have a lot more 6SL7s! I believe I can jumper the two input stages internally by connecting the grids together.

I'm going to fight the urge to mod until I've lived with the stock circuit for a while, though. It's all-too-easy to stare at the schematic and think "wouldn't it be cool if...?"

- Scott

[celeste]

Cool build Scott.

A word of warning about grid leak bias, Grid leaks hate boosters. Any kind of grid current and they make bad noises. In theory, LED bias should give you the same gain.

[ThermionicScott]

Not a lot of progress to report, but I did get all the sockets in. You'll notice that the chassis holes are a little too big for them. That's fine, I'll just have good upward ventilation around all of the tubes.



Wait -- what's that one on the end?? Yep, I've thrown an old-school curveball into this build: a 4-pin rectifier tube! I have an 80 (4-pin 5Y3), two 5Z3's (4-pin 5U4), and two of the mysterious 83 mercury-vapor rectifier:



With a 5Z3 plugged in, it will function exactly as it would with a 5U4.

Here's a minor thing I did... I didn't feel like drilling holes large enough to accommodate the grommets I have on hand, so I cut "sectors" out of each grommet to make them smaller. I dunno if other people do this, but it can work pretty well.



More pictures in a minute!

- Scott

[ThermionicScott]

Some more ST-bottle porn!

Here are my 6L6GA's. I would use the larger 6L6G's, but even the sketchy ones are commanding big bucks. GA's are much more reasonable.



BTW, I'm still kinda tempted to wire the thing for my 6BG6G's (which are very close in spec to 6L6's, with slightly higher transconductance -- they might actually be rebased 807's). Look at 'em! (I put a 6L6GA on the right to show the size difference between ST-16 and ST-14 bottles.)




Okay, back to work!
- Scott

[hal]

And you are going to send this to me when you are done, correct?

[celeste]

Scott, the 6BG6 is the development link between the 6L6 and the 807, so you are correct , they might be rebased 807's. If you want to check, just pt 1200v on the plates

[eugenedunn]

Oh my freakin' G..... this is the coolest project, Scott!

The choice of the tubes is fantastic and vibey....... best of luck with the build. Keep the photos coming..... This is one for the masses.

Geno

[ThermionicScott]

Now it's starting to look like an amp! I've got all but the PT and rectifier socket wired up. If you've seen some of my other builds, you'll note that I've been working toward the "style" of the early Fenders more and more.




Couldn't resist the urge to throw in some glass and snap a pic!



- Scott

[eugenedunn]

Bountiful Bevy of Bulbous Pron

[guitardedzen]

Wish I understood all the technical jargon (guess that comes with time and experience) but it sounds like it's going to be a bitchin' project. Will stay tuned in for progress. Do you have pics of the cabinet it'll be going into?

[ThermionicScott]

Yeah, it'll be this Bandmaster cabinet with a 1x15" baffle installed: