12BK5 Hybrid Amp

I have enough projects waiting for me to work on and I am not short on amps but I really need to build a new one. I have not used the 12BK5 and I have a bunch of NOS ones collecting dust. I am hoping that I can get them to play nice with a MOSFET phase inverter, 70V line transformer, a laptop power supply and a switching high voltage module.

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The 12BK5 biases up at -5V in single ended duty so it does not need a lot of signal prodding it so I might get away with a single 12AX7 with a Bassman styled tone stack.Whether the negative feedback remains depends on the amount of gain and the low end through the output transformer.I have used the IRF830 for the PI so I know that part will work, will be trying the LND150 to see how it will do.
 
Bought a tabletop radio today for $20. It is a battery operated one, 1R5, 1T4, 1U4, 1U5 and 3V4 tubes running 8.4V heater string at 50 mA, 90V plate supply for 0.25W. The 1U4, 1U5 and 3V4 are all pentodes and look like they may be an interesting project. I thought a 5W SE transformer for this cabinet and a 6x9 speaker might be useful.



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Seems someone thought the same thing with the speaker and it looks like they upgraded the output transformer. Thinking about it and looking at the wires, it might be a line powered version and the tube complement would have been a more common configuration.



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I think I see a pattern why it takes me a while to complete projects.
 
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So much for working on the 12BK5 amp, cleaned up the radio for future consideration.


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Going to see if I have any available seven pin tubes. I never expected to do a whole amp with them so I didn't stock up. As an idea.


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I expect the volume and master volume will go in the volume and tuning positions and a input jack as well as treble and bass controls in the tuning window on the plate.
 
Nothing fancy, may give it a shot of paint later.

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Is that a length of steel C-purlin as used in building construction? ( Purlin C






- Purlins and top hats Supplies











| Steeline
)

Might I ask what you used to cut it so cleanly? Cold saw? Band saw? Something else?

There was a construction project right next to my work place, and when it ended, the workers thoughtfully abandoned a pile of construction rubbish right outside the side door of the room where I work. Buried in between scraps of drywall, dozens of loose nails, and chunks of thermal insulation, I found a couple of short lengths of steel C-purlin, which I rescued and saved for possible use as chassis.


-Gnobuddy
 
I grabbed some C-purlin once, have it around here somewhere. No, this is just a scrap off cut from work. We have a small metal working shop at work. So just a shear and brake for this one. There will be wood blocks in the ends to which the screws that hold the chassis in the cabinet fasten to. So it will be fairly sturdy. This galvanized sheet is a little stronger than what homes have, can't remember the gauge.

I did a photo example of cutting and bending a similar chassis at one time, probably still at Photobucket but I still have issues with them so won't be visiting there soon. The thickness is still thin enough to cut with a hand sheet although it takes some effort. Can cut it with a fine blade hacksaw or a bandsaw if you have one. Scribe a line and with a file you can get an edge that would not be much worse than done with a sheer. I showed how to bend the metal using some angle iron and bar stock along with a piece of wood and a hammer. The more patience you have and going slow creates a better look than a get the job done style.

The chassis pic with the sockets mounted has a piece spot welded in where I had a hole for a socket and now the output jack. I thought it was a better arrangement, I won't be having speaker magnet issues. I am thinking of wiring it up tonight or tomorrow. That or my Telecaster project. The guitar will be more useful to me, the amp is just something I wanted to do because I have not done any electronic project for years. Just a cheap and dirty project to scratch an itch.
 
I did a photo example of cutting and bending a similar chassis at one time
I remember seeing your pics some years ago, around the time I first found your posts about your 2W 6AK6 mini 5E3.

I want to use the C-purlin I found, rather than fold up my own chassis from flat sheet. That means I have to find a way to cut it cleanly without bending or collapsing the C-shape in the process. I do have access to a small metal-cutting band saw, but I think the weight of the saw coming down on the purlin might bend the metal and make an ugly cut.

I'm thinking, maybe if I cut a block of wood to fit tightly inside the C-Purlin right where the saw cut is to be made, it will keep it from collapsing.

I like working with aluminium, but steel is a different matter. It usually seems to involve bloodshed (my own blood, that is!), iron filings that go everywhere, and lots of time.


-Gnobuddy
 
I like aluminum myself, don't come across a lot of it free though. Speaking of cheap, found a radio today for $20 that was built in Vancouver in the 1950's. I was not going to bother as the picture of it showed the grill area with a height for an 8". I thought the last thing I need is another five tube line powered amp with crappy tubes. But I took a chance on it because it was five minutes away and it is twenty degrees warmer than it was two days ago.

Painted top, who cares, didn't even look at the turntable. Looked at the back and I saw the 10" speaker and that is all I needed to decide to get it. Saw it had octal tubes, another plus, also a power transformer although I have enough of them. When I got it home found out it is suppose to have a 6K6 output tube, had a 6W6 in place. So I got a decent speaker and a output transformer for another SE amp, I have about 20 6K6GT's.

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No I am not a horder, I could not help myself on this one it looks so cute. A small floor standing radio, I actually works and I payed $75 as well as drove an hour for it. Other than recaping the power supply will leave it stock. It does have a phono input and switch on the chassis which I can hook up a guitar preamp to. The person that had it refinished it and it looks pretty good.

http://pacifictv.ca/schematics/viking51-45data.pdf

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Somehow lost the image in the above post.


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I have wired up some more of the 12BK5 amp but the trip to get the radio and changing the arrangement of my workshop is taking up my time.
 
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I want to use the C-purlin I found,

I found a bunch of similar looking stuff, but it was a thinner gauge steel, used for running bundles of cat 5 cables through a building between the ceiling and roof. It was about the right size for small amp chassis, so I grabbed several of the smaller pieces.

I made a wood buck from cheap pine 2 x 4's that fit tightly inside and then put a clamp across the outside to squeeze it for a tight fit. Position the buck so that about 1/2 inch gets cut off each time you use it. I could then cut the the whole thing on a band saw without much blood loss. A big old speaker magnet near the cutting area helps collect the steel filings, but also makes some cling to the material.

I found that punching or drilling a clean tube socket hole in the stuff was a bigger job than cutting it, and I wound up leaving it all behind when I moved.
 
...wood buck...clamp across the outside...punching or drilling a clean tube socket hole in the stuff was a bigger job than cutting it...
Thanks for sharing your method!

You have me wondering whether it's worth putting time into trying to work with this steel C-purlin stuff, or whether I should pitch it and just go hunt down some appropriately sized aluminium U-channel.

I've been using a set of three step-drills to make all the larger holes in my aluminium chassis, and the plywood boards I used for some builds. The step drills work wonderfully well in aluminium and wood, but I don't know how well they fare with sheet steel.

-Gnobuddy

...I am not a horder, I could not help myself...
Now that's a very convincing juxtapositon. :D

Cute radio!


-Gnobuddy
 
So I was buying online again and was looking at some 6AV6's for that little radio. While I was there I looked up some 35L6's, a reasonable price for them. I picked up a RCA Victor Model 45-EY-4 record player once and it had two 35L6's in P-P, which was unusual.

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It would make for a nice little low voltage amp, the only problem I have is with the 35V. I could use a laptop brick and a dc-dc converter to boost it up to 35V. Or I can go with a 12L6 or the like but for some reason it just seems to me a waste of capabilities. Mind you the 12L6 's might just last forever in the amp. I am going to have to think about it.
 
35L6...Or I can go with a 12L6
You know the 35L6 has lower plate and screen dissipation ratings than the 12L6, yes?

35 volts is an odd one all right. But there are inexpensive 36V DC switching power supplies, and you can easily knock off the extra volt with a series 1N4007 diode or two. How about this one: https://www.digikey.ca/product-detail/en/curtis-industries/L6R36-360/364-1783-ND/7681532

With a 1A rating, that power supply can also run three 12.6 -volt heaters in series, in addition to a pair of 35L6s. Maybe some 12AV6 or something similar?

If you want to go old-school and have lots of hum and heater-wiring grief, there are old-fashioned 60 Hz, 18V-0-18V transformers, like this one: VPP36-560 Triad Magnetics | Transformers | DigiKey


-Gnobuddy
 
Yeah I saw the differences in the tubes, I was going back and forth comparing them and looking at miniature tubes that are similar to the 35L6. Then afterward I had a thought that I am not sure of the speaker impedance, it might have been 4 ohm and I would be using an 8 ohm, I need to measure the impedance ratio. I may have found a small oddball transformer for the 35V heaters, I'll have to see what it does under load. The advantage of hoarding, sometimes you get lucky.