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PP 300B amps
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Author:  Grover Gardner [ February 9th, 2016, 11:38 pm ]
Post subject:  PP 300B amps
Well, here they are, the little guys I was building for my friend Jim. Fifteen glorious watts. :-). The layout is based on a Heath WM5, and in fact I was going to use the same chassis, but after getting them built I couldn't deal with the metal, and so I tore them down and redid them with Delrin and oak. They sound pretty good. The circuit is very basic--6J5 direct coupled to 6SN7 long-0tail phase splitter. Cathode bias with the Heath balance pot for the output tubes preserved. 5AR4 rectifier. James 6.6K outputs, Angela power trannies (if I were to do I again I'd order some Edcors). The Heath chokes also survived to serve duty. The 6J5 plate voltage is adjusted by a pot on the cathode. There's precious little headroom for the phase-splitter/driver so the voltages have to spot on. I'd include a shot of my patented poinht-to-point wiring but it won't let me and besides, you'd throw up. Take a good loook because one of you may be asked to repair them down the road. :-).

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Author:  Stuart Polansky [ February 9th, 2016, 11:42 pm ]
Post subject:  Re: PP 300B amps
Very nice Grover.

Thanks for finding out about the importance of chassis construction material, and sharing that with us.

Stuart
Author:  Grover Gardner [ February 9th, 2016, 11:49 pm ]
Post subject:  Re: PP 300B amps
Thank Charlie. :-). He had the goods on Delrin a long time ago. I thought I'd get away with repurposing the Heath chassis, but not only did it sound crappy, but I was coincerned abut the reliability of such a kludged build.

Minus tubes, I figure these cost around $1000 to build. Not bad.
Author:  ratbagp [ February 10th, 2016, 8:43 am ]
Post subject:  Re: PP 300B amps
How thick is the Delrin and how rigid? Do you use some additional support for the iron?

During the discussion on soapstone as the plinth for the turntable, I was pondering whether it could be used to hold the iron and tubes. Soapstone apparently cuts fairly easily and should be reasonably inert.

ray
Author:  Stuart Polansky [ February 10th, 2016, 9:18 am ]
Post subject:  Re: PP 300B amps
ratbagp wrote:
How thick is the Delrin and how rigid? Do you use some additional support for the iron?

During the discussion on soapstone as the plinth for the turntable, I was pondering whether it could be used to hold the iron and tubes. Soapstone apparently cuts fairly easily and should be reasonably inert.

ray


Ray,

I've used and continue to use Delrin in my electronics projects, ever since Charlie turned me on to it.
It is very rigid and hard (in fact, it's used as a bearing material in some race car suspension applications. I've broken off many a 6-32 tap in it. I've used thicknesses as thin as 1/4" for rear panels, up to 2" thick for mounting bases under circuit boards.

I look for cut pieces on eBay, since it is quite expensive. One of the best, if not the best sounding turntable I've ever heard has a Delrin plinth. It is superb. I dare not tell Roscoe what to do, but there is zero doubt what I'd do for a plinth.

As they say "highly recommended"!

A couple of pictures attached of my phono stage in a junk box, but with a ~0.75" piece of Delrin in the bottom and 0.25" Delrin as a mounting plate for the jacks.

Stuart

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Author:  Grover Gardner [ February 10th, 2016, 10:39 pm ]
Post subject:  Re: PP 300B amps
ratbagp wrote:
How thick is the Delrin and how rigid? Do you use some additional support for the iron?

During the discussion on soapstone as the plinth for the turntable, I was pondering whether it could be used to hold the iron and tubes. Soapstone apparently cuts fairly easily and should be reasonably inert.

ray


I use 1/4". It's plenty strong for iron, at least for a modest design like this. There's no extra support except on the sides. Also, it's the perfect thickness for sockets, pots and such, everything sits just about flush. It's not at all *difficult* to drill but it can be tricky. Regular drill bits "catch" at the end and you can get a nasty surprise. For anything larger than a quarter-inch hole I recommend a good set of Forstner bits.

Stuart, thanks for the tip about eBay. I never thought of buying it there. I just saved about $100 on my next two projects. :-)
Author:  Guy [ February 11th, 2016, 9:42 am ]
Post subject:  Re: PP 300B amps
When I drill Delrin, I use a sharp bit, and go at it in short plunges: drill and retract, drill and retract in short intervals - probably less than a second at a time. This helps keeps the bit cool and allows the flutes to clear of shavings. I haven't snagged a workpiece yet doing it this way. Don't let the bit get warm or it will start grabbing - Delrin has a melting point of 350°F, so it can start getting doughy at temperatures less than that. If you can control the drill speed (like on my drill press,) drop the speed to 200-400 RPM.

So peck at it with a slow, sharp bit and then make sure the flutes are clear in the bit - repeat. :)
Author:  tomp [ February 11th, 2016, 9:59 am ]
Post subject:  Re: PP 300B amps
You might want to take an old set of drill bits and grind them flat on the tip. It keeps the bit from pulling into the plastic. They will still drill into the plastic even with the flat ends. I started using them when I had to drill plexi but it works on other plastics as well. I know it sounds crazy but I have found a number of strange situations that work well, for example putting a fine tooth circular saw plywood blade on backwards to cut aluminum siding. It also prevents grabbing. Creates a real racket though.

Tom
Author:  SoundMods [ February 11th, 2016, 10:01 am ]
Post subject:  Re: PP 300B amps
Or drill with lubricant to limit the heat generated by drilling friction.
Author:  tomp [ February 11th, 2016, 11:31 am ]
Post subject:  Re: PP 300B amps
The lubricant will help keep the plastic cool and prevent galling but will not help with the pulling in with a standard bit.

Tom
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