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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 17th December 2019, 10:19 PM   #1
Steve Morley is offline Steve Morley  Canada
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Join Date: Jan 2015
Location: Georgian Bay
Default SPP Build

I coloured outside the lines a bit.

The PC board is suspended about 3/4" below the top plate. Tube sockets and some resistors are on the top of the board, all caps and majority of the res are on the bottom. "Iron" was salvaged from an old Heathkit EL84 amp. The power trannie put out a bit too much HV so the rectifier tube was mounted off-board and a pre-filter and choke were added to drop the B+ a bit. Like most old power trannies they run kind of high on the 120+ volts on the AC lines now so I put in the equivalent of 5 ohms in series with the primary. The value needed determined by keeping the filaments at or below 6.3 volts.

OP tube bias is adjustable. I copied the idea from the Parks Dynaclone. A 470 ohm res is bypassed with a trim pot. A 10 ohm res in series with both of these is used to set 35mA draw per tube. Adjustment is accessed from under a cover on the top panel.

Initially a resistor was used between the pre-filter and the main board but a Hammond 1.5H 200mA choke killed the last of the PS buzz.

I didn't install C101, the cap in the NFB loop. The question for all of youse wise guys is what measurement(s) will help me determine the value for C101?

It fired up fine right away except for an intermittent resistor in the 120 volt feed. It was replaced with the trio seen in one of the pictures.

Sound? Better than the Parks Dynaclone with vintage Dynaco OPTs.

Broskie style grommets will be installed around the 9 pin tubes to clean up the looks a bit.

Cheers, Steve
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File Type: jpg P1040445ab.jpg (994.7 KB, 130 views)
File Type: jpg P1040446a.jpg (843.0 KB, 126 views)
File Type: jpg P1040450a.jpg (923.4 KB, 123 views)
File Type: png Feedback cap a.png (9.6 KB, 123 views)
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Old 7th January 2020, 02:21 PM   #2
OldHector is offline OldHector
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Join Date: Jun 2018
Location: Täby, Sweden
Looks very neat with clear, modern lines!

How did you deal with the metal surgery on the top cover?
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Old 7th January 2020, 05:41 PM   #3
Steve Morley is offline Steve Morley  Canada
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Join Date: Jan 2015
Location: Georgian Bay
I cut the tube holes with a metal hole saw, see pic. Covering the top with painter's tape first helped mitigate damage. There will be Heyco grommets later, see pic. I ordered the wrong size but will get the correct ones with my next Mouser order. A good size centre drill divot helps keep the saw from wandering and a piece of wood below for the same reason too. The old trannies I used required the help of a local tool and die shop (got a deal, paid cash) for the large rectangular openings. If building with new "iron" I'd get the type like the Hammond shown below that only require small holes for the bolts and wires. The hole for the volume pot was made with stepped sheet metal drills. They're not really meant for thick plates but the aluminum front panel was quite soft and I used lots of lubricant. The pot mounts to a small 3mm plate that was epoxied to the inside of the front panel. The chassis came from China via eBay.
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File Type: jpg P1050311b.jpg (640.4 KB, 20 views)
File Type: jpg Hammond a.jpg (72.9 KB, 18 views)
File Type: jpg P1050607a.jpg (591.0 KB, 16 views)
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