steve
Rank: Trio
Posts: 206 
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Post by steve on Dec 30, 2015 13:04:37 GMT
OK this is my my home - built valve amplifier. As requested, the following blog will show how it was built from a piece of 18 x 18 sheet steel and a couple of bits of solid hardwood.   |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 13:07:49 GMT
We start with a piece of sheet steel, marked out with punch, scriber, engineer's square and steel ruler. Metal was drilled and punched with appropriate sized holes for valve sockets, transformers and capacitor bank.  |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 13:10:47 GMT
Metal was then bent into a chassis shape.  Chassis is fine adjusted with a rubber mallet until sits four square with no rocking. It is now ready for cleaning, rubbing down with wire wool and priming. |
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Post by MikeMusic on Dec 30, 2015 13:13:19 GMT
Very impressive Steve
I take my hat off to you
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 13:18:03 GMT
Cheers, OK the prepared chassis has now been primed and ready for the final colour. Camera angle makes the front look bowed, but in reality it is perfectly straight.  |
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Post by pre65 on Dec 30, 2015 13:20:19 GMT
Yes, Steve's not one for breadboarding, straight into the finished project, and very nice too.
My technique is what I call "deluxe breadboarding", usually a mix of MDF and alloy with the MDF painted white.
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Post by MikeMusic on Dec 30, 2015 13:20:41 GMT
I'm totting up the skills I don't have.
Although I have O level Metalwork. I couldn't do the plate and the bending
Spraying - don't have that.
Then there's the woodwork which I might fare better with and
The electrics - not a chance
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 13:21:08 GMT
After much flatting, re-priming and re-spraying with a metallic gold paint. The chassis is about as good as I can get it:  Transformer end bells have been sprayed a nice shade of raspberry red. That completes the 'fifties look as far as the paint job is concerned. The gold will be sealed with clear lacquer, |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 13:23:13 GMT
Chassis, transformer end-bells and choke have been clear-lacquered to give a deep shine.  I got four NIB Canada General Electric, 12SJ7GT input/driver pentodes cheap, which was nice. |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 13:25:23 GMT
The iron; all covers fitted:  L-R...choke, mains, OPT, OPT OK, that's all for the moment. Later, we will look at mounting all the bits and starting the wiring. |
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Post by John on Dec 30, 2015 16:19:06 GMT
I love seeing builds like this. Thanks for sharing Steve
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 18:47:52 GMT
Hardwood cheeks are on.  Mains transformer, choke, input stage balance pots and on/off switch fitted. Cap bank is now at full strength. |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 18:51:08 GMT
Mains, HT and heater leads terminated; rectifier board fitted.  Small toroid is 12V supply for the 12SJ7 heaters. Close-up of rear of delay relay socket,  After the allotted time, the relay contacts close, shorting out the resistor and allowing the caps to charge to the full HT voltage. Although it's nothing new, I nicked this simple idea from Merlin Blencowe. I wouldn't have thought of it on my own; I'd have just bunged a thermistor in. This seems a nice little way to get a slooooow start. |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 18:52:37 GMT
Power supply is done:  |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 18:53:13 GMT
And tested:  |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 18:54:03 GMT
All busbars installed and output transformer transformer wiring done:  |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 18:55:41 GMT
Screen supply (brown wires) and screen balance pots are done for input stage.  |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 18:57:26 GMT
Amp is almost finished.  |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 19:00:47 GMT
 Blue wires are the screen supply for the output valves We can see a set of, Russian K75-10 coupling caps. I have wired them in so they are easily accessible and removable for cap rolling purposes. The feedback resistors are also designed to be easily accessible, so that the feedback can be tweaked up if necessary, by paralleling suitable values up with them. |
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steve
Rank: Trio
Posts: 206
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Post by steve on Dec 30, 2015 19:04:36 GMT
Here's a pic of the bias supply and HT fusing. Board is elevated on a pillar:  Grid bias is set by the lower pot in each pair. The upper pot nulls out the current difference between individual valves in each output pair. Current is set by direct reading of mV across 1R resistors in each cathode, which converts to mA directly. Then nulled out across the pair and voylaa. Bias supply has since been upgraded, with zener regulation and higher quality adjustment pots. |
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