Williamson 6L6 Monoblocks (Finally!)

[sorenj07]

Well, I finally got all 99% of all the parts to put together a pair of 6L6 monoblocks, probably with a Williamson topology. Here's a tentative layout:



Another option:



The tubes are 4 Sov 6L6WXT+'s, 4 Sov 6SN7GT's, and 2 NOS 5U4GB's (forget which type..)

The big cylinders are 80uF 370VAC motor run capacitors for the first L filter with the 193J. The wide, flat caps are 40uF 330VAC motor run caps for the second
L filter, in conjunction with a 156G (9H 40mA, left out of picture). Finally, the white plastic cylinders are 22.5uF 280VAC film capacitors for the input and splitter stages.

I'll probably have to have a 3" tall chassis (boo) to fit the 40uF and 22.5uF caps and the small choke. I have most of the nitty gritty coupling caps and resistors and whatnot, and am just waiting on picking up a pair of aluminum 17x10x3" chassis. I'd love to be able to sub in KT88's or something given that each chassis gets its own 282X, and the 6.6K 60W OPT would probably work decently.

One of my concerns is that the 80uF caps are only rated to 70C (the 40uF is 90C and the 22.5's are 100C) so I'm a bit leery of putting tubes so close to it. Is that far enough?

[TomMcNally]

That's why I went with a 4" deep chassis (yay!) to hid the caps underneath. I don't see any beauty in capacitors and transformers (except the cool James jobs) so I like to hide them underneath when possible.

[sorenj07]

As long as they're not excessive, I think that iron and caps can look OK. Here's an excessively labeled diagram showing my new layout. A big flat motor run will go directly beneath the choke for the 2nd LC filter, and not shown are the two plastic caps per chassis that make the final RC filters for input and splitter. It took me an awfully long time to "find" this layout. Building starts as soon as I get some more of those amazing 2w 100R precision potentiometers, 10 turns :)

[TomMcNally]

That layout looks nice - the staggered tubes definitely look good. You have the caps away from the transformers, near cooler tubes. That should work well. Do you have brackets for the capacitors, or are you going to mount them through the chassis and hold them in place with aluminum ?

[sorenj07]

Thanks! I got a bunch of mounting clamps from Mouser so I'll probably mount the big guys on top and just drill holes for the terminals. I don't think I could make a very clean cut with a 2.5" hole saw by hand, though my 4.5 amp drill could probably beast through it.

[TomMcNally]

Yeah, holesaws are only good for things that cover the holes. The meter on the front of my 300B amp was cut by a $ 22 hole saw that only got used one time. I started collecting Greenlee chassis punches a few years ago, I just checked, the biggest I have is 1 7/8" so that won't help you. They are great for tube sockets, especially in the Hammond steel chassis, they punch through quickly and cleanly.

[sorenj07]

I'm going on Monday to see a local metal shop. They mostly do simple 1-plane cutting and some bending, so I'm not sure if they can fabricate a box and put the big holes, but if they can, it'll be in nice, thick .1" or .125" aluminum :)

[TomMcNally]

I've never tried that, although a friend used to make me some stuff in the metal shop at the local FAA base (thanks taxpayers !)

You could always hide those big caps underneath ...

[sorenj07]

Well, I already am hiding a few big caps underneath, but the biggest (two 2.5" diameter by 5" or so motor runs) definitely won't work. I don't like the look of a really tall chassis... I'm giving up the possibility of using 1.5" tall chassis by using motor runs, and it wasn't a very easy decision Yellow_Light_Colorz_PDT_13

[sorenj07]

Well, these have been sitting on a shelf and not getting much done. They're clunky and not very useful at the moment. I have 98dB-efficient speakers and no linestage to adjust volume! Plus they BARELY fit on my little stereo rack.

I was debating gutting the expensive custom chassis and rebuilding them in a more compact fashion, but I think I could still get some utility out of the amps as they are. I'm interested in changing the schematic, adding a legitimate fixed bias source, implementing feedback, and going back to 6L6GC tubes (I'm using horribly crooked KT77's at the moment). Here are some ideas, mainly gotten by ripping off the Eico HF-87. This also gives me the excuse to drill some nice ventilation holes after gutting all the old wiring :)

I also put together some dummy loads on some heatsinks, two 16R 35W aluminum-housed resistors in parallel. My signal generator is on the fritz, I think, but hopefully I'll be able to tune FB for pretty square waves!

Signal


Power


Bias



What do you think?

Note that if I keep the same chassis, I've got the following motor run capacitors bolted in there: 100uF 330VAC, 80uF 370VAC, and two 22.5uF 280VAC. I'd probably grab a 22uF 500V 'lytic for that first cap in the PSU, and then use the 100uF can for the main B+, the 80uF to feed the 6SN7's, and the last 22.5uF to feed the (input tube). Does that seem all right? A 5AR4 can handle being in that position, right?

P.S. thanks to Eric for the tip-off regarding the (input tube)s... I'd rather not name names because doing so has a way of sending eBay prices climbing. I'll wait till I have a healthy stash :)

[EWBrown]

I've been making a practice of "hiding" the iron and caps under the chassis lately, when the chassis size allows for this, and it makes for a very nice, clean look, and the best part is that with everything under the "hood" is that the amount of drilling and punching is reduced considerably. Yellow_Light_Colorz_PDT_06

I've been using toroid trannies, too, they only need one drilled hole, around 1/4 to 5/16 diameter. Life is good Yellow_Light_Colorz_PDT_03

/ed B in NH