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Old 16th Mar 2017, 7:56 am   #141
Diabolical Artificer
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Default Re: 807 (maybe) amplifier build.

Slowly plugging away with this amp. I've been looking into the power supply side of things which will mean winding my own tfmr. I've wound my own before but this one being about 400VA is taking some research. I found this article - http://ludens.cl/Electron/trafos/trafos.html - that has a spread sheet that has helped.

I also have to decide whether to make a seperate PSU for two monoblocks, this means a massive tfmr, or use separate tfmr's. I also have to decide whether to use smoothing chokes or just lot's a capacitance. I'd appreciate some guidance here, what are your thought's?

As it stands one monoblock tfmr would need a 320v HT winding at around 1A, a 6.3v htr wdng @ 6A if I use 807's or 6.3v @ around 10A if using EL34's. There is a lot to be said for running the OP stage htr's in series, it keeps the heater wiring neater. Front end htr's could also be run in series @ 12.6v DC @600mA. then there's the fixed neg bias and CCS supply - 80v @ 50mA. There is also the supply to any protection circuit's and other add on's. See attached screenshot of a similar PSU.

I've also been tweaking the OP stage as I found I had an asymetrical OP so needed to pop in a couple of pot's to be able to balance the two "sides". Have also matched the OP valves to within 5%. all these tweak's resulted in less THD and more OP power, not much, but an improvement non the less.

Andy.
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Old 16th Mar 2017, 9:52 am   #142
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Default Re: 807 (maybe) amplifier build.

Hi Andy, the Ludens link has some very useful tips, he alsohas a small project that allows the saturation of a choke with DC to be simply measured.
I suspect that you may need more than one transformer for your monster PSU, heaters would certainly be a candidate.
Note that some valves have a low value for the H-K voltage that can be applied, some twin triodes for cascade operation even have different values for each section of the valve. This may present problems if you series connect the heaters.
For high current windings you can get rectangular copper strip and square section wire. These have a better packing factor for winding.
The other approach is to multi-filar wind the secondary. At these voltages there is no insulation problem and you could then also run the heaters on separate wiring runs. This may help voltage drops.
Power valves and high signal level stages will run OK on AC heaters, but you may want to use regulated DC for the low level stages. These should be proper regulators, not a bridge and a large amount of caps, so the winding will need to be increased in voltage to give sufficient headroom.

Good luck, Ed
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Old 17th Mar 2017, 7:02 am   #143
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Default Re: 807 (maybe) amplifier build.

Thanks Ed, forgot to mention the seperate htr tfmr for the cathode followers. Yes I'll run the the front end htrs on DC and use a LM317 or similar regulator.

Forgot about the copper strip, good idea.

Andy.
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Old 17th Mar 2017, 8:35 am   #144
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Default Re: 807 (maybe) amplifier build.

Once you get up to a few amps, you are getting up to the sorts of currents where the packages which regulator chips get built into start to reach their limits, so don't work 'em too hard. Stay under 50% of their current and power ratings, or use a reg with a separate power transistor. Alternatively have a group of regs and split the load amongst them.

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Old 17th Mar 2017, 3:33 pm   #145
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Default Re: 807 (maybe) amplifier build.

What I've done in the past when building transmitters/linears is to 'split' the power-supply.

The amplifier shelf has a small low-power PSU that feeds HT/LT to all the signal-frequency stages in that shelf, and also supplies the bias for the high-power stage (and auxiliary voltages for things like cooling blowers and antenna changeover-relays though these won't be of relevance to you).

Then there's a separate 'high power' shelf that carries the big HT/LT transformer/rectifiers/smoothing.

Doing it this way reduces the total weight that any shelf has to carry [it can be inconvenient in some situations if the gear is so heavy it requires a "two-man lift"] and also the "two supplies" approach makes possible effective interlocking to prevent the "big HT" side ever being powered-up if the bias system for the output stage is not up-and-running.

The only real downside is the need for suitably mains- and HT-rated interconnecting cables/connectors between the two slabs.
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Old 18th Mar 2017, 7:02 am   #146
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Default Re: 807 (maybe) amplifier build.

Spent yesterday putting tentative spec's on paper. The DC supply for paralleled htr's on the front IE 12.6v @ 600mA is do-able with a LM317AHV say, but as you say David a regulated 18.9v (19v) at 1.5A is cutting it close for most regulator's.

You mean splitting it physically? That would mean separate tfmr's too really. Ehm, more weight, more boxes, will have a think about that one.

Lot's to think about, especially the safety aspect, possible fault scenario's, idiot proofing ( I might sell em one day) not to mention the fine art of tfmr winding - flux density, losses and what have you. Not as much fun as making the noisey bit but I quite enjoy making PSU's.

Andy.
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Old 18th Mar 2017, 10:12 am   #147
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Default Re: 807 (maybe) amplifier build.

Hi Andy, you can use the same supply and bridge plus caps if you want, but separate chip regs for the various heater chains.

Ed
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Old 30th Mar 2017, 11:19 am   #148
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Default Re: 807 (maybe) amplifier build.

Progress report. I've wound the mains transformer using a toroidal core out of an amplifier I replaced the tfmr's on. As money is always tight and I like to recycle it made sense to re-use it. The toroid is rated for 625VA which is more than adequate as I calculated I'd need a 500VA tfmr. I overwound it using heavier gauge wire than needed.

So we have winding's as follows - actual DC/wot's needed in bracket's) - a 370v 700mA bifilar wound HT winding tapped at 350v (430/450 @ 640mA), 60v 80mA (40v 10mA and 80v 11mA) for neg bias and the CCS for the LTP, 15v 1A (12.6v 300mA) for the front end and LTP htr's, 6.3v AC 600mA and finally two 18.9v 4A AC (18.9v @ 1.5A). I'll have to use another smaller tfmr for relay/metering/whatnot.

After much thought I decided on an unconventional way to house the amplifier. I think the "whacking everything on an upside down pudding tin" approach with valve's like god's on an alter isn't for me. With a big amp like this the pudding tray/tin or chassis would have to be huge. the big amp's I've seen like this look c*** IMHO. I love how Tek built their old 500 series scopes and have a scrap 545A so decided to build the actual amplifier like a Tek plugin with the PSU etc sitting behind. The amp with be able to be pulled out for working on (but will need an extender cable).

I've put in a lot of thunking on aspect's like layout, cooling, access, grounding, neatness etc. I've built a mock up - see pic's - to iron out any possible problem's. I think better with my hand's and am not one to sit and draw a precise technical drawing. This way if I drill holes in the wrong place it won't matter.

As you see I've re used some of the ceramic wafer's out of the Tek scope ( there was no way it could be easily restored, I thought hard before taking it apart), this way it will live on in some form.

Pic one iis the underside of the amp, the opt instead of being central will go to one side.

Pic two 120w ish amplier a la Tek Plugin.

Pic three, as amp "plugin" will sit in chassis; two will fit in, PSU's int back connected by plugs/connectors.

Pic 4 PSU at present, more work to do.



TFL, Andy.
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Old 30th Mar 2017, 1:19 pm   #149
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Default Re: 807 (maybe) amplifier build.

Nice concept and design, Andy. Very practical, too.

I'm just wondering, there are a lot of sprawling wires in your power supply design. This is not ideal, but will be a lot more critical in the preamplifier and pose amplifier stages - definitely something to give consideration to now.

How are you grounding things in your power supply?

There doesn't appear to be a ground-plane or have I missed something?
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Old 30th Mar 2017, 2:29 pm   #150
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Default Re: 807 (maybe) amplifier build.

*power amplifier *- typo!
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Old 31st Mar 2017, 12:51 am   #151
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Default Re: 807 (maybe) amplifier build.

I like the Chinese 10 turn precision pots you have in the underside view!! I use precisely the same ones. Cheap as chips and beautifully made. I even pulled one apart to check quality, which is excellent.

I'm glad you will be moving this monster around, not me

Er hmm silly question, but why 120 watts? That is HUGE gobs of valve power!!

Good work, Joe.
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Old 31st Mar 2017, 5:02 am   #152
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Default Re: 807 (maybe) amplifier build.

Thanks Al. The PSU has wires all over the shop at the mo as it's in a test stage; making sure the tfmr wind is ok, checking ripple etc.

I'll use a star ground back at the PSU, but there will have to be a two buses either side of the octal socket's for the six cathode current "sense" R's to join, no way round it.

It will make sense in the end when everything's tidied up.

Andy.
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Old 31st Mar 2017, 9:05 am   #153
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by Dr Wobble View Post

I'll use a star ground back at the PSU..,
Ahah but it looks like your zero volts are wires suspended above the chassis - I'm looking for example at the wires coming off those electrolytics?
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Old 1st Apr 2017, 7:18 am   #154
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Default Re: 807 (maybe) amplifier build.

Didn't see your post when I posted Joe...

The pot's are 10 turn precision NOS Spectrol, am not aware of the Chinese pot's your on about Joe, where do you get them from? A cheap source of high wattage 10 turn pot's would be useful as they average price for a 3w jobbie is £30-50! I won't use those pot's as they're too good for the application, I've got a Bourn's 2 gang 10k linear 10k 1 turn pot that will do the job of balancing the IP and bias to the two sides of the OP stage.

All ground's at present come off a fuse which is tied to the bus bar that connect's the 5 "bottom" caps. this is all temporary as I said Al. I've fused both ground and HT for the moment as I'm still mucking about making changes. As it is there is very little hum.

Andy.
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Old 9th Apr 2017, 7:39 am   #155
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Default Re: 807 (maybe) amplifier build.

Proper chassis built using 2mm ali sheet given to me by Rob AKA Glowing Anode who doesn't seem to have been on the forum for a while. If you see this Rob, you'll see I'm finally putting the sheet to use. It was a bit of a sod to bend, but I scored the underside of the bend will an angle grinder 2mm cutting disk, then ran a triangle file down the score to debur and put an angle on it.

Feels good to be cracking on with building it, what with having been stuck in the "development" stage for a long time.

Progress pic's....
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Old 9th Apr 2017, 7:48 am   #156
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Default Re: 807 (maybe) amplifier build.

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Old 11th Apr 2017, 9:00 am   #157
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Default Re: 807 (maybe) amplifier build.

Very neat work, Andy. That looks so promising!! I love your waxed cotton work there on the (ground?) green wires, too. You have obviously found a way to keep hum problems right down.

I love the ceramic standoff/terminal blocks (white ). What are they called and where did you source them from ?

Keep it up , looking great!
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Old 11th Apr 2017, 9:22 pm   #158
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Default Re: 807 (maybe) amplifier build.

Hi Al, they look like the ones used in some Tek scopes. If they are then you need to use a silver loaded solder on them.

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Old 11th Apr 2017, 11:24 pm   #159
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Default Re: 807 (maybe) amplifier build.

(He's) BTDT

http://www.vintage-radio.net/forum/s...d.php?t=135355
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Old 12th Apr 2017, 6:51 am   #160
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Default Re: 807 (maybe) amplifier build.

Thanks Al, the waxed cotton you mention is really synthetic cow gut. It's a cotton fibre with some sort of waxy plaggy substance on top.

The ceramic wafer's are from a Tektronix 545B that was past saving, mentioned it in post #148, but this thread is as long as the Forth bridge, you can't read it all.

Re soldering Tek ceramic wafer's, video here - https://youtu.be/RpB5JqGo1co (Yanks can't say solder) I managed to get some 4% silver solder off Rob AKA Whiskas, I've found it doesn't run well and fill the joint properly on it's own even with a long dwell time and a very hot iron. So I mixed in a bit of 60/40 solder to get the joint right. I unsoldered all the old components and wires using 60/40 with no problem's of the silver contacts becoming loose. I think you'd get away without silver content in the solder to be honest.

I've gone a bit OTT on neatness and the look of the thing and had a bit of a problem matching resistor sizes on this amp. I ordered some 2w R's ( the red ones) to match the size of the big grey 2w 1% metal film R's I had and used, but contemporary 2w R's are the same size as 5 year old 1/2w carbon comps. Obviously resistor manufacture has come on a bit in the last couple of years, so now to get the same size as the 2w grey jobbies ( L 15mm) you have to get 5w rated R's.

Am not too worried to keep most of the R's the same size and colour, if it's correctly rated for it's job it will do. More pic's to follow later today hopefully.

Andy.
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