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Old 19th May 2020, 6:57 am   #1
Diabolical Artificer
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Default Stereo 50w 807 PP amp.

Been working on this for the last few months trying out various OP stages. The amp is based around two OPT's made Joe from Oz, 3k4 pri Z, UL taps at 43%, enough laminations for a bigger 100w commercial OPT, these were designed for EL34's but I'm using 4 807's.

So far tried AB1, AB2, and extended class A, though I prefered AB2 it only uses two valves and the Z pri is a bit low for best power OP etc, so AB2 amp will have to wait. Therefore the amp OP stage will either be AB1 or Class A extended. This is an interesting configuration, two 807's are wired as triodes, with a big 1k R between anode and g2, two are wired as per usual, with a well regulated 300v for the screen grids. On low volume the triode section does the work, with the two 807's as beam tetrode are biased in near cutoff, as the OP is increased these switch on. For more info on extended class A see attached and - https://www.ampbooks.com/mobile/clas...ts/extended-a/ .

I did try class AB2 ultra linear, which resulted in one 807 going purple inside, arcing and the it died. AB2 has to have a well regulated screen grid supply, V should be within 5%, same goes for HT, max 200mW, with HT = 420v no regulation this killed the 807. To deal with the old chestnut "but the 807 g2 max is 300v" ; look at the STC 807 application notes, it says "triode strapped 807 HT 400v etc, etc" also I've run 807's lots of times in UL with g2 at 400v +, as long as you make sure the screen grids are within current/power limits, they're fine.

So, to get to the nub, still not 100% how I'll do the OP stage but have nearly finished the PSU etc. This amp will be different in that all the "iron" sits underneath, valves on top. I did this for various reasons, but the main one is that if I'd have laid out the amp in the traditional way, EG OPT's and power tfmr at the back, valves in front, all on a metal chassis, this makes the amp very badly balanced from a weight POV. Pick the thing up, it goes tits up, not a problem whilst it's sitting on a sideboard or whatever, but an issue non the less.

Also as the mains tfmr is homewound, it would have meant finding a saucepan or other cover for it, the same cosmetic considerations influenced the OPT's, they came with one bell housing, meaning I would have had to sit them drop through, I didn't want to do this, they would have taken up more space, so to keep expenses as low as poss I went with this layout.

To mount every component in a convention chassis was a non starter as outlined previously, I considered a separate PSU and amp, but the amount of fabrication needed would have taken a week. I was hunting around for boxes etc in my sheds when I noticed an old scientific instrument, it's been sitting for years doing nowt, so I nicked the box, everything fit in just.

The amp circuitry will fit on the lid, PSU and OPT's underneath. OPT's are screened from any possible magnetic field by two shields made from 0.75mm ali sheet, covered in copper tape, which will be grounded. HT is limited to around 240v by a big 50w 1k9 resistor, after 10 secs using a simple RC time constant a relay shunts out this R. There is also a protection relay, if any transient or OP valve takes the power drawn over a certain limit, a triac trips, energises the relay which cuts the neutral going to the mains tfmr.

Delayed HT is a little kinder on valves and also means I can use lower voltage caps in several places, they don't see full HT on power up so I can away with 400v rated caps.

Most valve amps don't have protection other than a fuse, now we have easily sourced, cheap silicon devices it seems to me a no brainer to include a simple protection circuit. I fitted this to my other amps and am glad I did. For instance I dropped the needle onto a record the other day whilst the amps were at high volume, the protection kicked in, saved the speakers. We also get big mains spikes here out in rural Lincs, again the protection is a boon.

Right, enough waffle, here's some pics of the nearly finished supply etc, jus have to tidy the wiring using lacing. Andy.

PS, Extended Class a file too big to attach.
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Old 19th May 2020, 7:30 am   #2
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Default Re: Stereo 50w 807 PP amp.

If you have a triac to open the neutral connection to the mains transformer, and it is controlled by protection circuitry in the amplifier side of things, how do you arrange isolation between mains L/N world and the earthed world of the amplifier?

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Old 19th May 2020, 11:29 am   #3
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Default Re: Stereo 50w 807 PP amp.

Don't the relays do that bit?
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Old 20th May 2020, 7:09 am   #4
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Default Re: Stereo 50w 807 PP amp.

Ooops, thyristor not triac. Thyristor switches relay which is in NC position to NO position hence cutting N to mains tfmr.

Cut out the top yesterday and replaced with sheet ali, wasn't happy with it as was. It took hours to get those holes in the wood too, still if some'ats not right it has to go.

This in part was to solve the solution of mounting the valve bases. As was the valves sat about 3/8" down, this being the thickness of wood top. It didn't look right and also left little room for holes to secure the bases.

Talking of bases, I have two versions of the UX5 base, one vintage McMurdo, similar to an octal in that a simple round hole is all that's needed to mount it, it can be mounted on top or under the chassis easily.

The other type is a modern Chinese jobbie, thick ceramic with pins sticking out the bottom at angles. I've never liked this style, it's a bu*ger to mount, there being only one way, IE under the chassis, this means you have to somehow make a hole as big as an 807 valve base, so the valve sits on the base correctly. If you mount said base on top of the chassis, you then have the nightmare of trying to accurately and neatly cut around the pins, see pics.

Andy.
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Old 24th May 2020, 1:50 pm   #5
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Default Re: Stereo 50w 807 PP amp.

Hi Andy,

I had to deal with exactly this socket-problem when i build my 2C34 push-pull class AB2 amplifier long ago.
My valves uses another variant of the ceramic socket, a Medium Size 7-Pin (U7B), but the problem is the same as yours.
As you suggest the Chinese ceramic sockets are for sub-chassis mounting, only.
Doing a chassis cut-out around the connecting pins is REALLY a bu*ger and should be avoided imho.
So, I did mount them sub-chassis and that actually works well.
Yes, you have to cut a hole big enough to let the 807-base pass through the chassis and it doesn't look bad at all.
Here's a picture of the U7B ceramic socket in place w/dust and all .

Rgds,

/Torben
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Old 5th Jun 2020, 7:08 am   #6
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Default Re: Stereo 50w 807 PP amp.

A bit of progress... Built a SS preamp, i wanted tone controls, but didn't want to use either passive or FB type using valves. I used Rod Elliott's design using 5532 opamps - https://sound-au.com/project97.htm with R113/213 swopped for a 15k I get a tad over unity gain, tone controls set to 12 o'clock (0.5v IP/0.5v OP), and about 1.5v out full boost.

Working out the layout for this board was a nightmare, didn't have my mojo working and struggled a bit, so there are several bodge wires underneath. After de-bugging which took a good while, the preamp works very well - lovely clean low noise, accurate boost/cut ( used 1% R's and C's and selected caps).

This will go into a common cathode gain stage, but I'll design this to have high 2nd 2H distortion if possible. The PS and DC conditions of the OP stage will also be adjustable to get high THD of the right sort. So the amp will be badly designed but in the right way, which kind of negates the low noise, low THD all singing and dancing preamp. It's only an idea at this stage though, will probably change my mind : )

Hit an interesting problem with the htr wiring, IE V drop. There's a run of about 8", not ideal, I was loosing about 400mV, partly down to wiring losses, partly down to the fuses. Ripped the fuses out and beefed up the wiring. Spent hours yesterday trying to find a good bit of heavy duty wire to wire the valve bases. I either had gurt big or not big enough. Wired it using the gurt big stuff, didn't look right and not enough twists to cancel AC hum. Ripped that out after spending ages drilling out valve base pins big enough to accept said wiring and used doubled up thinner more easily twisted wire. not 100% happy with the result but happier.

Also had to half dismantle the PSU in order to put on a 15v 0v 15v wndg for the preamp PSU. I looked at using a separate tfmr but several small 10 0 10 to 13 0 13v types I had either had not enough VA or not enough volts. I find getting this right tricky, EG supplying enough V for 78/79 fixed V regs, but not too much so they have a hard time and over heat. Typical, hundreds of tfmr's but never the right one, hence more windings on the big toroid. this is one aspect of designing a valve amp on the fly and having no fixed final design in mind.

Andy.
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Old 5th Jun 2020, 11:44 am   #7
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Default Re: Stereo 50w 807 PP amp.

Hi Andy
Great thread & fascinating pics
Guy
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Old 5th Jun 2020, 6:09 pm   #8
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Default Re: Stereo 50w 807 PP amp.

Thanks Guy.

Andy.
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Old 23rd Jun 2020, 1:44 pm   #9
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Default Re: Stereo 50w 807 PP amp.

Still plugging away.... wired up the OP stage and tried a combination of cathode & fixed bias. I never got to really test this as the amp started acting up big time. With a LTP/driver, driving the 4 807's, with an IP of only 0.5v RMS into the LTP, the delayed start up circuit started unlatching and then latching, effectively oscillating at a few hz's.

The slow start circuit is basically an RC timer, V reaches 12v, turning on a tranny through a 12v zener. This latch's a relay which shunts out a big resistor which keeps HT at 280/300v for 8 seconds.This is powered by a separate winding from the HT off the mains tfmr.

This was an odd fault, lets say the OP stage was oscillating, then how would this effect the separate slow start circuit? A duff relay? A short between windings, or some other short? I tried powering the amp from a big PSU, with the HT fuse out. Bringing up the HT slowly, all was well until at about 385v, the relay started chattering; this with no sinewave into the amp.

I disconnected the slow start circuit and power R, ripped out all the cathode resistor and connected all cathode to ground via 10r resistors. Amp works fine. Puzzler. Have still to get to the bottom of this, I plodded on with some other jobs.

I designed a circuit, built it and am now in the process of getting to grips with the adjustable negative bias circuit. Here we have a problem, we have to keep the grid leak R within 100k, but AFAIK, that should be half that, were running 2 valves in parallel. However any adjustable bias potential divider has to be of sufficient impedance so we don't load the bias supply down. After some experimentation and calcs I came up with the circuit attached.

This will give us Rg = 109k, so not perfect. If we make this any less, it means the LTP driver is going to be loaded down. The bias adjust can't go any lower, 50k is about right, it needs a 20k pot to give us -25v to -45v approx, 10k or 5k doesn't give us enough adjustment. Lowering the fixed R values loads the neg bias PSU too much. I'm not sure whether this needs by-passing with a cap (C? on schematic), some bias supplies have one, most don't, am pretty sure it needs one.

Anyroad, here's some pics of progress so far, any thoughts welcome. Andy.
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Old 23rd Jun 2020, 2:21 pm   #10
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Default Re: Stereo 50w 807 PP amp.

Quote:
Originally Posted by Diabolical Artificer View Post

OPT's are screened from any possible magnetic field by two shields made from 0.75mm ali sheet, covered in copper tape, which will be grounded.
I may be wrong, but I think that both aluminium and copper, not being ferromagnetic, won't shield from magnetic fields, only electrical fields. I would have thought that you needed to use steel, but not stainless, because austenitic stainless steel is also not ferromagnetic. That's why mass spectrometers are made from non-ferromagnetic materials.

Colin.
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Old 23rd Jun 2020, 3:17 pm   #11
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Default Re: Stereo 50w 807 PP amp.

Looks like fun and also a bit of work at the same time, it doesn't take much to lose half a volt in the heater chain. What are you using for speakers ?
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Old 24th Jun 2020, 6:01 am   #12
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Default Re: Stereo 50w 807 PP amp.

Will see Colin, so far the scope picks nothing up, ripple is about 10mV. I do have some mu metal if I do hit problems.

"it doesn't take much to lose half a volt in the heater chain" your right there, getting heater windings right on toroidal tfmr's is tricky, or on any tfmr come to that. My main speakers are mission 720's, one of the best models they made, but in my workshop I have a big test speaker comprised of a 15" Celestion G15 100, which AFAIK is a guitar speaker. The mid is a Celestion 10" and the tweeter duty is a 2 x 75w Fane horn cab. All this goes through a Kef Xover and sounds surprisingly good. One day I mean to use the drivers and make up two cabs. 15" woofers do sound tasty.

Andy.
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Old 24th Jun 2020, 9:35 am   #13
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Default Re: Stereo 50w 807 PP amp.

Hello, Hello !!!

NEVER lose sight of the horizon whilst looking for the sea.
Cheers Andy

Joe
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Old 24th Jun 2020, 1:22 pm   #14
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Default Re: Stereo 50w 807 PP amp.

Ahoy there shipmate! Good point, by which you mean stop worrying about niggling details and get on with it? Point is I'm wired to worry about details, it's the making sure the details are right, that things look smart but at the same time are correct from an EE standpoint. Or have I missed your point?

As someone on Down the Line ( a UK radio 4 satirical comedy on radio phone ins) said "what is point?"

Andy.
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Old 24th Jun 2020, 5:59 pm   #15
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Default Re: Stereo 50w 807 PP amp.

Quote:
Originally Posted by Diabolical Artificer View Post
My main speakers are mission 720's, one of the best models they made, but in my workshop I have a big test speaker comprised of a 15" Celestion G15 100, which AFAIK is a guitar speaker.
Andy.
While Celestion were indeed known for their speakers for guitar amplifiers, it may be that a 15" item might have more often been used in bass amplifiers, rather than guitar amplifiers, Andy.

Interestingly enough, if I remember correctly, one manufacturer (Fender, Vox? - I've forgotten) decided against using one 15" speaker over using four 10" ones. The reasoning behind this is the concept of "moving air" and it is reasonable to assume that the surface area of four 10" speakers (314 sq.in.) is greater than one 15" (177 sq.in.), so the four smaller ones will "move more air" .

Colin.

PS: I think it was the Fender Bassman amp that used one 15" in its original form, then changed to four 10" speakers.
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Old 24th Jun 2020, 11:20 pm   #16
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Default Re: Stereo 50w 807 PP amp.

You have it in one Andy. Wifey constantly tells me I lose sight of the project worrying over the smallest details. Like aligning screw head slots !!!. Nobody ever notices them apart from me.

Speaker sensitivities are probably more important than size. AS Colin says, moving more air faster
( after all its much easier to move 4 ounces of cone in a 10", than 1 pound of paper in a 15") sounds smarter.
If you look around today most "hifi" systems have quite small speakers and give pretty excellent sound. My son has a Sony system that has ( I think) 5" woofers, and comes close to my 12" Tannoys albeit without the depth. Actual volume I think he beats my 7 watts.

Finish the amp first!!. and dont forget 4 807's, even in pure class A is gonna give some pretty large output power and you should be able to drive a one metre length of 12 guage fence wire.

Joe
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Old 19th Jul 2020, 5:06 pm   #17
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Default Re: Stereo 50w 807 PP amp.

Well, what a roller coaster this amp build has been. Finally, I've got the bu**er working & bar a few minor
tweaks I hope that's it.

I had one channel working beautifully, tested great, low THD, good frequency response and sounded lovely too, i then built the RHS channel, bunged in some valves and the fun started. The working channel was now distorting badly, the LHS was oscillating even though it's identical to the LHS and no NFB was applied. Checked my test gear, moved grounds, ripped it to bits, put it back together, still playing up but now with a whole new set of faults.

I gave up for a few days and then went back to basic's, testing voltages, noticed bias V was low, so tried paralling the new bias winding with an old one I wasn't using - nada. Tried putting the two windings in series, big mistake, acrid smoke and a nasty smell out of the mains tfmr. Result burnt out old bias winding, shorted turn, so, completely stripped and rewound mains tfmr.

I wound it better than last time & bunged it back in, only this time I had a massive 10v Picassoesque waveform on the OP's, ***! After days of fault finding I noticed that when put the meter probe on one part of the CCS circuit, the crazyness stopped. I also had one half of the front end acting like a synthersiser, long story short chucked a 2200p cap across one CCS's dropping resistor, the whole amp fell quiet, all fault's disappeared, weird.

That was this morning, tydied up the mess I'd made and put the amp back together for the 3rd time, chucked a load of valves in, amp works like a charm. There's a few minor issues, a little bit of oscillation on SW off with a speaker load, but nothing a zobel won't fix.

Due to happenstance and not design distortion is mainly 2nd H, the SS hifi preamp is especially good, it's really nice having tone controls, though don't need bass boost as bass response is grand, though at same time I have plenty of top end, it's a well balanced amp, I'm quite chuffed with how it's turned out. however I'm not smitten with the look of the thing, it looks a bit empty up top due to all the iron being underneath. BTW there's no issue with stray EMF, quiet as a proverbial mouse.

A bit of tidying up and the jobs a good un. Oh one last thing, thanks to Joe i have some nice new 807's to pop in the amp, though I couldn't get a matched octet out of the 12 Joe sent. Got four reasonably matched, two are toast having shorted screen grids, maybe they got bashed about on the trip from Oz. The rest are have a pretty wide spread anode current wise with two having anode current off the scale . I also found a weird 807 amongst the set, it has like an extra getter, a grid like structure on one side of the anode, see pics.This is odd as all are made by Radiotron in Oz.

Andy.
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Old 19th Jul 2020, 5:16 pm   #18
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Default Re: Stereo 50w 807 PP amp.

It seems you have 1122 for the twisted heater wiring, you could try 1212 instead like quad microphone cable, it may reduce hum a bit.
 
Old 20th Jul 2020, 12:04 am   #19
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Default Re: Stereo 50w 807 PP amp.

Its finally finished then!! Sorry about the busted 807,s, but as you know they were packed in tropical packaging and dipped in beeswax. I didnt know, sorry Andy. Im glad you like the transformers. I did promise you excellent results using them.

Cheers
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Old 20th Jul 2020, 5:57 am   #20
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Default Re: Stereo 50w 807 PP amp.

"It seems you have 1122 for the twisted heater wiring, you could try 1212 instead" Sorry, not familiar with that nomenclature. I used the gurt big thick stuff for the OP stage heaters to get it as far as the amp proper, there was too big a voltage drop else, but it looked wrong and laid wrong for interbase wiring, I had no wire of the right size so used doubled up smaller wire.

For the IP & PS stage htr wiring I used solid core magnet wire covered in sleeve, then bog standard copper core. It's like with caps, draws full of wire, but often not the right type/colour. The choice of box to put this amp in presented some challenges, a lot of things aren't optimum, but with a bit of thought & a prayer to the gods, i winged it and got away with it. If i'd had some screened thick cable i'd probably have used it.

Hey Joe, no need to apologise, if you think about it's a miracle any of em work, they've been in god knows how many military stores, moved, clonked about, experienced high differences of temperature, sat for years, clonked about a bit more and just when they thought they could have a nice sit down some b*gger put em on a plane to fly 1000's of miles again with more clonking and temp change, the tails they could tell if they could talk : ) Two dead out of 12 aint bad.

Yes, the OPT's are sh*t hot. It's funny I've tried them in about four of five designs, all the way from Extended Class A to AB2, they've sounded underwhelming, amazing, so so, then back to bl**dy brilliant.

Anyhoo, nuff rambling, a big thankyou Joe for supplying the iron and bottles and banter. Andy.
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