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Old 1st Oct 2018, 7:43 pm   #21
astral highway
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Default Re: 120W PP EL34 amplifier re-born.

That's a satisfying outcome, Andy!

Nice one!

Have with with the woodwork,

Cheers
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Old 18th Oct 2018, 5:29 pm   #22
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Default Re: 120W PP EL34 amplifier re-born.

Making some progress. Monoblock #1 nearly finished, I just have to fix a couple of issues, EG the clipping indicator LED isn't working and I need to sort out the meter face. After struggling to rescale a VU meter face, I gave up, I havn't a decent printer anyway, and as the meter driver isn't 100% correct for proper VU meter ballistics, I had a brainwave to use runes and "mystic" sygils instead. So now the VU meter is an entropy meter, see pics.

Not sure I'm 100% happy with it, it was a first attempt. Have also been experimenting fitting a light to the meter. My spares box is a bit bare, no white LED's and no suitable bulbs. To be honest there isn't really the power anyway to run a lightbulb, so all I had was a dark blue LED. Waiting for it to get dark to have a proper butchers.

I finally found a finish for the woodwork I was happy with. I was after something to bring out the grain, and slightly colour the sycamore. I ordered some mellow brown Chestnut wax, but it was too dark. After watching some lads build boats on Youtube, I bought some raw linseed oil. So with sander/grain sealer and two coats of raw linseed oil, it looks good.

All I need to do now is drill four holes for some 4" brass handles that will fit on top, so as to make lifting out the amp "module" easier. That and find a volume control knob I'm happy with.

Monoblock #2 is underway, I've cut, planed and routed the wood and just need to plane the mitre joints. Getting them bang on 45 degrees is very tricky with just a Wilko saw and old plane that wobbles. I've part finished the PSU, bent, shaped and painted all the brackets etc.

TFL, Andy.
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Old 21st Oct 2018, 7:05 am   #23
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Default Re: 120W PP EL34 amplifier re-born.

Here's the schematics if anyone's interested and some more pics. Not shown are PSU and aux circuits.

Yesterday was spent improving the clipping indicator by putting a diode and cap on the base of Q1 as recommended by Argus in another thread. One thing that's problematic is that the amp behaves differently when testing with a sinewave say to when playing music, EG the clipping indicator behaves differently with music. It's a minor issue, one easily solved.

One other phenomenon of notes is OP power. When testing with a sinewave OP is 33v before clipping, when playing music OP rarely goes higher than a few volts, so therefore higher than a watt or so. Obviously it will go higher instantaneously on peaks, but I haven't the test gear to capture this.

This is where measuring amp performance is tricky, and the standard nomenclature deceiving, using a SPL meter might be better. Folk like to see numbers on amp specs, but I bet most don't understand what they mean. So it's understandable when subjective terminology creeps in. You can hardly put "bl**dy loud without sounding cr*p" on the amp spec sheet.

The bottom plate was a pain. Marking and drilling the air holes wasn't too hard, but deburring said holes cleanly was a nightmare. In the end I couldn't do it, so had to use a file and sandpaper thus destroying the good finish so I had to paint it.

Andy.
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Old 21st Oct 2018, 7:38 am   #24
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Default Re: 120W PP EL34 amplifier re-born.

Really enjoying this Andy.When its finished and you have had a rest will you knock me one up please.
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Old 21st Oct 2018, 10:42 am   #25
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Default Re: 120W PP EL34 amplifier re-born.

Hi Andy,

Sorry it didn't work out with the 807's and the AB2-drive.
I still plan to go there some day and I'll let you know how it turns out.
Your present circuit seems to be done along more traditional lines and it should definitely produce very good results.

Best of luck,

/Torben
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Old 21st Oct 2018, 11:05 am   #26
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Default Re: 120W PP EL34 amplifier re-born.

Nice work Andy.. 120W of valve audio
I hope the neighbours have braced themselves for the full power test!
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Old 21st Oct 2018, 5:52 pm   #27
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Default Re: 120W PP EL34 amplifier re-born.

Thanks Mick. You can have them when they're finished : ) Lord knows what I'd sell em for. I'm hoping someone with shed loads of cash will take a shine to them.

The prototype is sitting on the shelf here waiting till I finish these T, see pic. I have to be patient and plow on with these. Been planing, chiselling, drilling etc the wood case for amp #2 since 8am, it's slow old work.

Ta W, I'm a lot happier with this version; waiting for the right time when they're out : ) I have cranked em a few times, briefly. I'd need to take them outside to really, the windows in the house rattle and things fall off the shelves.

A.
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Old 22nd Oct 2018, 4:57 pm   #28
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Default Re: 120W PP EL34 amplifier re-born.

Lovely work Andy!
By eck you don't do things by halves.
Well I think that should shame me into rebuilding my baby EL34 amps a mere 35W! I was holding back in the expectation that the house would be "rebuilt" by now but I should have known better and its scheduled to start next march with the erection of a "tent" over the whole building while the roof comes off. So I have this winter in the garage (brrrrrrr) to do any project work rather than my planned warm and dry upstairs study/work room.
Mine won't be as complex as yours, I'm sticking with tried and trusted "conventional" circuitry that I know will work and work extremely well. I will be doing the rework as an "all in one" stereo amplifier as we discussed earlier which should simplify matters. My main bit of nervousness is the need to use DC heaters on the EL34 as well as the signal valves due to the winding being 9V on that lovely big toroid of joes. Although I am tempted to bung in a separate 6V tranny for the power valve heaters maybe rated at 8A so that if I get a notion to redesign for KT88 types there'll be enough there. It'll be a heavy beast! I have a nice piece of aluminium here for a top plate but the main issue for me is not so much carving holes in that for transformers and valves etc but making the wooden framework for the chassis. I doubt I will be able to do rebates like you. Top marks!
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Old 22nd Oct 2018, 5:14 pm   #29
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Default Re: 120W PP EL34 amplifier re-born.

I noticed on your circuit diagram that the input valve ECC83 has no grid leak resistor to ground and is capacitance coupled to the wiper of the pot via a rather high value 100k grid stopper.
Hmmm I admit I would have used something more like 10 to 22k as grid stopper at most and omitted the DC blocking cap on the input as unnnecessary as all my sources have output caps to block any DC offset on their outputs.
Of course i don't know what you feed that animal with and you may have a DC component on the signal coming in. Nonetheless I'd still have a grid leak resistor of around 1meg.
Another point to watch is that 100k grid stopper will shunt the volume pot and at maximum volume you have a potential divider that is shunting 2/3 of the signal to ground if I have understood it right, unless that series capacitor somehow negates the potential divider even at AC?

Andy.
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Old 23rd Oct 2018, 6:39 am   #30
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Default Re: 120W PP EL34 amplifier re-born.

Thanks Andy. sorry to hear your house is still in bits, hope you get started on your amp build soon." I'm sticking with tried and trusted "conventional" circuitry" I can understand that approach, this amp has taken a hell of a lot of work, with many steps backwards, however if I'd built an amp from an existing schematic I'd have learned nowt.

This amp started from a Hammond 1650TA OPT I got for a good price on ebay, at that point I knew next to nothing about about electronics, valves etc. I had read quite a bit on valve amplifier's, but it wasn't clear in my head. Once you try and build an amplifying stage from scratch, EG draw a load line, calculate all the R's and C's, light dawn's and another hundred question's arise.

" My main bit of nervousness is the need to use DC heaters on the EL34" I think you from recent experience that a 9v AC winding won't be sufficiant for 6.3v DC heaters. 9v rectified will give you around 12.5v unloaded, once loaded it will drop around 2v, so say 10v. So now it needs regulating, lose 1.2v ish for the reg and lose another volt ish for series pass transistors and you have not much "headroom" I found you need a winding of 12v AC to be on the safe side. Joe will probably tell me I'm talking out of my hat, just saying what I've found from building a 6.3v reguated 2A PSU. Anyhoo, you don't "need" DC heaters really on a PP OP stage. You could run them series connected at 25v (25.2v) @ 2A. ( four in series) or 2 x 12.6v This is what I did on this amp, IE 18.9v. It makes wiring the heaters neater, no big thick twisted wires. Just a thought.

Don't talk to me about rebates, although I thought mine were mitre joints, it's been a nightmare trying to get bang on accurate, square, 45 deg joint's with an old saw and plane. I made a shooting board which didn't work, and have had to do them by hand. These joint's, were a bad choice, they slip when assembled, so I had to put temp nails in, leaving six holes on each side.

Re wood fettling, I'd advise getting someone with a work shop and prefably long experience of woodworking to do them for you. To get 100% solid, no gaps joint's is not easy and takes far too much time. Find the wood, draw some plans and pay someone is my advise.

Re the IP stage, the 100k grid R is part of the FB circuit, so can't be messed with. I'm not 100% aux fait with this circuit, so I might be wrong, but AFAIK, it doesn't need a resistor to ground, and the coupling C is needed to stop as you DC getting to the grid and removes the pot from the DC equation.

I was in two minds as to whether to include the pot, these are power amps, so strictly speaking one isn't needed, but I'd already drilled the hole for one on the front panel. Also I like having one as I don't have a preamp.

This last issue highlight's one aspect of this amplifier that has been a problem, IE as it's taken two years to build and my knowledge has evolved over those two years, certain circuit's have needed changing. The OP stage, phase splitter and cathode followers haven't changed much, but I need to change the IP stage as it's frequency response was dire; HF rolled off at below 5khz, the amps sounded muddy and bass heavy as a result. Therefore I've been unable to drastically change some aspects without undoing weeks of work, so there are some fudges.

I had a bog standard common cathode gain stage at first, 220k Ra, 1.1k Rk, 1M Rg with a 39k grid stopper. The first thing that improved FR (frequency response) was removing the grid stopper, it went from 5khz ish to 10khz ish straight away. After then trying 7 different ways to provide cathode bias and improve the IP stage, I found the circuit for the IP stage with FB, it knocks every other stage into a cocked hat. On it's own it has a perfectly flat FR from 10hz to over 30khz. It massively improves the amp.

The downside is stability, I found that it worsened stability with especially bad oscillation on switch off. After experimentation fitting the RC's on the OP stage, IE 2k2 and 2200pf, the 220n and 4R7 on the OP but especially the 4k7 and 680p across the IP stage anode resistor improved stability immensely.

However the Mullard 3-3 use local FB, as do quite a few other commercial valve amps, so unless I experience problem's it stays. At some point I will do a test loading the OP with a capacitive load only, to really test stability.

Thanks for your input mate, Andy.
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Old 24th Oct 2018, 12:57 pm   #31
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Default Re: 120W PP EL34 amplifier re-born.

OK Andy,
That ECC83 input stage had a familiar look about it and I remembered using it in an adapation of the Mullard mixing preamplifier circuit. From the Mullard Valve circuits book. I have reminded myself of the circuit and the local feedback on the ECC83 is applied exactly as yours is, a low value capacitor in series with a large resistor from anode to grid. There's no grid leak, but the circuit shows a large grid stopper of 470k in series with a 500k pot so the effect is pretty much the same as having a grid leak of some description. There's no dc blocking cap on the input but obviously the previous stage is capacity coupled.
Interesting though that you applied a bit of local feedback around the first stage rather than rely on the global feedback which is also coupled in via the cathode in the usual way.
Gives me an idea for reducing the overall gain a bit on the new design as i don't need to use an EF86 or similar pentode strapped as a triode and can use a double triode valve in the first stage, either ECC81/2/3 or 6SN7/6SL7.
On the other hand I do have lots of small signal pentodes doing nowt....................
Yeah the challenge of designing your own circuit, I started down that path a few years ago only to discover I had reinvented the wheel! But I was keeping the circuit as simple as possible and decided against things like CCS on the cathodes and once you do that there's not really that many ways of wiring a valve to amplify.
I love my valve amps, theres no logic to it at all. It is very much an emotional rather than rational thing. And if i want a very high power, low distortion power amplifier that will provide low end thrutch down to DC then really only silicon will do it at any sane cost! But wheres the romance?
I've been using that 1980's mitsubishi power amp a lot in the stereo and given that its got, even by todays standards, a good sound (pun?) design and seems built like a battleship, I have been very pleased and surprised with it's performance. But i will still use the KT88 35watt amp even if the reasons are more sentimental than logical!
Love to see the finished job when you get there.
A.
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Old 25th Oct 2018, 7:41 am   #32
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Default Re: 120W PP EL34 amplifier re-born.

Interesting, about it being in the Mullard book, I lifted the circuit from the Valve Wizard. I'll do some testing of the IP stage, though so far I've no complaint's apart from it caused a loud squeak and thump on SW off which I cured. Your concerned about the grid having no reference to ground I presume, will check. It does have less gain less than 9x, and less IP Z, about 1M5 from memory, but wow, FR to die for.

"Interesting though that you applied a bit of local feedback around the first stage rather than rely on the global feedback" This is another instance of the amp evolving rather than being designed from scratch. When I applied global NFB I was limited as to how much I could apply as the amp took off with a speaker load, so I had throttle back a bit and use a 5k NFB R instead of a 4k. That might have been better as far as FR is concerned, but I doubt it would have been better than as is.

Using NFB early in the front end really makes a big difference, another way is to apply FB from the following PS stage, but I was unable to do that here without altering the PS, as it was the IP just bolted on.

The thing with front end's is that to get very low THD, you have to use a big anode resistor, so low THD, too much gain. Local NFB solves this problem.

Yep, if you a have a reasonable selection of signal pentodes and triodes your spoilt for choice. I bought some EF37A's a while back that I've been itching to use, but havn't had a chance.

Which brings me to building valve amp's. The Eco Hippy in me hates wasting 40W ish just to warm the cathodes, but then they look nice, daft. Oooh, should I use octal or B9A type valves, again not a good sound engineering choice, but that's the beauty, it's part creative art work, part hard no nonsense engineering. good craic.

Thanks for your IP and interest, Andy.
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Old 12th Dec 2018, 7:53 am   #33
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Default Re: 120W PP EL34 amplifier re-born.

Finally finished the amps bar a few niggles, a bit of testing and a few other minor jobs, see pics. I'll hopefully get to listen to them over the hols, I will of coarse be playing Jingle Bells, Here it is Merry Xmas and Walking in the Air at full blast.....not : )

It took me a little longer than anticipated as I had to redo the ali chassis for amp #2. This amp "module" was the first one I did, so it wasn't quite square and in fact was smaller than the other. It didn't sit well in the wooden case, so not happy, start again. Progress was slowed by my only electric drill dying half way through drilling out the big octal socket holes, I had to file the rest out by hand and drill all other holes by hand drill, all 100 odd of them.

I made a big boo boo in that I should have made the monoblocks a mirror image of each other, with one on/off SW on the left, tother on the right. I started to do this on the wooden case, then realised that as the PSU was already made, I was committed to making them as is, otherwise the layout would have been wrong. I'm bu**ered if I'm starting again : )

Another SNAFU I encountered was no ground connection from the PSU to amp chassis on amp #2. I found this out when I nearly had kniption's after reading 70v on the heater PSU! What the flegnog? Turn's out the McMurdo red range chassis connector locating pins which I was using as ground connect have a 13 thou gap between pin and outer, so OC. I got a coke can and made a round shim, to fill the gap and now have a good ground connection.

I have a problem to solve on both amps, they could be quieter and amp #2 has hideous 100hz hum. Hopefully this mean's redoing the decoupling of the IP, PS and cathode follower and checking for any ground loops. One last job is to do the meter faces which I'll try and do on a bit of software called meter.

TFL, Merry Xmas, ho ho ho, Andy
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Old 12th Dec 2018, 9:27 am   #34
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Default Re: 120W PP EL34 amplifier re-born.

A suggested brand name when you go commercial:
"Tenacity Audio plc"

Absolutely bl**dy fantastic.

Cheers
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Old 12th Dec 2018, 10:41 am   #35
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Default Re: 120W PP EL34 amplifier re-born.

ALL good 'tother Andy,
its workin!!!

A few niggles, as you suggest, to fix.
That should be easy-peasey.
My best regards
Joe
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Old 12th Dec 2018, 2:03 pm   #36
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Default Re: 120W PP EL34 amplifier re-born.

Thanks Guy and Joe (email ont way), some more pics ....

Been listening to them for a few hours, they don't sound too bad; cracking bass and good stereo separation, obviously. I do need to build a simple pre-amp though as they need about 1.5v to drive them to full whack. It'll be solid state though.

Cheers, Andy.
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Old 12th Dec 2018, 2:39 pm   #37
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Default Re: 120W PP EL34 amplifier re-born.

Excellent work, Andy!


If you have a couple of 6J5ís hanging around, I can heartily recommend my simple low voltage circuit to increase the drive level slightly 😜👍


Cheers. Simon.
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Old 12th Dec 2018, 5:55 pm   #38
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Default Re: 120W PP EL34 amplifier re-born.

I am not going to post any pics of my feeble DIY effort now!
Had a frozne fingers moment in the garage this afternoon and I can tell by the cock eyed holes i drilled for the input sockets and speaker posts. Its too ****** cold to work outdoors up here! And i won't have the indoor workspace until next june at the soonest.
I received everything except the toroid cover today to start the job so i still need to mark out and drill the holes to mount the tagboard standoffs. I'll be happy if it works as per the design brief and without annoying hum.
The woodwork there looks very nice, I wish I had the tools and ability to make joints like that but 2 left hands with 10 thumbs leaves me rather useless!

They look like they mean business and I guess your heating will be supplemented!

A.
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Old 12th Dec 2018, 6:12 pm   #39
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Default Re: 120W PP EL34 amplifier re-born.

Quote:
Originally Posted by thermionic View Post
Excellent work, Andy!


If you have a couple of 6J5ís hanging around, I can heartily recommend my simple low voltage circuit to increase the drive level slightly 😜👍


Cheers. Simon.
interesting low voltage circuit. Great if you don't have a ready source of HT.

Andy if you use this, just use one of your 6SN7 as they are the same valve in a dual triode package.

Myself I'd use 2 x 6SN7 or maybe a couple of ECC82 with the first valve as a normal common cathode amplifier and the second triode as a cathode follwer and then you can have the power amps away from the main sources as the CF would drive a fair old length of wire if needed.

Or make a mu follower from a couple of 6SN7 which will give you a nice lo Z output but the gain will be around 20x from that valve. which would be a 2 volt drive from a 100mV input, then again some older sources can be that low.

If you had the PSU and mono blocks closeish to the preamp then I'd just pinch a bit of heater and a few milliamps of HT off that.

A.
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Old 12th Dec 2018, 10:55 pm   #40
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Default Re: 120W PP EL34 amplifier re-born.

I can see Andy wandering around with nowt to do !!!
They are finished WWHHEEEEEE.
Took a lot of work!! I can see that. I do like the Tektronics type tagboards!! I have not been able to find them at a "normal" price, although as far as I can I do point to point in my builds. I still have my chassis to repaint , and then I will start my build. I have all the bits n pieces so really I have no excuse. Well we do have the first cyclone predicted to blow directly over us as it travels from the Gulf of Capentaria back to the east coast of Queensland.

In the meantime I have been doing a few odd jobs like building my first ever stereo stand. Its only el cheapo pine salvaged from a large cable drum ( like 10 feet in diameter) so I guess its called pallet wood. BUT its seriously solid and has reduced noise by a very large factor. I have also come up with a new preamp design. As I have a standard 19" rack case laying around I will start building this soon.

I figure that Andy will be playing with a couple of output transformers ?? that I know he has laying around. PERFECTLY suited to push/pull parallel 807's, more EL34's, also in push/pull parallel, How about something really exotic like 6 6V6's ?? Would look like Baird amps .

Anyway good to see its done, and with excellent workmanship.
Cheers Andy
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