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Old 30th Oct 2015, 7:20 pm   #41
TrevorG3VLF
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Default Re: EL34 power amp project

There is a resistance in series with the rectifier diodes, it is called the transformer wire resistance, both primary and secondary. An extra resistance outside the transformer will reduce the peak current and reduce the dissipation in the transformer.

Valves differ one from another. If the current is a bit high, then the EL 34 cathode resistors should be increased, not reduced. Try 560 ohm.
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Old 30th Oct 2015, 9:15 pm   #42
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Default Re: EL34 power amp project

thanks Trevor.

I had a bit of a think and as I am using a transformer that was originally designed for a 5-20 I revisited the mullard circuit. Sure enough it was as I remembered and it uses anode resistors on the GZ34. So I put a couple of 100R in on a tagboard where there was a convenient bit of chassis. Bingo! Ht now bang on the button everything measures right.
So I wired it into the Hi Fi and had a listen. Very nice indeed. A very slight hum only discernible by putting an ear to the speaker (Kudos Cardea1) and certainly no more than my leak stereo20 so I guess I got it mostly right on the heater wiring.
My main gripe is the noisy mains transformer got a noticeable 50 hz buzz so i guess its not magnetostriction as I believe that comes out as 2ndharmonic 100hz but I'll look that up again in Jones. The transformers are on a home designed compliant mount i.e. plasticky rubber cable entry grommits to break the magnetic coupling on the steel chassis also. I may try tightening the mounting bolts, otherwise I cant think of a way to stop it. I did notice that I could damp out a bit by pressing on certain areas of the top cap. Theres worse things that could have happened.
But I am rather chuffed with meself, this is the first large scale project i have built from scratch even though I built a KT88 amp from a kit it's hardly the same thing.

Andy.
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Old 30th Oct 2015, 9:18 pm   #43
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Default Re: EL34 power amp project

Ok now to examine the amp with the oscilloscope. That will be fun as I have never used a scope in anger yet. But as it sounds ok I am hoping I wont find any nasties.

Thanks to everyone so far.

A.
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Old 30th Oct 2015, 9:23 pm   #44
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Default Re: EL34 power amp project

and heres the finished product.............
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Old 30th Oct 2015, 10:19 pm   #45
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Default Re: EL34 power amp project

Looks good, and it works... well I assume you have a working pair.

Magnetostriction does frequency double, so you'd hear 100Hz. Loose windings are a possibility. Have the transformers been vacuum-impregnated? I had hellish problems with acoustic noise from the transformers in the amp that's been sitting in my lounge for the past geological age. Vac impress helped, but in the end I potted them in a thermally conductive silicone.

You get a lot of people posting on hifi issues. Very few of them have even built a kit, and a scratch build to an existing design is even rarer. So you're now in a fairly exclusive club.

Once you've got this finished and polished, what do you do next?

You could decide that this amplifier is it for you and get on with using it to listen to years and years of music.

You could try the next rung on the ladder and build something to your own design. It's a biggish step up - you have to learn how to choose bias points and quiescent currents and circuit topologies, then calculate values. The feeling of achievement is proportionately larger. The big pay-off is that you can use the skills acquired to do all sorts of other things.

You could have a go with transistors. There are some very good solid-state amplifiers, but they do tend to have more complex designs and higher component counts. Valves work remarkably well in much simpler designs and their unremoved characteristics aren't objectionable, but this certainly isn't true with transistors.

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Old 30th Oct 2015, 10:49 pm   #46
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Default Re: EL34 power amp project

Great to hear its working.

while you are chasing down that hum, have a check of where you have run the wiring to the valve bases. Keep grid wires away from plate ( anode) wires. By that I mean sometimes the orientation of the valve base means we have to cross over the two wires.
try and route the wiring to avoid this, or even turn the valve base around to achieve this. My phono stage didnt only have wrong resistors in it, It also had poor layout.
I actually ripped it out completely from my first build and totally changed the layout.
I now have hum free audio. I also have damaged paint from the changed position of the valve bases.
Although I have been building for more than 50 years, Im far from a "good" builder!
So if at first you dont succeed leave it alone for a week or so, then go back and have a real critical look.
I wish you the best results.

Joe
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Old 30th Oct 2015, 11:27 pm   #47
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Default Re: EL34 power amp project

thanks David and Joe. Appreciated very much.
I have cut my teeth on preamp projects over the last few years and that taught me a lot about layouts and earthing strategy. Morgan Jones' books are my bible on this.

I still have to build the other monobloc David, I figured that I should get this one right and then the other will be a simple task of building a carbon copy I hope.

I think the way oversized output transformer has contributed a lot to the overall perceivedbass performance in as much as its not being asked to work at its limits. Danbury advise this transformer can be used with a pair of KT88 in Class A-B giving 50watts, so a pair of EL34 banging out at most 25watts shouldn't worry it. Mind you I dug out some data for Leaks TL25+ ("Firsts in high fidelity" by steven Spicer a marvellous tome for those who admire and enjoy HJ Leaks products) and the TL25+ can give 30 watts at clip which is a tad more than the 5-20 but I am not certain on that point.

However, (theres always a However), I have made the transformer buzz WORSE by tightening the mounts down really tight. I suspect its the corners of the lamination stack resting on the screw heads that hold the "gasket" in place on the chassis, meaning that theres no compliance left in themounting grommets. Its obviously a mechanical problem so I shall mull it over. ****** annoying though. Its not an unknown issue to me, my KT88 kit amp had a nasty little buzz which in the end i fixed by the rather prosaic use of a blob of blutack on the chromed top cap. I suspect the top cap may be a suspect here as I got too clever cos i wanted nice dome head nuts on the transformer tops and i think i should revert to the original mounting method with the bolts going through, its just a chew to remove the caps when the bolt goes in from the top, and i may want to repaint them as the finish isnt as good as i would like. And I think Black would be a good colour for heat dissipation.
Andy.
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Old 30th Oct 2015, 11:46 pm   #48
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Default Re: EL34 power amp project

Quote:
Originally Posted by joebog1 View Post
while you are chasing down that hum, have a check of where you have run the wiring to the valve bases. Keep grid wires away from plate ( anode) wires. By that I mean sometimes the orientation of the valve base means we have to cross over the two wires.
try and route the wiring to avoid this, or even turn the valve base around to achieve this. My phono stage didnt only have wrong resistors in it, It also had poor layout.
Got you Joe.
I spent a lot of time thinking very hard about the heater layout and how to run signal carrying wiring. To which end i did the best I could to get the heater pins facing back into the chassis edge and bring the heater wiring in at Right angles.
I think I can improve the input "plumbing" I reckon moving the RCA socket to the front of the amp and thus avoid a run of screened cable from the back of the chassis will be a bonus as explained in a previous post. Theres a hole I can easily ream out to take a socket. I never saw the point in a LED to tell me a valve amp is switched on, its kind of obvious innit?
The other trick, well 2 tricks I guess, is to sort out DC heaters for the ECC81 and EF40/86. For a 500mA load I think I could get a small 9V transformer and use a 3 legged regulator rather than just rectifying the main heater supply and bunging a 10kuF capacitor across it. My kit amps use this technique to good effect and they are silent under no signal. Then again the hum is so quiet as to be undetectable more than a foot away from the speaker.
The other side of the coin would be to rewire the EF40 as a triode, knocking the sensitivity down from 125mV to a more sensible 500mV or so. That should help with the background noise, I noticed a very small amount of hiss when my lug'ole was against the speaker. Possibly partition noise from the pentode?
So still lots to do Joe, as you say, this isn't the end result, it's the beginning of the next phase. I'm treating this as much as a learning aid than an "Ultimate Amp" I got one of those already............... maybe.............

Andy.
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Old 31st Oct 2015, 6:34 am   #49
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Default Re: EL34 power amp project

Sorry matey!!! Ultimate is the next village over thar! ->.
Once you have the bug, it dont go away!!
I have retired from amp building about 25 times, Im already more than halfway thru a new design. Push pull parallel 7C5's ( wonderful 6V6's in an octal base) Origional yank order by US Army Signal Corps 1942. AND, AND, I have a big box of em ,so matching should be easy. I have coupled these into one of my own transformers. 5K : 8 ohms, Ultra Linear, BUT with 43% screen taps rather than the preferred 20% that 6V6/7C5's are supposed to like. So assuming I get the HT right it should fetch me about 20 watts. I was/am/canbe an audiophool, so the transformers are made on 1 1/2 X 2 1/4 inch cores ( about 250 watts for 50 Hz ) 9 section with staggered winding arrangements. They weigh 4.7 kilos each.
Power supply for heaters will be ALL DC as the heater winding is 15 volts @ about 5 amps, and I will wire in series to use 12 volts overall to the glass. Twice the voltage = half the current so hum SHOULD be lower than ever. The power transformer Im going to use came from a Royal Flying Doctor 100 watt base station transmitter made by AWA in the late 50's, a sprightly 7.3 kilogram fully potted and Mu-Metal screened baby with resin cast in turret tag outlet board. Phew its a mouthful but its also a wonderful transformer that I have been sitting on for 20 years.
8 7C5,s need 200 milliamps give or take, so Im going to use TV damper diodes as the rectifier, 6AU4GTB's with a separate heater transformer for them. I have a left over HT Delay board that I produced a few years ago that will apply HT after 1 minute. Where am I at ? Im trying to decide weather to make a separate phono stage, or incorporate it into 2 chassis.
That is power supply chassis with umbilical cord to power amp!!. WITH or WITHOUT phono stage is my conundrum .

Ill keep you up to speed.
Joe
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Old 31st Oct 2015, 2:19 pm   #50
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Default Re: EL34 power amp project

6V6s are octal base

7C5s are loctal base.
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Old 31st Oct 2015, 10:29 pm   #51
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Default Re: EL34 power amp project

Ok, heres the puzzle.
The transformer is silent both under full load and with no load if its just "nipped up" on the isolating grommets without the transformer "cap". As soon as I fit the dratted top cap it buzzes. Admittedly nothing like it did now I got some countersunk screws so the base of the laminations is clear of any protruding bits once the thing is bolted down tight. I dont like the idea of not having the transformer screwed dead tight to the chassis as i have read the small amount of vibration from a mains transformer can shake a bolt loose in time. The buzz is much less than at first so for now I'll live with it while I think matters through.
It's currently playing through the domestic system and sounds damn good. Leak's design still cuts it with modern kit. Although I think the oversized output transformer and improved PSU (50uF psu caps instead of 16uF/8uF and a choke instead of a 100 ohm resistor) is having some influence on this. bUT leak used very good output transformers and lamination mass is only part of the story. If the primary and secondary windings arent done correctly then you'll never get a good result. The danbury OPT does look well thought out and care seems to have been taken to interleave the windings for better coupling. Just wish it had some impedance tappings for at the very least 3-4 ohms as well as the 8 ohm tap. I usually use the 4 ohm tap on my leak amp into my (nominal) 6 ohm speakers.
All the same I decided to abuse it a little bit and am running 2 speakers in parallel and at normal listening volumes it sounds very pleasant indeed. Not the optimum way to treat a pair of EL34 but I think they aren't being run too hard.
The second mono is under construction and should be working by the middle to end of next week.
I probably shall continue tweaking it. triode connection of the first valve is on the cards along with moving the input to the front to be next to the first valve.
DC heaters of the preamp valves also but only if I feel the hum is intrusive and its pretty darn low now. If it can't be heard at the usual listening position its probably not an issue.
And I have an idea to try CCS on the EL34 Cathodes. There's a simple way using a 3 legged regulator as a constant current source but one has to be careful that the maximum 35V across the regulator isn't exceeded. There's ways around this.

All for the future, I just want to get them working as good as possible in the original circuit for now.

Andy.
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Old 31st Oct 2015, 10:30 pm   #52
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Quote:
Originally Posted by Herald1360 View Post
6V6s are octal base

7C5s are loctal base.
I think Joe knew that, slip of the fingers on the keyboard?

A.
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Old 31st Oct 2015, 10:32 pm   #53
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Default Re: EL34 power amp project

Correct I was typing too fast.

Joe
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Old 31st Oct 2015, 11:17 pm   #54
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For the transformer hum there is one trick I use that works well.
When mounting a kitchen sink into a benchtop, a type of foam tape is used. It cost a few cents per metre at most decent hardware stores. Its black and very mildly sticky.
Make a "gasket" out of it, cutting JUST the four mounting holes out. Leave the excess sticking out from under the cover. Attach the cover and do up the bolts. TIGHT. Then using a razor blade carefully trim the excess gasket off using the very edge of the cover as a guide. I actually slide the razor blade along the edge and if you are careful it wont damage the paint on the cover. The razor blade MUST be sharp!!!.

If that doesnt help, I dont know what you can do, mebbe try adding some "dampening" material to the inside of the cover ? A layer of silastic perhaps ( let it dry before refitting).

Regards
Joe
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Old 31st Oct 2015, 11:41 pm   #55
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Default Re: EL34 power amp project

Goodness knows Joe. I suspect part of the problem is the fact I had to make a "spacer" in order to fit the mains transformer to the chassis. I felt I'd be ok as the weight is borne via the grommets onto the steel chassis. Maybe there's some sort of resonance going on? I have an option, albeit expensive, which would be to obtain a pair of transformers that fit the chassis. Majestic do one that will go in the hole with only a small amount of opening out of the bolt holes.
I'll persevere for now. If it means using it without the cap, so be it.


A.
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Old 31st Oct 2015, 11:50 pm   #56
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Default Re: EL34 power amp project

Excellent, very satisfying to spend time making something and then have it work well.

Following this all with interest! The pics of chassis with those classic tagstrips and twisted pair layout, and discussion of commoning and hum reduction precautions took me back rather a long time to building up a 5-20 (well, 8-20, really, with stereo chassis and solid-state rectifier). I'd been mucking about with things that used 250VDC or so for a long time, but something that used nearly double that seemed frightening, even foolhardy. Anyway, it worked nicely and was satisfyingly hum-free. I was so wary of the electrolytic voltage limit that I used 2x 47uF 400V in series as reservoir and 2x 220uF 385V ditto as principal smoothing. Probably overcaution in retrospect.

I was also wary of the magneto-striction problem and stuck strips of those flight-case labels that have a hard plastic side and a softer, expanded sticky side to the drop-through transformer lam stacks where they rest against the chassis, which seemed to work. Something that has appeared on the scene since are silicone rubber heat transfer patches, available in several thicknesses (0.5, 1, 2mm), that would probably give excellent hum damping and would help transfer transformer heat to the chassis better than my plastic strips. 100 x 100mm patches can be had on eBay (and doubtless elsewhere) for less than £2 delivered.
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Old 31st Oct 2015, 11:54 pm   #57
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Default Re: EL34 power amp project

I dont think your spacer is the problem! I have had your trouble with "cheaper" transformers that have thin covers, and its usually the cover itself. The gasket idea doesnt actually stop the hum/buzz but isolates it from the main chassis thats being used as a diaphragm. My 6BW6 amp has a similar idea in the form of an adaptor plate cut from 1.6 mm steel that is held in place with six 3mm countersunk bolts. It doesnt vibrate !! I dont have a gasket between the adaptor, OR the transformer to adaptor plate,
its just bolted straight down. The transformer comes from a 1960,s Japanese amp, and is beautifully made. I have forgotten the brand name.

Joe
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Old 1st Nov 2015, 1:10 am   #58
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Default Re: EL34 power amp project

Its just a noisy transformer.
I had another play without the top cap and it was just as bad. if not worse. I loosened the bolts a bit and that only made it noisier. It vibrates when its in free air, just don't ask how i tested that............... It vibrates when its bolted down.
I can see what the other one does soon as its installed in the chassis, if that buzzes then i will know its a design fault, or it's the way I have mounted them.
I feel a phone call to majestic coming on.
Doesnt matter what I do, the blinking thing buzzes. I can feel the chassis and output transformer vibrating with it.
Shame really, as the Danbury output transformers are rather good.
And as I didnt buy them from the official supplier, I don't have any comeback or warranty.
I'll have a word with Ed tomorrow at golborne, if anybody knows whats happening he will.

A.
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Old 1st Nov 2015, 1:52 am   #59
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Default Re: EL34 power amp project

There is one other thing to do, but its a PITA.
Thats to have the transformer vacuum varnished.
it will increase winding capacity a little but it should shut it up.
I looked at all your photos again, and although you havent given any closeups of the transformer itself, I cant see any varnish on either transformer.
When I started winding transformers under my Dads instruction, I couldnt varnish them!!
When I got better at winding them Dad took them to work and vacuum varnished them as a foreign order ( SSHHH!!). I never had noise or loose lamination problems again.
I dont have a factory anymore, but when I hand make transformers now, I vacuum them in an old transformer case from a neon sign. Basically a "bucket" with a lid that is screwed on with 24 0BA screws. Its totally airtight ( origionally to keep the PCB coolant INSIDE !! ) I removed one of the HT insulators and fitted an air socket with one way valve, and connect it to my air compressor running in reverse. By reverse I mean I connect a hose to the air inlet manifold that would normally have the air filter connected, and connect that to the vacuum bucket. its not a very low pressure that I acheive, but its certainly far better than simply hot dipping, or not varnishing at all!!.
I put a 1 gallon container of ordinary baking varnish in the bucket and heat up my hand made transformer in my old kitchen oven oven, that I have for this purpose, to 105 degrees. I drop the tranny into the varnish and quickly screw the lid back on, and connect it to the compressor. I then let the compressor run with the outlet valve open so that it keeps pumping ( sucking) for about 2 hours, so that the transformer cools in the baking varnish.
I then unscrew the lid again, remove the transformer and allow it to "drip-dry" overnight
I then bake it in the oven again for 24 hours.
JUST like a bought one

Joe
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Old 1st Nov 2015, 4:31 am   #60
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Default Re: EL34 power amp project

One thing that came to mind:

With drop-through transformer mounting and an end-bell on top... Have you got electrically-insulating top-hat bushes on all the bolts? You otherwise complete a shorted turn, cut by part of the core flux

It would fit with less noise with the bolts less tight.

This goes for both of the transformers.

David
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