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Old 1st Nov 2015, 6:54 am   #61
joebog1
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Default Re: EL34 power amp project

I did detect red fibre washers under some of the screws!!
I didnt check them all, but biker I think would know about that.
BUT its a good point David, as I havent discussed that point with him.
I automatically do it without thinking, so I overlooked that simple thing.
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Old 1st Nov 2015, 10:26 am   #62
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Default Re: EL34 power amp project

Morning all,
If the hum/vibration problem persists, try stainless steel bolts for the transformers, or aluminium bolts if you can get hold of some. Non magnetic material. Also bare in mind, if the transformer remains exposed it must have earth continuity for safety reasons.
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Old 1st Nov 2015, 7:27 pm   #63
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Default Re: EL34 power amp project

Yes Joe, theres fibre washers under the topcap
I wonder if I should have put a set on top, underneath the blind nuts too?
The grommets on the base should isolate the base laminations, But I have earthed one Bolt to the Chassis underneath. The transformer doesn't have an Earth Screen tag which i find odd.
Anyway I have had a word with Mike Barker today, and came away with a veritable education on matters transformical!
The buzz aint too bad today and I am fairly sure now that its down to loose windings rather than anything else as i strongly suspect the windings arent varnished/impregnated.
The transformer isnt mounted any differently than the substantial beastie on my Kit KT88amp and that doesnt rattle. Its from that I got the idea to use the grommets to isolate the transformer.
I rather think also that the base mounting method isnt best either.
I also think I am being rather fussy, my idea of noisy is probably not a the same as others. Comes from being cursed with good hearing even at my advanced years. Unlike my eyesight................
Anyway, the transformers are good enough to continue developing the circuit. The amp really does sound good, possibly due to the output transformer being well over specified for its task. I always had a lower regard for EL34's compared to El84's and the KT88 and this shows my prejudice was unfounded.
Interesting to note guys, I haven't used any "fancy" parts, just the BVWS shop polyprop coupling caps, mostly Mr Maplins metal film 1% resistors and the el cheapo cement "Coffin" power resistors. Wiring is solid core copper 0.6mm and stripped out twin and earth lighting cable where I wanted better cureent handling. Its good enough for me. No fancy silver wire insulated with special mongoose hair or capacitors with rare earth metal foils and olive oil dielectrics etc etc etc.
One of the above statements I have actually seen in a claim for a component, I'll leave you to guess which.

Anyway, I think so far not bad for a first go at a highish power amplifier and It's not fully tidied up and tweaked yet.

Andy.
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Old 1st Nov 2015, 9:17 pm   #64
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Default Re: EL34 power amp project

Quote:
Originally Posted by ValvoStef View Post
Morning all,
If the hum/vibration problem persists, try stainless steel bolts for the transformers, or aluminium bolts if you can get hold of some. Non magnetic material. Also bare in mind, if the transformer remains exposed it must have earth continuity for safety reasons.
Correct me if I am wrong here, but the material of the bolts makes no difference as its the concept of a "shorted turn? i.e. a conductor having a voltage induced into it by the alternating magnetic field?

Tomorrow will tell me if it's the transformers or something else.
I also suspect the steel chassis won't be helping if there are stray magnetic lines of force?

Future projects will be built on a non magnetic chassis.

I will report when I have any significant findings. The buzz seemed less today, rubber feet seemed to help too on the chassis. I'm not aware of it when the musics playing so fingers crossed i've tamed it.

A.
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Old 1st Nov 2015, 11:36 pm   #65
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Default Re: EL34 power amp project

The sound of the buzz will depend a lot on your mains waveshape and that varies over the day and the days of the week depending on what dominant loads are on.

If they've not had vacuum impreg then that's your next step, I think. You ideally want to hang then in vacuo in a container with hot varnish in the bottom half, held up in the upper half to outgas for a while, then they get dunked in the hot varnish, then the vacuum is let up to ordinary ground-level pressure which pushes varnish into the evacuated voids in the transformer. They're left swimming for an hour before being brought out to cool and drip off excess. The whole purpose is to get the air out of voids and then to allow returning air pressure to force varnish into them.

Ideally you should use top-hat bushes, not simply flat washers for bolt insulators. Only one end of a bolt need be done. You might need a ground wire from the cover bell which is exposed. Or leave one screw unbushed.

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Old 2nd Nov 2015, 1:04 am   #66
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Default Re: EL34 power amp project

Something's ringing a bell here about tophats.
I will check the build instructions for my Kt88 amp, I have an odd feeling that the base of the mains transformer was mounted on inverted top hats through the chassis with a grommet to act as a compliant seat. Cant be certain though.
makes sense though.
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Old 2nd Nov 2015, 5:41 am   #67
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Default Re: EL34 power amp project

It wont do any damage, put it that way!!
If the traffos are not varnished you are really p---ing in the wind!,
you wont fix the problem. I have done the following though.
Buy a can of circuit board lacquer that comes with a long tube like WD40, RP7 etc.
you will have to remove the transformer yet again, but, spray circuit board lacquer through every hole that you can see/find in the bobbin, until its saturated!
Leave it for a day, then do the same again. If you can heat the traffo GENTLY and SLOWELY up to about 100 degrees C, all the better. Electric oven please !!! if you must go that way!! Gas releases a fair amount of water for some reason.
Heat the traffo untill its hot all the way through, then spray the gunk INTO the traffo, NOT onto the outside.
Cheap and nasty fix, but should work!!
joe < waves his hands and says the magic words
Nobody saw me say this

Best of luck
Joe

Last edited by joebog1; 2nd Nov 2015 at 5:43 am. Reason: additional reason
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Old 3rd Nov 2015, 12:59 am   #68
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Default Re: EL34 power amp project

Hi again..........
Well, after a bit of "Sleeping on it" I thought I'd have a final go at the job.

Into the garage, out with the spanners and the biggest Stanley screwdriver that will fit the screw slots.
I then "wellied" the mounting bolts up as tight as I could, and i mean TIGHT, down onto the grommets.
It ain't silent, but its inaudible from where I sit in front of the speakers. I have come to the conclusion/realisation that the problem lies in the mounting in as much as I had to make a "Gasket" to fit a smaller footprint transformer into the hole that existed in the chassis. I would bet good money that if mounted well in the chassis that came with the transformers i wouldnt be pulling my hair out.
I also found that stick on rubber feet made a huge difference, isolating the chassis from the hard floor surface. I haven't yet made the necesarry base covers, and this gives me some thought, I think I'll use the hardboard style perforated board instead of a bit of aluminium sheet. that should damp any ringing along with some nice compliant feet. I was looking at some of my hi fi components and my venerable Rega Planet 2000 Cd player has a set of clever feet that are supposed to help isolate the case from external vibrations. I figure the reverse should apply. I'm reasonably content with the results, I really can't be a*sed with fiddling about with varnish or boiling the transformers in wax etc.
I am sitting listening to the "Finished" monobloc and its behaving impeccably and sounding good.

Andy.
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Old 3rd Nov 2015, 1:30 am   #69
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Default Re: EL34 power amp project

Ok and now for the test results....................

I used the oscilloscope in anger today and i actually got meaningful results! Thanks Ed Dinning for the 1 to 1 tutorial earlier this year.

Ok here goes.

At rated sensitivity of 125 mV : 25 real world watts into an 8.25 ohm resistive load. 1kHz sine wave.

At Clip : 37 Watts! I have to say the poor EL34's were under a bit of stress at this point.

I tried an amateurish frequency response test. I set the sig gen to give 1 watt into the dummy load.
I wound it up and the sine wave stayed in shape and amplitude until around 24 kHz. I figured the -3db point looked to be at about 35kHz.
Then I did the scary stuff, I took the sig gen down to 50 Hz, and there seemed to be no attenuation, I did expect the response to be falling off at this point as the Leak circuit does use quite low value coupling capacitors.
Anyway the sinewave didn't attenuate as I went lower and by 10Hz I was getting a bit jumpy.
At 4 HZ things started happening inside an EL34 and I figured one of the grids must be moving in sync with the signal. I will need to have a look at the theory here, as if the amp is responding more or less down to DC, then there's a problem. I didn't check any phase shift, cos a) i don't know how to, and b) it didn't occur to me.
Anyway in terms of power output it's doing exactly what Harold Leak designed it to do, the max output at clip will no doubt have had a fair bit of harmonic distortion but at 25W it should be pretty reasonable still.
My conclusion?
Using modern close tolerance and stable resistors and capacitors shows that this old "outdated" design can cut it in a modern stereo system and I'd recommend the leak circuit over the Mullard in as much as it seems less fussy over layout. That doesn't mean you can just "glue" components in willy nilly, you will still have to consider sound construction techniques and pay particular attention to Earthing strategy.
The CD has just finished and from where I am sitting I can't hear the mains transformer buzzing.
I'll live with that.

Andy.
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Old 3rd Nov 2015, 10:35 pm   #70
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Default Re: EL34 power amp project

No2 amp up and running.
Voltages check out after some rectifications around the input valve, I discovered I'd got my g1 and g2 connections transposed. doh! Anyway sorted that and the voltages all checked out on spec.
But it's a hummer. Tha'ts disappointing as I followed the same plan as Amp No1.It hopefully will be a simple fix, I shall start by looking at the input wiring and see if I have an earth loop anywhere. I haven't used twincore mic cable for the input wiring from the RCA socket, it seemed ok on the first amp, but thats got to be first point of looking at. Then check my earthing routes again in case i missed something, but the earths checked out both visually and metered back to the star point.
the good news is the transformer noise is down and inaudible at the sofa.
Andy.
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Old 4th Nov 2015, 3:58 pm   #71
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Default Re: EL34 power amp project

Got the hum sorted. mainly refinements to layout. It seems to be mainly heater hum and a rearrangement of the heater wiring and removal of the tagboard which was close to the PSU components and input socket, and hardwiring the wires to the flying leads and tucking them into the back and side of the chassis opposite the DC bit of the PSU seems to have worked.
A good education in layout.
I have a few ideas to improve matters, and i will post later for any experts who haven't lost the will to live reading my blurb.......................

Andy.
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Old 5th Nov 2015, 1:23 am   #72
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Default Re: EL34 power amp project

Ahh Andy, its fun reading
Layout is a fickle thing!! Try moving the input wiring around while amp is on. just a few mm and its amazing what you hear. A classic is to tap the wire
a: with a screwdriver
b: with a wooden dowell

ALL screwdrivers are magnetic!!!
( be it a tiny field, or one made by dragging the screwdriver over a LARGE speaker magnet, or in todays world a voice coil drive magnet from a hard drive)
and it induces enough energy into input wiring to make large outputs from the amplifier.

Perhaps the best layout is one that follows more or less closely, the circuit diagram that has been very neatly drawn. that is, with the signal circuit "above" the centreline of the drawing, and the power supply below the drawing. The one thing that isnt shown in the circuit diagrams ( mostly, anyway) is the heater wiring, hence my earlier suggestion to use balanced cable for the input valve and voltage amplifier, and then the phase splitter and cathode followers ( if used) with a separate connection direct to the the 6.3 volt output on the transformer. You can also float the supply above ground by tapping off the HT supply, or as used in many commercial amps, biassing the heater volts to the cathode of the output pair. I have seen eccentric amp designs with thin steel tubing being welded to the chassis, and the heater wiring being run inside the said tubes.

My comments will spark a flurry of suggestions, arguments and comments, about my lack of experience, wealth of experience, or total ignorance of all things electronic Nevermind, its called experience!! I have read audiophool articles where the pitch of the twists in heater wiring are specified.
Whatever the answer is, is that you have progressed far, by building these amps!
THAT is the most important thing.

My best regards
Joe
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Old 5th Nov 2015, 10:20 am   #73
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Default Re: EL34 power amp project

Quote:
Originally Posted by joebog1 View Post
ALL screwdrivers are magnetic!!!
Tell that to my ceramic ones!

RF engineers need trimming tools which don't affect coils being twiddled. The guys who defused wartime bombs had an even stronger interest in non-magnetic stuff.

David
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Old 5th Nov 2015, 12:06 pm   #74
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Default Re: EL34 power amp project

If you can afford them, the BeCu or Titanium ones used around MRI magnets are also non magnetic AND usable as real screwdrivers.
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Old 5th Nov 2015, 11:18 pm   #75
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OK I stand corrected!
I think my thoughts were in the right direction though!!

Joe
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Old 5th Nov 2015, 11:57 pm   #76
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Notwithstanding the fascinating comments about screwdrivers and the cheap ones I bought that seemed to be made of plasticine........

Ok folks back on topic.

Refinements today.
I was still unhappy with the level of background hum and also partition noise (hiss) from the input pentode. This I figured was due to the unnecessarily high sensitivity of 125 mV for rated output of 25 watts. Both specified and actually measured on these amps.
Also the Steel Chassis I think wasn't helping matters much, I'd rather have had an aluminium one but it wasn't to be.
So, back to the textbooks and the valve data sheet.
I checked the heater wiring run and there was very little else I could do to improve matters. I had already earthed the metal spigotty thing underneath the B8A socket as advised by Phillips in their data sheets.
So I figured the way to go was 1) Reduce the sensitivity and overall gain (same thing?) and get the noise floor down, and 2) investigate DC heaters for the signal Valves as I was fairly certain the hum was pretty much induced heater hum.
After Re-reading the tomes in my possession, mainly the Morgan Jones books, "Valve Amplifiers" 2nd and 3rd Editions, and "Building Valve Amplifiers" I decided the way to go was to wire the EF40 as a triode. This pretty much would reduce the overall gain by a factor of 4 and get the sensitivity down to about 500mV.
This necessitates a reworking of the global feedback loop, and once again the hard work had been done for me as there is an example in the books so I was happy to use the "Rule of thumb". This is to reduce the feedback resistor by a factor of 4 and increase the compensation capacitor by 4x to keep the feedback ratio as close as possible. I then calculated the value of resistor to use in parallel with the existing one and like wise the capacitor. This allows me to quickly swop between the values by using the tags next to the feedback components and thus only 1 wire link needs moving. Likewise I moved the screen grid resistor and decoupling capacitor junction from the valve base to a spare tag on the input board and thus a wire from the valve base can be moved from the anode connection to that position in order to switch between pentode and triode mode.
Still with me?
15 minutes work and I put the amps back into the system, after a quick verification that I hadn't disturbed any other wiring and that the voltages checked out on the bench.
Amps warmed up................... silence......... hurried move to the speakers and mk1 Lughole up against the speaker grilles. I can only detect the slightest amount of hiss with ears right up to the speakers and the small amount of hum likewise can only be heard if the room is Library quiet with ear up to the speakers.
At this point I would say that the amp is as quiet as it needs to be.

Anyway to the listening test. CD in player, and off we went. Yep it sounds just as good as before except now I have to turn the wick up a wee bit more as desired.
Its a cracking sounding pair of amps, and if gentle readers, you were expecting something soft fuzzy and woolly in the bass then you'd be better off nicking the grandkids teddybear.............
No fancy boutique components. Just garden variety 1% metal film resistors mostly from Mr Maplin, "Cooking" electrolytics and polypropylene coupling capacitors from the BVWS shop.

Tomorrow when I get a free hour I'll get them on the oscilloscope and see what measurements I get.

It doesn't really need them but I fancy a play around with regulated DC heaters for the signal Valves just to see if it does make a difference.

A.
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Old 6th Nov 2015, 3:17 pm   #77
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Default Re: EL34 power amp project

Well done Andy for your perseverance
It's years since I've built a new valve amp but looking at your progress in this thread brought back quite a lot of memories of the trials and tribulations of what should be a simple venture!
As you have discovered layout is all and subtle changes , even moving things up and down an earth bus bar by an inch or two you can affect the noise level.
I think you've adopted the right method in reducing the gain of the first stage. these days 125 mV is way too high and reducing the game also reduces the noise so a win-win.
To tackle overall noise the method I used to adopt was to bypass the feedback loop and hear the noise in all its glory , using necessary methods to reduce it to a minimum and then reapply the feedback , thus ensuring at that stage it's probably inaudible !
To prove whether hum on a stage is heater borne, the suggested technique is to be able to remove the heater feed from that stage while the amp is operating , Preferably with no feedback so you can hear what's going on.
If the hum stops immediately on disconnection, then the heater is to blame, if the hum fades away as the valve cools, then it's something else.
Generally, DC heaters shouldn't be necessary in the power amp and only very occasionally in a preamp, such as a sensitive RIAA stage
Well done again for getting not one, but two of these beasts working !
Enjoy listening to them.

Andy
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Old 6th Nov 2015, 10:05 pm   #78
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Default Re: EL34 power amp project

Thanks to everyone for the positive remarks and encouragement.

I'm very happy now with the way it performs. Hum and noise is so far down that it's probably pointless chasing it further, But i'll have a go at yestertechs method with the feedback disconnected. This technique is ringing a bell in the murkiest depths of my memory. That way I will know if I have got the layout optimised.

I hope my attempt and experiences haven't put anyone else off from having a go, rather that it will encourage people to get their noses into the books and get the satisfaction of achieving something.

"with a little help from my friends"

Andy.
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Old 7th Nov 2015, 12:06 pm   #79
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Default Re: EL34 power amp project

Well done Andy, and a most interesting thread. I'm just about to embark on my first valve amp build from scratch. It's a bit scary but I have bits of knowledge floating about in the noggin, time to try and get it to coalesce.

Andy.
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Old 7th Nov 2015, 8:35 pm   #80
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I think you'll have an easier time of it than myself methinks.
I spent hours just thinking and scribbling layouts and looking at the chassis. "dry fits" of large components and the tagboards which helped get an idea of how and where things would fit.
I went for the "star earth and tagboard sub assembly" method because it's one I am familiar with and the shape of the chassis lent itself to it. Plus, I feel star earthing is possibly the better way to go about things in audio circuits. It can possibly look a bit untidy compared to a nice geometric pattern with a busbar.
I can't speak for radio thats a whole different animal. And much trickier in my view.
Please let us know how you progress Andy, I will be most interested.

A.
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