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Old 13th Dec 2018, 8:05 am   #41
Diabolical Artificer
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Default Re: 120W PP EL34 amplifier re-born.

Ta Simon. I had to reduce gain as in the original design the amp went into protection and switched off at about 1 o'clock on the vol knob, but I'll bear your circuit in mind.

There are loads of mistakes on mine Andy, I cocked up on a regular basis and made loads of blunders and those joints took weeks, just keep at it and it'll be reet. My workbench is an old window sill, one end screwed onto the extension window sill outside, tother end on a table sitting on a baulk of wood, with an old vice screwed on, tools = chainsaw, an old Stanley plane that cuts wonky, Wilko tenon saw and some old chisel's and a few clamps. I did most of the planing kneeling on a cushion with the wood against the door jam, so it's possible to do reasonable work with not much. Before I made this, my woodwork was more a la' Homer Simpson than Leanardo di Vinci. i think the secret is keeping your tools sharp and checking everything with a good straight edge.

It is parky mate, I'm in a T shirt, norgy and two jumpers and am still cold and that's with the fire on. I have to put a coat on in the workshop and fire up the Tek 585A heater to remove the chill in the morning.

Talking of hum #2 still has hideous hum, I reduced it a lot, found a ground wire grounding one side of the EL34 heaters off, now suspect the ECC82 PS.

There is quite a bit of heat off them - 6 x 6.3v x 1.5A = 56.7 + 2w ish about 60w heater power, times two, strike a light, 120w! That's nuts! They do supplement the Xmas light's though.

Re post # 39, thanks for the explanation mate.

To be honest after listening to them for a bit, I'm a bit underwhelmed. Bass is good and without using too many superlative's is rich and natural sounding, IE a double bass sounds more like a double bass than on my tranny amp, but apart from that I don't know that a valve amp is any better than a tranny amp. Obviously I know there's a lot of BS and hype about hifi, but still... I won't be using them as my everyday amp, my lekky bill has doubled this year, it's likely if I used them all the time I'd be chucking more £'s to the lekky Co.

I've also found this with record deck's, a £5 cheap 70's belt driven jobbie sounds the same to me as a
£500 one. The only time I've been truly amazed by the sound of a bit of hifi kit, is with a Cambridge Audio Azur CD player with twin DAC's; I put a Gorillaz CD in and the sound was a million times better than playing said CD on a PC.

That said although building this amp has been a bit of a trial, I've learned loads and had some fun and am now itching to build another.

Andy.
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Old 13th Dec 2018, 8:49 am   #42
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Default Re: 120W PP EL34 amplifier re-born.

Hi Andy,

Good to see you made it home
There a blunder in you schematic for the output-stage.
Take a look at where you feed the signal from the Cathode-Follower stage.
It isn't Grid-1 but that of course has to be a schematic error and not the actual wiring.
Then I'm confused about why to use the CF-stage at all ?
Usually you put an amplifying stage between the Phase-splitter and the output-stage to have enough voltage swing for the grids.
Thankfully EL34 doesn't need that much and in you circuit they don't get any more than is leaving the Phase-splitter anyway.
I can understand using the CF when DC-driving the EL34 grids but you have coupling cap's to the grids.
Low driving impedance source is good but is it needed when you are not driving the output valves into Grid-1 current as in Class AB2 ?

/Torben
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Old 13th Dec 2018, 9:57 am   #43
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Default Re: 120W PP EL34 amplifier re-born.

Hey Andy.

If you really want to build a simple, excellent sounding tranny amp, then it has to be a Class A, in my opinion.

Ever since I built myself the Linsley Hood Class A, I was so impressed with the sound quality. Iíve since built an updated version which provides up to 40 Watts RMS, and itís great.


So, what am I comparing the sound to? Iím lucky enough to own a Radford STA25, and it compares very favourably, in fact, to my ears, sounds as good as, if not better!

It also heats the workshop well at this time of year!


Cheers. SimonT.
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Old 13th Dec 2018, 12:25 pm   #44
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Default Re: 120W PP EL34 amplifier re-born.

I have to say if I was contemplating A homebrew Silicon amp I'd go straight to the "Gainclone" chip amp. Very simple as I imagine the clever electronics is in the chip but the ones I have heard sounded just like any other good decently powerful solid state amp. Which is very good.
I'm using a Mitsubishi silicon power amp that was GIVEN to me by the next door neighbour, they were going to chuck it on the recycling! It cleaned up lovely and it sounds pretty darn good to my tired old ears. I have no ideological bias towards vacuum toobs or silicon apart from I find a certain "romance" in an old school valve amp which for me is as good a reason as any.

You say you can't hear a difference between a £5 turntable and a £500 one? dare I say it thats because your £5 used deck would likely have cost the equivalent of £500 in its day and is therefore no mean machine. Also a lot of people don't take the time to set their turnatbles up in respect of a decent vibration free support, levelling and fetting arm and cartridge alignment. You can get really good results from very modest gear if a bit of time is invested getting it working right.
Not surprised your Cambrideg CD sounds better than a PC drive, it only has one job to do, probably has a decent linear power supply or if SMPS it'll have been designed to have low noise etc. I know I'll get pilloried from some sections but I do think the current crop of good well designed and made CD players squeeze a lot more off the much maligned CD than was previously thought possible. I have tried Hi res downloads and I do not feel i am missing out on anything when I listen to old fashioned (!!!) CD. Anyway thats off topic, hope you sort yer hums out, nothing more frustrating.
I'll dig out some circuit diagrams i have lying around for preamps, when you see them you'll think hmmm yep I could have designed those! Do you want tone controls? Theres an update on the baxandall circuit in the 4th ed of morgan jones book.

A.
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Old 14th Dec 2018, 8:11 am   #45
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Default Re: 120W PP EL34 amplifier re-born.

Well spotted T, what a clanger! Some good point's there also about the design, I can only say that any design faults are due to it being the first amp I designed, I was feeling my way, trying out this or that circuit till I arrived at one that worked. It's too late now to change anything, the thought of having to start again is unthinkable.

My problem at the time of design was getting full whack out of the OP stage. I remember being able to get about 90w out it (before application of NFB), but nothing more. I was advised to stick at this, but decided to ignor that advise as it would have been a waste of 6 OP valves etc. Two things got me over that hurdle, 1) using the 12BH7 as cathode follower's and 2) reducing the OP stage grid R's to 47k from 100/200k ish.

As I said I was a complete novice when I started both in terms of electronics and valves. I mostly used the Valve Wizard site for how to draw loadline's etc, but there are very few sources of "How to build a multi/parallel valve OP stage", I used three main sources of info. 1) GEC An approach to audio frequency amplifier design - especially the 400w amp. 2) This amp - http://www.turneraudio.com.au/100w-monobloc3-2014.html and other info on his site. 3) This amp - http://www.chambonino.com/construct/constwire2.html

A borrowed a bit from each design/source till it worked, in retrospect not the best way of going about things, but the result hasn't been too bad.

Aye Simon, a big Class A tranny amp is on my bucket list, but I'll start with a little one first. I'll have a crack at JLH's design obviously, as well as others.

Will have a look at the Gainclone Andy. You got a good score there as you know, the Mitsubishi amps have some good review's apart from going for oodles of £'s.

"You say you can't hear a difference between a £5 turntable and a £500 one?" No, maybe my hearing is a bit knackered, I havn't looked after it, using power tools with protection, standing in front of PA stack's at Motorhead gigs etc, however I went to Wigwam hifi show where there were numerous setup's of "top end" hifi, I left unimpressed having heard nothing that bought on an epiphany.

Ta for looking out some preamp design's, this one looks about right so far - http://sound.whsites.net/project02.htm - a bit of switching with gain, tone/vol control's.

Andy.
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Old 14th Dec 2018, 11:25 am   #46
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Default Re: 120W PP EL34 amplifier re-born.

Rod Elliots stuff looks good to me, no airy fairy nonsense. It's been around a while now.

Some how I feel it would just look more "right" with a "toob" preamp, pinch a bit of HT from your offboard PSU if you have the slack in it.

here's a link and this guy has a fair bit of info as long as you don't mind his "old American Guy" opinions ha ha. But the bit on tone controls is informative.

http://www.angelfire.com/electronic/.../Amp-Tone.html

A.
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Old 25th Dec 2018, 1:30 am   #47
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Default Re: 120W PP EL34 amplifier re-born.

Looking at the circuit in post #23, would it possible to change the current source to an EF91?
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Old 29th Dec 2018, 7:59 am   #48
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Default Re: 120W PP EL34 amplifier re-born.

There's some good stuff on there Andy, I've seen it before but thanks anyway.

Just when you thought you'd finished..... I did a bit of testing on one amp as it had quite a bit of hum and was stunned to find what I thought were two 120w mono block's were in fact 40w monoblocks. I really doubted my sanity there for a minute, I KNOW I saw 33v RMS on my bench meter lot's of times but as this has been playing up on AC volts the last few times I used it, it is now suspect.

This and a few comment's from more experienced builders like Tricomp above has made me look again at the circuit. Firstly the front end with local FB cause instability at SW off with a speaker load, this can be sorted by the zobel on the OP, but it would be better if the instability wasn't there.

Next the direct coupled LTP to cathode followers has been giving gyp from day one. Because it is direct coupled the CCS just doesn't act like it should. It should be possible to adjust it with a ECC82 to bias the LTP to around -10v Vgk, it isn't. Somehow the following stage is affecting the LTP I think, though I haven't sussed out how yet.

So that aside, I'm going to re-design from scratch, lost the cathode followers, the big problem being that there are three B9A valve sockets on the chassis, so if I use one or two triodes as the front end, into a LTP, i have one socket doing nowt, seems daft to build a design just to fill that base. Might be room for a EF91 CCS Richard?

Another issue is the PSU de-coupling, which needs improvement. If there is any slight ripple on the front end HT, it's amplified by a high gain stage and goes on to be further amplified. So lot's to think about.

I've had a break from working on them over the hols as it was doing my head in.

Whilst we're here I'd like some opinion's on this blokes MO of using twin triodes in parallel or triode strapped EL84's to drive his EL34 amps, see attached.... This may be a good way to drive the OP stage though I'm a bit wary of strapping two unmatched triodes together; problem?

Andy.
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Old 29th Dec 2018, 6:26 pm   #49
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Default Re: 120W PP EL34 amplifier re-born.

Andy
Looks a great piece of work - well done indeed

Rob
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Old 30th Dec 2018, 8:03 am   #50
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Default Re: 120W PP EL34 amplifier re-born.

Thanks Rob. It is like a wayward child, a source of pain and pleasure. Yeh, the way of the amp builder is long and hard and full of rocks, though the sun doth shine occasionally, amen. : )

Andy.
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Old 11th Jan 2019, 7:16 pm   #51
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Default Re: 120W PP EL34 amplifier re-born.

I've cracked most of the amplifier but still have a hum issue. At present I'm using a bog standard common cathode triode gain stage as the IP stage to bump the IP voltage up a bit to "drive" a LTP, with a sensitivity of 1.5v RMS in, 3v RMS out, open loop gain. Am using a very small Ra - 6k8 on an ECC82, biased at 5v, HT 250v, so not a high gain stage. Htr's are DC powered.

IP wiring is as shown, attached. Basically a short length (6") of good quality shielded cable grounded at one end only, into a coupling cap etc. I've tried routeing it in various ways, well away from htr wiring etc, flat to the chassis and only crossing DC HT cables at 90 deg. The amplifier is separated from the PSU by 2mm thick ali plate.

With nothing attached to g1 there is only 20mv P-P of 50hz AC waveform on the OP, however as soon as I connect the inner core of the shielded cable to the coupling cap, OP AC goes up to several volts P-P.

Have tried various values of resistor on the LHS of the coupling cap to ground, grounding the LHS inner, like a pi filter, as well as numerous other fixes, I still have unacceptable 50hz AC hum though. The only thing that does reduce hum is to lift or elevate the shield ground with a 22r or 100r resistor, a 1k increases hum.

Am really puzzled how and why spurious 50hz AC is not shunted to ground. I'm hoping someone will point out a glaring error, but at the mo I'm stumped.

Andy.
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Old 11th Jan 2019, 8:20 pm   #52
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Default Re: 120W PP EL34 amplifier re-born.

daft question Andy,
Wheres the earth return from the input socket running?
You show the screen earthed at the grid but not at the input so I have to assume that you have an earth return somewhere.
Theoretically earthing a screen at one end shouldnt create a hum loop, and the convention is usually to earth the screen at the input end rather than the output end although It wont really make a difference I think.
Now then, you should have your signal earth return from the socket to as near as possible to the earthed end of your input valve ie the bottom of the cathode resistor for a direct path. Otherwise the return path could be anywhere along the zero volt rail?
I'd use a bit of twin core screened microphone cable, connect one inner conductor to the screen at the socket outer (Earthy end) and then connect the 2 inner wires up at the valve grid and ground respectively if that makes sense to you? If you havent got twin core, just use the screen as your earth return from socket to input valve, it might be enough. And make sure there are no other earth runs between input socket and inputs. Use insulated phono sockets as if its got electrical continuity with the chassis you are asking for trouble using the chassis as an earth return.
It sounds a bit like the mild hum issue i had with my EL34 stereo power amp which I cured by re wiring the earth runs from the preamp section as a single wire to the star earth rather than two separate ones back which had created an extarnal hum loop via the connecting cables. Still baffles me.
I can't remember but are they monos or a stereo amp with separate PSU still?
I generally don't use a DC blocking capacitor on the input grids as It's not really necessary unless you know that your source has a DC offset on it. All my preamps have output coupling caps or are designed for zero DC offset so I dispense with the DC blocking cap. None of my commercial Valve amps use an input cap either. It doesnt do any harm though other than altering the LF response. I'm wondering if you're picking up noise via the capacitor too.
Cheers.

Andy.

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Old 11th Jan 2019, 8:28 pm   #53
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Default Re: 120W PP EL34 amplifier re-born.

Another thought, what happens when you short circuit the grid to ground? on the grid side of that coupling cap and then on the source side? somethings not sitting right and it could be something in that area? i usually do initial testing with a shorting plug in the input socket to shunt any extraneous induced hum etc to earth. If you have simply connected the inner of the coax up and left the socket open circuit you're pretty much guaranteed to get a bit of hum.
20mV p-p quiescent hum seems rather a lot to me too, I got my amp well below 1mV.
I also see there's no grid stopper resistor, I'd put at least a 1k there as a standard safeguard against HF oscillation. My memory may be playing tricks here but I seem to remember being told that HF oscillation can result in an audible hum as a side effect? I need to look it up in the textbook. Anyway using a grid stopper means you can safely short the grid to ground as would happen with a normal volume pot wiper connected to the input and turned to minimum.
A

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Old 12th Jan 2019, 11:25 am   #54
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Default Re: 120W PP EL34 amplifier re-born.

Thanks mate, didn't think about using 2 core shielded. Anyhoo, took the coupling cap off, did as suggested, quiet as the grave, whereas before it sounded like mad Uncle Jack drilling holes with a hammer drill.

I've had this before, grounded a stage 4" away along a ground bus from previous point = oscillation, return to previous ground = all good.

Going to do a few experiments, think that coupling cap wasn't helping things. The subleties of amp building eh? Especially grounding.

Andy.
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Old 12th Jan 2019, 12:26 pm   #55
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Default Re: 120W PP EL34 amplifier re-born.

ali will not shield from magnetically induced hum.
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Old 13th Jan 2019, 8:27 am   #56
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Default Re: 120W PP EL34 amplifier re-born.

"ali will not shield from magnetically induced hum." Thought it did? Was under the impression steel didn't as it can become magnetised. Seems I'm wrong - "the 'shield' needs to be of a material that has high permeability or low reluctance (they are reciprocal like resistance and conductance). The permeability of iron is .25 and the permeabilitu of aluminum is 1.25 x 10>-6. Nearly a million times less able to reduce magnetically induced hum."

This galling as I am pretty sure I read that ali was a good shield in a chapter on chassis building in toob amps. Oh well, you live and learn.

Andy.
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Old 13th Jan 2019, 10:34 pm   #57
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Default Re: 120W PP EL34 amplifier re-born.

But It should be good at reducing electrostatically induced noise so as long as its got good contact with Chassis ground it isn't completely wasted effort.
You're using a toroidal mains transformer if I remember? You have a naturally low magnetic field surrounding it. Given a reasonable physical separation from the mains transformer(s) then the main source of hummy stuff is likely to be the AC heater wiring, power valves consuming between 1.5 to over 2A each can cause issues from the heater wiring. You already know about tightly twisted wires going to the bases and not having the wires causing a loop around the valve base where the wiring continues to the next valve. And good earth referencing, possibly using a humbucker pot rather than the 2 x 47 ohm resistor pseudo centre tap trick.
Now I got to thinking here, are you still using the EL34 heaters in series with 18.9V feed per 3 valves? The issue here is that although the current through the chain is kept at 1.5A, because the first Valve in the chain has the full 18.9V across it, is there an increase in the hum field? Or is it purely current related, in which case its a clever idea. Again heres a daft maybe idea.If you are only drawing 1.5A per wiring run at 18.9V then an LM338 regulator will comfortably handle that current and regulated DC on the heater wiring will virtually eliminate hum. It can be argued that it doesnt matter on a push pull output stage due to self cancellation, but if the heater wiring gets close to other components then I guess there could be some small hum pickup off the heaters? I may have misunderstood here Andy and you already are running DC heaters to your EL34 valves?

And it seems you have a satisfactory result now, so I was chucking a discussion topic on the heap as I am sure I may have some of my ideas muxed ip.

A.
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Old 14th Jan 2019, 12:01 am   #58
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Default Re: 120W PP EL34 amplifier re-born.

So while reading the other day, I found this gem.

Joe
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Old 15th Jan 2019, 7:22 am   #59
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Default Re: 120W PP EL34 amplifier re-born.

Bang on and some good thoughts there mate. Yep, I think the problem is the EL34 htr wiring, I'm using a 16 way chassis connector to connect PSU to amp, the PSU sits on said 2mm ali plate, so is serarated from amp, as you say, electrostatically. Htr wiring are twisted and run on the RHS of amp chassis, tucked in a corner, but as they're series connected - 18.9v @1.5v, part of the htr cable run is single strand, IE not twisted. However it is away from the sig in cable. Htr's wiring is grounded on one leg to the ground bus bar, wonder if it'd be better grounded to chassis instead?

Though the twisted pair has reduced hum dramatically I'll still have about a cm of unshielded wire at the chassis connector, unless I wrap copper tape round the pins, which I'll do. Will also look at using humbuckers too, thanks Joe.

Andy.
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Old 15th Jan 2019, 7:44 pm   #60
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Default Re: 120W PP EL34 amplifier re-born.

Have you got room for a chunky rectifier? say a 25A bridge, and if enough spare volts if only drawing 1.5A then its well inside a LM338 reg or you could get fancy and use a big pass transistor. But for the brass washers a 5A reg costs I'd use those and keep the current at 3A or less per device.
this all assumes spare volts on the winding and enough current to allow for loss through the regulator.
If you,ve got 18-19V to play with and can supply the total current for them, I imagine 12 x EL34 ? then wire em in series as pairs at 12.6V.
DC heaters so your single runs won't create a fluctuating field and away you go.
I'm personally a bit against series chauffage of valves as one heater goes pop and nowt lights up and then you have to find the duffer.
One of my trusty Sovteks developed an intermittent contact on a heater pin, probably due to being pulled out and plugged back in too many times during testing on very tight sockets. It could have been tricky if wired in series.
The Idea seems tempting but overall I think it opens more cans of wriggly things than it seals.
OK keep up the good work I'm off out. My gear is getting salted away and boxed up for a few months until the building work and dust subsides.
I have a couple of refinements to add to my much simpler EL34 project when I get it back out but in the main it's pretty darn good. If a bit wimpy at 30W compared to your beasties.
If it drives a pair of ESL57 or 63 with no dramas and it should, then I will be a happy camper.
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