Simple 3 valve stereo amplifier - peer review sought

[Croozer]

Thanks for the HT heads up guys. The mains TX has only one HT winding and no centre tap (rated 120mA at 130V) It measures 160v off load which seems a little high, but the supply is over 250v here. I only need 90mA HT so it shouldn't be too hard worked.

The 6.3v heater winding is centre tapped; however, contrary to the drawing, there is no 90v heater winding; it's just a tap on the primary. Rather than let this float free, should I earth the neutral side of the string (and thus mains) to chassis with a class x capacitor ?

Andy, the link to the goldbourne thread was useful, esp. the discussion of input impedence. I didn't realise that modern devices were as low as 50k so some of my assumptions are wrong and might need experimentation.

Th circuit design began cribbed from a Dansette record player and an old WW article on negative feedback, it's undergone considerable change as a result of guidance from here, and as a consequence of having a tape recorder to raid for parts. My knowledge is limited, and I wanted a project that was basic enough that I could understand each component.

The chassis will be ready on Monday so hopefully will have positive progress to report next week.

PS - re 'Harry Beck underground map' circuit style - there is uncanny insight here. I have a background in railway infrastructure, where possible 'conflict' points between trains were important on track diagrams.

[bikerhifinut]

Just leave the 9oV tap on the primary alone and if not using it. make sure that its not exposed to any prying fingers or wiring.
I'd be very wary of using that tapping to fire up heaters. they'd be on the live to the mains side nuff said.
Now I'd just use the 6.3V heater winding but not knowing what its current rating is I don't know if it would heat a power pentode as well as the EF86.
At this point you can consider using different output valves unless the point of the exercise is just to use up stuff lying around in which case fair enough. If you need a beefier heater supply just get a cheap 6V transformer from one of the usual suppliers, the difference of 0.3V will make no difference to the performance.
I hope what I have said won't put you off, it looks interesting.

[Croozer]

Thanks for the quick reply, and apologies, I haven't been clear in what I was asking. The transformer and the output valves have all come from a tape recorder, which used three 30A5 valves, two as output valves and one as the oscillator. I've never come across these valves, but the literature shows they have a low anode voltage but a 30v heater (maybe designed for American ac/dc radios? ), hence the 90v transformer tapping for a series string. I thought that this tapping was a secondary winding, but measurement shows that it's not; it's just a tap on the primary.

My intention was to use the 90v tap with two of the valves and a 220R 7W resistor to create a separate series heater string for the output valves (which does have the consequence of requiring to dissipate 4.5W heat!)

Although all other circuitry will be isolated, this means that the heater string will be connected across 90v of the mains. The chassis will be earthed, and I was wondering whether I should tie the neutral side of the heater string to the chassis with a class x capacitor with the aim of taking unwanted noise off the heater circuit, or whether to leave it as a totally isolated string.

...another question that probably has an obvious answer.... Given the (nowadays) low cost of those little bridge rectifier packages, is there any advantage to using a (smoothed) DC heater supply to reduce mains hum?

[Diabolical Artificer]

It would be a class Y cap you'd need for mains to chassis.

If you use regulate the htr supply converting it to DC you'll end up with about 140v Dc, which means you'll have to loose some of this by using a resistor or regulator such as the TL783 but you'll need a series pass transitor as well for this regulator or you could use a floating LM350, LT117AHV, OM7620ST so you'll need a heatsink for the regulator. This all adds complexity. You could of coarse use aliminium bodied resistors and bolt them or the regulator onto the amp chassis. Or you could use a seperate 20v tfmr and connectt the htr's in parallel, much easier to regulate with an LM317AHV or similar. Just thinking aloud here.

You might be better to see if hum and noise is a problem first before going down that route.

You may be better ditching the 30A5's and using EL84's with a seperate htr tfmr. Firstly because then your htr supply is isolated, second EL84's are probably easier to get than 30A5's, thirdly a 6.3v high power supply is easier to make than a 100 ish volt one should you want to go down that route. The only fly in the ointment is your OPT's Zpri which may be a bit low for an EL84.

Andy.

[bikerhifinut]

Whats the anode resistance of EL84 connected as a triode?
Or how about a ECL/PCL86? triode connected may be ok into the 2.5k primary.
I've got a slack handful of NOS PCL86 (14V 300mA heaters but would work well off 12.6V or even down to 12V at a push) if you want to give them a go and I have a circuit using them triode wired for about 1.5watts.

A

[Croozer]

Thanks for the replies guys. It's great to have access to experience that sees the obvious issues that my naivety would miss until after construction. I hadn't considered the voltage increase. I suppose it could be countered by using the taps between 200 and 240V , but this would have the undesirable consequence of raising the whole chain 200V above earth to tackle a problem which I don't even know exists. I will therefore stick with the original plan and note the difference between class x and y capacitors if I need to address this.

I have been looking for some material on dc heater supplies and came across a recommendation for the EF86- http://www.r-type.org/articles/art-124.htm - however the 6.3v heater winding is centre tapped to earth so again I will leave as AC unless I find a problem.

I have finished construction of the power supply and will test tomorrow (fresh eyes in the morning more likely to reveal errors) and will post images of progress.

Apologies that this is turning into step by step reporting, but the advice here really is invaluable; every query results in improvement and my only relevant qualification is Higher physics 25 years ago.

PCL86 sounds like a good alternative if results are poor; the 30A5 is listed as having an output of only 2.1W but I have three of them to hand and it seems a shame not to give them a chance!

[Croozer]

Collected chassis from blacksmith - it should clean up quite nicely. Power supply complete and delivering 200V HT off load without explosion, spark or smoke.

Pics of the ransacked tape recorder, construction to date and the drawing for the power supply including amendments from here (and from Andy's thread).

A little metal bashing and I can progress to construction of the interesting bit..... Thanks for the support to date.

[Croozer]

Quick update: metalwork complete (though not necessarily tidy). Ditto heater wiring.

[Diabolical Artificer]

Unusual chassis design, i would never have thought to do it that way, interesting. I thought that was a fire extinguisher top right at first but see it's go juice. : ) Good to see your making progress.

One tiny point, noticed the red/blue htr wiring has the blue going around the red wire rather than twisted like it's yellow cousin, it's not significant though.

Keep up the good work, Andy.

[Al (astral highway)]

Hey Croozer,

Nice progress!! Just an idea , if you get the chance, try to take a detailed look at the underside of a commercial valve amplifier or the late sixties to early 70s by Leak, Quad, or Mullard or similar -- starting with the power supply. I notice some quite long leads floating about in your power supply build , whereas they should all be much closer to the chassis.

Also, are the cans of those vintage electrolytic the common -ve of the caps? If they are, it is arguably better to make sure they are electrically and mechanically well connected to the chassis and return your power supply ground through the chassis and get rid of some floating wires. Hum will be less of an issue if you take some steps at this early stage ...

[Al (astral highway)]

Hello again , also, following your 'as built' diagram , please note, ouch!!! As you realise and point out a few posts up there, your 90V transformer tap (you show this is for one of the heater windings - white and red) is not isolated from the mains...it is merely a tap on the primary side and so is not galvanically isolated.

I am not sure what others think about this , but from where I'm standing, this is an inherently dangerous low- impedance path straight to the mains, meaning a very high instantaneous fault current is possible and is only limited by the size of the windings and by your fuse and RCD.

You have a 1A fuse there but you really don't want close to this current at 90V to have the potential to pass through you or any component in a fault condition . It also introduces a lot of hum problems that would not be such an issue with an isolation transformer, as stated eslesewhere .

For these reasons it is far from an ideal arrangement...I'd change this as a matter of priority, on safety and performance grounds . I sometimes find safety -oriented posts tedious but as you appear relatively inexperienced , I hope you will understand my concern. And if the safety side doesn't sway you, perhaps the 'listensbility ' of the end result might?? Please don't be discouraged, keep up the great work !! ������

[Croozer]

Again, thanks for all of tge advice and guidance which has shaped thos project and apologies for the delay in reply; this project had to take a back seat for a couple of weeks, however here is a brief update and an outline of my errors which will be obvious to some, but might be of use to others.

1. First Power
The power supply and the amp were built as separate projects, connected by a 12-way socket. Using a phone as source and the 8 ohm speakers from the tape recorder, switch-on (much to my ill-concealed surprise) brought Samsung's 'over the horizon' from both speakers. A bit of motor-boating was eliminated with the pots on the feedback loop.

My good cheer was tempered by the considerable HMMMMMMMMM which needs to be addressed.

2. Valve heater supply
This project has been built around salvage parts, including (uncommon) 30A5 output valves. These have 30v heaters, a low anode voltage with a listed 2.4k load resistance (matching the 2.5k o/p transformers). The heater supply posed an unforseen problem. I had assumed (from the trader sheet) that the tape recorder mains transformer supplied a separate 90v secondary for this purpose. I had worked (for quite some time) on the tape recorder on this assumption. But measurement of the transformer revealed this only to be a tap on the primary side. The accessible chassis of the tape recorder wasn't earthed, the heater chain followed a tortuous route around and across chassis and pcb, and the machine was supplied mains via reversible two pin plug. Thus, chafed wiring or careless handling could have put either 90v or 240V mains on the chassis- a situation which wouldn't reveal itself until it was too late!

The points which have been made about the heater arrangements in my design (using the 90v primary tap) are valid concerns and I acknowledge them. I spent some time seeking alternate arrangements, but my design and chassis were made before I discovered the problem, and I don't have space for the additional Transformer necessary. Changing power valves to EL84s was considered but they require a better o/p transformer with a 5k primary, and having three 30A5 valves and not using them almost seemed to defeat the purpose.

I have therefore proceeded with caution. The project is isolated from the mains with the exception of the 90v heater string. It is safer than the original; the chassis is earthed and it's hard-wired to avoid the 'high' side of the mains ending up on the string. However the condition is sub optimal and the hazard is noted and will be highlighted on the final document.

3. Hummmmmm
The first approach to reduce this was because I had a 6v lantern battery to hand, which I used to supply the EF86 heaters. No change, therefore this heater supply( 0v centre tap) isn't suspect.

I then cheated. I took the whole affair to my Dad, who would be able at a glance to see the obvious faults it would take me a month to find. Firstly he pointed out that by using the 12 way power supply plug to carry earth from one chassis to the other I was introducing all sorts of hum pick up opportunities; so this was cut and replaced by an earth strap.
Secondly I had missed the point of the decoupling arrangents for the EF86 HT by installing the capacitor and resistor on the mains transformer chassis. This was an obvious error, come about because I had built amplifier and power supply separately; in my head was a spec, 'three separate HT supplies, smoothed and decoupled from each other' , neglecting the fact that the by the time the supply reached the valve it would have coupled to all available hmm en route.

4. Now ..........
I have changed the earthing arrangements, and shifted the EF86 decoupling condenser to the tag strip for the EF86, and fed this from the screen grid of the 30A5. HT for the LHS EF86 being fed from the RHS EF86. Hummmmmm is considerably reduced.

5....And next
LHS hum is considerably worse than RHS, so I will apply the same HT arrangements with a supply from the 30A5.

Hopefully this will reduce hum to a minimal andd acceptable level.

After solving hum, I'm inclined to experiment with an increase in the resistance between the EF86 grid and earth (currently it's only a 220k pot) I'm not sure about the compromise between trying to match input impedence, and the covention of minimising grid leak with a high (1 or 2 Meg) resistance.

The feedback loop is also more guesswork than mathematics so experimentation here may be beneficial.

The gain isn't great, but it's not bad; certainly useable. The maximum quoted output of the 30A5 is only 2.1W, so a certain amount of trial an error will be needed to get the best out of what gain is available.

Thanks again for all your assistance guys. This is my first construction project in 20 years, and your support and advice to this point has been invaluable.

Couple of pics to date. I'll give a final circuit diagram and some voltages when I finish tweeking. ..

[julie_m]

Ideally, every earth return should be a separate connection back to one common point; so no section of circuit has its earth return in series with another section's earth return. And where this is unavoidable, the "shared" connection should have as little resistance as possible. Hence the use of a metal chassis as a return conductor.

For instance, the connection from the bottom of the output valve's cathode bias resistor to the 0V reference point is going to be carrying the full cathode current of that valve; and there will be a voltage drop across it which depends upon that current. If that connection is several centimetres of 16/0.2 hookup wire, the voltage developed there might be sufficient to upset something. You certainly would not want to take the bottom of the the preamplifier valve's cathode bias resistor to the bottom of the output valve's cathode resistor; that voltage would effectively be added to the input voltage. The voltage won't change much, because of the capacitor in parallel with the resistor; but it doesn't need to change much, because the preamplifier has gain. It would be much better to run a separate connection from this point back to the power supply.

Likewise, the connection between the reservoir and smoothing capacitors -- if they were physically separate parts -- would be carrying the full ripple current. Only one end of that connection should ever be connected to anything (except perhaps a power-on LED).

Anyway, it sounds like you're on the home straight now, having identified the problem and a likely fix! Getting your first proper homebrew valve amplifier to work is one of those incomparable feelings, just like climbing a mountain because it was there .....

[Diabolical Artificer]

Well done, good to see you got your amp running. Good layout of point to point wired valve amplifier's is one of the first lesson's I learnt when I first built a valve amp. Now you have it working you can get your head round some of the subtleties to get it working better.

As Julie says getting sound out of your first amp is a good buzz, you must be well chuffed.

Andy.