Simple 3 valve stereo amplifier - peer review sought

[Croozer]

I was recently very impressed with simplicity vs quality of the two valve UL84/UY85 amplifier found in the basic dansette type record players, especially the way the UL84 was capable of being driven by a crystal pickup. Having raided the circuit and some contemporary literature I have drafted a basic three valve stereo amplifier using two EL84s each driven by half an ECC83, with the aim of using a phone/mp3 player etc. It has a basic negative feedback loop and a half-hearted effort of a tone control. The aim here is simplicity.

My design skills and maths are limited, I wondered if you good and experienced people might take a quick look at my proposal before I start construction to see if I have made any obvious errors.

Hi if it won't be - but it should be better than the tinny sound of a phone.

Circuit diagram attached. Apologies about the pencilled scrawl. I don't have any CAD skills either.
Thanks in advance,
Rob.

[TonyDuell]

Maybe it's because I'm a coward, but I really do have a dislike of non-isolated power supplies. If I were building something like this, I would use a double-wound transformer to get the HT+, not straight off the mains.

[Refugee]

All I can say is that the circuit you are showing will be fine with a high output cartridge.
A lot of people have found the gram inputs on valve radios do not have enough gain to get reasonable volume from a headphone output.
I do own a pair of valve mono blocks that have an extra pentode preamp that do have enough gain to work with an MP3 player or phone.
If you want a fuzzbox just plug in the signal from the RCA SCART sockets on something and that is what you get.
I hid the 33 ohm load resistors in the quarter inch jack plugs. They sound nice.

[Cobaltblue]

I have to say I agree with Tony.

Obtaining a suitable mains transformer from a Scrap Radio Chassis from the late 1940's shouldn't be too difficult.

Something that had Octal valves should be big enough.

Cheers

Mike T

[Radio Wrangler]

If you're going to build something, you have to consider the possibility that it doesn't go first time, and that (with a bit of help) you'll need to debug it.

This will mean making some voltage measurements with the thing running. With the voltages that valves run at, this involves some danger and care is needed.

The design you've posted runs a chassis connected to one side of mains and that is a big step more dangerous still. Most of us on here tackle these sorts of things, but we usually power it via an isolating transformer. Once you've got it going, the chassis is going to have to be mounted in a cabinet which provides insulation of a grade to which you are prepared to trust your life and those of your family and friends. Even the control knobs and their grub screws have to be safely insulated.

It is much easier to get a transformer and to provide the chassis with a solid earth connection to the plug.

The worst case risk is unlikely to happen, but if it did, boy would you give anything to nip back in a time-machine to now and change your approach.

These low gain amplifiers tie you to crystal cartridges because of their high output. Crystal piezo-electric materials are hygroscopic and take in water from the air in order to turn to sludge. There aren't many of them left and prices are moving in th inevitable direction associated with rarity. Another gain stage would allow the use of a ceramic cartridge, and there are more of these surviving, but supplies are still dwindling.

There have already been discussions on here about adding little transistor preamps so that magnetic cartridges (very low output, but still in production) can be used if you have a gentle turntable.

If you really want the sound of one of the Dansette amplifiers, then it would be awfully easy to suggest just getting a Dansette to restore, but that wouldn't be helpful, Dansettes for some reason have become particularly sought after and the prices have gone silly. But you could consider one of the machines made by Bush, Ekco, HMV and several other makes who built somewhat better players than Dansettes but are currently a lot more affordable.

Oh and Welcome!

David

[Herald1360]

I'll join the chorus for isolated HT. You'll get less hum issues that way too. I'd also suggest taking the tone control outside the feedback loop and move it to the input. No high voltage on the pot and no odd phase shifts to worry about.

A simple valve amplifier, it will work but be a bit on the hazardous side. I would start with a Mullard 3-3, just as simple and hi-fi to boot.

https://www.vintage-radio.net/forum/...ad.php?t=78559

[Croozer]

Wow ! Thanks for the prompt replies guys.

Point taken about the unnecessary risk of taking HT direct from the mains. My lazy assumptions from a start point of where old TVs derived their HT, and this being conveniently within the working parameters of an EL84. From memory such TV sets used a series heater string and avoided a mains transformer altogether, but in this case there are so few valves a heater transformer will be necessary, so it would be as well to use one with a suitable HT winding and benefit from the isolation.

The tone control was something of an afterthought- Herald1360, David, you are absolutely right - this is going to be trial and error rather than (in modern parlance) 'plug n' play' as my maths isn't up to design!

The motivation was because I was so impressed that a single valve in those old record players could deliver so much gain; my thought process was to add a single high-gain valve in front and try to reduce the impedence missmatch a bit with the aim of getting a result from the output of more modern technology. On reflection it might be that my enthusiasm was misdirected; I should have perhaps been more impressed with the output from those old cartridges!

If my fundamental assumption is flawed then the Mullard 3-3 is maybe the best course.

Again, thanks for taking the time to reply to my less-than-educated workings. At the very least it's improve safety!

[Radio Wrangler]

We don't like losing new members... it looks unprofessional!

David

[julie_m]

If you are going to build a valve amplifier, the hardest thing is going to be the transformers -- or "the iron", as pretentious idiots like to refer to them. (If anyone starts talking about "irons" in a valve amplifier, back away slowly right now .....) Think whether you plan to buy new (yes, companies are still supplying output transformers for valve amplifiers -- for the right price) or try to salvage something second-hand (a tape recorder that is mechanically Beyond Economical Repair will usually yield up an ECC83 and one of EL84, ECL82 or ECL86, together with speaker matching and power transformers).

Half an ECC83 driving an EL84 will, by similarity of characteristics, result in something a bit like a single ECL86. So maybe consider leaving room (and heater power) for one more valve; that will allow you a whole ECC83 per channel, just in order to have spare gain for emergencies. Better to be too cold and able to put on more clothes, than too hot!

And if you are going to build an amplifier which gets its HT supply straight from the mains, please consider taking the audio inputs through transformers, not just Y-rated DC-blocking capacitors. Modern equipment with low-impedance outputs will not mind looking into a low-impedance input. You're not a big manufacturing concern looking to save a few pence on each unit sold -- and if, absit omen, anything went amiss with the amplifier, the victim would not be just another one of tens of thousands of anonymous customers .....

[joebog1]

"Iron" is not that expensive!!!

http://www.vt4c.com/shop/program/mai...041&group_id=7

I know it is in UK and Australia, BUT it doesn’t need to be.

Being a "pretentious idiot" with about 50 years experience, I post a site that makes very very capable transformers, for small money.

I DO agree with using a mains transformer, Death is permanent!!!


Joe

[bikerhifinut]

I've posted this link before and can confirm from personal experience that this design does work and can sound very nice within its performance envelope.

http://rh-amps.blogspot.co.uk/2013/0...sion-2_26.html

You can replace the 317 device and 27 ohm resistor with a 270R resistor and the zener with a 2k2 resistor for a more conventional circuit which would work just as well. Which I'd personally recommend as you can always add the constant current sink and zener screen grid supply later once the bugs if any are ironed out. easy to reverse if you don't find any improvement.

Andy

[bikerhifinut]

http://iol.ie/~waltonaudio/pcl86.html

http://www.iol.ie/~waltonaudio/pcl86...hematicbig.jpg

That little amp piqued my interest, and I have 10 PCL86 that I bought for peanuts a couple of years ago. "Polam" polish jobs. You'd be welcome to a couple for the price of a pint to cover postage. And you can use recovered output transformers from old tape recorders/record players etc although I wouldn't condone stripping a repairable device just to get parts................... ever.
I'd also probably run that circuit with the output section wired in pentode as the power output is likely to be puny in triode, and try applying a bit of negative feedback. Tone controls if desired and many of us do could be added too. Personally I'd justify the use of an additional valve and make an active baxandall type.

A.

[Al (astral highway)]

Welcome !

Originally Posted by Radio Wrangler

If you really want the sound of one of the Dansette amplifiers, then it would be awfully easy to suggest just getting a Dansette to restore, but that wouldn't be helpful, Dansettes for some reason have become particularly sought after and the prices have gone silly.

Absolutely right ...they sell in a local 'hipster' shop here for £150-£180 - remarkable, given the tiny component count and low-fi output. I fix one of these regularly every few months as a favour for my friend who runs such a shop.

If you're committed to a three valve design, I'd say there's a strong case for looking for an established design as your first build... Also one where the results are (subjectively, to me, anyway ) going to sound a lot more pleasing.

Whatever you decide , you're in good hands here and if you heed all the safety advice you can safely enjoy any debugging , if , almost inevitably , it comes to that.

Safe and happy construction.

[Restoration73]

This works well
http://www.r-type.org/articles/art-007.htm

[Goodizzy]

Another option might be to take a look at the Mullard 3-3.

The great thing is that you can build them with second-hand OTs and they still sound fairly good. (With high-efficiency speakers) With the design down, you can then upgrade the "iron" at an economical pace.

Also, I don't find the term "iron" to be pretentious- and if it is, I question my broadcast friends' liberal use of it as they certainly have decades more "professional" experience then myself.

[Radio Wrangler]

On a forum where there are complete beginners as well as experts, I usually take the extra time to type a few more letters and write transformer when I mean transformer and inductor or choke when I mean those. In a field where the general population is ageing, beginners are terribly important people and clarity helps them.

David

[bikerhifinut]

Originally Posted by Restoration73

This works well
http://www.r-type.org/articles/art-007.htm

Oh that looks interesting.
It will be a sensitive amp I reckon with two gain stages, even taking into account the attenuation from the passive tone network. So suitable for a low output ceramic cartridge I would guess.
ECL86 are becoming a very rare breed these days though and when you do find them they cost eyewatering amounts. So I think I'd use PCL86 which is the same valve but with a 300mA heater. It is in the Mullard book as a 14.5V heater, some data sheets say a bit less. It could still work perfectly well at around 12.6V so this opens up the opportunity to use a UF86 which has a 12.6V heater. Might be tricky to source one but I have seen a few in various swapmeets/jumbles. Otherwise there are Octal pentodes with the same or similar characteristics at much better prices than EF86. Russian variants of the 6SJ7 spring to mind. I'd probably be building a stereo pair so I reckon I could use PCL86 and then wire the input pentode heaters in series if using 6.3V versions.

[davew001]

I am also a fan of the Mullard 3-3 design. The DC coupling between the starved EF86 configuration and EL84 make for a very sensitive design. Just 100mv input is required for about 3 watts of output.
In depth circuit info can be found here
http://www.electrojumble.org/DATA/Mu..._Amp-Mixer.pdf

[bikerhifinut]

It's good, it's proven and its sensitive as you say so can be used straight off a ceramic cartridge.
The only downside for me is the price of EF86's these days, they go for silly money even new eastern European ones.
However, I reckon if you don't mind using an octal pentode, you can use a 6SJ7 from Russia (or USA if you feel posh) which is similar enough to an EF86 as to not cause any aggro in the 3-3 circuit. I saw some on the "bay" a while back at around £2 each, you had to buy 5 though.........
A