Valve amplifier build

[hifiltdstaffs]

Hi, I have wanted to try a simple valve amp for some time and have been considering a kit or sourcing the parts from old equipment to build one.

I like the idea of using old stuff for a number of reasons, not leas of all cost. I also enjoy tinkering with stuff.

I am capable with a soldering iron and can follow a schematic etc.

Recently I have been looking at old mono, valve record players on eBay such as Dansette and have been wondering about using the amp's out of 2 of these as a mono-block set up. Ideally I'd like to rebuild them onto a more attractive chassis and replace all the capacitors other components likely to have deteriorated with age with new.

There seems to plenty of these things offered for sale from various manufacturers in pretty rough cosmetic condition for very little money.
I could get 2 matching models with the same amp.

My question is, how good could I expect the sound reproduction from such amplifiers to be? The power output isn't of much concern as I don't listen to music loud anyway.

Thanks

Thanks

[GrimJosef]

It depends a bit on which ones you choose, but bottom-of-the-range ones are typically not very good. They will be single-ended, which puts them at a disadvantage (except in how cheaply they can be made) to begin with. They may also have limited, if any, negative feedback, and they will probably use a cheap-and-cheerful output transformer. These things make them prone to distortion, hum and noise and limited bandwidth, particularly in the bass.

You'd get a better result building something from scratch - say something from Mullard's Circuits for Audio Amplifiers selections of which are available here http://www.r-type.org/articles/art-003.htm.

Cheers,

GJ

I would forget the monoblock approach, you need two power transformers for a start. You can't beat the Mullard 3-3 design for simplicity and sound quality. Here is my interpretation https://www.vintage-radio.net/forum/...ad.php?t=78559

[hifiltdstaffs]

Thanks for the reply's.

I'm not too concearned about keeping the circuit 'as is', I'm really just trying to repurpose as many of the bits as possible from the old circuits. Especially the more expensive stuff like the transformers, then maybe build them into a different circuit.

I would start with a proven circuit, saves a lot of hassle.

[Radio Wrangler]

A dansette should be your last choice.

THey were very popular back in the day because they were amongst the cheapest and could be afforded by teenagers. So a whole generation has grown up automatically thinking 'Dansette' in association with record players. Now this generation are feeling reminiscent and 45s and LPs are 'in' again, so they want a Dansette. Consequently dansette prices have inflated far higher than the much better models of their era.

Odd reversal!

So look instead for something a bit more substantial by Bush Ekco etc. You'll get a better amplifier and cheaper, too.

For stereo you'll need two the same. The first one you can pick up whatever turns up at a sensible price. For your second one you'll be hunting a specific make and model. That can take time and cost more dearly.

Personally, I'd suggest just building one of the Mullard designs. Well proven, sure-fire workers. Known quantity, and about as good quality as that sort of amplifier ever was.

David

[Ed_Dinning]

And the 3-3 and its ilk have the advantage of a non-live chassis, unlike the Dansette's and similar.

Ed

[hifiltdstaffs]

Any ideas what sort of stuff I should look out for that might provide bits that could be used in a 3-3 circuit?

[AC/HL]

A redundant tape recorder would probably supply just about everything needed for a 3-3, although the OPT wouldn't be as good as a genuine one.

[hifiltdstaffs]

Doing a bit of reading up mains and output transformers for a 3-3 amp.

As a reference point I'm looking at a brand new MT which is sold specifically for these amps:

Stated output: 600v (300v CT 300v), 6.3v (3.15v CT 3.15v) and 6.3v.

So, in something out of an existing piece of equipment, would this transformer be suitable? https://www.ebay.co.uk/itm/VALVE-POW...4AAOSwgoNec0We

Stated output: 650v (325v CT 325v), 6.3v, 6.3v.

Just reading on the Wikipedia page for the EL84 and it states maximum power is 300v?
Also, I am trying to understand what the centre tap between one of the 6.3v outputs is for on the brand new secondary winding, is this to give the option of the using the output in different ways for other circuits and other valves?

I'm not necessarily looking to buy this transformer, more just trying to get an idea of how to look for them.

[hifiltdstaffs]

Originally Posted by AC/HL

A redundant tape recorder would probably supply just about everything needed for a 3-3, although the OPT wouldn't be as good as a genuine one.

I've thought about tape recorders and this would seem to be a good option.

When you say the OPT wouldn't be as good as a genuine one, what do you mean by genuine? Sorry, I am learning!

Cheers

[AC/HL]

Official Mullard spec.

[GrimJosef]

Originally Posted by hifiltdstaffs

... I'm not necessarily looking to buy this transformer, more just trying to get an idea of how to look for them.

We have a strict forum rule about not discussing individual eBay listings. But as far as mains transformers go:

1. Does it supply the voltages you need ? Easiest is just to read the numbers on the transformer, but things to remember here include the fact that the voltage is almost always what you'll get when the attached circuit is drawing the current that the transformer's rated for. If the circuit draws less current then you'll get more volts. The amount varies, but 10% extra volts when no current at all is being drawn might be typical. So if, say, you had a 300-0-300 transformer rated to supply 150mA, it might deliver 330-0-330 when no current was being drawn and 315-0-315 if you were only drawing 75mA from it. The output voltage also depends pretty much linearly on the mains voltage. So if the transformer's mains winding (called the primary) is rated for 230V but you happen to live somewhere where the voltage out of your wall is more like 241.5V (i.e. 5% higher than the transformer is built for) then 300-0-300 at full current will become more like 315-0-315.

EDIT: I should add that in the case of the EL84 valve it is very tightly specified, and running it at higher voltage than it's designed for can quickly get you into serious trouble. As at least one major amplifier manufacturer found out to his cost and embarrassment back in the day ! I'd strongly recommend not raising the high voltage supply by any more than a very few volts. If you have to then there are some Russian military near-equivalents to the EL84 which can withstand a bit more voltage.

2. Can it deliver enough current ? A Mullard 3-3 amplifier draws a little over 50mA from its high voltage supply. So two of them, for stereo, might draw 105mA say. Your transformer needs to be capable of delivering this. Normally the current rating is printed on the label, alongside the voltage, or is given in the associated literature. In the case of a second-hand transformer it may not be though. Then you either have to look at the equipment the transformer was powering previously, or make an estimate, based on experience and the odd rule of thumb. There's lots of experience here if you're not sure (and some of us have days when we're all thumbs ). If you try to draw more current than your transformer is designed to deliver then its output voltage will be less than you were expecting and, much more seriously, it will overheat and risk being destroyed or, worst of all, starting a fire.

3. Is it electrically safe ? Very old or mistreated transformers may have important insulating elements damaged. You need to be 100% sure that the mains voltage can't come into contact with anything it shouldn't.

4. One seemingly trivial consideration is whether the transformer hums or buzzes electromechanically when it's running under load. A surprising number do. If it's powering a microwave oven this won't matter much. But if it's running a hi-fi audio amplifier then any noise at all from the amp itself can slowly drive you crazy. I have a pair of very nice amps indeed that I hardly ever use because their mains transformers buzz. I am seriously considering whether to try to get them re-impregnated with varnish to see if I can shut them up.

Cheers,

GJ

[rontech]

Personally I would not consider a live chassis type. Certainly not for a beginner.

[Herald1360]

Simply(?) add a 1:1 isolating transformer to feed a live chassis amplifier and you're back into the (relative) safety zone......

[Herald1360]

Thinking about HT supply for a 3-3....

A simple 240V 25VA isolating transformer (about £20 new) silicon bridge and 47uF 400V reservoir will give around 300V on load at 50mA or so. Voltage could be dropped a bit if required by adding some series resistance between bridge + and the 47uF. No need for EZ80 so only one 6.3V supply needed. A 12VA 6V transformer should do that nicely- the centre tap to 0V (required for minimum heater induced hum) can be done using a couple of 47R resistors or a 100R wirewound pot across the 6V. (Called a "humdinger".)

[ionburn]

I would concur with others who have suggested one of the Mullard circuits. There would be the big advantages of being circuits that many are familiar with and parts are readily available. It would be easy to get advice, if required, whereas with a rarer circuit knowlege may be rarer. For a first amplifier project it is far easier.

Another point which would put me off is that many parts in old equipment may be iffy anyway. As we know much old equipment needs substantial refurbishment before it is safe to use. It takes time and test equipment to properly check used valves and transformers etc before re-implementation. My take again would be to use NOS or known tested parts for an initial project to build experience and confidence.

[hifiltdstaffs]

Plenty to mull over there!

Thanks for the input 👍

[GrimJosef]

If you did want to go with something 'used' then the chassis from a stereogram might be a better bet. You wouldn't then have to integrate two units which were designed to be separate. And any controls that came with it would be stereo ones. The additional complexity and the presence of a radio section (which you could just disconnect) tends to put buyers off, so prices may be lower, although the same thing also means that sellers are less inclined to think they'll sell, so they list them less often.

Again, depending on the make, the output transformers are likely to be closer to the budget end of the market than would be optimum for true hi-fi. But if there's room on the chassis then it's quite possible to upgrade a transformer to a nicer newer one when funds allow. You might well still need to replace some components (any carbon composition resistors that have drifted too far, capacitors which have become electrically leaky or, in the case of electrolytics, dried out) but you'd at least be working with a unit which was known to have worked once upon a time.

Cheers,

GJ

[Radio Wrangler]

An alternative view might be to look at whaich valve types are still in current production.

A look at the RS website as a guide to what is available, new, from a reputable distributor who supports what they sell showed these types:

12AT7 = ECC81 Dual triode, small signal aimed at RF uses
12AU7 = ECC82 Dual triode, aimed at general purpose uses
12AX7 = ECC83 Dual triode, very low power high gain audio
EL84 Power beam tetrode, 5W each 12W/pair very popular but are rated such that they often have a hard life. Not the most reliable. Higher gain.
6L6GC An older generation power beam tetrode, quite robust in a bigger package.
EL34 Later generation power beam tetrode, higher power, higher gain.
KT88 High power beam tetrode two would do 100W of audio if pushed.

Most current production valves are those needed by the valve hifi and guitar amplifier markets.

So include the 6V6 being a somewhat smaller cousin of the 6L6, about the same power as the EL84 but lower gain in a larger package... good reputation for reliability. Produced for some classic guitar amplifiers.

These valves you can buy, new, nowadays. Anything else means you have to find good ones second-hand hopefully in good condition. The snag is that supplies are running down and free markets mean increasing prices and possible trouble finding replacements in future.

Some rectifier valves are still made for the guitar market - they drive their amplifiers so hard to get distorted sounds that power supplies are pulled down and valve rectifiers produce a different effect than do solid state rectifiers.

Valves for radio purposes aren't currently made. Valves like the ECL86 which is a beam tetrode a bit like an EL84 in the same bottle as a triode a bit like half an ECC83 aren't made.

A lot of people think that the currently produced valves are inferior in reliability to those made in the day. There is some rationale behind this as modern producers don't have the turnover to develop their art that the old firms had.

A lot of people also think that currently produced valves don't sound as good as those made back in the day. You have to form your own opinion on this one. Their demand for irreplaceable, limited stocks forces the prices up, though.

My preference is to use new valves for new construction, and to save period valves for keeping the period radios going that they were made for.

This argues against the Mullard 3-3, and towards one of their more powerful designs.

It's a decision with no right answer, just a mixture of advantages and disadvantages, and inevitably ends in some sort of compromise.

David