Build stereo valve amp

[Ian - G4JQT]

I have been tasked with building a stereo valve amplifier for display/use in the living room. (Not my idea, but anyway...)

Our present 1986 Technics amp claims 60W per channel, but that's more than we've ever needed. It is used 95% of the time for the sound from the TV - just using the output from the headphone jack (so the remote can adjust the volume).

Valve amp requirements:

• 20 - 40 watts RMS per channel is probably sufficient.
• Reasonable audio quality - we're not audiophiles, but I'll be in trouble if I get: "It doesn't sound as nice as the old one does it?"
• Needs to avoid any ridiculously expensive valves.
• Needs to have transformers that are obtainable, and preferably in current production. (And not those costing hundred of pound each either.)
• Apparently it's supposed to be a 'a talking point' so the valves have to be mounted on the top.
• Needs to dive our present 8 ohm speakers.
• Metalwork isn't my strong point so a design with a chassis layout will be a great help.
• Cost say up to £200.

Is that reasonable?

There are lots of designs available, but I'm sure for one reason or another there are some that are best avoided. I know there are valve amplifier experts here who could point me in the right direction.

Thanks.

Ian

[Herald1360]

Just a thought.......

If you hide a separate solid state subwoofer amp somewhere, you could probably come down to 10W or so per channel on the valve job which would save a bit and also make the output transformers less of an issue as they won't have to handle the lowest frequencies in the audio. Quad IIs were only 15W and have plenty of oomph for domestic use.

For £200 you might even round up a couple of distressed 5-10s to sort out.....

If you go for a pair of horn loaded speakers, 10W will blow your head off

[terrybull]

I totally agree with Herald1360. I have a Mullard 3-3 and it is quite loud, surprising so and as the name suggests that's only 3 watts. Even without the sub woofer Herald suggests, 10 watts will probably be more than sufficient. Don't think 10 watts will be 6 times quieter than your current 60 watts. It won't be.

What speakers will you be using?

The Mullard designs are very good and worth serious consideration.

[Sean Williams]

I built an amp a few years ago designed with 6V6/EL33 type valves - I think with a little junk box scavenging I only had to buy the output transformers - cost was around £100 all in.

It isn't the most powerful amplifier in the world, but certainly enough oompf to get SWMBO moaning.....

The thread is on here somewhere.

https://www.vintage-radio.net/forum/...t=68131&page=2

Sean

[mole42uk]

I have recently bought transformers from VVT which are very nicely made and not overly expensive.

[PE9ZZ_JO22KI]

It primarily depends on your taste. Or whim. If you don't have either, better stick to your fine Technics. Example: I came across a dual ECL86 amp (Hacker RV19) and now I'm hellbent on trying it. Puts out 10W or so. Do I really need it? Of course not! But it's just nice to have a go at it.

Tjerk, 9ZZ

[Diabolical Artificer]

As others have said the Mullard 5-10 is a good tried and tested amp. Theres also the Elektor version of this amp as well as a few other designs on the Valve Museum website.

For a very interesting looking amp using cheap valves, have a look at tri-comps 2C32 triode PP amp which you'll find on this forum in this section.

You may have or be able to find a second hand mains transformer for less than £30 on ebay, therefor your main expense will be the OPT's which you should be able to buy for around £100. As Richard says VVT are a good source for these. As is Ed Dinning here on this forum for all things iron.

You can get blank chassis off ebay for about £25, or ask a local engineering firm if they can knock you something up.

So, with a couple or so of big smoothing caps at around £20 you should be able to make either a couple of monoblocs or one stereo amp for around £200.

Look forward to seeing your project, Andy.

[k_yller]

hi
hmm.....mission impossible
but let's try to find any idea
20-40 W on cheap valves
PP on EL81 ->20W
PP on PL504 -> 40W
PP on PL36 -> 30W

anyway output transformers will cost over 120£ and it will be the most expensive parts next expensive part will be good looking housing

calculate yourself chances to go under 200£ (for good looking ready amp ) -check what u have in drawers

[k_yller]

new idea

for 10W output you will under 200£
PP on EL90 or russian 6P1P (electrically equivalent up to 250 V anode voltage) + russian 6z1P (chinese 6J1P or 6J1, but russian are much better) and are easy accesible
output transformators from vvt

valves 6 pcs. x 5£=30
output transformer=75
main transformer=20

[Ian - G4JQT]

Thanks to all for your suggestions. Two Mullard 5-10 amps are looking as if they will be more than adequate. And a nice classic design too.

There's a lot of design data for them so the only tricky bit is going to be obtaining a good-looking chassis when it's all done.

As it needs to all fit on one chassis, I'll need to do some research into beefing-up the power supply for the two amps - unless there's a very good reason to have two separate power supplies.

Ian

[mole42uk]

VVT have a stock transformer for this purpose.
Bluebell sell some Hammond chassis which are suitable - I have been happy with the ones in black painted steel.

[Ed_Dinning]

Hi Ian, the Mullard book has details of the transformers if a stereo pair is to be built as well as details of pre-amps and other auxiliaries that can be fed from the PSU. The 5-10 uses RC smoothing so it does not need an expensive choke.
Have a look at some of the adaptions on the web for this circuit; I believe the fixed bias mod that uses a neg supply for bias helps the sound as it removes the need for cathode bias components. The extra low voltage/ low current power is easily supplied.
In the absence of centre taped heater windings a 1K ww pot is easily fitted across the windings with wiper to E and adjust for min hum.
If you do this it is also easy to add pots to balance the drive as well, plus a meter on the panel to show valve balance.
Or will that make it look too hi tech?

Ed

[GrimJosef]

£200 will be tough if you want to use new mains and output transformers, valves, controls, PSU electrolytics etc. Second-hand valves aren't too hard to come by but as for the rest of the bits I'm going to make a suggestion which some may find upsetting viz. you could look around for an unloved valve stereogram and scavenge parts from that. The fact, like it or not, is that these are often considered incompatible with modern living, both in terms of their size (too large) and their styling (too old-fashioned). Provided you pick one which isn't rare or pristine you won't be hastening the disappearance of an endangered species. Indeed you may be saving the bits from going to landfill. And if you extract carefully the parts that you don't need (deck, cartridge, radio valves, speakers, maybe even the tuning capacitor) you might raise a pound or two by re-selling them. Just a thought.

Cheers,

GJ

[mole42uk]

Great idea GJ

[jay_oldstuff]

I did this a few years ago using the chassis form a large stereo Dynatron Radiogram bought for under £20 the cabinet having been re-purposed by the seller. I used the mains transformer push-pull output transformers and ECL82 and EZ81 valves i even re used the basic circuit as it worked very well, i just rebuilt it in my own chassis it produces about 8W per channel and is more then enough for the living room driving my KEF cadenza speakers. I do have the speakers from the radiogram and one day i will make some cabinets for them and see how they sound with the amp.

Jay

[AC/HL]

Some of the Bush radiograms have a useful looking stereo P-P amplifier and PSU, conveniently built on a separate chassis. Once again, little valued these days.

[Anthony Thomas]

Good idea Bill to use / repurpose otherwise uncherished parts.

I have a jukebox amplifier that has some decent transformers and some radiogram chassis that are gathering dust.

Tony

[Durn2014]

Try finding a copy of the Mullard Circuits for Audio Amplifiers ( First published in 1959) I have a very well used 2nd Ed copy which was used to build the 7 watt Stereophonic amp which is based on the ECC83 and ECL82 valves. I still have the amp and it provided a warm smooth sound into a set of WB HF 912 speakers all of which I still own. The amp has not been fired up for many years but I am sure that it still would function. The valves are still readily available but you may have problems obtaining such items as the output transformer and Mains Transformer. However the Mullard book is a great place to start giving a lot of constructional detail on a range of valve amplifiers.

[Richard]

Originally Posted by Durn2014

Try finding a copy of the Mullard Circuits for Audio Amplifiers ( First published in 1959)

This is available in PDF free here: -

http://basaudio.net/blog/wp-content/...Amplifiers.pdf

Richard

[Richard]

Originally Posted by Ian - G4JQT

Apparently it's supposed to be a 'a talking point' so the valves have to be mounted on the top.

• Metalwork isn't my strong point so a design with a chassis layout will be a great help.

Hi Ian

This article has all the construction details etc in it: -
http://www.r-type.org/articles/art-003f.htm

Have you considered a wood case, the only metal work needed then is a flat plate, I think some of them look very nice, and "softer" on the eye than a metal thing.
https://www.google.co.uk/search?q=va...w=1536&bih=770

Richard