Mullard stereo 5-20 build

[EF80TVVALVE]

Hi all,
For some time this has been on the list of things to do and I'm now getting round to starting it off. I thought I would make a thread to hear peoples opinions on the 5-20 or any tips and modifications that will improve the performance. I am intending to build the PSU onto a seperate chassis with the two amplifier circuits on another, HT will be provided by 2x GZ34 in parallel. I have had a bit of a mooch online but there is so much on these amplifiers that I thought it would be easier just hearing peoples different experiences and modifications with these amps instead. The most I have to hand is the original 5-20 circuit along with some additional papers with the ultralinear mod but other than that I'm interested in any other improvements that can be made.

Cheers!

[Sideband]

Golden rule....build the standard Mullard circuit first, get it working, use it and become familiar with it. Then if you feel it necessary, try some of the mods then check it again and compare. The reason for doing this is that if you build a 'modified' version and it doesn't work well, you don't know if it's the mods or something else that's wrong.

Personally I'd just build the original. Mullard knew what they were doing and the 5-20 has stood the test of time. Built with care using the best quality components (that means a good output transformer in particular...and they don't come cheap) it will work first time and you'll have an amplifier that stands out with the best.

[lesmw0sec]

I built the 5-20 many years ago. It was a very good amp. One secret being to use an output transformer correctly specified for the original design.

[ian_rodger]

Some years ago I built a 5-20 but with solid state rectification (4-20?). I then modified the front end based on Claus Byrith's suggestion in "Power Amplifiers with valves" (http://www.lundahl.se/wp-content/upl...fier_30wpp.pdf) although I did not change the output stage to fixed bias as he did. I can't recall much difference in audio quality, although the reduction in gain was useful for more modern line level sources. Its worth reading through his suggestions.

[G8HQP Dave]

There are other threads on here about the 5-20. It has quite high feedback, so can be difficult to keep stable unless you either build it exactly according to the Mullard layout (with an appropriate OPT) or accept the need to debug and investigate loop stability yourself.

It will have far too much gain for modern sources, so most people nowadays run the first stage as a triode and redesign the feedback.

Here is my take on it https://www.vintage-radio.net/forum/...ad.php?t=80273 listening to it now (to be pedantic listening to the sound from the 'speakers fed from it).

The output transformers are the important bit, not cheap (I think a shade less than £100 each).

[bikerhifinut]

Do you need 2 x GZ34?
The Leak Stereo50 and Stereo60 EL34 amplifiers managed with a single GZ34, granted it took the GZ34 to its limits but they were substantially similar enough in their output stages.
Alternatively, as you are considering 2x GZ34 I shall assume you'll specify a mains transformer capable of 4A at 5V for the heaters. (Ok 3.8A).
Why not save yourself the expense of a valve and an amp of heater current and use a 5U4G which would be up to the current draw of 4 x EL34. They are also an awful lot cheaper than GZ34 and there are some bargains out there of NOS russian ex military versions. The GZ34 is not a cheap valve and NOS or even used Mullards etc will cost you nigh on as much as the output transformer.
I'll agree with anybody who suggests using Silicon rectifiers as a quad of UF5408 will cost you pence rather than pounds or there are some really good encapsulated bridge rectifiers for beer money prices. And you arent restricted to 60uF reservoir capacitor off the cathode, whats not to like?
I am assuming you havent specced the mains transformer yet.

I agree with the advice to stick with the original circuit, after the PSU, it was designed in the days when high voltage electrolytics weren't readily available in higher values, and then you can tweak it about.
The triode wiring of the EF86 is an easy mod once you have built the amp and takes the sensitivity down to around half a volt which is much easier to live with and you can still get away with a pot in a box volume control.

Pay close attention to your earthing and layout, keep it spacious and keep it neat. This will pay dividends when the inevitable teething troubles occur and you need to trace faults.
I prefer to use a "star" earthing technique rather than busbars but both are valid as long as the golden rules are adhered to.
And this goes witout saying on this forum, avoid wasting money on boutique resistors and capacitors and any sort of fancy wire. Although i would counsel the use of metal film 1% resistors throughout apart from areas where you need high power wirewounds. I used the standard polywotsit 630V capacitors from the BVWS shop on my EL34 amp build and have never felt the need to "upgrade".
Output transformers are the thing to get right, mine arent probably the best out there but are rated to 50W and so they are pretty much well inside their performance comfort zone and I think it makes a huge difference.

mains transformers............ BEWARE! not all manufacturers can make a buzz free mechanically silent big mains transformer. I have gone through 2 sets and now use a custom wound toroid from a helpful forum member here.
There really is nothing more frustrating and annoying than a mains transformer buzzing away and distracting you from a quiet passage of music or just when you are listening late at night at low levels.
If the 2 gents don't mind me suggesting, why not consult Ed Dinning or Mike barker about a custom wind? You'll get sound and sensible advice. There may be other equally competent transformer gurus on the site so apologies for not knowing those members.

And finally, I know you are keen on a 5-20 design, but there are others out there. And one using a double triode first stage will not only keep the valve count down but could be a bit easier to tweak around later.
I confess to being a committed (maybe i should be committed!) fan of the leak valve amplifier designs, compromised in some respects they may be but they are good reliable commercial designs and give a good measured and sonic performance.
I built my take on the TL25+ (stereo version is the Stereo60), but used the 5-20 output stage, there's no real difference there as the 440 ohm cathode resistor used by leak is inside the 10% tolerance of the 470 ohm specified for the Mullard design.


Good luck, have fun, and apologies if i taught granny to suck eggs and theres loads of genuine practical assistance amongst the site.

Andy.

[ToneArm]

Curious why you want to make a stereo amp if you’re going to the trouble of having two chassis? Personally i’d do monoblocks, unless there is a performance reason for your choice. I’m not aware of any hum or noise resulting from the transformer placement, but who knows.
Good luck with the project, interested to follow it.
Glyn

[bikerhifinut]

I missed that bit tone arm, and I agree. it's also going to save a bit of money.
if you build a stereo job on a single chassis. It's perfectly possible to get a quiet hum free amp with careful layout and attention to earthing.
If you go down the separate psu/umbilicals route you are opening up a large can of worms and could end up creating more problems than you would solve.

i was going to do this with my monobloc EL34 amps but have changed tack, there were too many pitfalls and issues and safety concerns for my liking.

joe bog would disagree but I am sure he wouldnt mind my saying this.

Andy.

[bikerhifinut]

Originally Posted by EF80TVVALVE

F The most I have to hand is the original 5-20 circuit along with some additional papers with the ultralinear mod but other than that I'm interested in any other improvements that can be made.

errrr the 5-20 IS an Ultralinear design so you can't mod it to ultralinear.

A.

[joebog1]

Tonearm!! It could be that he is using two chassis for the same reason I am. WEIGHT!!
I built a beautiful single chassis stereo amp, but I needed 4 blokes to carry it ( well, two anyway ). It is a big problem if you use good quality iron. Good quality generally means larger than a normal stack, larger laminations in the first place so that larger wire can be used, reducing DC resistance. Power transfomers are similar, in that bigger is better making a stiff power supply that doesnt sag as badly at high levels, and of course big fat chokes to calm the HT.

Just my two bobs worth.

Joe

[bikerhifinut]

Mods. Would this not be better in Home Brew thread?
A.

[GrimJosef]

Originally Posted by bikerhifinut

... Why not save yourself the expense of a valve and an amp of heater current and use a 5U4G which would be up to the current draw of 4 x EL34. They are also an awful lot cheaper than GZ34 and there are some bargains out there of NOS russian ex military versions. The GZ34 is not a cheap valve and NOS or even used Mullards etc will cost you nigh on as much as the output transformer ...

There's a reason 5U4Gs should be cheaper than GZ34s. The 5U4G has a much larger forward voltage drop for a given current. The datasheets here https://frank.pocnet.net/sheets/127/5/5U4G.pdf and here https://frank.pocnet.net/sheets/035/5/5AR4.pdf indicate that for a single anode to pass 250mA the 5U4G needs 63V across the valve whereas the GZ34 needs just 16V. Hot Rox has pretty rugged Russian GZ34s for less than £20 each http://www.hotroxuk.com/sovtek-5ar4-gz34-rectifier.html (their picture is out of date - current production valves are in the standard sized bottle as used by Mullard). The cost of an extra one of those is going to be lost in the noise compared with the cost of decent transformers, and the mains transformer can be smaller of course since you won't need the extra voltage for the rectifier.

Cheers,

GJ

[AC/HL]

Originally Posted by bikerhifinut

Would this not be better in Home Brew thread?

Most 5-20 threads have been in this section, and it's definitely a vintage design, so on balance it can stay amongst it's peers!

[EF80TVVALVE]

Thanks for all the input guys! Sorry I haven't been on in a couple of days. To answer a couple of questions, I have a few spare chassis in the loft, and without wanting to spend too much on transformers etc I want to run both amplifier circuits from a single PSU. I do have enough chassis to build a psu and two blocks but they are fairly big and will look quite sparse with just a single amplifier circuit on each. The choice of using two rectifiers is just a personal one, I want to build it in a more traditional manner and I have access to lots of valves/rectifiers etc so pricing isn't so much a worry in this area. I must have been mistaken with some of the supplimentry info I have on it, the modification was on the UL feedback circuit itself to help increase the power output. I will take heed of the advice given and construct it to the original specs, I am in two minds about a seperate PSU, the reason behind it is so it can be moved away from any other equipment if needs be. I don't have much space and the phono stage sits next to my current amplifier and even picks up hum from that so it is more to do with inteference on other equipment as I don't have much room to play around with. I have seen triode strapping of the EF86 before and this is something I will consider as I have read about sensitivity of these amplifiers sometimes being an issue.

[qualityten]

There is an Audiokarma member who has made a small run of PCBs intended to simplify the wiring of the front end of a stereo 5-20 (the gain stage direct-coupled to a long-tail pair phase-inverter/driver stage).

The valve sockets are not mounted on the board, but directly to the chassis. I can PM the OP with the details, if interested, or he could search 'Jason's All-Purpose Stereo 5-20 Front-End board'. I have no connection with the seller, except as the owner of one of the boards for a future project.