EL34 power amp project

[bikerhifinut]

Hi Gents,
The stranger returns!
I eventually got off my backside and have now completed a pair of monobloc EL34 power amps.
Anybody here who also haunts the Vintage wireless forum will likely be aware of the project in painful detail!
It all started when I bought a set of danbury mains and output transformers and some steel chassis from richard for this project.
Along the way the project developed and after much head scratching and collating the rest of the parts and valves I decided on building a "Clone" of the classic Leak TL25+, rather than the usual mullard 5-20. The Leak has appeal for me on a number of fronts, mostly personal and emotional.

I realised that the chassis werent going to suit the job I planned, at least not without a bit of extra surgery and metalwork as they were drilled out for Octal valves and needed a slot cutting for the output transformer. My metalwork skills are very poor so when a pair of chassis turned up at Tonbridge audiojumble that would fit the purpose all ready drilled etc I got them and an accquaintance took the original chassis for his EL34 project. he's an engineer and the metal work didnt faze him.
So far so good.
I painted the steel chassis up, never done spray painting and i did have to rub one down and start again but i got the hang of it and 2 coats of etch primer, followed by 3 coats of "plasti Kote" enamel in Antique gold, followed by a lacquer finish got me a pair of chassis looking very like the Classic Leak jobs.
heres the photos of the chassis.

Andy.

[bikerhifinut]

transformers and valve bases mounted. Note I had to make "gaskets" so the mains transformer would fit the larger slot. Transformers mounted on isolation "grommets" for accoustic and electrical/magnetic isolation.

[bikerhifinut]

top view and underneath, mains input and heater wiring installed. Twisted pairs pushed as hard as I could into the side and corners, brought straight ut to each set of heaters. Sockets aligned so that heater wires come direct to the pins. Ignore the resistor on the tagboard its there because i was checking how they fitted on the tagboards.

[bikerhifinut]

PSU wiring, star earth, and the tagboards stuffed and mounted. Notice I used the same principle as that Used in the WD KT88 amp, each major stage ground taken back to a "star" wire between the 2 PSU cans. Hammond 5H choke, chosen because it was the best compromise between fit and performance specs.

[bikerhifinut]

Amp running, HT at output transformer CT bang on the nose at 451V. And a view of the finished article.

Andy.

[bikerhifinut]

Ok thats a potted pictorial history.
Valve lineup is EF40 pentode into ECC81 Long tail pair driving a pair of EL34. Rectification by GZ34.
I plumped for the EF40 because they are generally a third of the price of a New russian EF86, and way cheaper than a NOS from Mullard or other western manufacturer, Quirky B8A base but actually nicer to solder on to than a B9A. EF40 is the predecessor of the EF86 and electrically identical. If I had the urge I could also use a 6SJ7 octal there. Also cheap as chips for NOS and a straight swap in for an EF86.
ECC81 are RFT NOS, DDR made but those in the know will tell you that Telefunken had a lot of valves made "over the border". They're good valves.
EL34 are 2 matched pairs of Sovtek that I snapped up. I hope better than the Chinese stuff but who can say.
GZ34, the only area where I splashed out and am using used but good Mullards.
they aren't stressed in this design but I felt happier with known good uns.
No fancy resistors or capacitors, just cooking variety 1% metal films and white "coffins" for the power resistors. Coupling caps are Vishay roedenstein 630V polypropylene film jobs I got from the BVWS store and they are pretty darn good.
I am fairly agnostic about "special audio grade" parts and before fiddling about with stuff like that I rather would get the amps working properly and then have a base to start experimenting, if I felt the need. Likewise connecting wire is standard 0.6mm solid core, except for earth returns and Ht feed and that is wire stripped from a reel of twin and earth lighting cable. it's nice for bending and shaping into a loom, keeps its shape.
I did have a few issues during the build, mainly noise and hum issues and more annoyingly the mains transformers are both very noisy, they buzz a bit, although "wellying" them down well onto the chassis has got the mechanical noise down to a level where its not an issue from where I sit to listen. They also run very hot. I think they're borderline on the HT current at 150mA and the amp should pull about 145 mA with no Load. I may upgrade these later for a heavier duty set and give them an easier time. The heater loadings are quite light, at 3.5A from a 5 Amp winding.
A.

[bikerhifinut]

I got the hum down to reasonable levels but still not terribly happy and also it had (to me) a rather high level of hiss (partition noise from the pentode input?).
So I have wired the input valve as a Triode. Instant result! Global feedback adjusted to take into account lowered gain.
Amp now needs 500mV to get 25 Watts which is just about right for my purposes.
Before the mods I ran the amps up on the oscilloscope.
I got 25W at 125 mV in on a 1kHz Sine wave. Thats right for the leak design.
I then turned the level up and at clip I was observing 37watts into an 8.25 ohm resistive load! The EL34's were at this point probably being punished, so i Didnt spend any longer than I needed to note the output volts on the scope.
Impressive results i felt.
So how does it sound?
In my system, currently a Rega Cursa3 preamp fed by a rega DAC (CD and squeezebox touch) and the michell gyro/RB600/NWA retipped ortofon quintet black, Speakers are Kudos Cardea1.
The monoblocs sound very good indeed. Loads of bass grip, lucid mid band and smooth treble. I suspect its a bit rolled off in the treble but I like what it is doing. Response falls off at around 25kHz, by my crude scope testing.
I think the overspecified output transformers are helping with the bass response. The grip on the speakers is really good, but I may just be mistaking less actual LF input to the speakers as a tighter bass? The amp measured nearly down to DC on the scope and the output valves were REALLY unhappy at that point. I could hear the grid moving in time to the input signal. I may need to modify the LF cut off but in normal use its no problem.
So for now i am going to enjoy them before even thinking about tweaks.
Andy.

[Greg.]

Excellent write up, Andy and a really good looking retro style build. I love the shade of gold you have achieved. Without knowing otherwise, could easily be mistaken for Leak amps, albeit a little (pleasantly) shinier.

You should be proud of what you have achieved or, in the style of another forum I visit, shoddy! 😄

[bikerhifinut]

Originally Posted by Greg.

Excellent write up, Andy and a really good looking retro style build. I love the shade of gold you have achieved. Without knowing otherwise, could easily be mistaken for Leak amps, albeit a little (pleasantly) shinier.

You should be proud of what you have achieved or, in the style of another forum I visit, shoddy! 😄

Not the same one I visit Greg under the same pseudonym?
yes the resemblance to a classic Leak amp was deliberate. I have a bit of a passion for LEAK valve amps, sad I know. And this was the only affordable way I have of getting close to the sound of the TL25+, which is rare as unicorn poo and when you see them they are too expensive to justify a purchase.
I think the leaks when new were quite shiny, the patina of age is upon any original condition survivors. Although cough cough I am not old enough to remember them brand new

[Greg.]

If you publish a new build on Audio-Talk, the membership tends to refer to the build as 'shoddy' when it is clearly excellent