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.