EL34 power amp project

[bikerhifinut]

Originally Posted by 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

Ah I understand now Greg!
I don't go there, I find this one, the Leak forum, and the Vintage forum to be quite plenty for me
Thanks for the kind words, it's always nice to feel encouraged, and this was my first scratch build of an amp of any size. Kits don't count here, someone else did the fancy thinking there! And I confess I took inspiration from the WD KT88 kit when planning the layout. Tagboard sub assembly and star earthing does it for me.

A.

[colin.hepburn]

Hi Andy
Nice Work Andy
Wish I could do it like that My J.E Labs Rebuild isn’t quit working out as planed that's the last time I use mirror acrylic

[bob orbell]

You have been a busy chap Andy, and it looks very nice, plus ,as Greg says, a good write up, I also think that it would be a good idea if WD could offer an EL 34 power amp. BOB

[Richard]

Hi Andy,

Yes indeed, a superb build (or should that be "well shoddy!!" ) and a great write up.

I think you're being a bit modest about the standard of your metal and paintwork! I had a late ST20 in good condition and the paint was laquered and deep looking when cleaned up with a gentle car polish. It's possible Leak changed paint type and colour faded over the years - yours look great

Yes it would be good to see KEL34 back in the range.

[colin.hepburn]

Originally Posted by Richard

Hi Andy,


Yes it would be good to see KEL34 back in the range.

How's about A WD EL34 SE Amp

[bikerhifinut]

Originally Posted by Richard

Hi Andy,

Yes indeed, a superb build (or should that be "well shoddy!!" ) and a great write up.

I think you're being a bit modest about the standard of your metal and paintwork! I had a late ST20 in good condition and the paint was laquered and deep looking when cleaned up with a gentle car polish. It's possible Leak changed paint type and colour faded over the years - yours look great

Yes it would be good to see KEL34 back in the range.

Indeed Leak did change their paint scheme and suppliers over the years. The paint got progressively "Cheaper" as time progressed. Cost engineering and at least money shaved where generally it didnt matter. Those old amps were usually meant to hide in the bowels of a cabinet. Think radiogram without the speakers and a hundredweight of 401 instead of a cheap BSR changer!

I'm going to throw my Pennorth in about EL34 amps and say there aint no point in a WD version. The difference between the KT88 amp and a decent EL34 amp would be so small as to not make much difference except cause stock issues and extra expense for Matthew.
The difference between 25W and 30 to 35 W isn't all that much in the real world.
I think that was part of the problem with the old WAD range. There were just too many different kits. In my opine Matthew has it right with the EL84 amp and the KT88 amp. And the EL84 circuitry is sufficiently different from the usual Mullard 5-10 clones to give it appeal.
Ok maybe a SE project, but what sort and really how many buyers would it attract?

Nothing to stop you getting some chassis from Matthew and stuffing them with your own bits. If Matthew would countenance it that is.

Thanks chaps for the compliments, I had a few frustrating moments but overall the problems I had were relatively easy to resolve.
I will say this however:
The danbury output transformer DB910 that I used is an excellent thing especially when only asked to shove 25W into 8 ohms as it is rated to be able to give 50W from a pair of KT88 at 500V HT (class A-B).
The mains transformer DB/VT 1038 is a bit weedy and runs very hot. I'd personally choose one that was good for at least 200mA per mono or 400 for a stereo pair to give some headroom. They also buzz like a demented wasp and nothing seems to really stop it. I dare not say more in case I get myself into trouble.
Suffice to say, I intend these amps as "keepers" in my system and will be replacing the mains transformers for something a bit more beefy and that will fit in the slots provided in the chassis. I have already made enquiries and £200 with carriage and VAT gets me what I want.
which will include a 9V winding at somewhere about 1A to use for regulated DC heaters on the signal valves.
I dare say the Danburys may not be so buzzy in a different chassis.
A.

[bikerhifinut]

Ok this is more a cry for help as much as anything else.
Anyone on "the other forum" will know about the saga of the EL34 build.
In a nutshell, circuit wise I think it's bloody brilliant. A "tweaked up" modern take on Harold Leaks classic circuit.
The issue I have is with mains transformers.
The original danburies were replaced with somewhat beefier jobs as I was hammering them at their design limit and I felt part of the hot running and vibes was due to this.
The trouble was that the replacements were as bad. Bells did ring. I sent them back to the manufacturer in Poole by their request as they were also baffled by the vibration issue. They tested them, said the noise was within "acceptable limits", gave them an additional vacuum impregnation and bake just in case, and sentthem back with a "simon frame" and a thick paper gasket between said frame and the base of the lam stack. The frame is basically a steel tray that the transformer sits on, bolted through to the Chassis.
I mounted it in the usual manner, using the fibre washers under the bolt heads on the top cap. the base I insulated from the chassis with some double sided foam tape.
All should be well you'd think but no. The darn things vibrate audibly and can be heard ten feet away in a quiet(ish) room. I tightened the bolts down hard but it just made it worse.
As I was about to hit the off switch i happened to touch a bolt head on the top cap. Ouch! Very hot and diagonally opposite another one the same. That one had no fibre washers as it is the one that carries the earth continuity from the top cap to chassis earth. (The transformer is supplied like that).
So is the vibration issue and the hot bolts connected in some way? Perhaps by overloading the laminations and primary windings?
Also, this is the first time I have used a steel chassis, and wonder if theres some sort of interaction going on even though the transformer makers tell me it shouldn't be an issue.
The WD kits I own and have owned are all built on Aluminium chassis and anything I made in the past with large E-I drop thru transformers was also on alloy top plates/chassis, and none of these have had a noisy transformer problem.
Everything else about the amps is excellent it's just the annoying (and now worrying) buzz from the mains transformers. I can only think I am committing an elementary error somewhere and just can't see the wood for the trees.

Andy.

[bob orbell]

Hi Andy, I had a similar issue with my first kit build, a WAD KAT 6550, at switch on there was a very loud thrum, and then the mechanical buzz, my cure was to remove all isolation mountings and bolt down hard. Worth a try. BOB

[Phil Y]

My KEL84 transformer is mounted on rubber grommets which makes it acceptable re noise. The transformer in my phono stage (Sowter)is also on grommets but is noisier and is worse some days than others.
All the old amps I have come accross, Leak, Quad etc with their wax filled transformers have been silent. I think it is modern manufacturing techniques that is the problem. I'm sure vacuum impregnating is safe and technically good but it simply does not compete regarding noise (in my limited experience).
I'm sure it is cheaper and easier for the makers than wax filling but given a choice of paying £180 for a noisy transformer or £220 for a quiet one, I know what I would choose.

I should add that this is all speculation on my part.

Phil.

[bikerhifinut]

Well these weren't cheap.
And Majestics in poole have been exemplary with their after sales and service.
Anyway to cut to the chase.
This morning I used fresh eyes and spotted the schoolboy error. I'd managed to "nip" the heater wiring under the 2 mounting bolt washers that were giving trouble. The resultant voltage fed via the erath lift resistor, cooking it well and blowing the 0.1uF bypass capacitor to bits................
So I fixed it.
Still vibrating.............
So "Andy's patent isolation washers" were used in an act of desperation. They are a hard rubbery substance and are actually tap washers from the plubing supplies place. Anyway the effect was amazing, I think a combination of slight compliance and lifting the frame off the steel chassis by about 3mm or so did the trick.
Theres a small residual "tizz" which I reckon is the windings vibrating. But I think its as good as it will get.
Phil Y I think you have a point, these days due to the safety regs apart from any other issues transformers have to be wound differently and its difficult to get a good tight wind round the lams and then encapsulate them.
I am going to try the "wax dip" with the original danburys as now I reckon it was the mounting issue that exaggerated the vibes.
And they will be useful in my long planned SE 6550 project.
Now to adress the "red plating" on the eL34 anodes. the transformer is over voltage as its got a 230V primary. the heaters are at 6.8 to 6.9V, our mains was 246V last night. And the Ht is way too high still so its a bit more resistance in the GZ34 anodes.
5U4Gs arent the answer on 2 counts. first of all the heater winding is only rated at 2A although I am not using the full VA rating of the primary. and secondly I need the slow voltage rise of indirectlyheated rects, my PSU caps are only rated at 500V which is fine for the 450V HT i should be getting. but until the el34's warm up the HT gets over 500V .

A.