EL34 power amp project

[joebog1]

A nice 807 amp Andy ?? they are superb sounding amps. I built one wired as triodes driving a 3k4 load. tightest sounding valve amp ever. The output transformers were hand made by me though, and I tend to make welding size transformers

Keep us posted
Joe

[bikerhifinut]

Right now the tools and instruments are packed away again as out of the blue we are getting a potential buyer viewing the house tomorrow.
After this power amp project I will be revisiting the preamp side of life.
I have a nice pair of preamplifier output transformers waiting to be put into use.
Yes you read right, output transformers for a preamp. They are used in a current kit and I quite like the idea of an ultra low output impedance pre that I can use long interconnect wires to the power amps with. And they can be configured to give a balanced output too if needed or desired.
Sit the monos behind the speakers drive them with a long cable from the preamp.
That will be another thread. So any comments/advice etc as a PM or in a new thread I think.
Thanks chaps.

Andy.

[joebog1]

Biker and I have lots in common !!! Owned over 50 cycles in my life, and have always loved audio.
As far as thinking goes heres a pic of my line transformers
12k primary 600 balanced or 150 single ended +23 dB level.
I usually hate lots of boxes in the setup, but a separate phono stage might be the go. Ive been dabbling with this for some time. We may have to compare notes!!
Joe

[bikerhifinut]

Just a quick postscript.
I have given up on the noisy mains transformers.
They're coming out and I am going to shell out on a pair of overspecified transformers from a reliable manufacturer/person. I'll be looking at a rewind of the originals for use in something else less demanding. (Thanks Ed for the words of wisdom).
These will have an extra heater winding of 9V at about 1A which will allow me to use regulated DC on the input and phase splitter valves. Overkill perhaps but if I ever think of circuit mods and Valve changes it will give me some options. I even thought about a 12V winding which would open more valve choices out.
Wotcher think Joe and others?
This ship is too good to spoil for a haporth of tar I really think the amps are that good.
Andy.

[joebog1]

I did DC heaters in the 6BW6 amp.
When I make just pre amps, they are always DC heated.
Be warned though, that DC heating in itself doesnt nullify the need for decent layout.
The old pencil and paper trick of actually drawing the layout is a good idea.
When you have the transformers rewound make sure you specify the DC current required for the heater circuit!!
Amps AC multiplied by .62 is the DC capability of the winding.
Although If you are conservative it wont matter too much.
The reduction of the current capability is because of the charging current peaks into the filter capacitor after the rectifier.

Joe

[bikerhifinut]

If you remember joe, i reckon i have good heater routing and layout, Theres almost indiscernible hum with AC heaters. i just figured if I can do it why not. And it gives me scope for future modifications.

[turretslug]

DC heating at least gives the option of using off-the-beaten-track low-level valves that don''t have the explicit anti-hum precautions of the EF86/ECC83 and their ilk but might otherwise be perfectly capable.

[bikerhifinut]

Got it in one turretslug. . Opens out a lot of options, and I am now thinking of specifying a 12V winding at say 1A which gets me into the world of some unpopular series heated signal valves. Just regulate down if they need less than 12V, otherwise AC or a simple pi filter. I have done the pi filter on a Kit Amp that I bought. The previous owner had done a popular modification to the DC heater on the input ECF80 by using Schottkies in a bridge. Unfortunately he never thought to check the output Voltage and due to the low Vf of a Schottky there was 7.5V across the ECF80 heaters. Unsurprisingly I had a valve gone noisy when I checked the amp over. The pi Filter (1ohm resistor in a CRC arrangement, adjusted on test.) fixed it. These Kit amps turn up from time to time and although generally they may be ok, I reckon by my experience they don't work properly quite often. usually by enthusiastic DIY'ers who half read an article on mods and then do a half assed one, in the process actually reducing the amps performance. My standard approach in these circumstances is to restore to completely original circuit status. Then test and measure. And only do mods if there's a point in it.
Going off topic a bit sorry guys.

A.

[bikerhifinut]

On the cusp of ordering a pair of bespoke transformers from "majestic" in Poole. They will make them with electrostatic earthed screens and If I want also the Copper band screen round the windings.
After a chat with their people on the phone about my requirements they assure me that a HT secondary rated at 200mA will be more than adequate. (The original Mullard 5_20 specifies 180mA and that includes capacity for a preamp and tuner, which I ain't going to do). I also specced 5V at 2A for a GZ34, 6.3V at 4A for a pair of EL34 (with 1A in reserve), and a 12.6V at 1A winding for the ECC81 driver and EF40 input valve. I figure I can either use a 1086/317 type regulator at 6.3V for both valves (0.5A DC) or run the ECC81 at 12.6V AC and the EF40/86 type at 6.3V regulated DC in which case I probably wont need a heatsink on the regulator as it will only be supplying 200mA.
Other options now include, amongst others, UF86 which runs a 12.6V heater (100mA series type but equates to 12.6V). This keeps a lot of options open. Just thinking about possible future mods/upgrades.

Andy.

[bikerhifinut]

I've done the deed!
We'll be living off beans on toast the rest of the month...........
I gave it a lot of thought and specified a 250mA HT, this cost me quite a bit more as Majestic will be using a higher quality lamination quoted as "Grain orientated M6.35" laminations. Apparently used in Audio output transformers. I also hope I have a bit of "Future proofing" as I now have an option to run KT88/6550 if I ever fancy it as the output transformers are 50 watt jobs and are very under run at the moment. but something tells me its this very fact that accounts for the excellent way the amps measure on the 'scope as I am not hammering the output transformer. But I'm probably hammering the EL34's as I got 37 measured watts into 8 ohms at clip, and no observed crossover distortion as far as i could see. Running a pair of KT88 at 40watts, they'll be well inside their ratings in Class AB.
It should run nice and cool and silent. Which was the aim.
I'll get back on that in 3 weeks when they arrive.

A.

[Brian R Pateman]

I have moved the interesting side discussion about transformer design to a new thread here in the Components and Circuits section.

[bikerhifinut]

The transformers have arrived today!
They are very meaty looking beasts, I reckon looking at the lam stacks about twice the weight of steel in them, a good omen i hope.
Going to remover the originals tomorrow and get em looked at and see if its possible to do something with them economically as they'd be fine in the next power amp I have planned as it will be using a lot less HT and LT current.

I wont get the amps running again until after WB, theres more to the job than a straight swap in and out as one secondary i specified as 12.6V which means a rectifier and regulator for the 200mA of 6.3 in the EF40. Easy enough. I cocked up in my spec though, I should have asked for 6.3-0-6.3 instead of 12.6V@1A and that would have got me my CT ground ref for the 12.6V ECC81 supply. Looks like a pair of 47R resistors is in order to make a CT ground reference, I'll float the DC heaters off that 12.6 supply.
Fingers crossed they are silent runners.

By the way I specced them so high on the HT current as they now will be up to running a pair of KT88 if the urge takes me to upgrade the amps, although I can't think why I would want to at the moment.

It's all been a very satisfying (even through the frustration) learning experience. I could easily have just trawled eBay and bought a pair of amps, but wheres the fun in that.

Andy.

[Diabolical Artificer]

They sound just the job, you could always dismantle them and put in a centre tap on the 12v winding as its bound to have been the last winding on. I guess the tricky bit is taking the copper band off, not something you want to do on transformer that cost £!!!

If you double up your smoothing/res caps and put them in series and as you said put two resistors across these in parallel that will give you a 0v tap for your DC regulated 12.6v. Are you going to use LM317/337's for regulation? If so I'd recommend using a small preset pot for R2 instead of a fixed R so you can adjust the OP. Different valves present a different load and its nice to get your DC smack on.

Keep up the good work, Andy #?

[Radio Wrangler]

If a transformer is noisy on low load, it will remain noisy no matter what the load power it is used with. So I'm not sure if Andy's transformers would be satisfactory in anything else used in a quiet domestic setting. I'm afraid they may just be expensive junk.

David

[ToneArm]

My noisy mains transformers (hammond 378X's) drove me mad for ages. In the end I used 10mm rubber bobbin mounts between them and the chassis with great results, mechanical noise is undetectable unless you stick an ear within an inch of the transformer itself. I think the same thing might work with drop-through transformers, although not so easy maybe. Maintain earth connection to the transformer core with a flexible wire if you try this.
Glyn

[bikerhifinut]

Ok chaps.
David is correct, I fear. I tried all the usual isolation/rubber mounting etc etc and I think It's possibly just a pair of bad uns.
They are Titchy when I compared them to the Majestics which are proper battleship build. I weighed them, 4.5kg against 2.5kg for the danburys. The actual windings look about 3x the physical size, bearing in mind I have an extra 50 VA on the spec, but will be under running the HT. Also, I found majestic to be very helpful when I explained what I wanted and the exact job I was using them for. I could have saved just over £100 by going for a 200mA design without the extra 12.6V secondary and they assured me that would run the amp fine, but I really wanted a belt and braces and a bit of upgradeability for the future as the chassis are roomy and the power valves are well spaced to allow KT88 cooling.
The lack of CT ground referencing is mainly just a slight nuisance, a pair of 47R would solve it nae problems. I have in the toybox one of mr Vellemans 1 amp regulated PSU (Lm317) which is very neat and small so I thought I'll use that instead of a veroboard lashup. But I will use the LD1085 regs I obtained last year off Keith, I think they are a bit better. O got a small heatsink for the reg too, That'll do it and for ease I am going to run the ECC81 as well as the EF40 off the 6.3 regulated DC. Oh and yeah the psu is adjustable via a trimpot so thats a result too .

A.

[bikerhifinut]

As an additional, I had to open the mounting holes a bit as the centres weren't quite in line with the pre drilled chassis holes but they slotted in fine. I opened out the Fibreglass bit of PCB gaskets I had made for the originals and used that to space the laminations off the steel Chassis. If the transformers dont vibrate then I wont need rubbery things.

Andy

[bikerhifinut]

Well the old adage, buy cheap, buy twice seems to apply.
That said, the price I paid and also the quotes i have had for other products from majestic compares very well with the competition, slightly more expensive, but you get exactly what you asked for as every transformer is custom wound to your specification.

Andy

[bikerhifinut]

I may have spoke too soon.
The new transformers are no better in terms of the racket they make on the chassis. I can hear them 12 feet away in a quiet room.
I am fairly sure I am getting a bit of bobbin rattle and there's a decent amount of lamination vibration, however that could be as simple as a physical transmission of bobbin rattle. And the top Caps seem to amplify the lamination vibe while muffling the buzz from the windings. Best way to describe the noise from the bobbin as similar to that heard from overhead HV lines on a wet day like a sort of "fizz" but not the sort caused by arcing I hasten to add.
The transformers are obviously made with care, very well vacuum impregnated also.
And the manufacturers are very approachable and if it appears that they are faulty they will fix them.
I am wondering if there's something I am missing here. I have used the best isolation technique I can muster.
But a little bell is ringing (or it could just be my tinnitus).
The chassis are made from ordinary Mild steel with a zinc plating and spray painted with acrylic enamel and lacquer. The transformers are a "drop through" design and I am wondering here if the strong field from the physically large primary winding is interacting with the steel chassis and somehow feeding back, notwithstanding my attempts at mecahnical isolation.
(Thick rubbery washers on each mounting point.) I did try with just the fibre washers supplied to break the magnetic circuit also but nothing seems to cut the annoying noise out.
If all else fails its going to be packing them up and back to majestics for a rework.
I feel that if I ask for them to be repackaged as an above chassis unit with side covers and fixing holes the same as the existing ones I will be able to retrieve the dogs breakfast that this project has become.
I have to admit that this issue has nearly reduced me to tears of frustration in the last few days.
Can I just say, I don't feel that its the makers fault as such. I accept that the odd product can sometimes fail or slip through a QC net. And they have been helpful all the way so far and there is no issue with me returning them to get the right solution. I can't help thinking it is something I have done or have failed to do.
I strongly feel that "drop thru" Mains transformers on steel chassis are a bad idea, based on this experience. I reckon If I get some thick 5mm or more "perspex" either opaque black or paint it to match and cover the drop thru slot and get the transformers made as "top mounts", given that the manufacturer will test them for vibes and noise before shipping out, It might be the best all round fix.

A.

[Radio Wrangler]

I've just used google images and tried Leak amplifier and all the transformers are upright.

Ditto Quad II

Dynaco ST70 has a drop through mains transformer, but ttheir monoblocks had them upright.

Radford.... all upright.

I begin to see a pattern.

I've used drop-through , but I always use aluminium chassis.

Upright mounting needing two bells and brackets and the need to protect wires between bell and chassis have to cost more... and those people would have saved money if they could.

Arthut Radford was also primarily a transformer manufacturer.

Hmmmm?

David